vvEPA
United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600/8-80-035
Laboratory August 1980
Cincinnati OH 45268
Research and Development
Urban Stormwater
Management and
Technology
Case Histories
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Soeioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the "SPECIAL" REPORTS series. This series is
reserved for reports targeted to meet the technical information needs of specific
user groups. The series includes problem-oriented reports, research application
reports, and executive summary documents. Examples include state-of-the-art
analyses, technology assessments, design manuals, user manuals, and reports
on the results of major research and development efforts.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/8-80-035
August 1980
URBAN STORMWATER MANAGEMENT AND TECHNOLOGY:
CASE HISTORIES
by
William G. Lynard, E. John Finnemore, Joseph A. Loop, and Robert M. Finn
Metcalf & Eddy, Inc.
Palo Alto, California 94303
Contract No. 68-03-2617
Project Officer
Richard Field
Storm and Combined Sewer Section
Wastewater Research Division
Municipal Environmental Research Laboratory (Cincinnati)
Edison, New Jersey 08817
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily reflect
the views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
11
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FOREWORD
The Environmental Protection Agency was created because of increasing public
and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land
are tragic testimony to the deterioration of our natural environment. The
complexity of that environment and the interplay between its components,.
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution and
it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from
municipal and community sources, for the preservation and treatment of public
drinking water supplies, and to minimize the adverse economic, social, health,
and aesthetic effects of pollution. This publication is one of the products
of that research; a most vital communications link between the researcher and
the user community.
As the nation moves closer to its goals of clean water by implementing
programs that control urban stormwater runoff and combined sewer overflows,
the experience gained from operating full-scale control facilities becomes
invaluable. Using this experience in future planning and design is an
essential step in implementing cost-effective systems for urban stormwater
pollution control.
Francis T. Mayo
Director
Municipal Environmental Research Laboratory
m
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ABSTRACT
This report is the third in a series on urban stormwater and combined sewer
overflow management. It presents 12 case histories representing most
promising approaches to stormwater control. The case histories were
developed by evaluating completed and operational facilities or ongoing
demonstration projects that have significant information value for future
guidance. Essential elements of the case history evaluations cover
(1) approach methodology, (2) design considerations, (3) costs,
(4) effectiveness, and (5) environmental and socioeconomic impacts.
Eight of the case histories assess Best Management Practices (BMPs) and
expand the data base on source control methodology, focusing principally on
planning and storage alternatives. Special considerations are given to flood
and erosion control measures also having a dual benefit of stormwater
control. The project sites evaluated are Bellevue, Washington; Montgomery
County, Maryland; Lake Tahoe, California; The Woodlands, Texas; Orange
County, Florida; San Jose, California; Middlesex County, Connecticut; and
Boulder, Colorado.
The remaining four case histories evaluate the control of combined sewage
overflows and document a systems approach in applying unit process
alternatives. The effectiveness and unit costs of storage and treatment
processes are presented, together with evaluations of areawide and systemwide
integration of these technologies. Storage, the key element of an integrated
approach, can involve storage/wet-weather treatment or storage/dry-weather
treatment, or both. The project sites are Seattle, Washington; Saginaw,
Michigan; Mount Clemens, Michigan; and Lancaster, Pennsylvania.
This report was submitted in fulfillment of Contract No. 68-03-2617 by
Metcalf & Eddy, Inc., Western Reaional Office, under the sponsorship of the
U.S. Environmental Protection Agency. This report covers the period December
1977 to November 1979, and work was completed as of March 31, 1980.
IV
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CONTENTS
Foreword - i i i
Abstract i v
Figures ° xi
Tables xvi i i
Acknowl edgments xxv
c *• Page
Section —a—
1 INTRODUCTION 1
Urban Stormwater Management 2
State-of-the-Art Studies 2
Needs 2
Case Histories 3
Si te Search 3
Project Monitoring 6
Report Format ", 6
Metrics 8
Cost Index 8
2 SUMMARY 9
Best Management Practices 9
Soil Conservation Service 10
Selection of BMP Strategies 10
Nonstructural Control s 11
Low Structural Control s 18
Erosion Controls 27
Combined Sewer Overflow Controls 30
Storage. « 31
Treatment Processes 34
Integrated Systems 38
3 RECOMMENDATIONS 42
Combined Sewer Overflow Controls 42
Best Management Practices 43
Program Needs 43
Samp! ing Procedures • • 44
PART 1 - BEST MANAGEMENT PRACTICES
4 PUBLIC UTILITY APPROACH TO URBAN RUNOFF CONTROL
BELLEVUE, WASHINGTON 46
Approach to Runoff Control 46
Area Characteri sties 47
Probl em Assessment 48
Approach 50
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Section
CONTENTS (Continued)
Runoff Control Facilities 52
Sediment Control s 53
Permanent Runoff Control s 53
Costs.* 55
Storage Pond Costs 60
Operation and Maintenance Costs '. 61
Impacts 62
Environmental Impacts 62
Soci oeconomi c Impacts 63
SOURCE DETENTION OF URBAN RUNOFF
MONTGOMERY COUNTY , MARYLAND 66
Regional Approach to Runoff Control 66
Area Characteristics 66
Problem Assessment 68
Source Control Approach - Regulatory Requirements 71
Implemented Controls 72
Design Considerations 78
Performance of Stormwater Detention Ponds 82
Peak Flow Reduction 82
Pollutant Trap Efficiencies 82
Costs. 84
Watts Branch Management PI an Costs 84
Off site Detention Facility Costs 84
Cost Estimating 85
Impacts , 86
Environmental Impacts , 86
Soci oeconomi c Impacts 87
LAND USE PLANNING AND EROSION CONTROL
LAKE TAHOE, CALIFORNIA 89
Regional Approach to Runoff Control 89
Basin Characteristics 90
Problem Assessment 91
Countermeasure Philosophy (208 Planning) 95
Assessment of Land Use Planning 97
Approach to Land Use Planning 97
Description of the Project Sites 98
Economic and Environmental Impacts of Land Use Planning 105
Erosi on Control s 107
Description of Demonstration Project 108
Evaluation of Erosion Controls , Ill
Unit Costs of Erosion Controls 112
Socioeconomic Impacts. 116
Impacts on Private and Local Facilities 117
Public Acceptance 117
Aesthetics 118
v.i
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Section
CONTENTS (Continued)
MANAGEMENT OF A NATURAL DRAINAGE SYSTEM
THE WOODLANDS, TEXAS 119
Project Description 119
Site Development , , 120
Probl em Assessment , 120
Countermeasure Phil osophy ,. 123
Implemented Countermeasures 123
Performance , 125
Runoff Qual i ty 125
Effect of Lake Impoundment 127
Porous Pavement 129
Impacts , 129
Environmental Impacts 129
Socioeconomic Impacts 132
BEST MANAGEMENT PRACTICES
ORANGE COUNTY , FLORIDA 136
Program Description 136
Area Characteristics „ ..• 136
Probl em Assessment ,." 137
Countermeasure Philosophy ,.... 139
Operation and Performance 148*
Operation 148
Mai ntenance 150
Performance 150
Cost and Resources 151
Capi tal Costs 151
Operation and Maintenance Costs 151
Cost Effectiveness 154
Impacts 154
Environmental Impacts 154
Socioeconomic Impacts , 157
IMPROVED STREET CLEANING PRACTICES
SAN JOSE, CALIFORNIA 159
Demonstration Project 159
Area Characteristics 159
Problem Assessment 163
Countermeasure Philosophy 170
Operation and Performance. 172
Project Operation ,. 172
Performance 173
Costs and Effectiveness 182
Costs 182
Effectiveness 184
Impacts 186
Environmental Impacts 186
Socioeconomic Impacts 191
vn
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CONTENTS (CONTINUED)
Section
10
11
12
BEST MANAGEMENT PRACTICES
MIDDLESEX COUNTY , CONNECTICUT .................................. 1 92
Project Description ............................................ 1 92
Area Character!' sties ......................................... 193
Probl em Assessment ........................................... 194
Approach to Stormwater Control ............................... 1 94
Imp! emented Facil i ties ....................................... 1 95
Performance ........................................ • ........... 202
Vol lime Reduction ............................................. 203
Pollutant Loading Reduction .................................. 203
Costs [[[ 203
Impacts [[[ 205
Environmental Impacts ........................................ 205
Socioeconomic Impacts ........................................ 206
STORMWATER RUNOFF CONTROL
BOULDER, COLORADO .............................................. 208
Project Description ............................................ 208
Area Character!' sties ......................................... 208
Probl em Assessment ........................................... 209
Countermeasure Phil osophy .................................... 21 1
Operation and Performance. .... ................................. 220
Operation and Maintenance ........................ ............ 221
Performance .................................................. 221
Costs [[[ 222
Impacts [[[ 222
PART 2 - COMBINED SEWER OVERFLOW CONTROLS
INLINE STORAGE CONTROL
SEATTLE , WASHINGTON ............................................ 224
Control of Combined Sewer Overfl ow ....................... . ..... 224
Local Characteri sties .................................. . ..... 225
Combined Sewer Overflow Problems ....................... . ..... 228
Inline Storage Methodology ............................. . ..... 230
Implementation and Design Considerations ............... . ..... 234
Operation and Performance ................................ • ..... 234
System Operation ....................................... « ..... 234
System Performance ..................................... « ..... 236
Operation and Maintenance .............................. « ..... 238
Economic and Environmental Impacts ....................... . ..... 240
Costs of Inline Storage ................................ . ..... 240
Cost Effectiveness of Inline Storage... ................ . ..... 243
Mul ti use Benef i ts ...................................... . ..... 243
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Section
Page
13 CONTROL OF COMBINED SEWER OVERFLOWS USING
STORAGE/SEDIMENTATION - SAGINAW, MICHIGAN 248
Control System Development and Description ?•: 248
Area Character!'stics 249
Probl em Assessment 251
Recommended PI an to Control" Combi ned Sewer Overf1ows 254
Hancock Street Storage/Treatment Facilities 255
Desi gn Cri teri a 263
Performance and Operation 265
Storage/Treatment Performance 265
Operati on 268
Economic and Environmental Impacts 271
Economic Impacts 271
Environmental Impacts.... 272
Soci oeconomic Impacts 273
14 MULTIUSE COMBINED SEWER OVERFLOW FACILITIES
MOUNT CLEMENS, MICHIGAN 275
Project Descri pti on 275
Area Characteristics 276
Probl em Assessment 277
Imp! emented Countermeasure 277
Desi gn •• 283
Operati on and Performance « 284
Operati on 285
Performance ••• 287
Costs 289
Capi tal Costs 289
Annual Costs 29°
Impacts 290
Envi ronmental Impacts 291
Soci oeconomic Impacts 291
15 -SWIRL REGULATOR/CONCENTRATOR DEMONSTRATION PROJECT
LANCASTER, PENNSYLVANIA • 294
Proj ec t Descri pti on 294
Area Characteristics 295
Swirl Demonstration Project 295
Swi rl Regul ator/Concentrator Desi gn 301
Operati on and Performance
Operational Problems
Performance
Costs
Construct!'on Costs
Operation and Maintenance Costs 309
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CONTENTS (Concluded)
Page
REFERENCES 310
GLOSSARY 323
CONVERSION FACTORS 328
TECHNICAL REPORT DATA SHEET 329
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FIGURES
Number Page
1 Variation of annual solids removal with .
number of equipment passes 15
2 Variation of unit cleaning costs with number of passes.... 17
3 Storage pond construction costs, ENR 3000 22
4 Onsite stormwater detention pond costs, ENR 3000 23
5 Comparison of Seattle's inline storage
efficiency under three modes of operations 32
6 Projected cumulative pollutant removal efficiencies
of Mount Clemens combined sewage treatment facility 36
7 Location of Bellevue, Washington 47
8 Urban runoff problem areas, Bellevue 49
9 Storage volume design curve for the
100 yr storm, Bellevue 53
10 Temporary sedimentation ponds for construction sites 54
11 Permanent stormwater detention ponds 56
12 Offline detention system to control runoff
volumes from about 15 acres of road surface 57
13 Offline detention system 58
14 Instream stormwater storage 59
15 Storage pond construction costs, ENR 3000 61
16 Multiple stormwater facilities 65
17 Location of Montgomery County, Maryland,
and the surrounding Washington, D.C., area 67
18 Flow, erosion, and sediment deposition problems 69
xi
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FIGURES (Continued)
Number Page
19 Onsite sediment detention pond for
developing county service park 75
20 Proposed Crabbs Branch detention basin
to control up to the 100 year storm flow 77
21 Montgomery Mall Lake offsite
storage/detention pond 79
22 Proposed plan of the Wheaton Branch
off site'dry detention pond 80
23 Onsite stormwater detention pond
cost curve, ENR 3000 85
24 Multiuse stormwater detention facilities,
Montgomery County 88
25 Lake Tahoe Basin and developed areas 91
26 Stormwater problems in the Lake Tahoe Basin 93
27 Location-of the well planned and poorly
planned project sites in the Tahoe area 99
28 Results of land use planning at Site 1 101
29 Site 2 at Lake Tahoe 103
30 Results of uncontrolled development at Site 2 104
31 Comparison of sediment yields from a well
planned and a poorly planned development 106
32 Location of the disturbed areas at Site 2 108
33 Erosion control measures at Site 2 113
34 Conditions before and after implementation of erosion
controls at Site 2 114
35 Site plan of The Woodlands 121
36 Effect of lake impoundment on storm
flowrates and suspended solids concentration 128
37 Natural drainage system at The Woodlands 134
xii
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FIGURES (Continued)
Number Page
38 Landscaping with natural vegetation
at the Woodlands 135
39 Orlando and Orange County, Florida.. '.. .- 137
40 Soil capability of Orange County, Florida 138
41 Hydrology of Orange County, Florida 138
42 Diversion/percolation pond at 8 Days Inn 141
43 Total capture percolation facility 142
44 Swal e/percol ati on 143
45 Residential underdrain 144
46 Vacuum sweeping study area at Altamonte Mall....... 145
47 Size, efficiencies, and cost of
diversion/percolation basins , 147
48 Self-activatinq stormwater controls 149
49 Cost effectiveness of BMPs in Orange County...., 155
50 Cost comparison of percolation ponds
on well drained and poorly drained soils * 156
51 Multipurpose stormwater facilities in Orange County 158
52^ San Jose and the three study areas , 160
53 Typical streets in the five San Jose test sites 164
54 Keyes Street buffer zone and test sites..... 165
55 Sawtooth pattern of particul ate
deposition and removal 169
56 Variation of annual solids removal with ""
number of equipment passes 175
57 Total solids removal by particle size,
from various street surfaces 176
58 Cumulative loading distributions across
streets wi th di fferent surfaces 177
xiii
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FIGURES (Continued)
Number Page
59 Redistribution of total solids from street
cleaning in three different test sites,
averaged for all equipment types .. 179
60 Effects of parking and street conditions
on solids loading distribution 180
61 Effects of parking restrictions during street cleaning
on solids removal from two different street surfaces...... 181
62 Variation of unit cleaning costs
with number of passes 183
63 Variation of unit labor requirements
wi th number of passes 184
64 The variation of sediment quality
al ong Coyote Creek 188
65 Middlesex County, Connecticut 193
66 Schematic of stormwater control at
an industrial site, Middlesex County 196
67 Stormwater control facilities at
an industrial park, Middlesex County 197
68 Industrial stormwater control
using percolation, Middlesex County 200
69 Residential development with stormwater
control facilities, Middlesex County 202
70 Vegetation around sedimentation pond
supporting wildlife displaced by the adjacent
industrial site, Middlesex County 206
71 Landscaped multiuse stormwater detention
pond, Middlesex County 207
72 Boulder, Colorado „. 209
73 Boulder Creek basin 210
74 Potential flood hazard areas affecting a
portion of the developed area in Boulder „. 210
xiv
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FIGURES (Continued)
Number Page
75 A portion of planned stormwater
control facilities, Boulder. 213
76 Drainageways in Boulder with 4:1 slopes 214
77 Stormwater detention facilities, Boulder 215
78 Onsite detention facilities 216
79 Sediment pond servicing new residential subdivision
developed on a steep erodible hillside, Boulder 218
80 Percolation pond with grass-covered swale and
stone-covered bottom receives runoff from
a commercial area in Boulder 219
81 Multiuse stormwater facilities, Boulder 223
82 Seattle and surrounding receiving waters 225
83 Combined and partially separated service
areas, Seattle 227
84 Comparison of the relative impacts on benthic
sediments from combined sewer overflows and
stormwater runoff in Lake Washington.. 229
85 Regulator and pumping station facilities, Seattle. 233
86 Seattle's computer facilities 235
87 Reduction of combined sewer overflow volume, Seattle 237
-88 Comparison of inline system efficiency under
three modes of operation, Seattle... 238
89 Denny Way regulator station/landscaped park 246
90 Estimated costs and overflow volume reduction
for future system expansion, Seattle 247
91 Saginaw, Michigan 249
92 Normal distribution of annual precipitation, Saginaw 250
93 Probability of occurrence of rainfall
producing runoff 251
xv
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FIGURES (Continued)
Number
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
m
Page
Characterization of combined overflow suspended
solids, Hancock Street storage/treatment facilities 252
Characterization of combined overflow BOD,-,
Hancock Street storage/treatment facilities 253
Schematic of the proposed combined sewer overflow
system and the Hancock facilities, Saginaw 256
Components of the Hancock Street combined
sewer overf1ow control faci1i ti es
257
Regulator station with motor-operated sluice gate 257
Hancock Street flood control and combined
sewage pumping station 258
Flow schematic of the Hancock Street
storage/treatment facil ities 260
Hancock Street storage/treatment facilities.. 261
Hypochl ori te feed system 262
Hancock Street flushing system 264
Schematic of pollutant load reductions
and process elements of the Hancock wet- and
dry-weather integrated systems 269
Two-story parking garage over the
storage/treatment basi n 274
Mount Clemens, Michigan 276
Mount Clemens, combined and separated sewer
areas, overflow points, and control facilities ,„.. 279
Schematic of Mount Clemens combined
sewage treatment facilities 281
Mount Clemens retention basin site components 282
Park treatment facility for combined sewage 283
Projected pollutant removal efficiencies of Mount
Clemens combined sewage treatment facility 287
xvi
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FIGURES (Concluded)
Number
112
113
114
115
116
Paqe
Comparison of estimated pollutant loads to the
Clinton River before and after project implementation 288
Present state of the Mount Clemens project 293
Lancaster, Pennsyvlania 295
Swirl concentrator demonstration
drainage basin in Lancaster.... 296
Schematic of the Lancaster
117
118
119
120
121
Conceptual performance comparison of the
Hydrobrake and a conventional orifice
Schematic of Hydrobrake operation showing
Details of the Lancaster swirl
Lancaster swirl concentrator comoonents
299
300
300
302
305
xvn
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TABLES
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Page
Summary of Potential Candidate BMP and Combined Sewage
Overflow Projects Identified in the Nationwide Site Search.. 4
Range of Costs for Low Structural Source Controls 11
Comparison of Pollutant Concentrations in
Runoff from Several Land Uses 12
Macroinvertebrate Sampling Results Above and
Bel ow a Poorly PI anned Devel opment 13
Comparison of Urban Runoff Pollutant Loading from
The Woodlands and Two Fully Developed Urban Areas 14
Pollutant Removal Effectiveness on
Di fferent Street Surfaces 16
Annual Street Cleaning Costs and Labor
Requirements, San Jose 17
Response of Peak Storm Flow Through a Detention
System Designed for Specific Return Periods 24
Typical Design Requirements for Urban
Runoff Control Facil ities 25
Hydro!ogic Design Approaches for Stormwater
Detention Facilities
Description and Cost of Temporary Soil Stabilization.
26
28
Summary and Costs of Permanent Slope
Stabil ization Methods 29
Summary of Unit Costs of Runoff Management
Source Control s 30
Summary of Storage/Sedimentation
Activation Events, Saginaw, Michigan 34
xvm
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TABLES (Continued)
Number Page
15 Performance of the Saginaw Storage/Sedimentation Basin „ 36
16 Summary of System Costs, Saginaw, Michigan 38
17 Summary of System Costs, Seattle, Washington 39
18 Summary of System Component Costs,
Mount Clemens, Michigan 40
19 1974 Land Use and Projected Future
Land Use, Bellevue 48
20 Estimated Increases in Runoff from
Devel opment Over Predevel opment Rates 49
21 Estimated and Projected Nutrient Loads
in Kelsey Creek 50
22 Estimated Storage Pond Construction Costs,
Kelsey Creek Drainage System 60
23 Portion of the Annual Storm and Surface
Water Utility Billing Structure 62
24 Phosphorus Loads to Lake Sammamish Before
and After Diversion of Sanitary Flows 63
25 Estimates of Annual Phosphorus Loads to
Lake Washington , 64
26 Projected Changes in Land Use in
The Watts Branch Drainage Area '., 68
27 Estimated Annual Storm Flow Loads in the
Watts Branch Drainage Area 70
28 Pollutant Concentrations in Urban Runoff for
Several Land Uses in the Washington, D.C., Area.'.,. 70
29 Storm Pollutant Yields for Several
Land Uses in the Washington, D.C., Area.. 71
30 Summary of Stormwater Controls by Land
Use in Montgomery County 72
31 Summary of Planned Stormwater Controls by
Drainage Area Size in Montgomery County 73
xix
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TABLES (Continued)
Number page
32 Comparison of Onsite and Offsite Controls
For the Watts Branch Drainage Area 74
33 Design Features of the Crabbs Branch
Offsite Detention Facility 76
34 Expected Hydraulic Operation of the
Wheaton Branch Offsite, Dry Detention Ponds 80
35 Response of Peak Storm Flow Through a
Detention System Designed for Specific Return Periods 81
36 Comparison of HydroTogic Design Approaches
for Stormwater Detention Facilities 81
37 Summary of Expected Flow Reduction Performance for
the Crabbs Branch Offsite Detention Pond 82
38 Median Pollutant Trap Efficiencies, Montgomery
Mall Lake Offsite Detention Facility 83
39 Predicted Trap Efficiencies from Storms
of Varying Duration and Frequency 83
40 Estimated Basinwide Costs of Offsite Stormwater
Detention for the Watts Branch Drainage Area 84
41 Estimated Capital Costs for Offsite
Stormwater Detention Facilities 85
42 Physical Characteristics of the Lake Tahoe Basin 90
43 Increases in Primary Productivity Rates in Lake Tahoe 92
44 Comparison of Mean Runoff Water Quality for Several
Land Uses and Activities in the Tahoe Area 94
45 Summary of Erosion and Drainage Problem
Areas in the Tahoe Basin 95
46 Summary of Planned Land Use Areas at Site 1 99
47 Summary of Land Uses and Impervious Areas at
Site 2 Before Erosion Control Project 102
48 Average Instream Suspended Sediment
Concentrations Above and Below Site 2 ,. 1Q5
xx
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TABLES (Continued)
Number Page
49 Macroinvertebrate Sampling Results Above and
Below the Poorly Planned Development, Site 2 107
50 Macroinvertebrate Sampling Results Above and
Below the Well-Planned Development, Site 1 107
51 Description and Effectiveness of Temporary
Soil Stabilization Methods 109
52 Summary of Permanent Slope Stabilization Measures 110
53 Estimated Cost for Permanent Slope
Stabilization Methods 115
54 Estimated Cost of Temporary Soil
Stabil ization Methods 115
55 Summary of Unit Costs of Runoff
Management Source Control s 116
56 Estimated Cost of Various Revegetation Methods 116
57 Runoff Quality 126
58 Pol 1 utant Loadi ngs from Runoff 126
59 Water Quality Analysis of The Woodlands
Lake System During a 1975 Storm 127
60 Mean Values of Fecal Coliforms «, 131
61 Pollutant Loading and Concentration Comparison
Between Land Uses and Natural Areas in Orange County 139
62 Design and Implemented Facility Comparison 148
63 Performance of BMPs in Orange County.. 152
64 Capital Costs of BMPs in Orange County 153
65 BMP Operation and Maintenance Costs in Orange County 153
66 Comparison of Costs for Removing BODg
and Suspended Solids in Orange County 156
67 General Characteristics of the Three Study Areas 161
68 Surface Area and Land Use in the Study Areas....... 162
xx i
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TABLES (Continued)
Number page
69 Estimated Daily Traffic Volumes in the Test Sites 162
70 Pollutant Concentrations in Storm Runoff 166
71 Pollutant Strengths in Storm Runoff 167
72 Runoff Water Quality Parameters Exceeding
Recommended Beneficial Use Criteria 167
73 Sources of Common Street Surface Pollutants 168
74 Types of Available Street Cleaning Equipment 171
75 Street Cleaning Performance During San Jose
Demonstration Project 174
76 Removal Effectiveness for Various Pollutants
by Test Site 177
77 Annual Street Cleaning Costs and Labor Requirements 182
78 Cost Effectiveness for San Jose
Street Cleaning Operations ' 185
79 Comparison of Urban Runoff and Advanced Secondary
Treatment Plant Effluent at San Jose 187
80 The Importance of Factors Influencing
Fugitive Particulate Emission Rates 189
81 Fugitive Particulate Emission Rates for
Losses of Total Solids From Street Surfaces 190
82 Relative Fugitive Particulate Emission Rates of
Various Pollutants from Three Test Sites 190
83 Design Parameters for Industrial
Stormwater Facilities in Essex 198
84 Design Parameters for Residential
Stormwater Facilities in Haddam 201
85 Representative Pollutant Concentrations in
Stormwater Runoff for Several Land Uses 203
86 Estimated Annual Pollutant Loading Reductions
By Stormwater Control Facilities, Middlesex County 204
xx 11
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TABLES (Continued)
Number Pa9e
87 Summary of Cost Estimates of Stormwater
Controls, Middlesex County ,-•• 204
88 Characterization of Base Streamflow,
Stormwater, and Snowmelt Runoff, Boulder „.. 211
89 Effect of Peak Snowmelt Runoff
Concentrations on Boulder Creek 212
90 Suggested Design Criteria for Major
Grass-Lined Drainage Channels 219
91 Capital Costs of Stormwater
Control Facilities, Boulder ••• 222
92 Land Use Characteristics, Seattle.. 226
93 Combined Sewer Overflow Pollutant Concentrations, Seattle... 228
94 Average Sediment Pollutant Strengths
in Lake Washington 230
95 Combined Sewer Overflow Impacts on Local
Community and Beneficial Uses, Seattle 230
96 Inline Storage Potential, Seattle 232
97 Summary of Regulator Station
Modification and Construction Costs, Seattle 241
98 Total System Capital Costs for Inline
Storage and Sewer Separation, Seattle 241
99 Estimate of Inline Storage Operation
and Maintenance Costs, Seattle 242
100 Estimates of Annual Operation and Maintenance Costs
of Inline Storage Facilities Based on
Actual 1978 Figures, Seattle 242
101 Cost Effectiveness of Combined Sewer
Overfl ow Countermeasures, Seattl e 243
102 Summary of Fish Increases from 1967 Through 1970 245
103 Quality Characteristics of Combined Sewer Flows
Entering the Hancock Storage/Treatment Facilities. 254
xxi i i
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TABLES (Concluded)
Number
104
105
106
107
108
109
110
m
112
113
114
115
Page
Hancock Street Storage/Treatment Design Parameters 263
Summary of Storm and Basin Activation Events
During the Hancock Street Monitoring Period „ 266
Performance of the Hancock Street Sedimentation Basin 267
Summary of Heavy Metal and Other Pollutant Removals
from the Hancock Street Sedimentation Basin 267
Summary of Costs of the Hancock Street
Storage/Treatment System 272
Combined Sewer Overflow Characteristics
of Two Drainage Areas in Mount Clemens 278
Combined Sewer Overflow Treatment Facility
Design Parameters, Mount Clemens 284
Capital Costs and Unit Costs of the
Combined Sewer Overflow Facilities, Mount Clemens..... 289
Estimated Operation and Maintenance
Costs, Mount Clemens 290
Characteristics of the Solids Used in Developing Swirl
Design Relationships through Model Simulation 301
Swirl Regulator/Concentrator Effectiveness, Lancaster... 307
« 308
Summary of Swirl Facilities
Construction Costs, Lancaster.
xxiv
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ACKNOWLEDGMENTS
The cooperation and assistance of key personnel from the headquarters and
regional offices of the U.S. EPA, and all of the municipalities contacted and
their consultants, is gratefully acknowledged by Metcalf & Eddy. Special
recognition is deserving of those communities and municipalities and their
consultants whose systems were selected for case study evaluation. The
cooperation and assistance received from the City of Saginaw, Michigan, and
Mr. Robert C. Dust, Director of Public Utilities, for the monitoring program
on the Hancock Street storage/sedimentation facility is particularly
appreciated.
Especially acknowledged is Richard Field, Chief of the Storm and Combined
Sewer Section (Edison, New Jersey) of the U.S. EPA Municipal Environmental
Research Laboratory, Cincinnati, Ohio, and Project Officer, who provided
valuable guidance and assistance during this project.
This report has been prepared in the Western Regional Office of Metcalf &
Eddy, Inc., by William G. Lynard, Project Engineer, Joseph A. Loop, and Robert
M. Finn, under the direction of John A. Lager, Vice President, and E. John
Finnemore, Project Manager.
xxv
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SECTION 1
INTRODUCTION
Over the past decade, the technology available for controlling urban runoff
and combined sewer overflows has progressed from state-of-the-art
demonstration projects, designed to test the feasibility of Individual
treatment processes, to prototype facilities using an areawide systems
approach. The systems approach integrates several technologies to achieve
effective control over large service areas. However, even with the detailed
planning studies, problem characterization studies, and process evaluations
conducted for these facilities, information on the receiving water quality
benefits and cost effectiveness of the control facilities has been limited
and is nonexistent on a nationwide basis for the potential user community.
Many of our larger cities with combined sewer overflow problems have embarked
on massive receiving water restoration and cleanup projects, as a result of
the Federal Water Pollution Control Act Amendments of 1972 (PL 92-500) and
the Clean Water Act Amendments of 1977 (PL 95-217). Many communities are
also constructing small-scale urban runoff controls, often combining multiuse
purposes of flood and erosion/sediment control to reduce the impacts from
urban runoff on receiving waters. Although no technology-based treatment
requirements exist for storm and combined sewer overflows, federal funding
for the study, design, and construction of control or treatment facilities is
contingent on evaluations showing them to be cost effective, within the scope
of receiving water quality improvements or benefits received., In other
words, the marginal costs of a proposed control system must be consistent
with the expected marginal benefits. Costs should also include both
socioeconomic and environmental costs and should be compared either
quantitatively or qualitatively to pollutant reduction, water quality
improvement, and improvement in beneficial uses.
Nationwide costs of over $28 billion (ENR 3000) to abate pollution from
combined sewer overflows and achieve recreation objectives for receiving
waters in the country's 1,600 communities with combined sewers were estimated
in the EPA's 1978 Needs Survey [1]*. This estimate represents about a two-
fold reduction from the 1974 needs estimate because the 1978 survey
considered assimilation capacities of receiving waters and use of a mix of
technologies reflecting cost-effective solution planning. Both estimates,
however, were based on simplified assumptions of system performance, costs,
and effectiveness, and are subject to continued refinement as actual
^References are listed by chapter at the end of the report.
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operating data and information become available. To establish better
estimates, it is imperative that the cost effectiveness of operating full-
scale control/treatment facilities be evaluated.
This report, through a series of case histories, identifies the most
promising control alternatives, reevaluates the performance, and assesses the
costs and effectiveness of selected, constructed and operating urban runoff
and combined sewer overflow control facilities. Additional goals of this
study are identifying receiving water, socioeconomic, and environmental
impacts and benefits.
URBAN STORMWATER MANAGEMENT
The urban stormwater problem is characterized by surface runoff events that
are unpredictable and highly time and location variable. Control strategies,
therefore, need to be planned, evaluated, and designed subject to local
stormwater characteristics. Guidance for alternative approaches,
methodologies, and processes used on a nationwide basis provides a starting
point and first-cut evaluation for developing local control programs.
State-of-the-Art Studies
Nationwide evaluations of urban stormwater management approaches have been
documented in a number of reports sponsored by the EPA Storm and Combined
Sewer Section (Edison, New Jersey 08817) [2, 3, 4, 5]. Two of these reports
document the advancement of the state-of-the-art of urban runoff and combined
sewer overflow technology. The first report, URBAN STORMWATER MANAGEMENT AND
TECHNOLOGY: An Assessment, completed in 1974, presented a compendium of
information on control projects, unit processes, user assistance tools,
evaluation procedures, and problem characterization [5],
The second report, URBAN STORMWATER MANAGEMENT AND TECHNOLOGY: Update and
Users' Guide, completed in September 1977, presented an update and more
detailed evaluation of the original report [4]. Data and information on many
projects either under construction or just completed and not available for
the first report were incorporated in the update to expand and normalize
the data base, evaluate and screen approach methodologies and user
assistance tools, and present more detailed and up-to-date evaluations
of control system operation, performance, and costs.
Needs
Several major conclusions and needs become apparent after reviewing the
state-of-the-art advancement of the past 10 years:
• Although many demonstration and full-scale projects had been
implemented, performance information was limited, particularly
for control systems integrating several control strategies.
• The cost data base was insufficient for accurately estimating
full-scale system costs or for evaluating cost effectiveness.
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» Detailed information on the design, performance, and cost of
source control strategies was lagging behind the information
available on combined sewage overflow strategies.
e The link between receiving water quality/benefits and
stormwater abatement programs was available in theory only and
unsupported by monitoring data.
Establishing a credible link between receiving water quality/benefits
and abatement programs is the key milestone to properly evaluate the
feasibility and cost-effectiveness impacts, nationally and locally.
Trends in the application of stormwater technology solutions have been
identified from reviews of the state-of-the-art literature and existing
facilities. These solutions are generally considered essential and most
promising elements in areawide/systems control approaches and are evident in
both urban runoff source controls (low/nonstructural solutions) and combined
sewage overflow controls (end-of-pipe/structural and source control
solutions).
CASE HISTORIES
As a continuing process of updating and disseminating information on urban
stormwater state-of-the-art technology, a nationwide site search was
conducted to identify those completed and operating projects representative
of a most promising control approach. Most of the selected projects
presented as case histories have been identified in the prior technology
assessments [4, 5], and are reevaluated in greater detail here to take
advantage of recent operating experience. Since many of the case histories
dealing with source controls or Best Management Practices (BMPs) have not
previously been reported, this subject is covered more fully in this report
to advance the state-of-the-art knowledge.
Site Search
A nationwide site search was initiated with 135 letters of inquiry seeking to
identify potential projects for in-depth case study and evaluation. These
letters were sent to local and regional EPA administrators, 201 and 208
project directors, state environmental officials, state water resource
research institutions, county and city public works departments, and private
consultants. Each respondent was asked to nominate candidate projects for in-
depth evaluation under three general categories:
1. Completed and operating BMPs with potential multipurpose benefits.
2. New and ongoing BMP projects including demonstration projects and
studies.
3. Completed and operating combined sewer overflow facilities.
A total of 41 responses to the letter of inquiry were received, of which 35
projects were identified as potential candidates for case study evaluation.
These projects are summarized in Table 1.
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Table 1. SUMMARY OF POTENTIAL CANDIDATE BMP AND COMBINED SEWAGE
OVERFLOW PROJECTS IDENTIFIED IN THE NATIONWIDE SITE SEARCH
Project location
Type of
control8
Project description
Gaithersburg, Hd. > BMP
Montgomery County, Hd. BMP
Baltimore, Md BMP
Occoquon Watershed, Va. BMP
Virginia Polytechnic Institute, BMP
Blacksburg, Va.
Prince William County, Va. BMP
Lynchburg, Va. BMP
Orange County, Fla. BMP
Concord, Mass. - BMP
Wallingford, Conn. BMP
Middlesex County, Conn. BMP
1-84, Conn. BMP
Marlboro, Conn. BMP
Brooklyn, Conn. BMP
Minneapolis, Minn. BMP
Rochester, H.Y. BMP
Arlington Heights, 111. BMP
Des Moines, Iowa BMP
The Woodlands, Tex. BMP
Northcreek Lake, Montgomery Village - 5 acre lake,
controls peak flows from 2 and 10 year storms,
302 acre drainage area
Spring Lake - 5 acre lake (25 acre-ft permanent
pool), 130 acre drainage area
Porous pavement grasscrete 2 acre parking lot
Urban runoff control demonstration of several non-
and low-structural controls in a developing urban
area, (source storage and infiltration), developed
land use/runoff pollution relationships
Runoff coefficient studies with various porous
pavement materials (laboratory scale)
Lake Ridge residential development - five stormwater
detention basins, 1,000 acres drainage area
Infiltration/inflow study on portion of
combined sewer system and retention pond
Several constructed and operating controls -
percolation ponds, swales, detention/sedimentation,
fabric filters, underdrains, and street sweeping
Porous pavement parking lot at Wai den Pond
Debris/siltation basins, erosion and sedimentation
control plans for subdivision and industrial park
Percolation basin, detention/sedimentation basn'n,
and dry-well infiltration system serving industrial
and residential developments
Debris basins, siltation ponds, temporary and
permanent seedings, and detention ponds
Stream belt zoning regulations
Stream belt zoning regulations
Harriet Lake - vacuum street sweeping for nutrient
removal; Lake of the Isles - first flush diversion
of stormwater to sanitary sewer
Study of source control practices - surface flow
attenuation, porous pavement, erosion controls,
chemical use restrictions, infiltration/inflow
control, regulator improvement, and friction
reduction and hydraulic improvement
Twelve detention facilities planned or built, multi-
use, largest is a 100 acre-ft reservoir serving
717 acre drainage area
Land use control, detention storage, and floodwater
control
Newly constructed town using porous pavement,
natural drainage infiltration, detention storage,
land use planning
and
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Table 1 (Concluded)
Project location
Type of
control3
Project description
Denver, Colo.
Boulder, Colo.
Davis County, Vt.
Stateline, Nev.
South Lake Tahoe, Calif.
South Lake Tahoe, Calif.
Lake Tahoe Basin, Calif.
Lake Tahoe Basin, Calif.
San Oose, Calif.
Bellevue, Wash.
King County, Wash.
Philadelphia, Penna.
Mattoon, 111.
Kankakee, 111.
Madison Heights, Mich.
New Boston, Ohio
BMP Source storage projects - rooftop storage, mall
depression storage
BMP Ordinances controlling runoff, onsite
detention basins, rooftop storage, drainage
construction or improvements
BMP Countywide ongoing program - detention and
siltation ponds, and instream drainageway storage;
emphasis on multiuse.; local financing program
BMP Low structural erosion controls on unstable soil
on owner-operated land parcels
BMP Heavenly Valley Ski Area drainage - slope
regrading, revegetation, and drainage improvements
BMP Street drainage improvements and erosion control
practices
BMP Unpublished study for erosion control methods and
institutional modifications to reduce runoff
BMP Erosion control demonstration project Northstar
Ski area and Rubicon properties - slope stabiliza-
tion, revegetation, land use planning, and erosion
controls; collected receiving water quality data
BMP Demonstration project on street sweeping effective-
ness and receiving water quality
BMP Low- and non-structural controls - source deten-
tion ponds, instream storage, and storrawater
utility approach
BMP Low- and non-structural controls with multiuse
benefits and public educational program
CSO Inflatable dams creating inline storage in combined
sewer systems
CSO Diversion structures and conversion of sanitary
treatment plant to provide primary sedimentation
and disinfection of up to two times sanitary flow
capacity
CSO Offline storage/sedimentation with screening and
disinfection
CSO Inline storage with 190 acre-ft of storage,
disinfection, pumping, flushing facilities
CSO Modified trickling filters to allow use of
inflow as a substitute for filter recirculation,
1.7 Mgal/d flow
a. BMPs may also include source controls used on combined sewer systems to reduce flow
volumes or pollutant loads; CSO controls are structural solutions to combined sewer
overflow problems.
.5
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In addition, a number of combined sewage projects identified in prior studies
were reviewed, screened, and contacted for potential case study evaluation
[4]. These projects are summarized in the following:
• Saginaw, Michigan
• Seattle, Washington
• Mount Clemens, Michigan
• Lancaster, Pennsylvania
• Oil City, Pennsylvania
• Milwaukee, Wisconsin
• Racine, Wisconsin
Project Monitoring
Storage/sedimentation
Inline storage
Storage/transport, primary treatment,
biological lagoons
Swirl concentrator
Sedimentation/dual-use microscreening
Storage/sedimentation
Screening/dissolved air flotation
Because of the limited amount of operating data available for stormwater
control facilities, funds were set aside to perform small sampling programs
on selected facilities. The monitoring was designed to fill data gaps and
develop a base of information for our case history evaluation.
A monitoring program was conducted in Saginaw, Michigan, at the Hancock
Street storage/sedimentation facility to provide performance and operation
data. Eleven storms were sampled between May and September 1978, in
cooperation'with the city. Influent combined sewage flow sedimentation
performance and system pollutant load reduction were evaluated from the data.
Sample analysis was performed for suspended solids, volatile suspended
solids, BODc, COD, lead, zinc, chromium, total nitrogen, and total
phosphorus. Evaluations of influent flow volumes, hydraulic loading rates,
and effluent overflow volumes were also made.
Report Format
Twelve stormwater and combined sewage overflow projects were selected for in-
depth evaluation. Each case history is generally organized along the
following outline of topics:
• Project description
• Performance
• Costs
• Impacts
The case history reports or sections are compiled under two major parts in
this report. Part 1 contains eight case histories dealing with BMPs, and
Part 2 contains four case histories describing combined sewage overflow
control approaches.
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Part 1 - BMPs--
The BMP case histories cover subjects including: legislation/ regulation,
land use planning, enforcement and funding approaches, and application of low
and nonstructural source controls. Although many of the case histories are
not limited, to discussion of single subjects, several case histories describe
demonstrations of single technologies or are representative of applications
of source controls as unit processes rather than systemwide or areawide
control approaches. The case histories highlight the most promising
technology applications. The sequence and major topic of each case history
section are as follows:
1. Bellevue, Washington
2. Montgomery Co., Maryland
3. Tahoe, California
4. The Woodlands, Texas
5. Orange Co., Florida
6. San Jose, California
7. Middlesex Co., Connecticut
8. Boulder, Colorado
Approach methodology (public utility
approach)
Approach methodology (regional
approach)
Planning, erosion controls
Planning, urban runoff controls
Control description (detention)
Control description (street sweeping)
Control description (onsite private
controls)
Control description (flood/erosion
controls)
Part 2 - Combined Sewer Overflow Controls—
The sequence and general topic of the combined sewer overflow control case
histories is as follows:
1. Seattle, Washington
2. Saginaw, Michigan
3. Mount Clemens, Michigan -
4. Lancaster, Pennsylvania -
Inline storage
Storage/sedimentation
Storage/sedimentation/treatment
lagoons
Swirl concentrator
With the exception of the Lancaster case history, which describes a full-
scale demonstration of a unit process, the case histories represent a
description of full-scale systems approaches for areawide solutions.
Summary and Recommendations--
The Summary and Recommendations sections of the report not only present the
major findings of the studies, but also serve as a state-of-the-art
continuation, update, and guidance document to present overall technology
assessment for BMPs and combined sewer systems. The information represents a
compendium of experience from operating full-scale systems that can serve as
-------
guidance for future planning processes, establishing priorities of control
versus benefits, and cost-effective system selection.
Metrics
Units of measurement in this report are metric units (SI) with U.S. customary
units following in parenthesis. All values in the figures and tables are
presented in U.S. customary units for consistency with the previous state-of-
the-art reports.
A list of conversion factors is also presented at the end of the report for
the reader's convenience in converting values between the U.S. customary and
the SI.
Cost Index
All costs contained in this report are based on or have been converted to an
ENR 3000 base (1913 = 100). Where applicable, these costs have been
regionally adjusted by city and do not simply represent an average nationwide
base adjustment.
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SECTION 2
SUMMARY
A large number of small communities and almost every major metropolitan area
are implementing programs that use storage to control stormwater. Many
controls have already been constructed and are currently in operation.
Although storage is the most important element in a stormwater control
system, other methods are also representative of most promising technology
and have been applied individually to correct specific stormwater problems.
This section presents the results and conclusions of the 12 case history
evaluations and highlights the approach methodology, design considerations,
costs, and effectiveness of alternative control measures. The selected case
histories represent a technology update of the most promising control
approaches for both BMPs and combined sewer overflow controls for future
guidance.
BMPs are gaining national prominence as a cost-effective solution to urban
runoff problems and may also help to correct combined sewer overflow
problems. However, information on the design, cost, and effectiveness of
BMPs is limited in comparison with the information available on combined
sewer overflow controls. Therefore, the BMPs cover state-of-the-art
developments, including unit process approaches and specific control
alternatives. The major emphasis is on source storage alternatives.
The presentation of combined sewer overflow controls describes the most
promising full-scale operating facilities from a systems approach. Specific
information updating costs and effectiveness of unit process alternatives
provides a basis for developing an areawide control strategy that combines
various control measures. Again, the presentation focuses on storage as the
most important element of a systems approach.
BEST MANAGEMENT PRACTICES
BMPs control urban runoff volumes and pollutant loads at the source or in
upland areas rather than at the discharge point to the receiving water, where
more conventional treatment methods are used. BMPs, such as street cleaning
and sewer cleaning, can also be used to control source loads in combined
sewered areas.
BMPs are either nonstructural or low structural controls. Nonstructural
controls involve planning, modifying maintenance practices, and controlling
development or natural land conditions to reduce runoff or pollutant
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potential. Low structural controls use natural land features with minor
modifications or small, simple structures to control volume and pollutant
generation at the source.
BMPs can control runoff from developing and developed areas. Planning is
generally limited to new development, but maintenance practices and low
structural controls apply to both.
Source controls often have multiple benefits. In addition to providing flood
and erosion protection, the controls can improve the quality of receiving
water. The flood and erosion controls developed by the Soil Conservation
Service are the most common, and many of the controls and concepts used for
urban stormwater management have been developed from Soil Conservation
Service designs and criteria.
Soil Conservation Service
The Department of Agriculture Soil Conservation Service (SCS) gives technical
assistance to individuals, organizations, and local governments to control
soil loss and provide water resource management in urban and rural areas [1].
The types of controls the SCS has promoted reduce erosion/sediment, flow, and
flooding problems. These controls often have another benefit, stormwater
pollution control.
The SCS assists many local resource conservation districts and county
environmental protection departments. It provides a tangible benefit to
stormwater control programs by sharing and using technology on soils, soil
loss, and water resource management. In Montgomery County, Maryland, the
local SCS reviews drainage plans and control measures as part of the county's
permit issuing sequence for new development. The SCS also has compiled and
produced a cost study for source detention ponds and has helped develop
onsite stormwater management policy and guidelines [2].
In Middlesex County, Connecticut, the SCS assists in an environmental review
of proposed development and control facilities. It has provided input on the
distribution, quantity, and quality of the natural resources of the sites and
surrounding areas and their ability to support the proposed development. The
SCS also has prepared a handbook on the erosion and sediment controls used in
Connecticut [3]. The handbook discusses how to design structural and
nonstructural controls and the principles of erosion, sedimentation,
hydrology, site selection, and planning.
In addition to local assistance, the SCS has prepared a design methodology
for evaluating runoff volumes and flows from different land uses and soil
types [4]. This manual is useful in estimating and evaluating the effects of
land use changes and structural measures on hydraulic and hydrologic
parameters.
Selection of BMP Strategies
The selection of a BMP or a combination of BMPs depends on the area's
characteristics and the problem to be corrected. For example, street
10
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cleaning may be considered for the control of source pollutant loads
generated on urban streets. But, detention or detention/treatment may also
be required to control peak flow volumes or treat urban runoff flows that
contain pollutant loads from offstreet areas. The type of detention selected
(onsite, offsite, inline, offline) depends on the availability of land, the
type of drainage system (sewers or open channels), and the land use. Other
considerations needing evaluation may include the potential downstream
effects resulting from upstream BMP implementation, such as altered timing of
stored flow volumes.
The costs of BMPs should also affect the control program selected.
Nonstructural solutions usually cost less and may have levels of pollution
control similar to low structural solutions. The costs of land use planning
for new developments may be as low as about $490/ha ($200/acre) [5], Street
cleaning in impervious areas like parking lots can cost as much as $3,200/ha
($1,300/acre) [6], and studies in San Jose, California, indicate that street
cleaning can cost $8.70 to $9.30/curb-km ($14 to $15/curb-mile) [7]. For the
more common types of low structural controls, the construction costs range
from about $2,500 to $9,900/ha ($1,000 to $4,000/acre) as shown in Table 2.
Table 2. RANGE OF CONSTRUCTION COSTS FOR LOW STRUCTURAL SOURCE CONTROLS
Cost, $/acre
Source control measure
Detention/sedimentation basins
Percolation ponds
Swales/underdrains
Erosion controls
a. Per acre of gross controlled
b. Per acre of disturbed area.
Average
2,200
2,600
2,300
area.
Range
1,000-3,800
1,800-3,500
1,900-2,600
1 , 500-38, 000b
The effectiveness of BMPs can range from total pollutant capture for systems
capturing and percolating the total runoff volume from a development, to
pollutant trap efficiencies for detention basins as high as 90%. The trap
efficiencies depend a great deal on the design criteria, i.e., multiple storm
frequency design to provide adequate detention volume for different size
storms, and use of large permanent pools to hold pollutants [8]. Large
permanent pools provide continued settling and oxidation of organic material
after storms are over.
Nonstructural Controls
Nonstructural controls can be the least costly BMP for the control of urban
runoff since they usually involve modifying maintenance practices or
enforcing ordinances that control development and planning practices. The
effectiveness of these measures, however, is not well documented, and their
benefits are therefore determined more by an intuitive judgment.
11
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The most promising solutions for nonstructural control are land use planning,
street cleaning, and establishing a public agency organized to adopt and
enforce ordinances, conduct areawide control projects, and levy a stable and
equitable source of funding. These solutions are not meant to be applied
exclusively, but rather represent alternative elements available for use in
an areawide control approach depending on local characteristics.
Land Use Planning--
Land use planning should be used as a first step to prevent urban runoff
problems from occurring during development because urbanization upsets the
natural hydro!ogic and ecologic balance of a watershed. The degree of
change, beneficial or detrimental, depends on the mix, location, and
distribution of the proposed land use activities.
The potential pollutant concentrations in runoff from different land use
activities in the Lake Tahoe area are compared in Table 3. Generally,
increasing density and intensity of use creates higher pollutant
concentrations. Commercial land uses have the highest pollutant potential,
particularly for lead and zinc; the pollutant concentrations can be over five
times the concentration from rural or undeveloped land.
Table 3. COMPARISON OF POLLUTANT CONCENTRATIONS IN
RUNOFF FROM SEVERAL LAND USES [9]
Pollutant concentration, mg/L
Oil and
Land use
Rural /undevel oped
Low density
High density
Commercial
residential
residential
SS
50
600
250
770
Total N
0.
1.
0.
1.
2
2
7
7
Total P
0.
0.
0.
1.
1
7
8
3
grease
0.
0.
20.
33.
6
8
0
0
Storm pollutant load yields from the different land uses show similar trends.
In the Washington, D.C., area, nitrogen and phosphorus storm yields from high
density residential and commercial land uses are over 10 times the load from
rural or undeveloped areas [10].
The goal of land use planning as a source control is to limit land use
activities with high pollutant yields to areas of the development that can
support the intended activity and protect the receiving waters. Planning
therefore may involve limiting development on steep erodible soils and in
flood plains and retaining undisturbed areas and using them as buffers to
development.
12
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The approach to land use planning should have the following four elements to
balance the economic impacts of a planned development with the potential
environment benefits:
• Physical analysis of the site: identification of the environment--
soils, geology, slopes, drainage, access, and types of development
suitable for the site.
• Market analysis: identification of the public needs and interests
in the types of activities and facilities to be developed.
• Economic analysis: examination of the costs and profitability of
developing facilities on the land available.
« Regulatory requirements: coordination with local, regional, and
state agencies to conform with environmental legislation and
ordinances.
A comparison of a well-planned and a poorly planned development in the Lake
Tahoe area showed a 100-fold reduction in sediment yield from the site using
land use planning criteria [5]. This difference was also evident in the
stream benthic sampling results. Below the poorly planned development, the
macroinvertebrate community showed sharp decreases in density, number of
families, and diversity, as shown in Table 4.
Table 4. MACROINVERTEBRATE SAMPLING RESULTS ABOVE AND
BELOW A POORLY PLANNED DEVELOPMENT [5]
Date of
sampling
Jul 1975
Dec 1 975
Jun 1976
Oct 1976
Density,
Above
1,542
1,321
2,125
1,560
, No./m2
Below
267
277
1,652
19
Number of species
Above
20
19
14
14
Below
9
12
14
4
Species
diversity index
Above
2.50
2.25
2.15
2.21
Below
1.91
2.08
1.85
1.35
a. A measure of the relationship between the number of species
and the total biologic community population by the Shannon-
Weaver index: diversity = -£(Ni/N) In (Ni/N), where
Ni = number of species and N = total community population.
Corresponding sampling results below the well-planned development, in most
cases, exceeded the quality of the above stream sampling at the poorly
planned development.
In the planned community at The Woodlands, Texas, sampling results showed, in
most cases, that the runoff water quality, particularly for nitrogen and
phosphorus loads, was better than the runoff quality sampled at two urban
areas near Houston, as shown in Table 5.
13
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Table 5. COMPARISON OF URBAN RUNOFF POLLUTANT LOADING
FROM THE WOODLANDS AND TWO FULLY DEVELOPED URBAN AREAS [11]
Rank (Decreasing pollution
Pollutant
Suspended
solids
COD
Soluble
COD
Total
phosphorus
Kjeldahl
nitrogen
N03
Area
Mean Ib/acre-in.
Confidence limits3
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
1
P-30
43
±13
HB
19
±7
P-10
10
±1
HB
0.28
±0.12
HB
0.95
±0.10
WB
0.088
±0.032
2
HB
38
±5
P-10
14
+1
P-30
9
±1
WB
0.24
±0.66
WB
0.40
±1.10
HB
0.037
±0.013
3
WB
14
±74
P-30
13
±1
HB
4.4
±2
P-30
0.021
±0.007
P-30
0.30
±0.06
P-30
0.020
±0.018
4
P-10
8.2
±2
WB
9.5
±24
WB
4.1
±28
P-10
0.014
±0.003
P-10
0.28
±0.10
P-10
0.012
±0.008
Note: P-10, P-30 - The Woodlands
HB - Hunting Bayou (Houston)
WB - Westbury (Houston)
a. 95% confidence limits.
The cost of land use planning associated with environmental protection is
extremely difficult to separate and assess. For the well-developed site at
Lake Tahoe, the entire cost of the planning was attributed to runoff
pollution control, which amounted to about $1,900/ha ($220/acre).
Street Cleaning—
Street cleaning can effectively control street-originating pollutants (heavy
metals), and is moderately effective in controlling oil and grease,
floatables, and salts. It is less effective in controlling sediment,
nutrients, and oxygen-demanding matter [7]. Therefore, street cleaning
should be selected and tailored to mitigate specific problems and may be
required as one of a combination of measures to provide areawide runoff
control.
14
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Street cleaning is practiced in most areas of the country; however, it has
not been used as a BMP. Modification to or the development of a street
cleaning program can be a most promising source control.
The recommended elements in designing an effective street cleaning source
control program are as follows [7]:
1. Determine an allowable street surface residual loading from the
city's street cleaning objectives. These objectives are determined
by environmental, aesthetic, safety, and public relations
requirements to meet urban runoff load allocations.
2. Measure or estimate the long-term average particulate accumulation
rate on street surfaces. This will vary with the street surface
and the cleaning frequency.
3. Determine the maximum allowable effective days of accumulation and
then determine required combinations of cleaning interval and
cleaning efficiency.
The area characteristics (street surface conditions and accumulation rates)
and the street cleaning program (number of passes and cleaning intervals) are
more important for effective cleaning than the type of cleaning equipment
[7]. The cleaning effectiveness, as a percentage of the initial solids
loading removed, ranged from about 40 to 60% for asphalt streets in good
condition and from 0 to about 20% for streets with oil and screens surfaces.
The effectiveness in terms of the cleaning interval on an annual basis is
shown in Figure 1. The average removals for oil and screens and asphalt
streets (both in good and poor conditions) in the San Jose study are
summarized in Table 6.
50.000 i-
•2 40,000 -
30.000
20.000
" 10.000
SMOOTH ASPHALT
STREETS IN
eOOO CONDITION
J
10
100
1.000
PASSES, number/yr
Figure 1. Variation of annual solids removal
with number of equipment passes [7].
15
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Table 6. POLLUTANT REMOVAL EFFECTIVENESS
ON DIFFERENT STREET SURFACES [7]
Removal ,
Parameter
Oil and
screens Asphalt
Total solids
COD
Kjeldahl nitrogen
Orthophosphate
Lead
Zinc
Chromium
Cadmium
9
9
6
7
5
12
9
8
30-40
30-45
30-45
30-40
30-40
30-45
30-50
30-50
a. Includes streets in both good
and poor condition in residential
and commercial areas.
Detailed annual costs for street cleaning and all support activities in San
Jose during 1976-1977 are presented in Table 7. The units costs are
$9.25/curb-km ($H.88/curb-mi) [7]. A generalized cost curve based on the
number of passes and the condition of the street is shown in Figure 2.
Local Approach—
Many local agencies have limited budgets and manpower to implement BMP
programs beyond the plan review, permit issuing, and construction inspection
stages. Since most BMP facilities are implemented at the local level,
information on the use and effectiveness of areawide controls has lagged
behind the information on federally funded combined sewer overflow control
measures.
The City of Bellevue, Washington, has successfully operated a separately
funded stormwater utility that may promote the use of areawide controls. In
addition, the utility monitors and enforces controls on privately, owned
developments to ensure compatibility with the city's overall water quality
goals. This approach is one of the most promising organizational techniques
for controlling urban runoff.
Public Stormwater Utility—The advantages of a utility approach to stormwater
control are a stable source of funding and a centralized technical and
operational staff that deals only with water resource related problems.
Charges are made on all property, developed and undeveloped, based on their
contribution to the runoff problem. The organization of the utility places
utility inspectors, plan review engineers, water quality technicians, and
maintenance personnel under a single operation, rather than drawing on
personnel from a public works department, where stormwater control may have
lower priorities.
16
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0.25
0.20
» 0.15
0.10
0.05
Table 7. ANNUAL STREET CLEANING COSTS
AND LABOR REQUIREMENTS, SAN JOSE [7]
Costs Labor
Item
Maintenance supplies
Operation supplies0
Debris transfer and
disposal d
Equipment depreciation
Labor6
Sweeper operators
Maintenance personnel
Supervisors
Total
$/yra
97,000
30,000
67,000
32,000
338,000
183,000
83,000
830,000
Percentage Percentage
of total Man-d/yr of total
12
3 — —
8 780 13
3
41 3,400 56
23 1,200 20
10 650 11
100% 6,030 100%
a. ENR 3000.
b. Includes broom replacements.
c. Tires, fuel, and oil.
d. Front-end loaders removed interim piles from streets, and
dump trucks transported them (maximum 15 miles) to landfill.
e. Includes administration, warehouse, secretary, and overhead
costs.
ASPHALT STREETS
•ODD CONDITION
OIL AND SCREENS
SURFACED STREETS
«R ASPHALT STREETS
IN POOR CONDITION
10
100
PASSES.NUMBER/yr
Figure 2. Variation of unit cleaning costs
with number of passes [7].
1.QOO
17
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Several problems, principally socioeconomic, can develop when organizing a
separate stormwater utility. The public may not easily understand the
services that a stormwater utility can provide. The public can relate to the
services for water, sewer, and garbage collection; the benefits of stormwater
control, however, are not readily apparent. This can lead to potential
problems in developing a rate structure acceptable and equitable to the
public.
Other Organizational Approaches--Other approaches to stormwater control have
been set up and used as management and funding agencies to carry out
stormwater policy and projects. These include (1) using a county or local
government as the managing agency supported by the property tax base, or
(2) creating a special improvement district with authority to levy a property
tax for stormwater projects.
Davis County, Utah, evaluated these two alternatives and chose to make the
county government the managing agency because, under Utah law, the county has
authority over stream channels in both incorporated and unincorporated areas,
can levy property taxes, and would, with voter approval, have authority to
issue bonds for capital improvements. In addition, the county government can
use planning and zoning powers to pass ordinances regulating development for
urban runoff control [12].
The special improvement district, while having the operational advantage of a
separate governmental agency exclusively for runoff control, would not have
authority for land use planning and regulating development. Another concern
was that the creation of a new and separate agency may be a difficult
approach, politically.
Low Structural Controls
Low structural controls require few structural facilities or modifications
other than berms, earthwork, outlet devices, or minor modifications to the
land surface features to control urban runoff. They are applied at the
source or in upland areas of a watershed, and control runoff in new
developments or mitigate existing problems in developed areas.
The most common low structural control in the country is storage. Storage
attenuates peak runoff flows, treats runoff (detention/sedimentation), or
totally contains the flow in combination with another treatment process
(retention/percolation).
Most of the storage facilities are usually constructed onsite, where the
runoff or pollutant problems begin and are wet or dry ponds. Approximately
42% of all the low structural controls in Montgomery County, Maryland, are
storage ponds used to control areas of 0.4 to 2.0 ha (1 to 5 acres) [13, 14].
For larger control areas, up to 200 ha (500 acres), storage ponds are used
almost exclusively and are constructed offsite to control entire developments
or small drainage basins [14].
-------
Erosion controls and other types of storage can control specific stormwater
problems, where the site or the condition of the soils make these solutions
particularly attractive. The low structural controls selected should,
therefore, be based on design requirements or regulations, cost,
effectiveness, and conditions of the site.
Storage/Rate Control —
Storage controls the peak runoff rate from sites that have become more
impervious because of development. By controlling the rate at which stored
runoff is released, potential downstream damage including flooding,
streambank erosion, and damage to vegetation can be lessened. Estimates of
annual damages from storm flows in a 1,170 ha (2,900 acre) watershed in
Montgomery County, Maryland, exceeded $400,000, or about $370/ha ($150/acre)
of gross watershed area [15].
The peak flow reduction is a function of the maximum usable storage volume
and the release rate of the outlet control. Storage basins designed to
control a peak flowrate from a single design storm frequency, for example a
design based on a 2 yr storm, can reduce the peak flow over 90% on the
average for that storm. This efficiency, however, drops significantly (50 to
60% or less) for storms that exceed the design storm [16], The effectiveness
of storage may be increased by using outlet controls to detain flows from
several design storms so that peak flows from storms up to the 100 yr storms
could be reduced by 70 to 80% [17].
Rooftop and parking lot storage facilities have been used for small onsite
applications to control flows from highly impervious areas. Rooftop storage
detains water on roofs, releasing the volume through roof drains. Parking
lot storage is created by constructing depressions in the parking lot to
store excess runoff. The released flow is controlled by limiting the drain
capacity from the lot. Other methods of onsite storage include underground
storage vaults or oversized underground pipes, but they often cost more.
Rooftop and parking lot storage are highly feasible alternatives for detaining
urban runoff from small sites in developed areas, where land, may be
unavailable for the construction of detention basins. Since most roofs are
flat, watertight, and structurally designed to withstand loads greater than
ponded water, it adds very little to the cost of a building to provide for
rooftop storage. Similarly, parking lots can easily be designed to store
water and still function as parking lots with little inconvenience to people.
Storage/Treatment— .
Storage can control stormwater pollution by providing treatment or total
containment (retention). Retention usually involves such processes as
infiltration. Storage can be used onsite or offsite. Storage can treat
runoff in the following ways:
Facilities that totally retain storm runoff will have no surface
water pollution loading from that site. Infiltrating water can,
19
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however, pollute the groundwater, and an analysis should consider
the filtering effect of the soil, depth to groundwater, and
groundwater use.
• The flow-controlling properties of detention storage prevent rapid
changes in flow regime, which is a form of pollution. Increased
stream velocity suspends silt, stirs up bottom deposits, and
disturbs the habitats of organisms living in pools or in contact
with stream banks.
• A receiving water has a natural ability to assimilate and neutralize
pollutants and is degraded only when this ability is overwhelmed by
pollutant loading. Delaying the release of runoff and extending the
pollutant loading over a period of time helps the stream assimilate
the contaminants.
• Onsite storage keeps rainwater from flowing over urban surfaces that
are potential sources of pollution. Runoff forced to travel over
urban streets picks up a large quantity of street surface
contaminants.
• The decreased velocity of storm runoff caused by detaining peak
volumes means less channel erosion in natural streams and earthen
conduits. The lower velocities also mean a lower sediment-carrying
capacity.
• The detention of stormwater in a pond for a period of time results
in settling and decreases the particulate loading of the outflow.
Biological oxidation of organic materials also occurs in permanent
pool detention ponds. This can include aerobic and anaerobic
(benthos) treatment or stabilization.
Onsite Detention/Retention Basins--0nsite detention uses simple ponding
techniques in open areas to accumulate stormwater. The basic design elements
are a contained area that allows the stormwater to pond and a release
structure that controls the rate at which the runoff enters the downstream
drainage system.
Onsite detention ponds can be instream or offstream facilities. Instream
facilities are flow-through controls that are a part of the drainage system.
With instream storage, a weir, check dam, restricted pipe, or other outlet
control uses the existing storage capacity in the drainageway. Offstream
detention facilities are usually natural or excavated depressions or swales
into which high flow volumes are diverted during storms and later drained
back into the drainage system.
Onsite detention is used for new, small, privately owned developments or
individual sites, such as industrial complexes, because it is simple to
construct and offers one of the lowest cost alternatives for stormwater
control. Another type of onsite detention is the temporary sediment pond
used during construction to prevent sediment from disturbed or unvegetated
soils from entering receiving waters. Most local authorities throughout the
20
-------
country require similar temporary measures as part of their sediment control
regulations and ordinances.
Several commercial, industrial, and residential developments have used
retention basins to capture the entire runoff volume on the site and dispose
of the water by percolation. These measures are most appropriate for small
sites, usually less than 2 ha (5 acres) of impervious area. The application
of retention/percolation systems greatly depends on the percolation capacity
of the soil. The costs of these systems are equivalent to or higher than the
costs of onsite detention/sedimentation basins, but these systems can reduce
flow and pollution load by 100%.
A 3 ha (7.5 acre) light industrial site in Connecticut uses a series of dry
wells to capture and percolate the runoff from the building and parking lot
at an estimated cost of $16,000/ha ($6,600/acre) of impervious surface.
Another industrial development using detention/sedimentation costs $10,800/ha
($4,400/acre).
Offsite Detention Basins--0ffsite detention can control several developments
or small watersheds and has been gaining in acceptance. While the
construction costs of these larger basins are higher, the cost per unit area
controlled is lower and the level of control is often increased by designing
the facilities to control storms of up to 100 yr return frequencies.
In many areas of the country, developers contribute funds to regional control
facilities in lieu of providing onsite controls. Maintenance costs can be
reduced by placing the maintenance responsibility on a public authority for
one large facility, rather than operating an equivalent number of smaller
onsite basins on the same area. In addition, maintenance operations on large
publicly owned facilities have a better chance of being properly carried out;
the required maintenance on small, privately owned facilities is often
forgotten after they are constructed. The possibility exists, however, for
increased conveyance or drainage system costs to the offsite control
facility.
Offsite detention basins are usually constructed in a drainage system. In
Montgomery County, large dams are built across streams below development, and
pool areas are formed either by the existing topography or by excavating and
reshaping the topography [17, 18]. The outlets are engineered to control
several storm frequencies and most maintain permanent pools. The usable
storage volume is, therefore, provided above the permanent pool elevation.
In Bellevue, Washington, the city plans to construct offsite detention basins
within the drainage system and has adopted local ordinances that require new
development to control onsite runoff during and after construction. The
offsite storage facilities will control runoff in the entire city, including
existing developments. The types of offsite basins range from simple
drainage channel or culvert modifications to create storage, costing $0.35 to
$3.50/m3 ($0.01 to $0.10/ft3) of created usable storage, to more expensive
facilities where major excavation and outlets are required, costing $7.00 to
$23.00/m3 ($0.30 to $0.65/ft3) of storage [19].
21
-------
Offsite regional control of existing development and existing runoff and
pollutant problems is one of the greatest values or advantages of this
alternative. Constructing onsite controls on existing developments is
prohibitively expensive and might encounter public opposition.
Construction Costs--
The cost of constructing onsite or offsite storage facilities depends on the
natural features of the proposed site. Storage basins created from natural
depressions, or in wetlands requiring only minor control structures and
limited earthwork, are up to 10 times less expensive than basins that needed
major excavation, as shown in Figure 3. The cost curves, developed from
planned offsite detention basins in Bellevue, show distinct scales of economy
in unit-cons true tion~costs for basins with storage-capacities ranging from
7,000 nr (250,000 ft3) to 113,000 m3 (4,000,000 ft5) [19].
.•Or
l.so
0.40
*•>
CD
CJ
- i.SO
u
8.20
• .10
-STORAGE FINDS REOUIRIN6 SUBSTANTIAL EXCAVATION.
EIIANKHENT, AND SPILLWAY WORK,
-STORAGE PONDS CREATED FROM EXISTING
WETLANDS AND NATURAL LOW AREAS
_L
JL
500 1000 1500 2000 2500
TOTAL STORAGE CAPACITY. 1000
3000
3500
4000
Figure 3. Storage pond construction costs, ENR 3000.
The cost of the land for offsite storage facilities should also be considered
in selecting a storage alternative, particularly if the land is not publicly
owned. In Bellevue, an estimated 86 ha (212 acres) would be needed to
construct detention basins in one watershed at an average cost of $54,000/ha
($22,000/acre) [19].
22
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A cost curve for estimating the construction cost of detention ponds with
storage volumes from 28 to 28,300 m3 (1,000 to 1,000,000 ft3) was developed
by the SCS using actual construction costs in Montgomery County, Maryland,
and is shown in Figure 4 [14]. These costs are representative of small
onsite detention ponds and are about 2 to 3 times the cost of offsite
detention ponds created from existing wetlands and natural depressions.
100.000
- 10.000
v>
•*.
o
C9
1,000
1.000
10.000
100.000
1.000,000
•ETENTION STORAGE VOLUME, ft3
Figure 4. Onsite stormwater detention pond costs, ENR 3000 [14].
Estimated costs of onsite retention/percolation facilities range from $10.60
to $15.90/m3 ($0.30 to $0.45/ft3) of storage capacity provided. These costs,
developed for the Florida area, apply to small facilities controlling up to
about 12 ha (30 acres) of contributing watershed area [20, 21]. The size of
.the basins ranges from 500 to 4,500 m3 (18,000 to 160,000 ft3) with an
average depth of 1.5 m (5 ft).
Operation and Maintenance Costs--
Information on the annual operation and maintenance cost of source detention
facilities is limited because many are small, privately owned onsite basins;
the accounting of costs has been incomplete; or the facilities have not been
maintained after their construction. Estimates of operation and maintenance
costs from several facilities, however, indicate the annual costs for low
23
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structural source detention. In Florida, the annual operation and
maintenance costs for retention/percolation systems ranged from about $740 to
$l,500/ha-yr ($300 to $600/acre-yr) of service area.
Operating and maintaining large offsite facilities can result in annual cost
savings over the use of many small onsite facilities. For example, the
estimated operation and maintenance cost for six large offsite detention
basins controlling a 1,170 ha (2,900 acre) area in Montgomery County,
Maryland, is $84,000, or approximately $72/ha-yr ($30/acre'yr) [15]. This is
at least 10 times less than the costs for the small onsite facilities in
Florida.
Design Guidelines—
Design guidelines for low structural controls vary from region to region and
can also vary within a local jurisdiction depending on the problem being
solved, i.e., stormwater quality control or flood control. Most local
jurisdications, however, have ordinances that require both urban runoff
controls for all new development and designs that limit runoff from the
development to predevelopment rates.
Storm Frequency—A minimum design requirement for control of a 2 yr storm
flow is common in areas that have comprehensive ongoing control programs.
Outlet controls on large offsite detention facilities can handle up to the
100 yr storm flow [17]. These large facilities protect downstream receiving
waters from peak flowrates more than would an offsite facility designed just
for a 2 yr storm. The impact of different storm frequency designs on flow
control performance is shown in Table 8.
Table 8. RESPONSE OF PEAK STORM FLOW THROUGH A
DETENTION SYSTEM DESIGNED FOR SPECIFIC RETURN PERIODS [22]
Level of control/design
Storm flow return
period, yr
2 10 100
Predevelopment flow, ft /s 9 29 77
Post-development flow through
design control, ft3/s
2 yr storm released at 9 22 42
2 yr predevelopment rate
10 yr storm released at 14 29 57
10 yr predevelopment rate
10 yr storm released at 6 9 27
2 yr predevelopment rate
100 yr storm released at 15 33 77
100 yr predevelopment rate
24
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Design requirements specifying maximum runoff or release rates from storage
have also been used [23], Typical design requirements for controlling urban
runoff are summarized in Table 9.
Table 9. TYPICAL DESIGN REQUIREMENTS FOR
URBAN RUNOFF CONTROL FACILITIES
Location
Design requirement
Bellevue, Washington [23]
Montgomery County, Maryland
[14,.15, 17]
Orange County, Florida [24]
Middlesex County, Maryland
[25]
Boulder, Colorado [27]
Limits runoff to predevelopment rates,
maximum runoff or storage release rate
at 0.2 ft3/acre-s, storage requirements
based on 100 yr storm.
Limits runoff to predevelopment rates,
2 yr storm frequency design. In
practice, regional offsite detention
facilities also provide controls for
the 10 and the 100 yr storm frequencies.
Detains first 1 in. of runoff and release
at specified rates for flood protection.
Has no specific design criteria for
control facilities, but requires
controls during and after construction.
Drainage facilities are designed for
25 yr storm. In practice facilities
have been designed for the 50 and 100
yr storm.
Limits runoff to predevelopment rates.
Has no specific design storm require-
ments other than that drainageways
must accommodate flows from the 100
yr storm.
Wet Versus Dry Ponds--Selecting a wet or a dry pond design depends on many
factors--desi gn purpose (pollution control/flow control), public safety,
maintenance difficulties, land area requirements, and appearance. In
addition to functional considerations, many of these factors are
socioeconomic considerations that may significantly influence selection.
A wet pond design controls both flow and pollution by maintaining a permanent
pool and a storage volume above the pool level. The permanent pool in the
wet pond traps pollutants. The pool volume maintains sufficient detention
times so that the pond can treat pollutants that would otherwise be drained
from a dry pond to the receiving water. In a dry pond, settled pollutants
from previous storms are resuspended and washed out of the basin with the
next storm flow.
The safety features of a wet pond depend on the potential secondary use of
the facility; a dual-purpose recreational lake cannot be fenced to prevent
access. Typical safety features of a wet pond are shallow bank slopes,
fences, and outlet guards.
25
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Maintenance of wet and dry ponds is often inadequate or deficient and should
therefore be considered or provided for during design. Maintenance measures
can include removing debris or sediment, landscaping, and maintaining the
outlet structure. Inadequate maintenance can adversely affect the operation,
efficiency, and flow characteristics of the stormwater control facility.:. The
two major problems are sediment deposition and vegetation growth in the
emergency spillway [22].
Both mosquito and algae problems can be eliminated in dry basins by ensuring
that the areas dry out completely between uses. For permanent pool ponds,
these problems are more difficult to control. Mosquito breeding can be upset
by removing the grass at the shoreline, varying the pond water depth every
few days, or stocking the ponds with larvae-eating fish.
The best way to overcome objections to setting land aside for a detention
pond is to recognize that the area can be an asset as open space. Housing
near greenbelts and pond areas usually has a higher market value if the open
space is aesthetically designed.
Dry detention ponds are most presentable when a grass cover is kept on the
basin slopes and floor. Grasses can be grown that will withstand periodic
flooding. If retention basins contain water for long periods or need to be
vegetation-free for better infiltration, appearance objections may be
overcome by sight barriers such as trees.
In most cases, a public agency should own and operate dry and wet ponds.
Public agencies usually have more equipment, manpower, and expertise
available than homeowner associations and developers.
Design Approach—The approach for designing low structural controls can range
"from graphical techniques to complex mathematical models. The stormwater
design approaches and important design criteria are summarized in Table 10.
Table 10. HYDROLOGIC DESIGN APPROACHES
FOR STORMWATER DETENTION FACILITIES [22]
Design approach
Evaluation criteria
Graphical Empirical
Unit-
hydrograph
Conceptual models
Simple Complex
Design elements
Multiple return period No Maybe Yes Yes Yes
Storm duration No Maybe Yes Yes Yes
Maintenance No No No Maybe Yes
Soil characteristics No No Maybe Yes Yes
Downstream effects No No Maybe Yes Yes
Accuracy Low Low Medium High High
Training requirements Low Low Medium Medium High
Relative design cost Low Low Medium Medium High
26
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Stormwater detention system design should incorporate intensity-duration-
frequency concepts to account for important variables of urban runoff events
that can affect the control's performance. The SCS method is an example of a
simple conceptual model and is regarded as one of the most appropriate design
approaches for stormwater facilities. Its principal features are a desktop
approach that has high accuracy-medium training requirements and is capable
of evaluating multiple return period storms, storm duration, and soil
characteristics.
Erosion Controls
Erosion controls limit the adverse impacts of uncontrolled stormwater runoff
or correct existing erosion problems resulting from poorly planned
development. They also can limit sediment and soil loss from construction
sites. Erosion controls include temporary soil stabilization, permanent
slope stabilization, runoff control, and revegetation.
Temporary Soil Stabilization--
Temporary soil stabilization methods are used on disturbed slopes or areas to
provide erosion control, dust control, mulch, or mulch protection. The
effectiveness of these methods is short lived. The cost of these methods
ranges from about $1,500 to $24,700/ha ($600 to $10,000/acre), as shown in
Table 11.
Permanent Slope Stabilization--
Permanent slope stabilization controls are mechanical methods that physically
change the disturbed slope area or provide physical barriers to support the
slope. The methods described in Table 12 do not provide mulch or surface
protection to bare slopes and require temporary slope stabilization methods
until permanent vegetation is established.
Runoff Controls--
Runoff controls are used in construction areas and are used in addition to
slope stabilization controls for increased effectiveness in mitigating
erosion. Runoff controls include diversion dikes, interception trenches,
pipe drops, chutes and flumes, sediment barriers, and berms. The unit costs
of these controls are summarized in Table 13.
Revegetation--
Vegetation provides the best long-term protection to sloped surfaces and is
the ultimate goal of providing erosion controls to disturbed areas. The
costs to revegetate disturbed areas can vary significantly depending on the
slope; the need for slope stabilization, reshaping, seed bed preparation; and
the method of application of seed or plantings. The costs range from about
$2,500/ha ($l,000/acre) for seed application (hydroseeding with mulch) to
over $67,000/ha ($27,000/acre) for plantings or rooted shrub cuttings. Seed
and fertilizer application adds about $500/ha ($200/acre) to the cost of
hydromulching.
21
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Table 11. DESCRIPTION AND COST OF TEMPORARY
SOIL STABILIZATION3 [5, 28]
Method
Description
Total cost,
$/acre&
Jute matting
Hatting in drainage
channels
Plastic netting
Wood excelsior
matting
Fiberglass
roving
Hydromulching
Chemicals and
tackifiers
Wood chip
application
Crushed gravel
mulches
Straw mulch
Mulch nets made of jute used for-erosion control 9,100
and protection of other mulches.
Application of jute matting or fiberglass roving 9,500
for dust and erosion control in very small drain-
age channels with flow velocities less than
2 ft/s.
Monolithic plastic cloth-like material used over 4,800
mulch, straw, or hydromulch.
Mat of wood excelsior fibers bonded to a paper 10,700
or plastic used for dust and erosion control.
Flows under mat should be prevented.
Matting of continuous strands of glass fibers 4,000
and tacking agent. Used for dust and erosion
control and as a mulch for seeded and unseeded
areas.
Mechanized rapid method for applying wood fiber 1,200C
mulch, and tacking agent with or without seeds
to large areas.
Plastics, organic seeding additives, asphaltic 600
tacking agents and other products used to tack
fibers to slopes for erosion and dust control.
Temporary mulch and surface protection using 850
chips of wood. Used for dust and erosion
control during construction and as a mulch
around plantings.
Application of gravel or crushed stone as a 800
mulch to stabilize soils during construction,
or for low-use dirt roads, driveways, and areas
of light vehicular use.
Application of staple straw as a protective 680
cover over bare or seeded soil to reduce
erosion and provide a mulch. Requires matting
or other methods to hold it in place.
a. ENR 3000.
b. Includes materials, labor, and equipment.
c. At 2.5 tons/acre.
d. At 2.0 tons/acre.
28
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Table 12. SUMMARY AND COSTS OF PERMANENT
SLOPE STABILIZATION METHODS3 [5, 28]
Method
Rock retaining
wall
Description
A low gravity wall constructed of rock
materials to provide an aesthetically
attractive method for physical stabi-
lizing a slope.
Unit 'cost,
27/1 fc
$b
Redwood retaining
wall
Gabions
Slope bottom
bench
Wattling
Slope steeping
Slope
serration
A retaining wall constructed of redwood 25/1 f
planking and posts to stabilize over
steepen or unstable slopes.
Large, single- or multi-celled rectan- 21/lf
gular wire mesh boxes filled with rock
and wired together for permanent slope
or drainage stabilization and erosion
control.
A gently sloping surface at the base 7/1f
of a steeper slope to retain eroded
material.
Bundles of live cuttings from willows 2.3/lf
to stabilize slopes and provide revege-
tation. Wattling reduces slope lengths
for surface runoff, increases water
retention, and provides additional
organic matter.
Continuous series of horizontal steps 570/acre
cut on the face of cut slopes to
interrupt slope length and provide
slope stabilization.
Construction of approximately 10 in. 420/acre
horizontal steps on the entire face of
a cut slope to provide stabilization
benches which can support vegetation.
a. ENR 3000.
b. Includes materials, labor, and equipment.
c. 4 ft high wall.
d. 3 ft high wall.
29
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Table 13. SUMMARY OF UNIT COSTS OF RUNOFF
MANAGEMENT SOURCE CONTROLS3 [28]
Method
Unit cost, $/lf
Diversion dike
Runoff interception trench
Strawbale sediment barrier
Sandbag sediment barrier
Filter berm
Filter fence
Filter inlet
Siltation berm
4.24
7.22
2.49
3.67
8.08
3.31
2.30
8.52
a. ENR 3000.
b. Includes materials, labor, and equipment.
COMBINED SEWER OVERFLOW CONTROLS
Combined sewer overflows occur when the combination of high storm inflow
volumes and sanitary sewage exceeds the capacity of the interceptor and
treatment plant to transport and treat those flows, respectively. Control of
combined sewer overflows can best be achieved by a systems approach that
includes either storage or treatment or both. The system approach is a most
promising solution to areawide control because it can (1) make use of the
existing collection system; (2) combine control technologies (storage/
treatment) to obtain a more cost-effective solution than the use of a single
technology (either storage or treatment alone); and (3) integrate combined
sewer overflow controls with dry-weather treatment facilities, providing
opportunities for higher levels of control for stormwater pollutants.
Storage, the key to a systems approach, is the fundamental element in each of
the three solution approaches. Storage of combined sewage, in addition to
reducing peak flows, treats the combined sewage by allowing heavier solids in
the detained flows to settle.
The treatment step in the systems approach is used to achieve direct
pollutant removal to meet water quality goals. Treatment can include both
physical and biological unit processes applied as end-of-pipe controls before
discharge to a receiving water. Physical processes are the most widely used
controls because of simplicity, ease of startup, and capability to handle
transient flow and quality characteristics of combined flows. Some physical
processes, such as the swirl concentrator/regulator, have been applied as
upstream control devices with the concentrated flow going to a dry-weather
treatment plant.
30
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Storage
Planning for storage in combined sewage control facilities should first
consider the use of excess capacities in existing combined sewer trunklines
and interceptors, then progress to offline storage or downstream end-of-pipe
storage basins. Inline storage and routing costs the least and potentially
has the greatest level of operational flexibility, not only for stormwater
control, but also for dry-weather system operation. Offline storage
facilities may be needed to provide the required level of flow control and to
optimize the sizing and/or the use of downstream treatment facilities. This
is particularly critical when a sanitary sewage treatment plant is integrated
with the wet-weather control system.
Inline Storage--
Inline storage is created by use of a flow restriction device either within
the transport conduit or at an overflow point (regulators). Pumping stations
in interceptors have also successfully controlled flows and used excess
storage capacity [29]. Various levels of operational control are available
for an inline storage system and can signficantly affect system effectiveness
and flexibility. These considerations need to be evaluated in terms of cost
and control requirements.
The cost of inline storage can range from as low as $250/ha ($100/acre) for
simple inline flow restriction to over $2,500/ha ($1,000/acre) for a system
that includes central computer controls, remote-controlled regulators and
pumping stations, and total system surveillance and data collection. The
Seattle system averages about $182/m ($0.69/gal) of inline storage capacity.
In addition to benefits of increased combined sewer control, increased system
control may provide benefits and cost incentives for dry-weather operations
that are not readily recognized. For example, in Seattle, Washington, the
computerized inline storage control system operates during dry-weather
periods to provide:
e Continuous system surveillance of sanitary flows, sewage levels,
and equipment operation.
» Continuous hard copy data base of system operation obtained by data
logging equipment.
a Flexibility and capability to route and store sanitary flows, which
allow maintenance crews to repair the system without bypassing
untreated flows.
» Capacity to reduce diurnal or unexpected flow variations to the dry-
weather treatment plant, potentially improving treatment
performance.
The effectiveness of inline storage depends on the volume of excess usable
capacity in the system and the level of control (simple local versus complex
automatic or supervisory control). The amount of excess storage capacity
directly affects the overflow volume reduction effectiveness. If the system
31
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configuration is such that inline storage cannot provide the required storage
volume for the desired level of control, then offline impoundments in upstream
or downstream areas may be required. In this situation, the inline storage
system can be used to connect and control offline storage facilities in a
common control network.
An example of the effects of various levels of system control is shown in
Figure 5. The total physical excess storage volume in the system is the same
for each level of control; however, by adding supervisory control of the
system operation, the overflow volume was reduced by 60% over that achieved by
local control of the regulators and pumping stations.
100
80
so
70
s BO
^ 50 —
40
30
20
10
0-
DYNAyiC LOCAL CONTROL
I
I
COMBINED
SUPERVISORY
AND COMPUTER
CONTROL
I I
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.6 0.9 1.0
RAINFALL, in.
Figure 5. Comparison of Seattle's inline storage
efficiency under three modes of operations.
The cost of adding the computer facilities and interfacing was about $5.7
million or approximately $940/ha ($380/acre) for the 6,080 ha (15,000 acre)
combined sewer area [29]. The Seattle computer system represents the most
sophisticated facilities used to control inline storage, to date. This level
of control, however, applies to larger systems because the basic cost of
similar computer facilities for a small system could significantly decrease
the cost effectiveness of the overall system. The unit costs and the marginal
benefits of the computer-controlled inline storage system become less
attractive as the costs equal or exceed the unit costs for offline storage
achieving a similar level of control.
32
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Offline Storage--
Offline storage, alone or in combination with inline systems, increases the
level of overflow control beyond the capacity of the inline system. Seattle
plans to use offline storage to reduce overflows from approximately 40 to
10/yr [30]. The offline storage would be connected to the inline system.
Saginaw, Michigan, uses offline and inline storage to control combined sewer
overflows; and Mount Clemens, Michigan, uses storage as part of the wet-
weather treatment system to reduce pumped flows entering the plant from 10.9
m3/s (250 Mgal/d) to an average of 0.2 m3/s (4 Mgal/d) before they enter the
biological treatment lagoons [31].
In most applications of offline storage, a storage basin also operates as a
sedimentation basin to handle flows that exceed the storage capacity. Most
small storm volumes are totally contained in the basins, and after the storm,
the combined sewage is released to the interceptor to flow to a dry-weather
treatment plant as capacity becomes available. This type of operation is a
most promising approach for both large and small systems, where the dry-
weather treatment facilities permit processing of the extra flow and combined
sewage pollutant load.
o
The cost of offline storage can range from about $105/m ($0.40/gal) of
storage capacity for simple earthen structures similar to those used to
control urban runoff, to about $530/m3 ($2.00/gal) for concrete storage
sedimentation tanks such as those used in Saginaw, Michigan [32].
The effectiveness of offline storage can be addressed in several ways:
(1) reduction of peak flows, (2) reduction of overflows, and (3) containment
of pollutant loads. In most applications, all three considerations are
incorporated into the design. Peak flow reduction provides a more uniform
flow to downstream treatment processes, preventing possible process breakdown
and flooding from transient flows and loadings. Similarly, depending on the
volume of storage provided, downstream treatment capacity can be reduced,
thus trading more expensive treatment costs for potentially cheaper storage
costs. The proper balance of storage and treatment for the optimum cost-
effective solution is one of the major design considerations in planning and
evaluating alternative system approaches.
Storage can decrease the number of overflows occurring in a combined sewer
system. Design considerations for sizing storage for overflow reduction
should include regulations or water quality goals, statistical analysis of
precipitation events, and storm duration. For example, a large storm volume
with a long duration may not result in an overflow from storage, while a
smaller storm volume with a short duration may. The effectiveness of the
Saginaw, Michigan, storage/sedimentation basin, which was designed to contain
approximately 1.3 cm (0.5 in.) of runoff from the contributing watershed,
reduced the overflow volume about 60% and the overflow frequency about 73%.
A summary of 11 storms, monitored during the summer of 1978, activating the
storage/sedimentation facilities is presented in Table 14.
33
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Table 14. SUMMARY OF STORAGE/SEDIMENTATION
ACTIVATION EVENTS, SAGINAW, MICHIGAN
Date
Average Average
precipitation, intensity,
in.a in./hb
Volume Overflow volume
pumped to treated and
basin, Mgalc discharged, Mgal
5/12/78
5/30/78
6/12/78
7/21/78
8/16/78
8/19/78
9/12/78
9/13/78
9/17/78
9/20/78
9/27/78
Total
0.21
0.68
0.44
0.42
0.50
0.52
0,48
1.35
0.32
0.90
0.39
6.21
0.11
0.14
0.19
0.15
0.29
0.15
0.11
0.17
0.13
0.28
0.11
2.62
4.19
3.60
1.08
3.70
5.12
3.68
16.79
0.90
9.66
0.89
52.23
0
0
0
0
0
1.60
0
13.27
0
6.14
_0
21.01
a. Average of two rain gage measurements.
b. Average rainfall divided by average duration.
c. Starting on 8/19/78, volumes determined from pump operation
logs. All previous values were estimated by water level
observations.
Storage/sedimentation basins also store and trap combined sewage pollutant
loads, in addition to treating flows that exceed the storage capacity.
Pollutant loads from small storms that are totally captured are contained and
released to the dry-weather treatment facilities for processing. On an
annual load basis, excluding the efficiency of the dry-weather plant, the
Saginaw storage/sedimentation basin removed approximately 89% of the
suspended solids load and 81% of the BOD load.
Treatment Processes
Treatment processes used for combined sewage range from simple physical
processes to more operationally complex methods using biological processes
such as treatment lagoons. The period of operation, operating costs, and
treatment response times are important factors in evaluating and designing
treatment alternatives.
The operation of combined sewage treatment facilities usually occurs only
during storms. Therefore, the facilities and equipment may sit idle during
dry-weather periods and may require special maintenance procedures to keep
the equipment operational. This is especially critical if the mechanical
equipment is in contact with the flows or in an enclosed corrosive
environment.
34
-------
Complex treatment processes usually have high operation and maintenance costs.
This is particularly true if the process scheme uses chemicals.
Combined sewage treatment facilities are also subject to transient loading
conditions. These can occur as first-flush loads, or varying loads, as flows
from contributing watersheds arrive at the plant. The principal transient
flow condition is a result of the storm characteristics. The ability of a
treatment process to handle transient conditions is a key consideration in
selecting a process. Process evaluations should also consider process
effectiveness limits and process recovery rates back to stabilized operation.
Maintaining a continuous, relatively uniform feed is particularly important
for biological systems and is a major problem in applying this process to
stormwater control. Successful biological applications use dry-weather
process units to treat wet-weather flows up to peak design loading rates
consistent with the process being used. Treatment lagoons can be designed to
provide a reliable biological process for combined sewage and can be main-
tained during dry weather.
A treatment approach should use the simplest, most mechanically free process
that can achieve water quality goals. Physical primary treatment processes
meeting these design and operational requirements are the most common type of
treatment used throughout the country for both large and small systems. They
usually have low hydraulic head requirements and sludge removal and washdown
can be achieved hydraulically, thus having no mechanical equipment in the
basin such as in Saginaw's storage/sedimentation basin [33].
The swirl concentrator/regulator is perhaps the simplest primary physical
process. The swirl has no mechanical parts and can be constructed.inline
either for service as a regulator at an overflow point, where the concen-
trated underflow is returned to the interceptor, or as an end-of-pipe treat-
ment device. At Lancaster, Pennsylvania, where a swirl concentration/regula-
tor is being demonstrated, the concentrated combined sewage underflow enters
a grit swirl and then receives further treatment at the dry-weather plant.
The clear overflow is chlorinated before discharge to the receiving water.
Sedimentation--
Sedimentation treatment efficiency for combined sewage systems is a function
of the influent solids concentration, particle settling velocities and the
hydraulic loading rate in the basin. Process removal rates for the Saginaw
facilities averaged 73% for suspended solids and 54% for BOD, based on
monitoring for three storms that overflowed the sedimentation basin during
the summer of 1978. The performance of the basin for these storms is sum-
marized in Table 15.
The estimated construction cost of the storage/sedimentation facility was
$6,910,000 or.about $494,000/m3-s ($21,400/Mgal-d) of peak design flow capaci-
ty. The sedimentation facility was estimated to cost about $10,600/ha
($4,300/acre) of combined sewer area. The estimated operation and mainte-
nance costs are about $50,000/yr or about $0.14/kg ($0.06/lb) of suspended
solids removed.
35
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Table 15. PERFORMANCE OF THE SAGINAW
STORAGE/SEDIMENTATION BASIN, 1978
Pollutant removal3
Storm overflow rate.
date
8/19/78
9/13/78
9/20/78
Average
gal/ftz-d
970
1,235
2,270
overflow rate,
gal/ft2- d
1,500
6,500
6,300
Suspended solids
Influent,
mg/L
896
149
420
Effluent,
mg/L
62
27
232
Removal
93
82
45
73
Influent,
mg/L
126
62
42
BOD
Effluent,
mg/L
40
20
31
Removal ,
*
68
68
26
54
a. From flow-weighted composite samples of influent and effluent.
Sedimentation/Biological Treatment Lagoons--
Based on projected pollutant removals of 95% for suspended solids and BOD
using a series of lagoons during an EPA demonstration project in Mount
Clemens, Michigan [31], a citywide combined sewage treatment system that also
included storage and sedimentation was constructed around the lagoons. The
treatment facilities are expected to meet effluent water quality criteria of
10 mg/L BODg and 15 mg/L suspended solids. The projected cumulative system
component process performance is shown in Figure 6.
250
PEAK FLI* FM SEWENTIAL WHT FMCESSES.
Figure 6. Projected cumulative pollutant removal efficiencies
of Mount Clemens combined sewage treatment facility.
36
-------
The estimated cost of the lagoon system is approximately $1,650,000 and the
cost of the sedimentation facility, including the storage basin, is
$6,930,000, for a total of about $18,400/ha ($7,460/acre) of combined sewer
area. The estimated operational costs are about $276,000/yr or about
$0.52/kg ($0.24/lb) of suspended solids removed.
Swirl Concentrator/Regulator--
The swirl concentrator/regulator is designed to remove 90% or more of grit,
with a specific gravity of 2.65 and an effective diameter of 0.2 mm, and
settleable solids, having a specific gravity of 1.20 and an effective diameter
of 1.0 mm. These solids characteristics were used in hydraulic model testing
to develop design criteria for prototype installations [34]. The solids are
separated by secondary fluid motion because of the long circular flow path in
the swi rl.
A full-scale demonstration project using a 7.3 m (24 ft) diameter swirl
concentrator was constructed in Lancaster, Pennsylvania. The swirl tank
design used the standard design details and relationships developed from the
model testing for an inlet diameter of 0.9 m (3 ft) and a design flow of 1.13
m3/s (25.9 Mgal/d) [34]. The project also includes a swirl degritter, a grit
conveyance system, instrumentation to conduct a full-scale monitoring and
evaluation program, a disinfection system, and a control building. The swirl
degritter is used to remove grit from the concentrated underflow of the swirl
regulator/concentrator to prevent downstream deposition, protect the pumping
station, and prevent grit from reaching the downstream treatment plant. The
construction costs of the facilities were estimated at about $690,000, of 3
which the swirl costs were estimated at about $168,000 or about $148,000/m -s
($6,500/Mgal-d) of design capacity. The swirl is the least expensive physical
treatment process for controlling combined sewage overflow, but removals are
not as good as sedimentation~or dissolved air flotation. Sedimentation, for
example, averages $480,000/m -s ($21,000/Mgal-d) and dissolved air flotation
averages $1 J64,000/m3-s ($51,000/Mgal-d) of capacity [32].
Data on the performance of the Lancaster swirl operation are limited; only a
few storms have been sampled [7], and fewer have flow data for mass removal
determinations. The sampling data collected may not be representative of the
flow at the monitoring locations because of low sampling velocities and long
piping runs to the samplers.
The estimated mass removal rate for suspended solids ranged from 17 to
with hydraulic flow splits from 15 to 48%, respectively. The size of the
storm inflow volume and the split between the concentrated underflow and the
clear overflow significantly affect the overall performance of the unit. For
example, a small storm could have up to 50% of its total flow volume diverted
through the foul underflow, thereby netting at least a 50% mass removal
independent of any concentrating effects. However, for very large storms at
the same underflow rate, a relatively small fraction of the total flow volume
is diverted through the foul underflow, and the corresponding mass removal by
the flow split is lower.
37
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Tests of a swirl concentrator/regulator in Syracuse, New York, indicated that
the mass suspended solids reduction ranged from about 30 to 80% and averaged
52% for 11 different storms. The corresponding mass removal for the same 11
storms by a conventional regulator was estimated at about 33%. Therefore, the
concentrator contributed about 20% additional removal [35].
Integrated Systems
Areawide combined sewage overflow control requires combinations of storage and
treatment technologies to provide a level of mitigation consistent with the
goals of water quality improvement and optimization of cost effectiveness.
The integration of these technologies may also involve existing dry-weather
treatment facilities. Storage and/or treatment facilities in several large
cities have been integrated with sewer separation projects.
Storage/Treatment Integration—
Saginaw, Michigan, uses an integrated systems approach that includes inline
storage, offline storage/sedimentation, and dry-weather treatment of the
stored combined flows. The dry-weather plant can handle the peak wet-weather
loads and flows, and the sludge dewatering system can handle the additional
solids from the combined sewage overflow events.
The unit costs of this combined sewage system indicate the benefit of
integrating technology. The total system unit storage costs are about 20%
less than the cost of the offline storage/sedimentation basin, alone, as shown
in Table 16. Similarly, without the cost-effective storage volume provided by
inline storage, the size of the storage/treatment basin required for
equivalent levels of control would have to be increased at the higher unit
cost rate; or if the size remained the same, the effectiveness would be
reduced.
Table 16. SUMMARY OF SYSTEM COSTS,
SAGINAW, MICHIGAN9
Component
Storage Area Storage Cost per
capacity, served, Construction cost, acre served,
Mgal acres cost, $ $/gal $/acre
Storage/treatment
facilities
Inline storage
Total system
3.52
1.07
4.59
1,600
1,600
1,600
6,910,000
370,000
7,280,000
1.96
0.35
1.58
4,300
230
4,530
a. ENR 3000.
38
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Integration of Sewer Separation Programs--
Sewer separation programs have been implemented to solve specific system'
problems often unrelated to quality control of combined sewage overflows, yet
sewer separation can be viewed as part of a total system approach. In
Seattle, for example, a program of sewer separation in selected areas of the
city was intended to reduce severe local flooding problems. A comparison of
the cost effectiveness of the sewer separation and Seattle's inline storage
system is presented in Table 17. For equivalent results of combined sewage
overflow reduction, inline storage is over 18 times more cost effective than
separation. However, each measure was unique to an areawide problem solution
(overflow control and local flooding). The integrated system of inline
storage and separation is approximately 3 times more cost effective than just
separation.
Table 17. SUMMARY OF SYSTEM COSTS, SEATTLE, WASHINGTON3
Control measure
Inline storage
Sewer separation
Combined inline
and separation
projects
Total capital
costs, $
15,753,000
147,810,000
163,563,000
Contributing
area, acres
15,000
23,000
38,000
Annual
combined
overflow
volume
reduction,
Mgal
600
300b
900
Excess
storage
capacity,
Mgal
22.79
--
— —
Cost,
$/acre
1,050
6,430
4,300
Overflow
volume
reduction
cost, $/Mgal
26,260
492,700
131,740
Storage
cost,
$/gal
0.69
_.
—
a. ENR 3000.
b. Still allows stormwater volumes and pollutants to be discharged.
Mount Clemens, Michigan, also has a combined sewage control system that
integrates sewer separation with intercept!'on/storage, physical treatment, and
biological treatment to eliminate all untreated combined sewage overflows.
Approximately half of the city was separated because of the configuration of
the collection system and the potential high cost of constructing interceptors
in these areas. The total cost of the system was about $26,000/ha
($10,500/acre). A comparison of the system component costs is presented in
Table 18.
Systems Approach--
Areawide control of combined sewer overflows should consider the potential
cost-effectiveness benefits of a systems approach. The combination of wet-
weather storage and treatment is an essential, most promising first step for
effective combined sewer overflow control. The impacts of the wet-weather
system on dry-weather operations is of extreme importance, particularly when
the dry-weather processes are integrated with such systems as inline storage.
39
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Table 18. SUMMARY OF SYSTEM COMPONENT COSTS,
MOUNT CLEMENS, MICHIGAN3
Control measure*
Sewer separation
Interception
Treatment
Retention basin site
Park treatment site
Total capital
costs, $
4,019,000
8,916,000
—
6,934,000
1,653,000
Serviced
area, acres
900
1,150
1,150
--
—
Annual
combined
overflow
volume
reduction,
Mgal
333b
426
426
—
--
Cost,
$/acre
4,466
7,753
—
6,030
1,437
Overflow
volume
reduction
cost, $/Mgal
12,070
21,000
—
—
~
Treatment
cost,
$/gal
—
—
--
0.016
0.002
a. ENR 3000.
b. Still allows stormwater volumes and pollutants to be discharged.
A common requirement for a successful systems approach is the selection of
storage. Inline storage should be developed initially because it is the least
expensive, $250 to $2,500/ha ($100 to $1,000/acre) of combined sewered area,
and is the easiest to implement. The inline storage volume in any system,
however, is limited by the size and physical characteristics of the collection
system. The effective use of this limited storage volume can be greatly
enhanced by automatic control operation. If more storage is required than
provided by the inline system, the inline system can serve as the link between
upstream offline storage and downstream storage/treatment.
Offline storage should be considered as the next step to providing more
storage capacity. Offline storage basins are the most common type of measure
used in large cities with combined sewer overflow problems, but the costs can
be up to six times greater than inline storage costs. In many situations,
offline storage is combined with a sedimentation process to treat flows
exceeding the storage capacity.
Another potential for storage, but which has not received great attention, is
the use of source detention measures in upland areas of combined systems to
prevent or delay the release of runoff into the combined sewer. The source
controls that could be easily applied in a systems approach are rooftop,
plaza, and parking lot storage in highly developed urban areas and source
detention and detention/percolation basins in less developed areas. These
measures may be as cost effective as large in-system structural controls.
The use of a dry-weather treatment facility as an element of the wet-weather
control system provides additional treatment for reducing combined sewage
overflow pollutants. For inline storage systems, it is the only treatment
step of the system. While dry-weather facilities may help to control combined
sewage (increased pollutant removal), unless they have been designed or
properly evaluated for the impacts of increased combined sewage flows and
loads, process operational problems could develop. This could result in the
40
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need for increased clarification capacity, sludge and grit handling
facilities. The key to a cost-effective wet/dry integration may therefore
depend on the dry-weather facilities capability, or lack of capability, to
handle stored flows.
In a systems approach, the type, mix, and size of alternative
storage/treatment technologies can provide various levels of control and
overall cost effectiveness. Several methodologies are available for
evaluating storage/treatment tradeoffs in planning combined sewage overflow
control systems [32]. One, developed by the University of Florida, is a
desktop approach for 208 level planning activities and can screen alternative
stormwater mnagement plans [36]. A simplified stormwater management model is
currently being documented by Metcalf & Eddy, Inc., for release by the EPA
[37]. This model is a 201 facilities planning model that is an inexpensive
and flexible tool for planning and preliminary sizing of stormwater
facilities. The model can assess the effects of alternative storage/treatment
balances developed for a systemwide control approach.
41
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SECTION 3
RECOMMENDATIONS
Over the past 14 years, millions of research and development dollars have
been granted by the U.S. EPA for the study of storm and combined sewer
overflows. The output of this program has served as the base for implmenting
large, full-scale, areawide control solutions. The emphasis of the program
has principally been to control combined sewage overflows. Recent studies,
however, have indicated that the potential pollution from urban runoff and
other nonpoint sources has caused major pollution problems in receiving
waters, ranging from nutrient enrichment, introduction of toxic materials,
turbidity, and sediment deposition [1-5]. The magnitude of the urban runoff
problem, therefore, may be of equal importance compared with the combined
sewer overflow problem, particulary when flooding and erosion impacts >are
considered.
Federal research and development (R&D) programs should continue to refine and
develop most promising approaches to areawide, combined sewer overflow
control systems for user guidance and to advance the state-of-the-art for
BMPs. Particular emphasis should be placed on program needs related to the
conduct of R&D program evaluations.
COMBINED SEWER OVERFLOW CONTROLS
Considering the large investment made in demonstration projects, and the even
larger potential investment for full-scale areawide control, ranging into the
billions of dollars for such systems as San Francisco's wastewater and
combined sewer overflow master plan [6], and Chicago's deep tunnel and
reservoir plan [1], more emphasis should be placed on obtaining information
on process and system effectiveness, costs, and receiving water quality
impacts/benefits from established and operating facilities. Several
considerations for addressing these data needs include:
• Monitoring programs need to be continued or reestablished on
existing demonstration facilities.that represent most promising
technology to fill data gaps and create a functional data base.
Many of the demonstration projects, once built and evaluated under
demonstration programs, receive little subsequent monitoring after
they are turned over to the local jurisdiction.
• The data collected during the initial phases of the demonstration
program evaluation period may be unrepresentative or inadequate.
During the first year of operation of any facility, operational
42
-------
changes, system debugging, and startup problems may interfere with
data collection and representative operational costs.
A number of facilities constructed under 201 programs may also^be
representative of most promising technology and would offer
excellent opportunities for low-cost monitoring programs without the
need for new costly construction of R&D facilities. Saginaw,
Michigan's storage/sedimentation facility is an excellent example of
a nondemonstration facility that was monitored with R&D funds.
BEST MANAGEMENT PRACTICES
Studies of source control measures for urban runoff should be increased to
provide an adequate data base for evaluating appropriate alternative control
approaches. The need for such studies is pressing. A builders' association
in Virginia, for example, has provided $34,000 for the study of urban runoff
control practices to identify the best, least expensive methods for
controlling urban runoff from new development, in actual field conditions.
The key objective is to determine which BMPs are cost effective before local
governments force builders to install and pay for such facilities [7]. The
objectives of this private study should be followed through in R&D ,
demonstration projects to identify and monitor the following:
• The costs and effectiveness—both flow control and pollutant removal
efficiencies, design relationships, and receiving water impacts
resulting from control implementation.
• The impact and benefits of multiuse facilities including those that
have flood and erosion controls and recreational uses.
• A monitoring strategy to produce data that, may be used .and
transferred for evalution of similar control methodologies.
PROGRAM NEEDS
The foregoing recommendations all indicate the need to collect and .compile
reliable data allowing users/designers/decision-makers to'evaluate and select
cost-effective controls or control systems. The data needs .are keyed to
receiving water quality, control measure performance, and costs. Specific
data requirements needing attention are summarized in the following tasks:
• Refine and expand systemwide cost data. This information would
include both unit process or control costs and resulting systemwide
unit costs where several controls are integrated to achieve the most
cost-effective combinations--"knee of the curve" analysis comparing
marginal costs to marginal benefits.
• Develop operation and maintenance costs from actual operating
experience, not affected by startup or debugging problems. In times
of high energy and labor costs, evaluation of the annual costs may
contribute more to the deciding criteria for alternative selection,
than just construction costs alone.
43
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• Evaluate impacts on the dry-weather treatment plant operation,
where they are integrated into the combined sewage overflow control
system. Specific data requirements include effects on process
performance from the increased flow and pollutant load, evaluation
of required costs to modify the plant to handle wet-weather flows,
and evaluation of sludge handling problems.
• Expand performance data base on the most promising control
technologies. This includes evaluation of performance under
transient-loading conditions, process effectiveness limits,
settling characteristics and changes in water quality in storage
facilities, and mass balances of pollutant removal in integrated
control systems.
• Evaluate receiving water impacts/benefits using existing data
(limited) and new data from monitoring programs designed to fill
these data gaps. These programs should emphasize both long-term
macroscopic effects and single-storm event analysis.
SAMPLING PROCEDURES
Monitoring the effectiveness and operational parameters of a demonstration
control facility is the most critical function of a facility evaluation. The
collection of representative samples is essential to document the process
being evaluated for future guidance. Development of a guidance program on
sampling procedures would be beneficial to the evaluation of storm and
combined sewer overflow demonstration facilities, and could possibly avoid
some of the following problems that can limit the value and usefulness of
sampled data:
• Flow velocities at the sampler intake and in the sample lines
should be similar to the velocity of the flow stream being sampled.
Sampling velocities of at least 1.2 m/s (4 ft/s) should allow
adequate intake of heavy settleable solids and keep them in
suspension; therefore, more representative samples can be taken.
• Both automatic composite and timed discrete samples should be taken
on at least the influent stormwater flows for complete flow
characterization. Composite samples should be adequate for the
effluent from a stormwater storage or treatment unit. The discrete
samples will identify first-flush or transient-loading conditions
that may influence system design or operation; i.e, using or
designing sufficient detention capacity to capture high initial
storm loads. The automatic composite sampler should be capable of
taking flow-weighted samples.
ft A uniformity of monitoring analysis and procedures between
different projects around the country is essential to compile and
extend a useful data base that is statistically significant, such
that an aggregate information source can be created and evaluated
as a homogeneous unit.
44
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Accurate flow measurement is essential for the evaluation of system
effectiveness. In addition to process efficiency, which is
characterized by pollutant concentration removal, flow measurements
are required to evaluate total mass removal effectiveness.
Analysis for particle settling velocities and associated pollutants
is essential to be able to select and design better treatment
facilities. This information is also useful in determining solids
transport characteristics.
45
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PART 1
BEST MANAGEMENT PRACTICES
SECTION 4
PUBLIC UTILITY APPROACH TO URBAN RUNOFF CONTROL
BELLEVUE, WASHINGTON
Establishing a public utility to control urban runoff in Bellevue,
Washington, has provided flexibility in developing and financing a water
quality improvement program to meet the needs of the community. In addition
to enforcing stormwater regulations and control requirements on all new
development, the utility is responsible for maintaining the city's drainage
system. This includes making improvements that affect the water quality and
the use of the natural streams as a water resource.
Financing of the utility is based on a service charge on all developed and
undeveloped property. The amount of the service charge depends on the
property's contribution to the runoff problem. The utility has been
successful in establishing a high level of commitment to local stormwater
problems and in maintaining and protecting the nature and quality of the
natural stream/drainage system environment.
Both controls for erosion and sedimentation from construction sites and
postdevelopment runoff management are required for the rapidly developing
urban area. Major drainage system improvements, including offline and
instream storage/detention, channel lining and cleaning, and stormwater
drains and bypasses, are part of a comprehensive drainage master plan. The
estimated costs of these master plan improvements average about $2,500/ha
($1,000/acre). The comprehensive drainage master plan is just being
implemented, but has increased public awareness of water quality problems and
goals, and has provided environmental and socioeconomic benefits.
An ongoing two year BMP evaluation in Bellevue is being jointly sponsored by
the U.S. EPA's Storm and Combined Sewer Section and the 208 Planning Section,
and the U.S. Geological Survey (USGS). The USGS will be primarily
responsible for data collection to evaluate storm runoff flow and
characteristics (wet-weather washoff and modeling). The EPA project will
evaluate BMPs on a macroscopic scale involving analysis of 40 to 90 storms to
determine basinwide effectiveness and long-term water quality impacts.
APPROACH TO RUNOFF CONTROL
Rapidly developing urban areas can create water quality and quantity
problems, and while controls on new development at least hold the problems in
check, regional or areawide controls may be required to improve water quality
in existing developed areas. Bellevue's Storm and Surface Water Utility is a
46
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unique, separately funded organization that regulates controls on new
developments and provides a citywide stormwater control approach.
Area Characteristics
Bellevue is a rapidly developing suburban community located about 9.7 km (6
mi) east of Seattle, between Lake Washington and Lake Sammamish, as shown in
Figure 7. The city encompasses about 7,700 ha (19,000 acres), of which
nearly 70% is developed [1]. Dynamic growth has occurred in Bellevue over
the past 25 years with almost a 16-fold increase in population: from about
5,000 in 1954 to 80,000 in 1979.
LAKE
SAMMAMISH
Figure 7. Location of Bellevue, Washington.
The topography of the area is rolling hills and valleys with elevations
varying from approximately 7.6 to 122 m (25 to 400 ft). The area is drained
by an extensive stream system that serpentines through the city. The 11
drainage basins in the city are generally small, averaging less than 610 ha
(1,500 acres), and most residential developments are near small streams or
creeks.
The soils are generally gravelly loam with high permeability, but there are
some peat-filled valleys in the lower drainage areas and wetlands. Peat
depths have been measured up to 30 m (100 ft).
47
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The climate of the area is moderated by the Pacific Ocean and is considered
maritime. The annual precipitation is about 102 cm (40 in.), of which 77%
occurs between October and March [2],
The land use in Bellevue is almost 55% single-family residential. During the
past 5 years, high density residential and commercial development has
increased, but single-family home construction has continued at a high level
as well. The 1974 land use and the projected future land use changes are
summarized in Table 19.
Table 19. 1974 LAND USE AND PROJECTED
FUTURE LAND USE, BELLEVUE [1].
Land use
Single-family residential
Multifamily residential
Commercial
Light industrial
Institutional
Parks, open space,
undeveloped
Freeways/roads
Total
Estimated
1974 land
use, acres
9,743
517
1,116
463
583
6,175
663
19,260
Projected
future
land use,
acres
10,280
1,173
2,862
772
713
2,680
780
19,260
Change
in land'
area,
acres
537
656
1,746
309
130
-3,495
117
0
Change
in total
land area,
%
2.7
3.4
9.1
1.6
0.6
-18
0.6
0
Problem Assessment
The rapid development in Bellevue has created stormwater runoff problems in
most of the natural streams draining the area, including:
• Floodi ng
• Erosion
• Stream sedimentation/siltation
• Water quality problems
Flooding, erosion, and sedimentation are a direct result of increased runoff,
extreme topographic relief (going from steep ridges to flat lowlands), high
groundwater table in the lowlands, and poor soil conditions. The water
quality problems created by the increased runoff include the effects of
pollutants entering the streams and receiving waters, and the changes in the
ecology or nature of the stream. Bellevue's principal problem areas are
identified in Figure 8.
48
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\REDMOND
EROSION
SEDIMENTATION
Figure 8. Urban runoff problem areas, Bellevue [1].
Quantity—
Most of the streams in Bellevue are still above ground and exposed to
development, although some channelization and underground pipes have been
installed. Flows in these streams are generally less than 0.08 m3/s (3 -
ft3/s);3however, during intense rainfall, flood flows have exceeded 11 m /s
(400 ft /s). Estimates of the increases in runoff rates above predevelopment
rates for several different land uses are summarized in Table 20.
Table 20. ESTIMATED INCREASES IN RUNOFF FROM
DEVELOPMENT OVER PREDEVELOPMENT RATES [1]
Land use
Runoff increase, %
Single-family residential
Multifamily residential
Commercial
Industrial
200
800
1,900
1,900
49.
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The flooding caused by these increased flows is the most visible problem and
occurs along stream channels and in low lying wetlands. Streets and
basements have also been flooded.
Erosion from cleared land under development and from stream channels has also
occurred. Besides loss of land and soil and damage to stream channel banks
and vegetation, sedimentation and siltation of the stream bottom can change
the biological structure of the benthic life in the stream and reduce the
flow capacity of a stream segment, possibly causing additional flooding.
Deposits of eroded sediment can cover fish spawning beds and kill the aquatic
insect community that supports the fish.
Qua! i ty~
Although water quality in Lake Washington has improved since the late 1950s
and early 1960s, mainly because of the diversion and regional treatment of
sanitary sewage, nutrient discharges from small streams can cause nearshore
water quality problems. The estimated nutrient loadings from Kelsey Creek,
the major stream draining Bellevue, are summarized in Table 21. The increase
in load is principally a result of increased urban development. Other
nearshore problems include increased sedimentation, turbidity, and toxic
inputs of oils, heavy metals, and pesticides. In addition to these urban
runoff pollutants, ambient stream dissolved oxygen, temperature, and
turbidity are prime water quality considerations because preserving the
chemical and physical characteristics of the streams and wetlands for fish
propagation is one of Bellevue's goals.
Table 21. ESTIMATED AND PROJECTED
NUTRIENT LOADS IN KELSEY CREEK [3]
Nutrient
Phosphorus as PO^
Nitrogen as NOg
1970 load,
Ifa/d
8.2
58.7
2000 projected
load, Ib/d
15.8
142.0
Increase,
%
93
142
Approach
Bellevue uses low and nonstructural source controls to control urban runoff.
Regulation, enforcement, and implementation of these controls is conducted by
a utility of the city government. The utility receives public financing
through the collection of service charges and is a major division of the
Public Works Department.
Utility Concept--
A sequence of events, beginning in the early 1970s over concern for water
quality in the streams draining the city, led to the adoption of an ordinance
regulating clearing and grading operations to minimize sediment and a
recommendation that a Storm and Surface Water Utility be established to
assume responsibility for all water resource related matters. The utility
50
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was established in 1974 and is a most promising approach for runoff control
on a local level. One of the significant problems of the utility was
developing-an equitable billing system and rate structure [4].
Utility Organization and Function--The responsibility of the utility is not
only to control the drainage system, but to protect the water resource
itself, as the drainage system affects it. The utility focuses on a number
of diverse issues, such as biological quality, aesthetics, and recreational
benefits as well as conveyance of stormwater runoff. To achieve these goals,
the utility was originally organized to provide the required technical and
operational staff in a single integrated group.
This organizational structure is significantly different from that found in
most public works departments and is often difficult to form within
established agencies, particularly those without funds earmarked for
drainage. Most public works departments are organized in a staff concept,
with an engineering division providing all engineering services to other
divisions, a maintenance division providing all maintenance services to the
department, and a construction division providing inspection services, and so
on. A staff concept can be unresponsive to the needs of a stormwater
management program because stormwater control is only one of several
responsibilities assigned to a division, and usually carries a secondary
priority to road construction, water and sewer installation, and construction
inspection services.
Bellevue's Storm and Surface Water Utility was organized to provide utility
inspectors, plan review engineers, water quality technicians, and maintenance
personnel under a single operation for controlling urban runoff and water
resource problems. This approach has been very successful in Bellevue and
has proved an efficient mechanism for enforcing and financing urban
stormwater management programs.
Uti1ity F1nancing--Financing stormwater management programs at the local or
city level has often been difficult, and limited funding has restricted the
effectiveness of program control and enforcement. In Bellevue, it was
recommended that storm and surface water activities be treated as a utility
and financed similar to other utility operations.
Storm and surface water services are, however, different from most other
utility services, such as water supply or sewage utilities where the public
receives a readily identifiable product or service. One of the biggest
problems faced in establishing the utility was convincing the public that
runoff from developed land entering a drainage system created problems and
that the service performed by the utility, although not readily visible,
could benefit the entire public and should therefore be subsidized by all
property owners.
The utility rate structure in Bellevue is'based on a property's contribution
to the stormwater problem with the level of charge commensurate with (1) the
property area, and (2) the intensity of development. A rate structure
determined solely on the property's contribution to the problem, however,
does not provide for such situations as oversizing downstream drainage
51
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controls in expectation of future upstream development. Owners of upstream
undeveloped property could argue that they were not contributing to the
problem, which would force downstream owners to pay for the overdesign.
Another important element of the rate structure is that the utility charges
apply to both public and private property. Even streets and freeways are
billed as developed, real property.
The utility is now well established and accepted by the community. The
residential bimonthly service charges for the utility average about $1.60.
About $600,000 is generated annually to carry out the urban runoff and water
resources programs and represents a stable source of funding.
Facility Design Criteria--
Stormwater control facilities are designed to limit the rate of runoff from
developed areas to predevelopment rates and store runoff in excess of this
rate. Infiltration potential and impervious surface characteristics are also
considered in the design criteria [5].
Allowable Runoff Rate--The allowable runoff rate is based on a citywide
average of predevel opment runoff from a 10 year storm of 0.014 m-Vha-s (0.2
ft3/acre-s), or about 0.5 cm/h (0.2 in./h). The runoff criteria also apply
to the release rate from storage facilities.
Storage Requirements—Storage requirements for runoff in excess of the
allowable runoff rate are based on a rainfall intensity/duration curve
developed for the 100 yr storm, shown in Figure 9. Impervious and pervious
areas have different storage requirements.
For impervious areas, the storage required is based on a 100 yr, 4 h rainfall
of 4.6 cm (1.8 in.) less the allowable release rate over the 4 h period, or a
total storage requirement of 2.54 cm (1.0 in.).
For pervious areas, the storage required is based on a 100 yr, 2 h rainfall
of 3.3 cm (1.3 in.) less the allowable release rate over the rainfall period
and the infiltration rate over the rainfall period. Storage would not be
required for pervious areas with infiltration rates in excess of 1.3 cm/h '
(0.5 in./h). However, because of the poor infiltration conditions in most
areas of the city, storage may be required in many situations. In the
absence of infiltration test data for a particular development site, an
assumed allowable infiltration rate of 0.5 cm/h (0.2 in./h) is used. At this
rate, the total storage requirements for pervious areas is about 1.3 cm (0,5
in.). Different infiltration rates would affect the total storage
requirement.
RUNOFF CONTROL FACILITIES
Runoff control facilities are required both during construction for erosion
and sediment control and after development for runoff control. A large
emphasis is placed on the use of nonstructural controls; however, in many
situations the use of low structural controls, such as storage basins, is
required.
52
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4 i-
3 -
^ 2 -
DESIGN STORM
DURATION, h
100 yr
RAINFALL, in
1 -
1 in. STORAGE
0.5 in. STORAGE
i
1
2
4
6
12
24
36
4B
96
1.0
1.3
1.8
2.1
3.0
4.2
5.3
6.3
7.7
12
18
24
DESIGN 100 yr STORM DURATION, h
Figure 9. Storage volume design curve for
the 100 yr storm, Bellevue [5].
Sediment Controls
Sediment controls are required in all new construction areas, where the soil
has been disturbed or vegetation removed to prevent sediment from reaching •:
the receiving streams. Simple sediment trap ponds are the most common and
are usually removed after construction is completed. These ponds are located
downstream from the construction site and runoff is directed to the pond
along protective berms or through drainage ditches. ?The required pond
capacity is at least 0.028 m3 (1 ft3) for each 4.6 m (50 ft2) of tributary
area. Temporary sedimentation ponds are shown in Figure 10.
Permanent Runoff Controls
,; ' . « i ' '
Permanent runoff controls use some form of detention with offline dry ponds
and parking lot storage being the most common. Other detention methods used
include rooftop storage (limited use), instream storage, underground vaults,
and underground pipes. Infiltration and recharge trenches can be used to
reduce the storage requirements.
53
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-- .:";->", ':'•;' --•"-'•'•--v •-'.-
Figure 10. Temporary sedimentation ponds for construction sites:
(a) runoff from large construction site channeled to pond,
(b) sediment trapped in pond, and (c) small sediment
pond for apartment complex under construction.
54
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A number of small offline detention ponds have been used for residential and
commercial applications. Most are dry ponds and are simple excavations, as
shown in Figure 11. They can have protected- inlets and inverted outlets to
prevent trash from entering the pond and to prevent fleatables and oil and
grease from leaving the structure.
In addition to detaining stormwater, they also act as sediment traps and
require cleaning and maintenance as the sediment deposits increase.
An example of an offline detention system used to control runoff volumes from
about 6.1 ha (15 acres) of almost totally impervious street surface is shown
in Figures 12 and 13. A series of five detention basins, separated by
control manholes, was constructed as part of a four-lane avenue
reconstruction project and was integrated into a roadside park setting.
Runoff from approximately 1,070 m (3,500 ft) of roadway is drained into the
pond system through a 68 cm (27 in.) pipe that interconnects the control
manholes around each pond. A base flow runoff is allowed to pass through the
pond system. However, as runoff increases, the control manholes impede the
flow and sequentially fill the ponds through an inlet/outlet grate, beginning
with the upstream pond. Weirs in the manholes prevent the storage depth from
overtopping the pond embankment and pass the flow to the next downstream
pond.
The pond system can be adjusted for various flow and operating conditions.
The base flow passes through the manholes through adjustable mud valves in
the floor to the next downstream manholes. The weirs controlling the maximum
pond elevation are also adjustable. The ponds can also be operated as a
storage/infiltration system using a 15 cm (6 in.) underdrain pipe connected
to the downstream side of each manhole. The underdrain system can provide a
high level of treatment in addition to volume and rate control.
Instream detention has also been used. Existing or excess capacity in the
stream itself or instream ponds provide additional storage volume, as shown
in Figure 14. Check dams with adjustable stop log weirs regulate the
available storage and also provide some stream aeration from the falling
water. Floating booms across stream channels have also been used to trap
floatable material.
COSTS
While a large number of stormwater controls will be constructed and financed
by private developers, the City of Bellevue has developed a drainage master
plan that identified needed improvements .to the drainage system over the city
and also recommended a number of regional detention facilities to control
runoff volumes and rates from several large drainage areas [1]. The
estimated total cost for the master plan improvements is about $21 million,
of which Bellevue's cost is about $19.6 million (ENR 3000). The difference
in cost is to be shared by surrounding county and city agencies. Through the
areawide 208 agency, Bellevue is developing intergovernmental agreements for
drainage control where the drainage basins cross jurisdictional lines.
55
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Figure 11. Permanent stormwater detention ponds: (a) detention pond
with fence in developing residential area, (b) bar rack inlet to
detention pond to trap debris, and (c) oil-covered detention pond
receiving runoff from a bus garage and storage area.
56
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INLET/OUTLET
STRUCTURE
DETENTION POND
(a)
27 in. STORM DRAIN
8 in. POND INLET/OUTLET PIPE
jr6 in. UNDERDRAfiH
INLET/OUTLET
STRUCTURE
6 in. UNDERDRA1N PIPE
Figure 12. Offline detention system to control runoff from about 15 acres
of road surface: (a) schematic plan showing five .offline detention ponds
in series; (b) detail of Pond 5 showing stormdrain, control manholes,
inlet/outlet pipe, and underdrain; and (c) section view of Pond 5.
57
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Figure 13. Offline detention system: (a) drainage area of
impervious.roadway, (b) detention ponds consist.of grassed
depressions that form a roadside park, (c) detention Pond 5
with control manhole in right foreground.
58
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Figure 14. Instream stormwater storage: (a and b) check dams in
relocated streambed, (c) floating boom to trap fleatables,
(d). adjustable sluice gates in stream, and (e) instream permanent
pool storage pond controlled by check dams in a residential area.
59
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Storage pond construction and drainage system improvements are planned over a
phased program:
• Phase I - immediate action. This phase focuses primarily on the
purchase of land for regional detention facilities. Land purchase
and/or construction improvements, which will take 4 years, are
planned for seven of the eight major drainage basins in Bellevue at
a cost of $8.4 million.
<
• Subsequent phases. The next level of land purchase and
construction improvements will be prioritized during Phase I for
subsequent phases as yet unspecified.
The drainage system improvements include channel modification, streambank
protection, channel cleaning, installation of parallel pipes and bypass
pipes, and culvert replacement.
Storage Pond Costs
Two types of storage ponds are planned: (1) excess storage capacity in
streams and wetlands is used; only minor construction of embankments or
modification to culverts is required; and (2) storage capacity is created
extensive excavation and construction of embankments. Construction cost
estimates of the 19 storage ponds planned in the Kelsey Creek system are
presented in Table 22.
by
Table 22. ESTIMATED STORAGE POND CONSTRUCTION COSTS,
KELSEY CREEK DRAINAGE SYSTEM [l]a
Kelsey Creek
subbasin
Ultimate storage
capacity, ft3
Kelsey Creek - 4 pond sites
1
2
3
4
Valley Creek - 4 pond sites
348,000
1,177,000
3,049,000
3,267,000
Construction
cost, $b
78,000
62,000
739,000
34,000
Unit storage
cost, $/ft3
a. ENR 3000.
b. Includes contingency, design, overhead, and administration.
0.22
0.05
0.24
0.01
West
1
2
3
4
Tributary - 5 pond sites
1
2
3
4
5
936,000
2,483,000
2,614,000
1,350,000
1,111,000
283,000
1,786,000
1,459,000
327,000
121,000
319,000
539,000
694,000
448,000
184,000
22,000
30,000
11,000
0.13
0.13
0.21
0.51
0.40
0.65
0.01
0.02
0.03
Richards Creek - 4 pond sites
1
2
3
4
2,309,000
828,000
2,831 ,000
1,699,000
22,000
19,000
506,000 '
15,000
0.01
0.02
0.18
0.01
Mercer Slough - 2 pond sites
1
2
675,000
871 ,000
15,000
91 ,000
0.02
0.10
60
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The average cost of wetlands storage, $1.06/m ($0.03/ft ), is about 10% of
the cost of storage ponds requiring substantial excavation, about $10.60/m3
($0.30/ft3). Using these estimates and estimates of planned storage ponds in
other drainage basins, generalized unit construction cost curves were
developed and are shown in Figure 15.
0.60 i-
0.50
P3
~ 0.40
^
00
CD
o
z
^0.30
C3
o:
i—
CO
z
« 0. 20
z
0. 10
STORAGE PONDS REQUIRING SUBSTANTIAL EXCAVATION.
EMBANKMENT, AND SPILLWAY WORK,
STORAGE PONDS CREATED FROM EXISTING
WETLANDS AND NATURAL LOW AREAS
500 1000 1500 2000 2500 3000
TOTAL STORAGE CAPACITY. 1000 ft3
3500
4000
Figure 15. Storage pond construction costs, ENR 3000.
Approximately 86 ha (212 acres) of land would have to be obtained for
construction of the ponds on the Kelsey Creek system, at an estimated cost of
about $4.6 million, or about $54,000/ha ($22,000/acre). This includes
associated administration and acquisition costs [1].
Operation and Maintenance Costs
The- annual operating budget of the Storm and Surface Water Utility is
approximately $600,000, which covers utility expenses and operation and
maintenance of the drainage system. This revenue, based on the utility rate
structure, is balanced to just meet the costs of the utility.
61
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The rate structure charges property owners based on total area and degree of
development. There are five categories of development [6]:
• Undeveloped - Real property undeveloped and unaltered by buildings,
roads, impervious surfaces, or other physical improvements that
change the hydrology of the property from its natural state.
• Light Development - Developed real property that has impervious
surfaces of less than 20% of the total property area.
• Moderate Development - Developed real property that has impervious
surfaces between 20 and 40% of the total property area.
• Heavy Development - Developed real property that has impervious
surfaces between 40 and 70% of the total square property area.
• Very Heavy Development - Developed real property that has
impervious surfaces of more than 70% of the total property area.
A portion of the service charge rate structure for several categories of
development and property size is shown in Table 23.
Table 23. PORTION OF THE ANNUAL STORM AND
SURFACE WATER UTILITY BILLING STRUCTURE* [6]
Property
size,
acres
0.25
0.50
0.75
1.00
2.00
Billing rate
Undeveloped
5.64
10.32
15.96
20.64
41.16
by development classification, $
Light
8.4
15.48
23.88
30.84
61.80
Moderate
11.28
20.64
31.92
41.28
82.44
Heavy
16.80
30.84
47.76
61.80
123.60
Very heavy
22.44
41.16
63.60
82.32
164.76
a. Based on 1977 rates.
IMPACTS
Bellevue's stormwater runoff control program is in its early stages of
development, and while new development is required to control runoff, the
master plan drainage improvements have not been completed. There has been,
however, increasing public concern over the water quality and the stream
environment. Local property owners can now identify the measures implemented
to control urban runoff and have recognized the environmental and
socioeconomic impacts of the drainage plan and the public utility,,
Environmental Impacts
The main goals of stormwater control are to protect the stream and wetlands
environment and reduce potential flooding and damage caused by increased
runoff from developed areas.
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During development of the master plan stormwater storage and drainage system
improvements, alternative plans were evaluated according to a number of
environmental factors. These included (1) potential for environmental damage
during high flow conditions; (2) alteration of or the need to change the
natural drainage capability of streams, lakes, and wetlands; (3) potential
impacts on the groundwater system; (4) surface water quality impacts
including temperature, dissolved oxygen, and turbidity; and (5) evaluation of
the impacts on wildlife, aquatic life, and vegetation.
With the application of low and nonstructural runoff control technology, the
appearance and nature of the streams in Bellevue have improved, or at least
not worsened because of the rapid increase in new development. While the
effectiveness of these controls has not been monitored, the water quality in
the large receiving water lakes surrounding Bellevue indicates that water
quality has stabilized.
Much of the water quality improvement in these lakes was a result of the
diversion of all sanitary wastewater flows. However, after sanitary flows
were diverted, the emphasis and potential quality impacts of urban runoff
increased, as shown in Table 24 for phosphorus in Lake Sammarnish. Further
water quality improvement would therefore necessarily be centered on control
of urban runoff.
Table 24. PHOSPHORUS LOADS TO LAKE SAMMAMISH
BEFORE AND AFTER DIVERSION OF SANITARY FLOWS [7]
Phosphorus load, Ib/yr
Percent of total
Source
Waste discharges
Runoff
Precipitation
Total
Before
diversion
16,500
25,300
2,200
44,000
After
diversion
1,100
25,300
2,200
28,600
Before
diversion
37
58
5
100
After
diversion
4
88
8
100
Similar trends can also be seen in Lake Washington. The phosphorus load in
the lake sharply decreased after sewage diversion in 1968, as shown in Table
25. Most of the phosphorus load after 1970 is a result of stormwater
discharges and stream inflows.
Socioeconomic Impacts
The socioeconomic impacts of the required runoff controls and the development
of the stormwater utility have been mixed; but through a public education
program, the stormwater program is well accepted and supported. The
principal socioeconomic impacts are aesthetics and multiuse potential of the
controls. Most of the major drainage system improvements are planned as
instream or wetlands controls; therefore, there will be no displacement of
people.
63
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Table 25. ESTIMATES OF ANNUAL PHOSPHORUS LOADS
TO LAKE WASHINGTON3 [8]
Lake inflow, Total phosphorus Dissolved phosphorus
Year million ft-3 load, Ib/ft2-yrt> load, Ib/ft2-yrb
1957
1962
1964
1970
1971
1972
1973
1974
1975
34.4
34.1
54.9
42.7
54.4
53.5
31.7
47.0
52.3
0.25
0.53
0.47
0.10
0.09
0.20
0.06
0.10
0.15
0.22
—
--
—
—
0.05
0.03
0.04
0.02
a. Does not include Seattle's storm sewer or combined
sewer discharges.
b. Based on a lake area of 943,100 ft .
Maintaining the natural appearance of the open streams is a key element in
the control and improvement of the drainage system. Controls on private land
and new developments are reviewed for consistency of scale and materials used
in relationship to the surrounding land use, topography, and general
aesthetics.
The city and a number of developers have incorporated stormwater controls
into multiuse facilities. These include permanent pool, instream detention
ponds at a condominium development and a series of dry, offline detention
ponds integrated into a recreational park along a major avenue, as shown in
Figure 16.
The development of the utility provided a public educational benefit that has
also aided in the control of stormwater pollutant loads to receiving waters.
During the creation of the utility, a public vote on alternative financing
methods indicated that the public was disenchanted about receiving a utility
bill for stormwater management services. Subsequent voter approval of the
utility rate structure occurred after a public education program explained
the financing alternatives and the expected water quality benefits of
controlling runoff. Because of the increased public awareness of potential
costs and water quality, there has been significantly less dumping of oil and
debris in catchbasins and neighborhood housekeeping practices, such as
cleaning up pet wastes and litter control, have increased.
64
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Figure 16. Multiple stormwater facilities: (a) stormwater detention
pond integrated into the development landscaping, and (b) detention ponds
integrated into a roadside park--ponds are grassed depressions.
65
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SECTION 5
SOURCE DETENTION OF URBAN RUNOFF
MONTGOMERY COUNTY, MARYLAND :
Montgomery County and the region surrounding Washington, D.C., have been
using source controls to limit urban runoff. In 1965, initial sediment
controls were established to correct erosion problems, and state and local
ordinances setting basic control requirements were developed and adopted in
the early 1970s. Montgomery County adopted an ordinance in 1971 stating that
the 2 year storm shall be stored and released at predevelopment rates. This
ordinance applies to all new development.
Over 800 source control facilities have been constructed in Montgomery
County. Most are small individual controls--wet and dry detention ponds;
underground storage vaults; parking lot and rooftop storage; and
infiltration/percolation systems. Detention ponds are the most common
control and are used extensively in residential and industrial developments.
The detention ponds were primarily developed for construction sediment
control and postconstruction volume (flood/drainage) control, but other
benefits, including pollution control, recreation, and aesthetics, are now
being realized.
The source control strategy in Montgomery County is moving toward larger
control areas, 80 to 240 ha (200 to 600 acres), representing basinwide
applications. A basinwide management plan and several source detention sites
are used as examples to illustrate control for large areas serving several
different land uses. These sites have incorporated, where possible, multiuse
features.
REGIONAL APPROACH TO RUNOFF CONTROL
The control of nonpoint sources of pollution in the Washington, D.C., area is
becoming increasingly important, particularly as dry-weather treatment levels
increase. Many of the existing water quality problems have been traced to
urban runoff. As rural areas become developed, the problems of and the
potential for flooding, erosion, and sediment deposition in the natural
streams draining the region have increased. Montgomery County, by adopting
and enforcing source control ordinances, has one of the most advanced
programs in the nation.
Area Characteristics
Montgomery County is northwest of Washington, D.C., and borders on and drains
into the Potomac River, as shown in Figure 17. The topography consists of
66
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rolling hills with slopes ranging from 5 to 10%. The soils in the uplands
are well drained and subject to moderate erosion. The soils along the
natural drainage courses are poorly drained and subject to high erosion.
WASHINGTON
/ D.C.
VIRGINIA
MARYLAND
UC v
A MS \ FAIRFAX
\rv^ CO.
PRINCE
GEORGES
CO.
Figure 17. Location of Montgomery County, Maryland,
and the surrounding Washington, D.C., area.
The mean annual precipitation in the county is approximately 104 cm (41 in.),
with about 57% of the precipitation occurring from April through September.
Thunderstorms occur on an average of about 30 days per year and 75 to 80% of
these occur during the summer months [1].
Montgomery County has been changing from a rural, agricultural area to an
urban area with single-family and high-density residential developments.
.Commercial and light industrial centers have also been constructed and have
also contributed to the change in runoff characteristics of the land. The
projected changes in land use for a 5,750 ha (13,200 acre) drainage area in
Montgomery County are shown in Table 26. The change in land use, based on
the ultimate development capacity using future land use projections and
existing zoning, could be representative of the long-range urbanization
process in the Washington, D.C., area [2].
Potential urban runoff impacts of development can be associated with the
increase in residential land use. Although changes in commercial and
.industrial land use are only a fraction of the residential increase, equal
67
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attention and controls should be provided because these areas are highly
impervious and have high use intensities.
Table 26. PROJECTED CHANGES IN LAND USE IN
THE WATTS BRANCH DRAINAGE AREA [2]
Land use
category
Total residential
Commercial
Industrial
Institutional
Open recreation
Agricultural, vacant land
Surface water (ponds)
Total
1975 land
use, acre
2,720
135
401
295
774
9,862
21
14,208
Projected long-
range land use,
acre
9,952
167
1,498
435
1,883
252
21
14,208
Change in ,
total drainage
area, %
50.9
0.2
7.7
1.0
7.8
-67.6
0
0
Problem Assessment
The increased urban runoff from developing areas creates both quantity
problems and water quality problems. Source controls were originally
implemented to limit flooding, erosion, and sediment deposition. However,
with increasing water quality goals and standards, source controls have also
been recognized as providing pollutant reduction benefits. Eroding soils and
pollutants washed off land surfaces are directly influenced by the rate and
volume of runoff from the urban area.
Quantity--
Channel and stream erosion and local flooding have been caused by increased
flow from developed and developing areas in Montgomery County. The flow
problems are intensified in the smaller, upper drainage areas and are
moderated as the size of the drainage area increases. In the upper portions
of the Watts Branch watershed, the increase in peak flow from existing
development was estimated at over 110% of the natural predevelopment rates
for the 2 year storm. Yet, for the entire drainage area, almost 12 times the
area of the upper portion, the increase in peak flow for the 2 year storm was
estimated at about 10% [2]. These increased flows in the upper watersheds
can cause substantial erosion, flooding, sediment deposition, and stream
damage.
Erosion from these flows has cut into natural streams and doubled their
original predevelopment width, undercut stream banks and structures, and
damaged vegetation. Massive sediment yields are washed into receiving waters
or are deposited in streams. Erosion losses as high as 3,800 m^/km^-yr
(8,000 yd3/mi2-yr) have been estimated for the Watts Branch drainage area
[2]. Results of stream erosion problems are shown in Figure 18.
68
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Figure 18. Flow, erosion, and sediment deposition problems: (a) stream
subject to increased flow from developed watershed, (b) stream channel
over twice natural width--steep eroded banks have been riprapped,
(c) sediment deposition of eroded material in half of bridge culvert,
and (d) undercut and damaged vegetation.
69
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Quality—
Substantial pollutant loads are associated with urban runoff, and eroded
material can potentially contribute most of the suspended solids and
phosphorus loads. Storm flows in the 5,750 ha (14,200 acre) Watts Branch
drainage area annually contribute over 1,485,000 kg (3,270,000 lb) of
suspended solids, or about 88% of the total annual load [2]. Estimated
annual
27.
storm flow loads for BODr, nitrogen, and phosphorus are shown in Table
Table 27. ESTIMATED ANNUAL STORM FLOW LOADS IN THE
WATTS BRANCH DRAINAGE AREA [2]
Constituent
Suspended solids
BOD5
Nitrogen
Phosphorus - P04
Annual
storm flow
load, Ib/yr
3,270,000
240,000
33,000
8,000
Storm flow,
% of total
annual load3
88
86
43
64
Unit loading,
lb/acre-yr
230
17
2.3
0.6
a. Total annual load = base flow load + storm flow load.
In the Washington, D.C., area, pollutant concentrations in urban runoff
increase slightly for land uses with higher impervious areas or a higher
intensity of use. Commercial land use, as shown in Table 28, generally has
the highest concentrations, particularly for lead and zinc.
Table 28. POLLUTANT CONCENTRATIONS IN URBAN RUNOFF FOR
SEVERAL LAND USES IN THE WASHINGTON, D.C., AREA [3, 4]
Pollutant concentration, mg/L
Land use
Total Total
COD nitrogen phosphorus
Lead
Zinc
Low density residential
Medium density residential
High density residential
High rise residential
Commercial
70-120
80-1 30
70-90
50-100
90-120
2-4
2-3
2-3
1-2
2-3
0.3-0.4
0.3-0.4
0.3-0.5
0.2-0.3
0.2-0.3
0.05-0.1
0.1-0.2
0.1-0.2
0.1-0.2
0.3-0.5
0.02-0.1
0.05-0.2
0.05-0.3
0.1-0.2
0.1-0.4
Pollutant loads vary significantly between different land uses with different
impervious characteristics. Generally, land uses in high impervious areas,
such as high density residential and commercial, yield the highest runoff
volume and the highest pollutant load. The mean and the range of pollutant
loads for sampled urban runoff are shown in Table 29. The loads are
presented on a daily basis and represent discrete storm loadings.
70
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Table 29. STORM POLLUTANT YIELDS FOR SEVERAL LAND
USES IN THE WASHINGTON, D.C., AREA [5]
Pollutant yield, Ib/acre-d
Land use
Number Suspended solids Total nitrogen Total phosphorus
of :
storms mean range mean range mean range
Low density residential
Medium density residential
High density residential
High rise residential
Commercial
Rural
Agricultural
23
42
64
21
50
6
32
15
21
31
18
43
29
83
0-110
1-270
0-190
0-120
0-320
0-170
0-620
0.2
0.7
0.6
0.8
1.0
0.1
0.6
0-0.7
0.1-5.1
0-2.9
0-3.7
0-6.5
0-0.3
0-4.9
0.04
0.12
0.11
0.10
0.12
0.01
0.18
0-0.2
0.01-0.8
0-0.5
0-0.4
0-0.8
0-0.1
0-2.1
Source Control Approach - Regulatory Requirements
The regulatory objective of source controls in Montgomery County, originating
with the SCS concepts of mitigating increased flooding, sedimentation, land
erosion, and accelerated streambank erosion, is to limit urban runoff to
natural predevelopment rates and volumes. Water quality benefits from the
control of urban runoff have also been recognized. Through a program of
regulatory requirements, a large number of source controls have been
constructed and design concepts have been improved.
All new development and construction, which increase the impervious area,
must have stormwater management facilities. These facilities can include
onsite controls, those located on the site being controlled, or offsite
controls, those that are located downstream from the area to be controlled
and can control a number of developments. Offsite controls, generally
located in a stream or drainage valley, are designed to control subwatersheds
or whole drainage areas and are used in place of many small onsite controls.
Certain exemptions are allowed under the regulatory policy for development
sites having minimal land disturbance or small percentages of impervious
area. Waiver applications for detached single family residential
developments may be allowed for the following minimum lot sizes within
certain subdivision sizes [6]:
Minimum lot size Maximum subdivision size
1 acre
0.5 acre
15,000 ft2
9,000 ft2
6,000 ft2
10 acres
5 acres
2 acres
2 acres
2 acres
71
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Exemptions are also considered for multifamily residential, industrial,
commercial, and institutional developments meeting developed area and
impervious area limitations. Developments that expand beyond the limits set
by a waiver would be required to provide stormwater management controls for
the entire site.
The regulations require the control or storage of stormwater runoff in excess
of the natural predevelopment flow from the 2 year storm. Other than
requiring source controls, there is currently no mechanism for enforcement of
maintenance once the facilities are built. The role of the county in
enforcing the stormwater management policy is limited to design review and
approval, permit issuance, and inspection during construction. On private
land, the owner must maintain the control facility.
Implemented Controls
A large number of stormwater management controls are used in Montgomery
County, including detention ponds, underground storage vaults, parking lot
storage, rooftop storage, and infiltration systems. Over 42% of all controls
used are detention ponds, 69% of which are associated with residential
developments. A number of infiltration systems have also been used, but
principally for institutional, commercial, and industrial developments.
These can include detention/infiltration ponds and infiltration pits (dry
wells). A summary of the distribution of 832 control measures by type and
land use is presented in Table 30.
Table 30. SUMMARY OF STORMWATER CONTROLS BY LAND
USE IN MONTGOMERY COUNTY [7]
Land use
Residential
Institutional
Commercial
Industrial
Unclassified
Total
Total
number
of
controls
256
274
112
93
97
832
Stormwater
controls,
% of total
31
33
13
11
12
100
Distribution by type, of control, % •
Detention
pond
69
26
25
32
42
42
Underground
storage
3
10
8
8
11
7
Parking lot
storage
3
2
'9
13
4
4
Rooftop
storage
9
14
20
25
6
14
Infiltration
system
16
48
38
22
36
33
Most of the controls have been constructed on development sites of less than
4 ha (10 acres). Of the planned controls shown in Table 31, the most widely
used controls for developments of 0.4 to 2 ha (1 to 5 acres) are infiltration
systems, underground storage vaults, dry detention/sedimentation ponds,
swales, rooftop storage, and parking lot storage. Detention ponds are used,
almost exclusively, for larger control areas, and dry ponds represent the
largest fraction of controls used for all drainage area sizes.
72
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Table 31. SUMMARY OF PLANNED STORMWATER CONTROLS
BY DRAINAGE AREA SIZE IN MONTGOMERY COUNTY [6]
Type of control
Infiltration systems
Underground vaults
Dry pond
Wet pond
Underground pipe
Swales
Rooftop storage
Parking lot storage
Total
Controls by area, %
Total
number
of
controls
47
29
53
10
9
19
16
10
193
--
Stormwater
controls,
% of total
24
15
28
5
5
10
8
5
100
—
Number
1-5
47
29
20
--
6
15
16
10
143
74
of controls by drainage area category
6-10
—
--
11
~
2
3
-- •
—
16
8
11-50
—
~
16
4
1
—
~
21
11
51-100
—
— .
4
3
—
1
~
—
8
4
101-500
—
--
1
1
-,-
--
--
2
1
, acres
>500
—
. ~
1
2
—
—
~
—
3
2
Some of the facilities under construction will serve larger drainage areas or
several developments to reduce costs and maintenance problems. Larger
facilities may realize some economies of scale, where small systems, less
than 4 ha (10 acres), would not. Most of the larger facilities being
constructed are detention ponds with permanent pools, which have better
pollutant reduction capabilities in addition to flow control. The permanent
pool volume acts as a sink that traps pollutants from the stormwater flows,
providing increased removals because of the long detention times in the pool.
An example of areawide source control planning and examples of individual
detention facilities in Montgomery County that represent the most promising
source control technology are discussed in the following.
Watts Branch Management Plan--
Three alternative stormwater management concepts were evaluated to control
urban runoff from the 5,750 ha (14,200 acre) Watts Branch drainage area.
These included (1) offsite headwater or tributary small-scale detention
facilities; (2) onsite detention for individual developments; and (3) land ,
use control within the 100 year flood plain, with no structural controls [2].
The offsite detention facilities would include a system of small-scale ponds
from 0.8 to 4.0 ha (2 to 10 acres) in size, designed to control flowrates and
volumes from the 2, 10, and possibly 100 year storm, at predevelopment
levels. The detention pond system would have permanent pools; to increase
pollutant removal. This type of detention can be used not only to control
runoff from developing sites, but also to control runoff from developed
areas.
Although onsite detention facilities can control peak flowrates for a design
frequency storm, usually a 2 year storm, they fall short of their intended
efficiency with flows from any storms different than the design frequency.
73
-------
Also, onsite detention is applicable only to developing sites; site
conditions, such as available area, size requirements, soils, or topography,
can limit the effectiveness of onsite controls. Most onsite controls are dr
ponds that are used to reduce erosion and sedimentation problems.
Maintenance responsibilities are usually passed on by the developer to the
owner, who may or may not maintain the controls.
dry
The recommended plan for the Watts Branch drainage area involves portions of
all three alternatives, but the principal effort is contructing small-scale
offsite detention ponds to control flows up to the 100 year storm. Where
offsite controls are not feasible, developers would construct onsite controls
within easements dedicated to the local agency or the county for maintenance.
The plan also recommends that the county continue to acquire flood plain land
for parks and prohibit filling or construction within the 100 year flood
plain [2].
The required facilities for onsite and offsite alternative plans, together,
with estimates of total costs and expected annual benefits, are compared in
Table 32. Although the costs of offsite controls are slightly higher than
the costs of onsite controls, the expected annual benefits are greater for
offsite controls.
Table 32. COMPARISON OF ONSITE AND OFFSITE CONTROLS
FOR THE WATTS BRANCH DRAINAGE AREA [2]
Alternative
Parameter
Onsite
flow control
Offsite flow
and quality control
Drainage area controlled,
acre
Storage
Flowrate control
storage, 10^ ft3
Hater quality control
storage, 106 ft3
2,960
6.97
0
6.97
2,900
9.19
2.66
11.85
106 ft3
Estimated number of structures
Estimated land requirement, acres
Annual costs^
Annual capital, $b
Operation and maintenance, $
Total annual costs, $
Annual benefits, $°
Net annual cost, $
105
80
907,000
125,000
1,032,000,
86.700
945,300
6
72
1,050,000
84,000
1,134,000 , ,
346,700
787,300
a. ENR 3000.
b. Capital costs include construction, land, and design, annualized
at 7% interest for 10 years.
c. Includes savings in water treatment plant costs, land loss and
sediment damage, damage to vegetation, and cleanup.
74
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Source Detention Facilities--
A number of individual detention facilities are planned or are being
constructed in Montgomery County. Most of these facilities are offsite
controls; however, some onsite controls are also being constructed.
Crabbs Branch Facilities--In the Crabbs Branch drainage area, two detention
facilities are being constructed: an onsite detention pond to control
stormwater runoff from a complex of county government warehouses and
maintenance depots and an offsite detention pond to provide overall control
for the developing watershed.
The onsite detention pond provides sediment control for an 18 ha (45 acre)
county service park, still under construction. The pond is a rectangular
basin, 122 m by 43 m (400 ft by 140 ft) with a pool surface area of,about 0.5
ha (1.3 acres), as shown in Figure 19. Runoff enters the pond at two points
through reinforced-concrete drains. The basin outlet is a 1.2 m (48 in.)
diameter, nonperforated corrugated metal pipe riser and barrel. The T.8 m (6
ft) high riser is topped with a closed lid and an antivortex hood equipped
with a trash rack to retain debris and other floating material. The riser
also has a 10.2 cm (4 in.) hole to maintain the pool level 0.46 m (1.5 ft)
below the crest of the riser. A grassed overflow channel is also provided as
an emergency spillway. The basin will be retained as a permanent flow
control pond after the service park is completed.
Figure 19. Onsite sediment detention pond for
developing county service park.
75
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Downstream from the service park, an offsite detention pond will be
constructed to control up to the 100 year storm flows from a 238 ha (590
acre) tributary area. The facility includes an earthfill dam, a permanent
pool to collect sediment and other settleable pollutants, a principal
spillway, and an outlet structure, as shown in Figure 20. The detention pond
is expected to reduce peak stormwater runoff discharge rates to or below
estimated 1973 levels for the 2 year, 3 hour; 10 year, 3 hour; and the 100
year, 24 hour £torm events. The spillway and outlet structure were also
designed to convey the Maximum Probable Flood without overtopping the dam.
The design features of the detention facility are described in Table 33.
Table-33. DESIGN FEATURES OF THE CRABBS BRANCH
OFFSITE DETENTION FACILITY [8]
Description
Value
Watershed area, acres 590
Storage pool
Permanent pool area, acres 6.8
Permanent pool volume, 1.4
TO6 ft3
Design storage pool area, 21.0
acres
Design storage volume, 7.5
10? ft3
Outlet structure
Riser dimensions, ft 22.0 x 20.0
Riser height, ft 19
Orifice size (2 yr flow) 2.17 x 1.0
2 each, ft
Orifice size (10 yr flow) 16.0 x 3.0
2 each, ft
Weir length (100 yr flow) 20.0
2 each, ft
Weir length (MPF) 22.0
2 each, fta
Outlet conduit size, ft 9.5 x 9.5
Outlet length, ft 145
a. Maximum probable flood.
Montgomery Mall Lake--An offsite stormwater storage/detention pond was
constructed to control a 60 ha (148 acre) subwatershed of Cabin John Creek in
Montgomery County. The drainage area includes the impervious area of a large
shopping mall, several apartment complexes, townhouses, a major highway, and
several secondary roads.
76
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EXISTING
STREAM
DESIGN
tOO yr
HI6H WATER
POOL
a. PROPOSED OFFSITE DETENTION BASIN
b. DETAIL OF OUTLET STRUCTURE
Figure 20. Proposed Crabbs Branch detention basin to
control up to the 100 year storm flow [8].
77
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Drainage from the area enters the pond at six different locations, with the
major inflow point serving the shopping mall and draining about 38 ha (95
acres) of the watershed. This overflow point enters the head end of the pond
through a 1.7 m (66 in.) pipe.
The pond has a 2.4 ha (5.9 acre) permanent pool, about 229 m by 107 m (750 ft
by 350 ft), and is used to limit peak storm flows and to reduce pollutants.
The level of the pool, which has about 45,600 m3 (1,600,000 ft3) of dead
storage capacity, can vary from 0.9 to about 4 m (3 to 13 ft). The primary
outflow is controlled by a 0.6 m (24 in.) corrugated metal pipe riser, topped
with a 0.9 m (36 in.) hood. The emergency overflow is controlled by a 12 m
(40 ft) grassed overflow channel and a concrete, riprapped crest about 0.9 m
(3 ft) above the riser crest [9]. The-detention.,pond, shown in Figure 21, is
designed to control an-inflow of 0.6 m /s (22 ft /s) and to release the
volume at about 0.06 m /s (2 ft3/s).
Wheaton Branch Facility—An offsite stormwater detention pond is being
constructed to control runoff from a 314 ha (775 acre), totally developed
area on the Wheaton Branch in Montgomery County [10]. The detention
facility, a dry pond, will be drained following a storm. The design flood
storage will cover about 6.3 ha (15.5 acres) and will have a total storage
capacity of about 138,000 m3 (4,880,000 ft3). The pond will control flows
from the 2 to 100 year storm. A summary of the expected operation of the
facility for various storm frequencies is presented in Table 34. The
facility is shown in Figure 22.
Design Considerations
The two different strategies for controlling urban runoff by detention,
onsite and offsite storage, are designed to control at least the peak
flowrates of the 2 year storm to meet regulatory standards and prevent
downstream erosion and sediment deposition. Most large offsite controls are
also designed to control several different, more extreme runoff events, up to
the 100 year storm flow, while most small onsite controls are designed for a
2 year return period. Designing for one return period storm may not provide
the desired level of control for return period storm flows that are different
from the design storm.
The results of a mathematical model simulation to evaluate how various
control designs affect the flows from different return period storms are
shown in Table 35. The values represent a control design—the
postdevelopment peak discharge for a given return period storm equals the
release rate for the specified predevelopment return year storm. The impacts
of different storm frequencies other than the design frequency are also shown
in Table 35.
Other elements that affect stormwater control performance and that should be
considered in design include:
• Storm duration
• Control facility maintenance
• Soil characteristics
• Downstream velocity and channel
78
erosion potential
-------
Figure 21. Montgomery Mall Lake offsite storage/detention pond:
(a) permanent pool lake with dam in background, (b) .principal 66 in.
inflow pipe at head end of lake, (c) emergency overflow spillway, and
(d) pond overflow point to receiving stream equipped with a flow recorder.
79
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Table 34. EXPECTED HYDRAULIC OPERATION OF THE WHEATON BRANCH
OFFSITE, DRY DETENTION POND
Storm
frequency
2 yr-24 hr
5 yr-24 hr
10 yr-24 hr
50 yr-24 hr
100 yr-24 hr
Design storma
Pond
surface
area,
acres
4.9
6.9
7.7
7.9
8.5
15.5
Storage
ft3
620,000
1,440,000
2,220,000
2,620,000
3,250,000
4,880,000
Watershed, in.
0.22
0.51
0.79
0.93
1.16
1.73
Peak flow, ft3/s
Inflow
840
1,270
1,680
2,250
2,690
4,050
Outflow
510
590
820
1,100
1,930
2,790
a. An 11 in. rainfall in 6 hours.
APPROXIMATE
10 yr STORM
STORAGE LEVEL
Figure 22. Proposed plan of the Wheaton Branch
offsite dry detention pond.
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Table 35. RESPONSE OF PEAK STORM FLOW THROUGH A DETENTION
SYSTEM DESIGNED FOR SPECIFIC RETURN PERIODS [11]
Storm flow return
period, yr
Level of control /design
3
Predevelopment flow, ft /s
2 10 TOO
- 9 29 77
Post development flow through
design control, ft3/s
2 yr storm released at 9 22 42
2 yr predevelopment rate
10 yr storm released at 14 29 57
10 yr predevelopment rate
10 yr storm released at 6 9 27
2 yr predevelopment rate
100 yr storm released at 15 33 77
100 yr predevelopment rate
The design approach evaluating stormwater detention controls should consider
all of these elements to more closely anticipate how a control will respond
to a storm. Several design approaches were evaluated by comparing their
accuracy, training requirements, relative design costs, and applicability of
using or determining the effects of the design elements previously listed;
the comparison is given in Table 36.
Table 36. COMPARISON OF HYDROLOGIC DESIGN APPROACHES
FOR STORMWATER DETENTION FACILITIES [11]
Design approach
Evaluation criteria
Unit-
Graphical Empirical hydrograph
Conceptual models
Simple Complex
Design elements
Multiple return period
Storm duration
Maintenance
Soil characteristics
Downstream effects
Accuracy
Training requirements
Relative design cost
No
No
No
No
No
Low
Low
Low
Maybe
Maybe
No
No
No
Low
Low
Low
Yes
Yes
No
Maybe ,
Maybe
Medium
Med i urn
Medium
Yes
Yes
Maybe
Yes
Yes
High
Medium
Medium
Yes
Yes
Yes
Yes
Yes
High
High
High
The method, outlined in Urban Hydrology for Small Watersheds, Technical
Release No. 55, 1975, by the SCS, is used in Montgomery County and is an
example of a simple conceptual model [12].
81
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Montgomery County uses a solid riser design rather than a perforated riser
design to maintain a permanent pool to maximize pollutant removal. Studies
indicate that peak flow reduction is a function of the riser characteristics
(height and size), and that sediment and pollutant trapping is affected by
the size and shape of the pool [13],
PERFORMANCE OF STORMWATER DETENTION PONDS
Stormwater detention ponds, originally used to reduce peak storm flowrates in
downstream segments, also reduce pollutants. Although performance data are
limited, some information on efficiency has been compiled and used in
predicting system response to various storm durations and frequencies.
Peak Flow Reduction
Both onsite and offsite detention ponds reduce peak storm flows. Flow
reduction efficiencies can approach about 90% for flows at or less than the
design storm flow.
Thirty-six storms were monitored during 1977 at the onsite detention pond
serving the Montgomery County service park. Peak flow reduction was
consistently about 90% for the small volume, short duration storms. Peak
flow reduction dropped to abouto60% for a larger volume, longer duration
storm flow that peaked at 3.3 m /s (177 ft3/s) [13].
The expected operating conditions for the Crabbs Branch offsite detention,
pond are summarized in Table 37. These larger facilities using multiple
stage risers can reduce peak flows from multiple return period storms to or
below base or predevelopment levels.
Table 37. SUMMARY OF EXPECTED FLOW REDUCTION PERFORMANCE FOR
THE CRABBS BRANCH OFFSITE DETENTION POND [8]
Peak discharge, ft3/s
Projected Projected
condition condition Pool elevation 1973
Flood flow 1973 flow without with ftbove permanent discharge,
frequency, yr conditions detention detention pool elevation, ft %
2
10
100
Maximum
Probable Flood
273
600
1,139
5,180
892
1,591
2,623
5,793
269
439
913
4,234
4.7
6.4
8.5
15.0
99
73
80
82
Pollutant Trap Efficiencies
Pollutant trap efficiencies were monitored at the Montgomery Mall Lake
detention facility and are presented in Table 38. The median trap efficiency
values suggest that pollutant removal can be high if the basin is properly
designed for increased pollutant removal, i.e., having large permanent pool
storage volume.
82
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Table 38. MEDIAN POLLUTANT TRAP EFFICIENCIES, MONTGOMERY
MALL LAKE OFFSITE DETENTION FACILITY [9]
Parameter
Inflow Outflow Trap
Units rate rate efficiency,
BOD5
BOD2Q
COD
TOC
Orthophosphate
Total phosphorus
Ammonia - N
Zinc
Cadmium
Lead
Iron
10"3 Ib/s
10~3 Ib/s
10"3 Ib/s
10"3 Ib/s
10"3 Ib/s
10~3 Ib/s
10"3 Ib/s
10"6 Ib/s
10~6 Ib/s
10"6 Ib/s
10"6 Ib/s
11.4
19.7
60.5
13.2
0.3
0.7
6.8
0.5
0.4
0.4
10.3
0.4
1.4
2.1
0.5
0.02
0.007
0.02
0.003
0.006
0.02
0.4
97
93
97
96
93
99
99
99
98
96
96
Sediment trap efficiencies for the Montgomery County service park facility
averaged better than 92%. Smaller storms produced better removals, but no
monitored storms produced less than 88% [13]. A determinisitic model was
used to evaluate the effects of different storm durations and frequencies,
using a nine storm data set monitored at the service park facility. The
predicted pollutant trap efficiencies of the detention basin for the 2 and 10
year storms are summarized in Table 39.
Table 39. PREDICTED TRAP EFFICIENCIES FROM- STORMS
OF VARYING DURATION AND FREQUENCY [11]
Percent
Parameter
BOD5
BOD2Q
COD
TOC
Orthophosphate
Total phosphorus
Ammonia - N
Zinc
Cadmium
Lead
Iron
2 yr
0.5
91
.- 59
76
84
69
22
92
98
79
97
92
storm duration, h
1.0
88
57
67
80
65
17
90
97
77
97
90
2.0
86
49
57
75
56
15
88
96
70
96
87
6.0
84
37
48
70
40
7
89
94
55
94
81
10 yr
0.5
86
54
57
76
62
17
85
96
76
97
89
storm
1.0
81
45
45
68
. 50
13
77
95
73
96
85
duration, h
2.0
79
37
37
64
41
10
76
94
68
96
83
6.0
80
27
36
63
29
3
82
93
55
94
80
83
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COSTS
Most onsite controls are provided by the developer during construction of the
development, and the capital costs, including land costs, can be passed on to
the eventual owners of the development. The .recent directions of Montgomery
County's stormwater management program toward larger offsite tributary
controls enable developers to contribute to the cost or construction of the
offsite facility that would control runoff from the development. Offsite
controls are less expensive and are easier to maintain than many privately '
owned, small structures. A planning study for the Watts Branch drainage area
includes six offsite detention ponds and has evaluated the control costs.
Individual cost estimates are available for several planned facilities, and
the SCS has developed cost guidelines and estimating procedures.
Watts Branch Management Plan Costs
The total capital cost to provide offsite stormwater controls for
approximately 1,175 ha (2,900 acres) of the Watts Branch drainage area is
about $7,375,000 (ENR 3000). The capital cost estimates include
construction; land acquisition; and planning, design, and supervision, as
summarized in Table 40. The average cost for the controlled area is about
$6,300/ha ($2,500/acre), and the storage cost is about $22/m3 ($0.62/ft3) of
total storage capacity, basinwide [2]. The estimated operation and
maintenance costs are about $84,000/yr, or about $72/ha-yr ($29/acre-yr) of
controlled area.
Table 40. ESTIMATED BASINWIDE COSTS of OFFSITE
STORMWATER DETENTION FOR THE WATTS BRANCH DRAINAGE AREA9 [2]
Cost component
Cost, $
Construction . 4,195,000
Landb 2,128,000
Planning, design, and supervision 1,052.000
Total capital costs 7,375,000
a. ENR 3000.
b. Estimate of land is at full value (about
$30,000/acre) even though much of the land
is publtcly owned.
Offsite Detention Facility Costs
The capital costs of the Crabbs Branch and the Wheaton Branch detention
facilities are summarized in Table 41. The costs are present estimated
program costs that include planning, design, supervision, land, site
improvements and utilities, and construction.
84
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Table 41. ESTIMATED CAPITAL COSTS FOR OFFSITE
STORMWATER DETENTION FACILITIES3 [14]
Facility
location
Crabbs Branch
Wheaton Branch
Service
area,
acres
590
775
Storage
capacity,
10° ft3t>
7.54
4.88
Design
325
162
Costs , $1 ,000
Site improvements
Land and utilities
635 62
68
Construction
1,166
818
Total
2,188
1,048
Service
area cost,
$/acre
3,700
1,350
Storage
cost,
$/ft3
0.29
0.21
a. ENR 3000.
b. Storage in excess of permanent pool capacity.
Cost Estimating
Unit cost estimates for stormwater detention ponds in Montgomery County were
used by the SCS to develop a first-cut method of estimating costs of onsite
stormwater detention facilities. A graphical presentation of the estimated
costs is shown in Figure 23.
100.000
CO
o
o
- 10.000
CO
CO
z
o
u
1 ,000
1.000
10.000
100,000
1.000.000
DETENTION STORAGE VOLUME, ft3
Figure 23. Onsite stormwater detention pond cost curve, ENR 3000 [6].
85
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Several variables were used in a statistical analysis to develop the costs;
these include (1) controlled drainage area, (2) storage volume,
(3) percentage of impervious area, (4) cost/impervious area, and
(5) cost/unit storage volume. There was no strong linear relationship
between any of the variables; however, there was a stong curvilinear
relationship (r2 = 0.856) between storage volume (V ) and cost/storage
volume. These were used to develop the following equation for predicting
costs [6]:
$/ft3 = 106 vs"°'517
(Equations are adjusted to represent ENR 3000 costs.)
Total costs can be estimated by rearranging this equation:
$ = 106 V +0'483
IMPACTS
With the exception of erosion and sediment deposition, few relationships
between stormwater runoff and adverse environmental impacts have been
supported by data collection programs. Socioeconomic impacts may actually be
more apparent; both onsite and offsite source detention practices have
reduced downstream flooding.
Environmental Impacts
In Montgomery County, runoff from developed and developing areas is
transported by receiving streams, and source detention controls potentially
reduce both water quantity and quality impacts to this environment. Quantity
impacts include (1) flooding; (2) land surface erosion; (3) stream channel
erosion; (4) downstream sediment deposition; and (5) reduced groundwater
levels and reduced base stream flow resulting from increased runoff from
impervious areas and less soil infiltration. Water quality impacts include
suspended particles (including sediment), nutrients, toxics, debris,
bacteria, and oxygen depletion.
Although no monitoring data are available to quantify the magnitude of the
quality impacts to receiving waters, estimates indicate that annual storm
flow pollutant loads for BOD and suspended solids are over seven times the ...
annual base flow loads. The impacts of these increased loads are
particularly important where the downstream environment or flow is
influenced. At a water treatment facility with the water intake influenced
by flows from the Watts Branch drainage area, an average additional 21,000
kg/d (46,000 Ib/d) of sediment is removed during moderate to heavy storm
flows. This condition occurs about 90 d/yr [2].
Environmental impacts of erosion and sedimentation can be seen in stream
channels draining developed areas without source controls. Undercut and
damaged vegetation, steep eroded stream banks over twice their original
width, undermined structures, and deposits of eroded material are evidence of
86
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the damage potential. In watersheds where controls are used, environmental
damage has been held to a minimum, and streams have retained their natural
state.
While many of the environmental impacts of uncontrolled storm runoff are
aesthetically undesirable, many also create hazardous conditions and have
significant socioeconomic impacts.
Socioeconomic Impacts
Properly designed detention controls, which consider downstream flows and
peak flow timing, can effectively reduce flooding, erosion, and property
damage. An estimate of the annual cost of damage to the Watts Branch
drainage area from storm flows (without controls) was approximately $430,000
(ENR 3000) [2]. This cost was attributed to the following:
Land loss and sediment-generated damages 27%
Cleanup and minor repairs 2%
Personal inconvenience 1%
Additional water treatment 56%
Damage to stream side vegetation 14%
Many offsite permanent pool detention ponds are designed to provide multiuse
benefits, primarily recreational and aesthetic in nature, as shown in Figure
24. Several detention ponds have restricted access for safety reasons;
however, even these ponds can provide aesthetic benefits and are frequently
integrated into developments as a part of the overall architectural setting.
Most of the planned offsite detention facilities are located on vacant
private land or on publicly owned land and do not require relocation of
residences or business. Onsite facilities planned within individual
developments require a portion of the developable land; consequently, land
costs and the loss of potential profits from the developable land are often
passed on in higher costs to the remaining development. Maintenance of the
facility also represents a cost to the owner(s) and is often neglected. The
movement toward larger offsite facilities lessens the burden of the private
developer/owner and potentially reduces the overall unit cost of the control
measure.
Proposed funding of offsite controls can come from the county and
contributions made by the developer in lieu of providing onsite controls.
Depending on the method of county financing, bond or tax, part of the
financial burden for the controls may be placed on the public, who benefit
from stormwater control.
Public acceptance of offsite multiuse detention ponds is favorable when the
pond is integrated into the initial development planning. The pond is
considered a selling point, with adjacent lots bringing a premium. However,
placing a stormwater detention pond in an existing development has sometimes
met with opposition.
87
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Figure 24. Multiuse stormwater detention facilities, Montgomery County:
(a) portion of a detention pond with a permanent pool to detain flows
from (b) the highly impervious parking lot of shopping mall;
(c) landscaped detention pond integrated into a residential
development; and (d) control outlet and dam.
88
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SECTION 6
LAND USE PLANNING AND EROSION CONTROL
LAKE TAHOE, CALIFORNIA
Controlling water quality and protecting the environment of receiving streams
and Lake Tahoe from nonpoint sources of pollution are of major concern in the
Lake Tahoe Basin. Over the last decade, the increase in population and
development in the Tahoe area has resulted in increased nutrient and sediment
loadings to Lake Tahoe. Although sanitary sewage is now exported from the
basin, land disturbances associated with development continue to create water
quality problems and result in increased rates of eutrophication of the lake.
Recent planning studies, demonstration projects, and wastewater management
programs have focused on controlling these adverse impacts through
implementation of BMP technology.
The description of stormwater runoff control using BMPs in the Tahoe area is
presented in several parts: (1) the regional approach using the result of
the Tahoe Basin 208 planning study to identify general problems, land use
planning, and erosion control measures; (2) a site-specific comparison of
land use planning practiced in the Tahoe area; and (3) a site-specific
description of implemented erosion controls.
Land use planning and erosion/sediment controls can limit the adverse impacts
of uncontrolled stormwater runoff. Land use planning, which places controls
on sensitive features of the site, can offer distinct economic advantages
over correcting existing problems in poorly planned, existing developments.
Comparing the impacts of a well-planned development and a poorly planned
development highlights the advantages, effectiveness, and potential
socioeconomic impacts of sound land use planning. Erosion/sediment controls
can effectively correct the results of poor initial planning or disturbed
land area; however, they can have high unit costs compared with the costs of
land use planning.
REGIONAL APPROACH TO RUNOFF CONTROL
The unique features of the Lake Tahoe Basin provide scenic and recreational
benefits that attract increasing numbers of people to the area. This growth
has stimulated rapid urbanization and development of the shoreline areas and
has also created the potential for adverse environmental impacts from
nonpoint sources of pollution. The water quality of both the tributary
streams and Lake Tahoe is of particular concern because of the increasing "
rate of pollution from stormwater runoff in these high quality waters.
89
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Recent 208 planning has identified pollutant sources and has proposed
countermeasure solutions and planning guidelines to curb uncontrolled
development and correct problems in existing developments [1].
Basin Characteristics
y y
The Lake Tahoe Basin includes about 1,300 km (500 mi) of rugged mountainous
terrain; approximately two-thirds of which is in California and one-third in
Nevada. Sixty-three major tributary watersheds drain into Lake Tahoe, which
covers approximately 40% of the basin area. Approximately 5 to 10% of the
basin land area is impervious. The only outlet from the lake is the Truckee
River at the north end of the lake. The flushing capacity of the lake is
limited. The total surface fluctuation or storage capacity is only 1.8 m (6
ft), which is about 0.6% of the total lake volume, and the estimated mean
annual change in storage volume of the lake is only 0.02% of the total volume
[2]. The physical characteristics of the Lake Tahoe Basin are summarized in
Table 42.
Table 42. PHYSICAL CHARACTERISTICS OF
THE LAKE TAHOE BASIN [2]
Total surface area of basin, im'2
Land surface area, mi 2
Lake Tahoe
Surface area, mi2
Surface elevation, ft
Length, mi
Width, mi
Length of shoreline, mi
Maximum depth, ft
Average depth, ft
Storage volume (top 6 ft), acre-ft
Total volume
mi
3
(acre-ft)
501
310
191
6,223 to 6,229.1
22
12
71
1,645
1,027
720,000
37.43
(126,000,000)
The mean annual precipitation varies from approximately 180 cm (70 in.) in
the higher elevations along the western ridge of the basin to about 50 cm (20
in.) on the eastern shoreline area. About 50 to 70% of the precipitation
occurs between December and March, in the form of snow. During the summer,
the area is subject to highly localized, intense thunderstorms.
The development around the lake is concentrated on the northern and southern
shorelines, with scattered development along the western shoreline. Most of
the recent development in the basin is related to tourist and recreational
activities, which attract over 15 million people annually [1]. The total
land area zoned for development is 18%. The existing development is only 12%
of the total basin land area [2]. The physical setting and developed areas
are shown in Figure 25.
90
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BASIN BOUNDARY
TRUCKEE
RIVER
DEVELOPED AREAS
Figure 25. Lake Tahoe Basin and developed areas.
Problem Assessment
The problems caused by stormwater runoff in Lake Tahoe are the transport of
nutrients and sediment to the highly pure water of the lake. Nutrient
loading to the lake is considered more of a potential problem than sediment.
91
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Nutrient Problems--
Lake Tahoe is naturally low in nutrient concentration and is considered
relatively infertile. However, where most lakes respond to nutrient
increases in the range of parts per million, algae in Lake Tahoe respond to
concentration increases that are at least 1,000 times lower (parts per
billion) [3]. Lake Tahoe is considered a nitrogen-limited lake—small
increases in nitrogen can trigger algal blooms. Long-term macrobenthic
studies and baseline water quality sampling indicate Lake Tahoe is of high
purity; however, they also show trends toward higher rates of eutrophication
as measured by increases in paraphyton levels and primary productivity.
During the period 1968 through 1971, the primary productivity increased by
about 25%. Compared with data from 1959-1960, the increase was over 50%, as
shown in Table 43 [4, 5].
Table 43. INCREASES IN PRIMARY PRODUCTIVITY
RATES IN LAKE TAHOE [4, 5]
Primary productivity, Percent increase over
Year mg C/m2-yr previous year, %
1959 - 1960
1968
1969
1970
1971
38,958
46,685
50,525
52,467
58,655
20% since
8.2
3.8
11.8
1959 - 1960
Stimulation of algal growth is evident along shore areas close to developed
areas of the basin. Visible changes are apparent; green algae cling to the
lake bottom and, during the spring, grow rapidly along the shore forming long
hair-like strands on mooring ropes, buoys, boats, docks, and on submerged
boulders [6].
Nutrient sources have been linked to stormwater and snowmelt runoff and to
groundwater inflow. High levels of nitrogen have been measured in the
groundwater entering the lake. The source of this nitrogen is not known;
however, this loading may be caused by old leach fields used before
construction of the sewerage system. One of the major sources of nitrogen
has been attributed to soil disturbances from construction and development
activities. Natural nitrogen fixation may also be another source of loading.
Sediment Problems—
Sediment transport to the lake also contributes to water quality degradation;
however, most sediments are sand sized with little clay; and turbidity in the
lake from these sediments is short lived. Sediment and particulates once in
the lake provide surface attachment sites for microbial communities [7].
Problems of siltation and sediment deposition in the receiving streams
draining to Lake Tahoe have occurred from erosion of unstable slopes and soil
loss from developed and developing areas. Stream siltation, as shown in
Figure 26, has changed the nature of many streams in the basin and has
92
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Figure 26. Stormwater problems in the Lake Tahoe Basin:
(a) stream siltation, (b) algal growth in nearshore areas
of the lake, (c) stormwater discharge to the lake, and
(d) stormwater sediment plume.
93
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affected the lake's appearance as well as changed the macronnvertebrate
species found in the streams and the lake.
Sources of Pollutants--
Pollutant loadings to the lake come from many sources around the lake; most
are a result of land activities and land disturbances caused by man. Land
disturbances that result in erosion and siltation include construction
activities, roadside drainage, sewering activities, unstabilized road cuts
and fills, land clearance, unpaved roads and parking lots, and channelization
of stream beds. Pollutant contributions from land use activities are
compared in Table 44.
Table 44. COMPARISON OF MEAN RUNOFF WATER QUALITY FOR SEVERAL
LAND USES AND ACTIVITIES IN THE TAHOE AREA [2]
mg/L except as noted
Land use or activity
Suspended Turbidity, Total Total Total Oil and
solids Ftu N03-N nitrogen phosphate iron grease
Unpaved parking lots
Bare areas
Unsurfaced roads and driveways
Paved parking lots
Dirt roadside ditches
Unstable dirt channels
Paved streets
Snow storage
Rooftop drainage
Roadway slopes
Construction sites
Corporation yards
Mobile home parks
Service stations
Stables
Land use types
Tourist commercial
General commercial
Public service
High density residential
Medium density residential
Low density residential
Recreation
General forest
General urbanized area
16,600
989
7,780.
320
648
613
680
136
30
443
8,630
435
5,680
281
71
4,020
773
323
249
489
613
48
66
482
1,000
319
5,060
107
175
305
280
90
7
304
764
142
931
112
27
1,084
832
105
' 92
52
169
21
6
242
__
0.3
0.9
0.6
• —
0.1
0.1
0.1
0.02
0.2
0.1
0.1
0.1
0.2
0.02
0.4
0.2
0.1
0.1
0.04
0.1
0.1
0.03 '
0.1
9.2
4.0
2.6
3.8
3.2
1.2
1.2
3.5
0.8
1.0 •
4.0
3.3
0.9
. 0.8
1.8
1.3
1.7
1.9
0.7
0.6
1.2
0.6
0.2
1.1
3.4
1.7
1.2
1.6
1.0
1.0
0.9
0.6
0.5
0.7
0.5
0.8
0,8
0.9
2.2
0.8
1.3
0.8
0.8
0.5
0.7
0.4
0.1
0.8
3.4
1.9
3.2
1.0
1.1
0.8
0.9
0.2
4.7
0.5
2.3
7.7
4.4
1.3
6.2
4.2
1.1
4.3
1.4
0.4
0.3
0.5
0.4
1.3
76.0
8.0
38.1
42.6
28.4
31.3
23.8
9.6
7.1
6.7
0.1
56.6
23.9
11.7
9.1
67.7
33.0
23.8
20.0
3.6
0.8
5.3
0.6
34.4
Pollutant problems in the lake have not yet reached major proportions, but do
indicate the need for preventive programs to arrest these problems before
irreversible damage and impacts result.
94
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Countermeasure Philosophy (208 Planning)
A basinwide countermeasure approach has been developed for water quality
problems through 208 planning. A portion of the plan addresses land use
planning and erosion control measures for limiting stormwater runoff. These
BMPs, enforced by ordinances and regulations, are applicable to developing
and developed areas of the basin.
The 208 planning goals for these practices include expected reductions of 80%
for suspended solids, 40% for nitrate-nitrogen, 70% for total nitrogen, and
80% for total phosphorus. Overall implementation costs to control stormwater
runoff in the Lake Tahoe Basin are expected to reach about $97 million (ENR
3000) [1].
Developing Areas--
By controlling development through land use planning, land use capability
features can be identified and restrictions of use imposed to prevent runoff
problems. Evaluation of land use restrictions is based on land use
capability: the ability of the land to withstand disturbance caused by
development.
Much of the Tahoe area is considered fragile and consists of steep slopes,
poorly drained soils, and areas of sensitive or relatively spare vegetative
cover. Three levels of land capability have been identified according to
risk or potential land damage or disturbance [33:
1. Lands that should remain in their natural condition
2. Lands that can permit certain uses
3. Lands that are most tolerant to urban uses
Developed Areas--
Erosion countermeasures are used to control stormwater runoff problems in
existing developments if proper planning and management techniques were not
used. In developing areas, they can solve specific problems that are
unavoidable or that cannot be controlled by land use planning programs.
Erosion controls include temporary soil stabilization, slope stabilization,
temporary runoff management, runoff collection and conveyance structures,
control of runoff from impervious surfaces, and revegetatiori of disturbed
areas [8].
Soil stabilization techniques are only temporary and should be used as
control measures during construction or until permanent stabilization, such
as vegetation, is established. Control measures include hydromulching or
application of wood chips or straw, with or without tackifiers, and netting
or matting to prevent soil loss and to provide a medium for vegetative
growth.
Slope stabilization involves reshaping erosion hazard slopes and can include
retaining walls, benches, or serrations to prevent erosion and soil loss.
95
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Examples of temporary runoff management measures are sandbag or straw bale
sediment barriers, filter berms and fences, and filter inlets. These methods
are particularly applicable to construction activities where runoff from
construction sites is to be controlled. Permanent measures to control runoff
from slopes include diversion dikes, subsurface drainage, runoff interception
trenches, chutes, or flumes, and level spreader areas.
For runoff collection and conveyance, available controls are catchbasins,
curbs and gutters, roadside ditches, storm drains, check dams, and source
detention facilities.
Use of pavements and proper design of parking lots, service aprons,
driveways, and corporation yards provide control for runoff from impervious
surfaces. Use of porous pavements and dripline trenches can also be
considered.
To revegetate disturbed areas, the following can be considered: selection of
vegetation types and planting techniques, seedbed preparation, maintenance,
and fertilizer use. Use of soil and slope stabilization measures or other
BMPs previously addressed are interrelated and may be used to assist
revegetation efforts.
The 208 study has identified potential erosion and drainage hazard areas in
the Tahoe Basin, as shown in Table 45, where source control measures could be
used.
Table 45. SUMMARY OF EROSION AND DRAINAGE PROBLEM
AREAS IN THE TAHOE BASIN [1]
Description
Erosion hazard rating
— Basinwide
High Moderate Slight total
Unvegetated roadway slopes, acres
Over-steepened roadway slopes, mi
Areas stripped of vegetation, acres
Eroding roadway shoulders, mi
Unstable drainage systems, mi
Eroding dirt roads, acres
201
51
114
68
21
63
84
18
32
101
30
61
169
15
75
293
67
53
'454
84
221
462
118
177
Legislation—
The use of land use planning criteria and erosion controls in the Lake Tahoe
Basin depends on the adoption and enforcement of ordinances and regulatory
programs. The regional planning agency has developed a number of ordinances
to enforce compliance with planning measures to control stormwater runoff
pollution [9], including:
• Land use ordinance - establishes land use districts, limits density
and land coverage, and provides procedural requirements for land
use matters.
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• Grading ordinance - regulates cuts, clearing of vegetation and
construction and maintenance of landfills, and sets revegetation
standards.
• Subdivision ordinance - regulates the subdivision of land and
establishes procedures required for such subdivision.
• Shoreline ordinance - establishes standards and regulates shoreline
development, filling and dredging, and the construction,
alteration, removal, and maintenance of shoreline structures.
Land use controls will be regulated through review and permit procedures for
new developments. For developed areas, compliance with water quality goals
may have to be enforced through legislative action. Economic: and social
impacts of the latter, however, may adversely affect individual landowners.
ASSESSMENT OF LAND USE PLANNING
Land use planning, source control concepts that can prevent and reduce
sources of stormwater pollution, is a most promising countermeasure approach.
The goal of planning is to preserve the natural ecological balance of an area
in terms of volume, runoff rate, and pollutant characteristics by recognizing
sensitive areas and restricting development in those areas.
An assessment and comparison of land use planning to show njiethods of
planning, effectiveness, impacts, and potential costs is made between two
developments in the Tahoe area, Site 1 and Site 2. Site 1 is a well planned
and constructed residential/recreational development built in the early
1970s. Site 2 is a residential subdivision constructed in the late 1950s and
early 1960s and lacks many of the planning and construction controls
necessary for environmental protection.
Approach to Land Use Planning
Site 1 was planned as an all-year recreational and resort community and
includes both a major ski complex and golf course. Planning activities were
guided, recognizing the qualities and limitations of the surrounding
landscape and environment to develop a harmonious combination of land use
activity and environmental protection. Erosion control considerations of
land use planning included (1) restricting land uses to suitable sites that
could support the intended activity, (2) minimizing land disturbances (road
cuts and fills), and (3) protecting stream environment zones.
Land Use Planning Elements--
The planning activities at Site 1 centered on a multidisciplined approach
involving an iterative evaluation of the following elements:
• Physical analysis of the site - identification of the environment--
soils, geology, slopes, drainage, access, and types of development
suitable for the site.
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• Market analysis - identification of public needs and interests in
the types of activities and facilities to be developed.
• Economic analysis - examination of costs and profitability to
develop facilities within the limits of the land available for
development.
• Regulatory requirements - coordination with local, regional, and
state agencies to conform with environmental legislation and
ordinances.
Implementation of Land Use Planning--
Implementation of land use planning at Site 1 involved identification and
mapping of the physical features of the site and screening of sensitive and
environmental hazard areas by applying land use criteria. Identification of
developable areas and development types having minimum conflicts with the
physical features of the area resulted after screening the areas with
sensitive vegetation types, slopes too steep for development, areas within
stream zones and flood plains, and areas of poorly drained soils. The most
important screening considerations were slope and drainage, with vegetation
type and density, soils, and exposure and snow depth being secondary.
Approximately 70% of the open space areas was to remain undeveloped. The
remaining 30% was planned for residential, commercial, utilities, roads and
parking, and recreational facilities.
Site 2, on the other hand, represents an uncontrolled development with little
apparent concern or controls placed on sensitive physical features of the
site. This development had no provision for open space or for limiting
encroachment of development in stream zones and drainage areas. Roads and
residential housing were constructed without regard to slope or erosion
potential and, therefore, required extensive erosion controls to protect land
features and receiving water quality.
Description of the Project Sites
Both project sites are on mountainous terrain in the Tahoe area. Site 2 is
in the Lake Tahoe Basin, and Site 1 is just north of the basin, as shown in
Figure 27.
The characteristics of each site, including type and method of development,
highlight the difference between a high degree of environmental planning and
no planning at all. The effects are reflected in the postdevelopment water
quality conditions and sediment yields from each site.
Well-Planned Development--
The development at Site 1 consists of 1,036 ha (2,560 acres) of a privately
owned, 10,500 ha (25,900 acre) tract north of Lake Tahoe. In addition to a
planned ultimate residential development of 585 single-family lots and 3 115
condominium units, the area includes a 68 ha (168 acre) golf course and a 132
5aJ32La?r^ ski area> A summary of land use areas at Site 1 is shown in
Table 46 [10].
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Figure 27. Location of the well planned and poorly
planned project sites in the Tahoe area.
Table 46. SUMMARY OF PLANNED LAND
USE AREAS AT SITE 1 [10]
Land use
Open space
Developed area
Residential
Commercial
Utilities
Roads and parking
Recreational facilities
Subtotal
Total area
Area, Percent of
acres total area
1,825
88
17
20
118
491
734
2,559.
71.3
3.4
0.7
0.8
4.6
19.2
28.7
100.0
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Land Use Planning and Erosion Controls--The development of the land use types
at Site 1 included preplanned erosion controls and provision for compatible
construction with the environment. Erosion potential from the ski areas was
reduced by proper site selection of the ski runs and facilities. In most
cases, ski runs were cut on less than 3:1 slopes, diagonal to the fall line,
to route runoff to undisturbed areas adjacent to the runs. The use of flow
barriers on steeper slopes and maintaining natural vegetation or implementing
comprehensive r^evegetation programs prevented erosion in cleared and
disturbed areas.
Cuts and fills for street and parking lot construction were limited to slopes
no steeper than 1-1/2:1 for cuts and 2:1 for fills. To reestablish native
vegetation, topsoil was stockpiled and replaced after construction.
Propagation of native vegetation and seeds by this method -proved successful
in most areas where slopes were not too steep.
Planned construction of residential and commercial buildings was limited to
slopes of less than,15%. This restricted the developable area of the site to
relatively flat ground on ridgelines and in valley floors. The area
conforming to this criteria was limited, however, and actual construction
extended beyond these planned limits. In these areas, erosion control
practices were implemented to correct potential problems. In all cases, no
development was constructed on slopes steeper than 25%, with most less than
20%. Other practices during construction included: (1) minimizing surface
disturbances, (2) using check dams and erosion baffles, (3) lining drainage
channels, (4) using filter fences and berms to prevent siltation from
contruction site erosion, and (5) using slope stabilization techniques.
These and other erosion control measures are covered later in this case study.
The development at Site 1 was constructed to blend in with the natural
surroundings and created minimal environmental disturbances, as shown in
Figure 28.
Hater Qua!ity--The surface water quality leaving the development at Site 1
showed slight increases in suspended sediment concentrations above estimated
background levels. Suspended sediment is the heavy solids that cause
sediment deposition or siltration in the stream as the flow recedes. The
suspended sediment concentration is proportional to three types of runoff
conditions and runoff rates: low or base flow, rainfall, and snowmelt.
Rainfall causes the highest concentration of suspended sediment with the
highest monitored values from the entire site in the range of 500 to 600
mg/L. The average concentration leaving the site during rainfall is 115
mg/L. The average concentration for low flow is 7.9 mg/L and for snowmelt
conditions is 28.5 mg/L [10].
Suspended sediment loads from the site represent?approximately a 100%
increase over predevelopment conditions, 12 T/km-yr (107 lb/acre-yr) to 24
T/km 'yr (214 lb/acre-yr).
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Figure 28. Results of land use planning at Site 1:
(a) condominimum development and undisturbed natural surrounding,
(b) revegetated ski slope, (c) revegetated gentle-sloped road
cut, and (d) rock-lined natural drainage channel.
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Poorly Planned Development--
The development at Site 2 is a single-family residential subdivision of 128
ha (316 acres), with 632 subdivided parcels. The upper portions of the site
are on steep slopes and have a high potential for erosion. A 24 ha (60 acre)
portion of this upper area was selected as a demonstration site for
implementation of erosion controls to correct the effects of poor initial
planning [10]. The site is shown in Figure 29.
The upper portions of this site have been under continuous development
without land use restriction controls. Currently, only 19% of the 24 ha (60
acre) site has been developed. Virtually all of the site area is on steep
slopes ranging between 30 and 60%, with poor soil drainage characteristics.
Regional planning guidelines rate the site in the lowest land capability
class and allow only a 1% impervious surface coverage [10]. At full
development of the site, it has been estimated that half of the area would be
impervious, coverage, and disturbed or unvegetated slopes. A summary of the
land uses and impervious areas before implementation of the erosion control
project is shown in Table 47.
Table 47. SUMMARY OF LAND USES AND
IMPERVIOUS AREAS AT SITE 2
BEFORE EROSION CONTROL PROJECT [10]
Description
Impervious coverage
on private lots
Road surfaces
Disturbed and
unvegetated slopes
Undisturbed area3
Total area
Area,
acres
2.8
6.7
8.1
42.5
60.1
Percent of
total area
5
11
13
71
100
a. Site 2 has no provision for open space.
Construction of homes and roads is continuing on the steep slopes of the
site. Embankments that were originally cut at 1:1 slopes or greater have
eroded to 1-1/2:1 or less. Several roads on the site have grades steeper
than 15% and present both maintenance problems during snow removal and
traffic hazards.
The steep slopes and development within drainage areas and in stream zones
add to the stormwater drainage problems. Drainage ditches,; curbs, gutters,
and culverts at the site are generally undersized for the high flows that
occur. These facilities are constantly clogged with eroded sofV'from the
disturbed areas. The subdivided area at Site 2 has no open space areas and
has lots adjacent to streambanks and in drainage areas. The development and
examples of poor land use planning are shown in Figure 30. Development in
stream or drainage zones allows runoff to enter receiving waters^directly
from the disturbed areas.
102
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Figure 29. Site 2 at Lake Tahoe: (a) and (b) development in
the upper portion of the site on extremely steep slopes,
and (c) resulting erosion problem from road cut.
103
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Figure 30. Results of uncontrolled development at Site 2:
(a) unstable road cut, (b) eroded road fill, (c) steep road
grades, and (d) sediment-laden stormwater entering storm culvert.
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Water quality problems from Site 2 are related to suspended sediments carried
to the receiving waters by stormwater runoff. Estimates of erosion rates
from unstable cut and fill slopes averaged 1,975 T/km -yr (17,600
lb/acre-yr). Unit sediment yields from the development represented^more than
a 100-fold increase over measured background levels: from 3.4 T/km -yr (30
lb/acre-yr) to 366 T/km -yr (3,260 lb/acre-yr).
Rainfall produces the highest suspended sediment concentrations from the
site. The highest monitored concentration was measured at over 15,000 mg/L,
with the average at about 1,800 mg/L. A comparison of the average suspended
sediment concentrations for various flow conditions entering and leaving the
developed area is shown in Table 48.
Table 48. AVERAGE INSTREAM SUSPENDED
SEDIMENT CONCENTRATIONS ABOVE
AND BELOW SITE 2 [10]
mg/L
Flow
Above
Below
condition development development
Low flow
Rainfall
Snowmel t
1.3
20.8
9.1
12.9
1,798.1
434.6
a. Represents background levels.
Economic and Environmental Impacts of Land Use Planning
The costs of land use planning for environmental protection are difficult to
quantify. Most costs are not directly related to a specific control measure
but rather to an overall development philosophy that includes aesthetics and
profitability, in addition to meeting regulatory requirements. Environmental
impacts are characterized by the quality of the receiving waters draining the
controlled and the uncontrolled development.
Costs of Land Use Planning--
Two cost components were identified to assess the cost of land use planning
at Site 1. These include predevelopment planning costs, $443,000, and costs
to construct erosion control measures where needed, $127,000. Assuming that
all predevelopment planning costs and the required erosion controls are
attributed to environmental protection, the total control costs representing
land use planning for Site 1 are $570,000 (ENR 3000), or $550/ha ($220/acre)
of total development area [10].
The average projected cost per condominium unit, at full development, is
approximately $360. This cost could be passed on to the purchaser to cover
the cost of planning and preplanned erosion control measures.
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Environmental Impacts of Land Use Planning--
The effectiveness of land use planning is shown by a comparison of the
sediment yields of the developed areas at Site 1 and Site 2. Although
development slightly increases sediment yields even in well-planned
developments, the disregard of land use planning criteria to protect
sensitive areas can produce the extreme results shown in Figure 31.
ui
>-
UI
09
4.000
3.000
2.000
1.000
POST-
DEVELOPMENT
PRE-
DEVELOPMENT
POST-
DEVELOPMENT
WELL PLANNED DEVELOPMENT
SITE 1
PRE-
DEVELOPMENT
POORLY PLANNED DEVELOPMENT
SITE 2
Figure 31. Comparison of sediment yields from a well planned
and a poorly planned development [10],
Environmental impacts can be characterized by the interrelationship of the
receiving water quality of the streams (sediment deposition) and the health
of the stream and macrobenthic communities.
Impacts of suspended sediment on the macrobenthic community increase as
sediment loadings increase. The changes in the macrobenthic community, at
sampling stations above and below the poorly planned development at Site 2,..
are shown in Table 49. These changes result from about a 100-fold increase
in sediment load over background conditions. .
Although a twofold increase in suspended sediment load was experienced at -,,.
Site 1, the impact of this increase on the macrobenthic comrnunity was
negligible. Several monitoring stations near areas of soil disturbance
showed some fluctuations in the number species, density, and diversity
106
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of species. However, for the development as a whole, monitoring downstream
of the development indicated only minor impacts compared with upstream
monitoring, as shown in Table 50.
Table 49. MACROINVERTEBRATE SAMPLING RESULTS ABOVE
AND BELOW THE POORLY PLANNED DEVELOPMENT
SITE 2 [10]
Date of
sampling
Jul 1975
Dec 1975
Jun 1976
Oct 1 976
2
Density, No./m
Above
1,542
1,321
2,125
1,560
Below
267
277
1,652
19
Number of species
Above
20
19
14
14
Below
9
12
14
4
Species
diversity index
Above
2.50
2.25
2.15
2.21
Below
1.91
2.08
1.85
1.35
a. A measure of the relationship between the number of species
and the total biologic community population by the Shannon-
Weaver index: diversity = -z(Ni/N) In (Ni/N), where
Ni = number of species and N = total community population.
Table 50. MACROINVERTEBRATE SAMPLING RESULTS
ABOVE AND BELOW THE WELL-PLANNED DEVELOPMENT
SITE 1 [10]
Date of
sampling
Sep 1974
Jul 1975
Dec 1975
Jun 1976
Oct 1976
2
Density, No./m
Above
619
785
1,002
995
1,212
Below
2,193
1,611
821
1,364
1,578
Number of species
Above
19
20
19
18
15
Below
20
22
20
14
20
Species
diversity index
Above
2.78
2.01
1.69
1.69
2.44
Below
2.26
2.06
2.29 "
1.55
2.08
EROSION CONTROLS
Erosion controls mitigate erosion and sediment problems where land use
activities have disturbed soil surfaces or created erosion hazard areas.
Even in the well-planned development, erosion controls were used; however,
they were implemented according to a planned strategy where land disturbances
were unavoidable. For developments with no land use planning, such as at
Site 2, erosion controls serve as the only mitigation option available to
correct sediment and erosion problems.
107
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The erosion controls at Site 2 were implemented as part of a demonstration
project to determine the effectiveness of a large number of different control
technologies and variations of similar technologies. Side-by-side plots were
used on most control areas where different controls were applied for
comparison [10].
Erosion controls can be temporary measures that are effective for short
periods until permanent controls can be established or until land disturbance
is minimized after construction. Permanent controls are long-term
countermeasures used to control erosion from disturbed areas, such as road
cuts and fills, that are physically unstable or exposed to rain or runoff
over long periods. These controls usually employ mechanical stabilization
methods.
Description of Demonstration Project
Approximately 2.9 ha (7 acres) of disturbed area was identified in the 24 ha
(60 acre) area of Site 2 for implementation of erosion controls. These
disturbed areas, for the most part, represent road cuts and fills, as shown
in Figure 32. The principal control measures were permanent and temporary
slope stabilization controls and revegetation. Other control measures were
temporary runoff and siltation control, runoff control on slopes, and
conveyance systems. Over 200 separate plots were used to demonstrate various
erosion control measures within the site [10].
O
LEGEND
PROJECT SITE BOUNDARY
DISTURBED AREAS
STREAM CHANNEL
EXISTING HOUSES
Figure 32. Location of the disturbed areas at Site 2 [10],
108
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Temporary Soil Stabilization--
Temporary soil stabilization methods are used on disturbed slopes or on areas
to provide erosion control, dust control, mulch or mulch protection, or a
medium for applying or holding seeds during revegetation. The effectiveness
of these methods is short lived. Sediment and nutrient yield control,
compared to bare ground, is most effective for 6 months to 1 year, with a
steady decrease up to 2 years, and a sharp drop in effectiveness after 2
years. A description of temporary soil stabilization methods and their
projected effectiveness is presented in Table 51.
Table 51. DESCRIPTION AND EFFECTIVENESS OF
TEMPORARY SOIL STABILIZATION METHODS [8]
Effectiveness,
Sediment
Nutrients
Method
Description
Initial 2 yr Initial 2 yr
Jute matting
Hatting in drainage
channels
Plastic netting
Wood excelsior
matting
Fiberglass
roving
Hydromulching
Wood chip
application
Crushed gravel
mulch
Straw mulch
Mulch nets made of jute used for erosion control
and protection of other mulches.
Application of jute matting or fiberglass roving
for dust and erosion control in very small drain-
age channels with flow velocities less than
2 ft/s.
Monolithic p.lastic cloth!ike material used over
mulch, straw, or hydromulch.
Mat of wood excelsior fibers bonded to a paper
or plastic used for dust and erosion control.
Flows under mat should be prevented.
Hatting of continuous strands of glass fibers
and tacking agent. Used for dust and erosion
control and as a mulch for seeded and unseeded
areas.
Mechanized rapid method for applying wood fiber
mulch, and tacking agent with or without seeds
to large areas.
Temporary mulch and surface protection using
chips of wood. Used for dust and erosion
control during construction and as a mulch
around plantings.
Application of gravel or crushed stone as a
mulch to stabilize soils during construction,
or for low-use dirt roads, driveways, and areas
of light vehicular use.
Application of staple straw as a protective
cover over bare or seeded soil to reduce
erosion and provide a mulch. Requires matting
or other methods to hold it in place.
70-90 40-60 50-70 20-50
50-90 20-60 30-70 10-50
Provides no control by itself
50-90 20-60 30-70 10-50
90-95 80-90 60-80 50-70
70-90 40-60 50-70 20-50
90-95 80-90 60-80 50-70
70-90 70-90 50-70 50-70
90-95 40-60 60-80 20-50
a. Effectiveness for most methods after 2 years usually ranges between 0 and 10%.
Permanent Slope Stabilization--
Permanent slope stabilization controls are mechanical methods used to
physically change the disturbed slope area or provide physical barriers to
support the slope. The following methods, described in Table 52, do not
provide for mulch or surface protection to bare slopes and require temporary
slope stabilization methods until permanent vegetation is established. In
109
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many of the corrective measures implemented at Site 2, one or more of these
methods were used to provide adequate erosion protection.
Table 52. SUMMARY OF PERMANENT SLOPE STABILIZATION MEASURES [8]
Methods
Description
Applicability
Rock retaining
wall
Redwood retaining
wall
Gabions
Slope bottom
bench
Wattling
Slope stepping
Slope
serration
A low gravity wall constructed of rock
materials to provide an aesthetically
attractive method for physically stabi-
lizing a slope.
A retaining wall constructed of redwood
planking and posts to stabilize over-
steepened or unstable slopes.
Large, single- or multi-celled, rectan-
gular wire mesh boxes filled with rock
and wired together for permanent slope
or drainage stabilization and erosion
control.
A gently sloping surface at the base
of a steeper slope to retain eroded
material.
Bundles of live cuttings from willows
to stabilize slopes and provide revege-
tation. Wattling reduces slope lengths
for surface runoff, increases water
retention, and provides additional
organic matter.
Continuous series of horizontal steps
cut on the face of cut slopes to
interrupt slope length and provide
slope stabilization.
Construction of approximately 10 in.
horizontal steps on the entire face of
a cut slope to provide stabilization
benches that can support vegetation.
For use on slopes which are steeper than
2:1 and cannot be regraded to achieve this
gradient.
Used on small slopes of loose material
underlain by rigid rock or firm subsoil
to securely anchor the wall.
Used as retaining walls to stabilize over-
steepened slopes, or slope facing (revet-
ments^ •••»•!••<;, channel linings, culvert
headwalls and aprons particularly where
seepage is anticipated.
Used to control erosion on small over-
steepened slopes (20 ft or less) that
cannot be regraded because of access and
or newly constructed slopes.
Used on slopes no steeper than 2:1 with
long slope lengths providing uninterrupted
paths for surface runoff. "Best applied to
moist sites and should not be a substitute
for retaining walls or mechanical stabili-
zation methods.
Used in new construction on large cut slopes
in soft rock which can be excavated by
ripping.
Serration is limited to slopes in medium
to highly cohesive soils or in soft rock
with a gentle slope (2:1 or less). Not
applicable on deposited soils or in
moraines.
Runoff Control and Temporary Runoff Management--
Runoff control and temporary runoff management measures should be considered
in construction areas or used in addition to slope stabilization controls for
increased effectiveness. Several of the following measures were used during
construction at Site 1 and were used during implementation of the erosion
control measures at Site 2:
• Diversion dike (at the top of cut or fill slopes) - diverts flows
from the slopes into stable areas
• Runoff interception trench - intercepts long slope faces on gentle
slopes (less than 3:1) and allows diversion and infiltration of the
runoff and retention of sediments
• Pipe drops, chutes, or flumes - conducts flows down unstable slopes
• Straw bale sediment barrier - allows flows to be filtered through
barrier and retains the sediment
110
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o Sand bag sediment barrier - diverts flows and retains sediments,
but does not allow water to filter through the barrier
• Gravel filter berm - removes sediments from stormwater runoff
• Filter fence (a barrier of filter cloth) - provides sediment
removal for water discharged from construction sites
• Filter inlet (a temporary berm) - protects a stormwater inlet that
retains sediment and allows flow to pass through
• Siltation berm (a temporary impermeable berm) - retains runoff from
construction sites
Revegetation--
Vegetation provides the best long-term erosion protection on sloped surfaces,
and is the ultimate goal when applying corrective measures to disturbed
areas. However, revegetation of disturbed or bare slopes by itself will not
stabilize over-steepened slopes; therefore, temporary or permanent slope
stabilization must be used before establishing vegetative cover. In many
instances, revegetation methods can be combined into single operations with
temporary stabilization methods.
The selection and use of native seeds and plantings for revegetation is a
critical factor in the success and effectiveness in establishing plant
growth, considering the adaptability of native vegetation to climate, soil
condition, and soil type. Success of establishing new growth is also
affected by slope and aspect of the disturbed area, with better growth on
less steep slopes and protected exposures.
Hydraulic application of seeds to the seedbed (hydroseeding) can often be
combined with the hydromulching step used for temporary slope protection.
Hydroseeding applies the seed and fertilizer in a water slurry, but requires
close vehicular access to the seeding area. Application of fertilizers
requires control to prevent improper or excessive use to protect water
quality and should only be used where soil nutrient deficiencies exist.
Evaluation of Erosion Controls
The use of straw mulch with a chemical or mechanical tackifier is one of the
most effective erosion control methods at Site 2. Using a straw mulch is as
effective or better than other techniques, some of which are more expensive.
Contour wattling is also an effective method to mechanically stabilize and
revegetate over-steepened slopes. Growth of other types of seeded or planted
vegetation is higher on slopes that received contour wattling.
Implementation of corrective erosion controls is extremely labor intensive,
particularly for slope reshaping and mechanical stabilization methods.
in
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Several maintenance practices at Site 2 have also increased the sediment load
from the site or have increased the development erosion problems [10]. These
include:
• Removing accumulated sediments from the toe of an eroding slope
that undercuts the stability of the slope and leads to an increased
erosion rate.
• Washing culverts and drains clogged by eroded sediments increases
the rate of downslope sediment transport. This practice, however,
does reduce upstream flooding.
• Improper disposal of waste earthen material, such as "over-the-
bank" practices, increases sediment transport and hinders the
proper establishment of slope stabilization measures.
• Negligence in providing revegetation or other stabilization to
areas disturbed for the connection of sewer and water laterals or
other underground utilities. Frequently, the disturbed surface
acts as a channel for upslope storm runoff or snowmelt runoff.
These erosion control measure applications, either singularly or in
combination, are shown in Figures 33 and 34.
Unit Costs of Erosion Controls
The erosion controls at Site 2 involved a combination of many unit control
measures to provide adequate slope protection, with an estimated average cost
of $93,400/ha ($37,800/acre) of disturbed area. The costs ranged from
$3,700/ha ($l,500/acre) for simple seeding and mulch applications to over
$249,000/ha ($100,800/acre) for extensive controls, including retaining
walls, wattling, plantings, seeding, and mulch applications. Overall costs
to individual landowners to provide these controls could be in excess of
$2,500 per lot [10].
Estimated unit costs for permanent and temporary slope stabilization methods
are summarized in Tables 53 and 54, and include materials, labor, and
equipment cost components.
Estimated unit costs for runoff control and temporary runoff management and
other controls are summarized in Table 55.
The costs to revegetate disturbed areas can vary significantly depending on
the slope, the need for slope stabilization, reshaping, or seed bed
preparation, and the method of application of seed or plantings. The costs
vary from approximately $2,500/ha ($1,000/acre) for seed application
(hydroseeding with mulch) to over $67,000/ha ($27,000/acre) for plantings of
routed shrub cuttings. Seed and fertilizer application adds about $500/ha
($200/acre) to the cost of hydromulching. Estimated costs of various
revegetation methods are summarized in Table 56.
112
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,»,™ - V-TX r*-f iy I
S
^Uft>-3g."*:'3
-ilOT**^
!**|£i'!^N^
H^iSiS^
Sf-v
Sfa^
«.--"•.
!',;:•
Figure 33. Erosion control measures at Site 2: (a) rock retaining wall,
willow wattling and netting, (b) willow wattling, (c) gabion retaining
wall and hydroseeded slope, (d) maintenance and cleanup operations near
stabilized slope, and (e) application of straw mulch.
113
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Figure 34. Conditions before and after implementation of erosion
controls at Site 2: (a) and (b) heavily eroded, unstable road-cut slopes,
and same slopes with erosion controls, including slope reshaping,
retaining walls, willow wattling, hydroseeding, and netting.
114
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Table 53. ESTIMATED COST FOR PERMANENT
SLOPE STABILIZATION METHODS [8, 10]
Method
Rock retaining wall,
4 ft high
Redwood retaining
wall, 3 ft high
Gabion retaining
wall, 3 ft high
Slope bottom bench
Wattling
Slope steping
Slope serration
Overhang removal and
scaling (manual)
Overhang removal and
scaling (backhoe)
Units
$/lf
$/lf
$/lf
$/lf
$/lf
$/acre
$/acre
$/yd3
$/yd3
Materials
5b
12
10
none
0.3
none
none
none
• none
Cost
Labor
12
10
8
5
1.9
320
300
39
5
Equipment
10
3
3
2
0.1
250
120
2
2
Total
cost
27
25
21
7
2.3
570
420
41
7
a. ENR 3000.
b. Assumes use of native material.
Table 54. ESTIMATED COST OF TEMPORARY
SOIL STABILIZATION METHODS [8, 10]
Method
Jute matting
Paper fabric
'Plastic netting
Wood excelsior
matting
Fiberglass roving
Hydromulching
Chemicals and
tackifiers
Wood chip
application
Crushed gravel
mulches
Straw mulchc
a. ENR 3000.
b. 2.5 tons/acre.
c. 2.0 tons/acre.
Cost,
$/acre
Materials Labor and equipment
2,700
2,900
700
1,700 '
1 ,900
500
400
750
620
150
, -p'
6,400
6,600
4,100
9,000
2,100
700
200
100
180
530
Total cost,
$/acre
9,100
9,500
4,800
10,700
4,000
1 ,200
600
850 ,,
800
680
115
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Table 55. SUMMARY OF UNIT COSTS OF RUNOFF
MANAGEMENT SOURCE CONTROLS [8]a
Method
Diversion dike
Runoff interception trench
Strawbale sediment barrier
Sandbag sediment barrier
Filter berm
Filter fence
Filter inlet
Siltatiqn berm
Unit
Materials
none
none
1.12
0.93
3.73
1.48
0.62
3.30
costs, $/lf
Labor and equipment
4.24
7.22
1.37
2.74
4.35
1.83
1.68
5.22
Total
cost
4.24
7.22
2.49
3.67
8.08
3.31
2.30
8.52
a. ENR 3000.
Table 56. ESTIMATED COST OF VARIOUS
REVEGETATION METHODS [10]a
Method
Willow staking
Rooted shrub cuttings
Bare root seedlings
Seed with hydromulching
Seed with tacked straw
Seed with jute matting
Seed with paper fabric
Seed with excelsior
Seed with straw and plastic net
Seed with fiberglass roving
Unit cost, $
0.75/stake
1.69/plant
0.78/plant
0.19-0.28/yd2
0.23/yd2
1.92/yd2
2.00/yd2
2.25/yd2
I.ll/yd2
0.83/yd2
Cost, $/acre
12,100
27,400
13,100
900-1 ,400
1,100
9,300
9,700
10,900
5,400
4,000
a. ENR 3000.
SOCIOECONOMIC IMPACTS
Land use planning and erosion control measures have improved the receiving
waters in the Lake Tahoe area. Land use planning is considered essential in
the initial stages of development, because of lower costs, greater
aesthetics, and greater continuity of overall environmental protection.
Erosion controls are also effective control alternatives for existing
problems resulting from poorly planned or disturbed areas. Erosion controls
116
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are, however, more expensive to implement than equivalent results of land use
planning. The cost of erosion controls used in developed areas is usually
borne by local government or private landowners.
Impacts on Private and Local Facilities
The costs to correct existing problems are greater than the costs associated
with preplanned environmental controls during development. The costs to
private landowners at Site 2 would be in excess of $2,500 per lot for the
implemented erosion controls compared with approximately $360 per lot at Site
1, where extensive land use planning was used. The costs for the erosion
controls represent a high financial burden for the individual landowner, and
erosion controls are difficult to implement without public assistance.
Although the costs of environmental control in well planned developments are
substantially lower to the individual lot owner, the developers are forced to
make concessions in overall profitability by reducing the area of land
available for development. This may increase the cost of individual lots, in
addition to the costs for environmental planning and controls.
Because of the difficulty of enforcing regulations and environmental controls
on private individuals in existing developments, local government agencies
are often financially burdened by ill-fated developments, half-finished
subdivisions, vacant lots, scarred land, and maintenance problems. The costs
to the county for eroded sediment cleanup and maintenance within Site 2 have
been estimated at $14,300 per year (ENR 3000) [10]. As a result of the
erosion controls, massive cleanup efforts and costs to the county following
rainstorms and snowmelt have been substantially reduced. The total cost of
the erosion control work could be amortized (at 8%) over a 12.,5 year period
at the annual budget spent for cleanups [10].
Land values and demographics are affected by overall land use planning. Land
use intensity controls, such as providing for open space, resulting in
increased attractiveness to build (aesthetics), plus a reduction of available
land to build on, can increase land values and the rate of development.
Long-term socioeconomic impacts can include a net population decrease due to
rezoning and management of high erosion hazard lands and stream environment
zones. Projections of ultimate potential population reductions in the Lake
Tahoe Basin have been made: (1) a reduction of about 2,400 persons through
rezoning, and (2) a maximum reduction potential of 35,000 persons through
management of stream environment zones [2].
Public Acceptance
A survey of 26,000 property owners in the Lake Tahoe Basin identified water
pollution, scenic destruction, and too much commercialism as priority
problems [2]. One of the main objections was high density developments.
Environmental planning through land use control addresses most of these ;
concerns.
117
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Several beneficial uses of the waters in the Lake Tahoe Basin that can be
adversely affected by poor land use planning and erosion sediment loadings
have been identified and include [10]:
Domestic water supply
Agricultural water supply
Water contact recreation
Nonwater contact recreation
Fresh water habitats
Fish spawning
These considerations, together with the aesthetic improvement to scarred and
disturbed areas through erosion control and land use planning, may be taken
as positive public acceptance.
Aesthetics
Aesthetics is probably the single most important factor for development and
tourism in the Lake Tahoe Basin. It has contributed to both the economic
growth of the area and its environmental problems. Land use planning
provides for open space and controls development. Erosion controls can
improve scarred and disturbed areas by grooming and revegetation of the land.
Mechanical stabilization methods, such as retaining walls, may be considered
aesthetic improvements in themselves. Erosion controls can also prevent
sediment loads from entering streams, allowing the stream to clean and scour
itself.
However, aesthetics is one of the main reasons for the problems developing in
the first place. The uncontrolled high density subdivision on the steep
terrain at Site 2 was developed to take advantage of the spectacular view
afforded by the location, and was probably an influencing factor in the
marketability of the property.
118
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SECTION 7
MANAGEMENT OF A NATURAL DRAINAGE SYSTEM
THE WOODLANDS, TEXAS
The planners of a new community, The Woodlands, in southeastern Texas,
attempted to minimize the water resources problems traditionally caused by
urbanization. The governing principle of this development's planning was
preserving both the natural drainage system and the predevelopment surface
water-groundwater balance. In addition to avoiding many of the problems of
urban runoff, the planners also hoped to decrease the site development costs
and create a unique new community with a natural forest setting that would
attract home buyers.
Urbanization of an undeveloped site changes both the quantity and quality of
runoff. The primary factors increasing the quantity of runoff are: (1) the
replacement of porous soils by impervious pavement and building sites, and
(2) a decrease in the infiltration of ponded water caused by both grading and
the replacement of natural drainage systems by storm sewers or lined ditches.
The change in runoff quality is due to the introduction of pollutants related
to an urban setting. Construction activity will increase suspended solids
loadings; intensive landscaping will increase nutrient, pesticide, and
herbicide concentrations; litter will increase the quantities of floating
solids; and various nonpoint sources will increase the concentrations of
trace metals, bacteria, and oil and grease.
At The Woodlands an effort was made to maintain runoff quantities at a
predevelopment level by planning to avoid site conditions that increase
runoff. It was hoped that the techniques used to prevent increased quantity
would also preserve the quality of runoff at pollutant concentrations close
to that of predevelopment conditions. The runoff quality had to be
acceptable for recreational lakes, irrigation, and groundwater recharge.
Studies were conducted by a team from Rice University to determine how
effectively The Woodlands' development preserved the water resources of the
site and to determine if changes in runoff water quality would make the
surface water unacceptable for recreational or aesthetic uses [1].
PROJECT DESCRIPTION
The Woodlands project began in 1971 with $50 million in loans guaranteed by
the U.S. Department of Housing and Urban Development. The project was one of
13 new communities nationwide begun under a 1970 federal program to encourage
the planning of self-contained, economically balanced urban centers that
would demonstrate modern planning concepts. The natural drainage concept was
119
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a part of The Woodlands planning process from the beginning and extensive
surveys of soils, drainage, vegetation, and water table levels were made to
provide background data for the drainage planning.
Site Development
The Woodlands is a 7,300 ha (18,000 acre) site approximately 40 km (25 miles)
north of Houston, Texas. The predevelopment vegetation at the site consisted
of a pine and oak forest with generally medium-to-heavy understory
vegetation. The site is flat with slopes commonly from 0 to 3% and
predevelopment stream bed slopes averaging 0.2%.
Soil s—
The soil types, mapped before the initial development of The Woodlands, are
fine sands and fine sandy loams underlain by an impermeable clay zone at a
depth ranging from 0 to 2 m (0 to 80 in.) below the ground surface. The clay
zone supports a perched water table at 0.4 to 2 m (15 to 80 in.). The soils
were categorized by the depth to the impermeable layer and thus the ability
to store storm runoff in the upper soils.
The shallow depths of permeable soil, the high water table, and.the flat
terrain of the site indicate that casual water ponding is common during rainy
periods.
Predevelopment Drainage--
The major portion of the site drains into either Panther Branch or its main
tributary, Bear Branch. Panther Branch and the remainder of the site are
tributary to Spring Creek and ultimately Lake Houston, which is a surface
water supply for the Houston metropolitan area. The three streams are shown
in Figure 35 as they relate to The Woodlands. Panther and Bear branches are
meandering streams with well-defined, low flow channels and very wide, flat
flood plains. The low flow channel varies between 1.5 and 6 m (5 and 20 ft)
wide and has several pools. The channel increases from approximately 1 m (3
ft) at the headwaters to 3 m (10 ft) at the junction with Spring Creek.
The average yearly rainfall for the north Houston area is approximately 115
cm ( 46 in.) from a combination of intense summer thunderstorms and more
prolonged winter rains. It is estimated that 10 to 15% of the rainfall
becomes runoff in undeveloped portions of The Woodlands; the remainder will
pond, then evaporate or infiltrate [1].
Problem Assessment
The challenge at The Woodlands was to develop a self-contained, new community
while maintaining the preconstruction quality of the land, water, and forest.
Obviously, trees would have to be cut for structures and roadways, but there
would not be a general clearing of tracts or lots. The drainage system
should blend in with the natural setting and yet serve the developing area.
120
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THE WOODLANDS
PROPERTY LINE
HIT LINt —7-
I /
Q
100 YEAR FLOOD PLAIN
LEGEND
PHASE ONE DEVELOPMENT
100 YEAR FLOOD PLAIN
LAKE B
SAMPLE SUE
Figure 35. Site plan of The Woodlands.
121
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Two major problems were faced in designing the drainage system for The
Woodlands. First, the groundwater at the site had to be protected and
preserved and second, the surface water had to be suitable for onsite use and
maintained both to preserve the quality of Lake Houston and not cause
downstream flooding.
Groundwater—
There are two groundwater reservoirs that are important to the development of
The Woodlands. The deeper reservoir provides the drinking water for the new
community; the need for preserving its quality and quantity is obvious. The
shallow reservoir is the perched water table found within 3 m (80 in.) of the
surface. The existing vegetation of The Woodlands has developed an
ecological balance based on this high water table.
Lowering the level of the water table would damage or at least change the
balance and create a new vegetation ecosystem. Conventional development
calls for the interception of surface runoff and its removal from the
vicinity of housing areas. Although central ponds or recharge wells could
preserve the deeper groundwater reservoir, local drainage would mean the loss
of perched water in any area drained by storm sewers. The problem at the
Woodlands was to preserve both levels of groundwater throughout the site.
Surface Water— ''•
i
Preservation of the surface water quality was required both for onsite
recreation and aesthetics and for downstream uses. Development plans call
for small recreational lakes in the neighborhoods and a large impoundment on
Panther Branch to be called Lake Woodlands. The primary purpose of these
ponds and lakes is to increase the recreational appeal of the new community
and enhance the aesthetic appeal of the housing sites near the ponds and
lakes. However, it would be desirable for the lakes to serve as part of the
water management system by storing stormwater peaks and providing a source of
nonpotable water for irrigation and recharge. The quality has to be • ' "
preserved to serve all of these goals.
Urban runoff is normally contaminated by significant amounts of pollutants,
including solids, oxygen-demanding substances, nutrients, pesticides,
herbicides, trace metals, and floating litter or grease. A major goal of The
Woodlands was to minimize the presence of these pollutants, then study the
runoff to understand the impact of the remaining pollutants on the water
resources system. Potential problems include the following:
• The growth of objectional lake algae or water plants fertilized by
nutrients in runoff from golf courses and other landscaped areas.
• A buildup of toxic pesticides, herbicides, or metals in the lakes
that could kill fish and pollute groundwater resources.
;- j
t An increase of the bacteriological indicator organisms in the lakes
to levels that would require prohibition of recreational
activities. :
122
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• A large increase in the quantity or a decrease in the quality of
water in Spring Creek due either to storm runoff or sewage
treatment effluent.
It was expected that any design would cause some decrease in surface water
quality--the problem was to develop and modify a system that would keep the
impact of the pollutants to a level acceptable for. the planned uses of the
surface water.
Countermeasure Philosophy
The water management planners at The Woodlands assumed that the
predevelopment stream system provided good quantity and quality control and
that a channelized urban system would not serve all the planning goals. The
resultant plan was to preserve the natural drainage system as much as
possible. The development was to be managed so that sites chosen for
buildings, roads, and other construction would have a minimum impact on the
natural drainage and that unavoidable impacts would be offset by improvements
to neutralize these impacts. The improvements would not be the common storm
sewer system designed to remove water but rather ponding systems designed to
infiltrate, runoff.
An interrelated planning goal was the desire to achieve a hydro!ogic balance
at the site by recharging the deep groundwater reservoir at the same rate as
water is withdrawn for the potable water system. The recharge would result
from infiltration of water through the lake systems. The lakes, in turn,
would be fed by storm runoff and sewage treatment plant effluent. A key
point to achieving this goal is an acceptable quality of sewage effluent
so that the lakes can be maintained during low rainfall periods.
Implemented Countermeasures
Planning--
The first step in preserving the natural drainage system was an extensive
survey of vegetation, soils, and slopes at the site. The, purpose was to
indicate portions of the site suitable for various housing densities,
shopping centers, community centers, and roads. Each hydro!ogic subbasin Was
to retain enough pervious soil to infiltrate the runoff from a 2.54 cm (1
in.) storm. The planned dwelling unit densities of from 2.5 to 37 per ha (1
to 15 per acre) required site imperviousness ranging from 24 to 56% and site
clearing from 37 to 93%. The high density developments were limited to
locations where the soil was already naturally impervious and the vegetation
considered less valuable.
Existing Drainage—
The existing drainage system was protected by prohibiting development within
the 25 year flood plain of tributary streams and controlling development
within the 100 year flood plain of the major streams. This was a major
commitment of land since approximately one-third of the site lies within the
protected flood plains of Bear Branch, Panther Branch, and Spring Creek. An
123
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important part of the ability to maintain flood plains, open space, and
drainage channels at The Woodlands was the location of a 36-hole golf course.
The course winds in and out among the neighborhoods using land unavailable
for housing because of planning restrictions.
Every effort was to be made to preserve the tributary swales in their
natural, forested condition. The planners considered that excessive use of
even broad, shallow (10:1), grassy, man-made swales would endanger the
perched water table and consequently the site's ecological balance. The
forested swales were considered ideal for percolation and the prevention of
soil erosion.
Natural Drainage—
In areas where the existing drainage had to be improved because of
development, natural systems were constructed. Natural systems, as defined
at The Woodlands, are'broad, grassy swales, instead of lined ditches or drain
pipes, and check dams with temporary ponding rather than permanently retained
water. The check dams were expected to maintain local perched groundwater
and yet dry out often enough to prevent mosquito breeding.
Infiltration into the deep groundwater aquifer was to be maintained by large
multipurpose lakes servings as recreational facilities, stormwater retention
basins, and recharge facilities. These lakes would be expected to offset the
quantity of water withdrawn from deep aquifers that supply the potable water
requirements of the residents. The lakes would also serve the drainage
system as peak flow retention basins to prevent storm flow surges in to
Spring Creek. It is anticipated that in spite of the best efforts to
maintain infiltration and percolation at the site, there would be increased
runoff during heavy storms. The lake systems would be able to capture the
surge, alleviating downstream flooding and allowing some pollutants to
settle out.
Other Counter-measures—
Two concepts expected to become part of the future planning at The Woodlands
are the reuse of sewage effluent and the construction of porous pavement to
preserve infiltration in locations where large parking lots are required.
Both concepts were to be investigated. An attempt was to be made to
determine the quality of treated effluent suitable for recycling to surface
waters by investigating disinfection, algae blooms, and the relative quality
of lakes, runoff, and reclaimed water. Porous pavement was to be tested for
maintenance of porosity and quality of the water draining through the
pavement.
The final countermeasure to be used at the development was to control the use
of fertilizers', herbicides, and pesticides, by making them unnecessary. The
planners hoped that by leaving the natural plant and animal life in the
predevelopment ecological balance, chemical substances would not be needed to
promote or control plant growth. Limits were placed on the amount of
clearing by developers and homeowners are encouraged to leave existing brush
and trees instead of planting lawns.
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PERFORMANCE
An evaluation of the performance of countermeasures at The Woodlands is based
on data collected by investigators from Rice University during the period
January 1974 to April 1976 [1]. Data were collected both at The Woodlands
and at two other Houston watersheds that serve as examples of fully developed
urban neighorhoods. The first watershed, Hunting Bayou, is a 800 hectare
(2,000 acre) area in northeast Houston. It is 48% residential and 46%
commercial-industrial; 28% of the area has storm sewers. Westbury, the
second control watershed, is an 80 hectare (200 acre) area in southwest
Houston that is 100% residential and 100% storm sewered.
During the studies, the population of The Woodlands was approximately 2,000
as compared with the originally predicted population of 150,000 by 1992.
Extensive construction was taking place at the site including excavation of a
borrow pit that will become a major lake. Therefore, the results reflect
only a small portion of the final development and are biased by construction
activity.
Samples were collected from four principal locations at The Woodlands, which
are shown in Figure 35 and described as follows:
0 Station P-10 is near the confluence of the Bear and Panther Branches.
The tributary area is 6,500 hectares (16,000 acres) of natural forest.
• Station P-30 is on the Panther Branch downstream of the Phase One
development. The area includes 8,700 hectares (21,500 acres) that is
90% forest and 10% developed or developing.
• Lake B is located at the inlet to Harrison Lake. The 135 hectare
(355 acre) tributary area was undergoing development.
• Lake A is located at the outlet to Harrison Lake. The 195 hectare
(485 acre) area includes the Lake B tributary area and some areas
directly tributary to the lake.
Runoff Quality
The runoff quality data from January 1974 through April 1976 for dry-weather
samples and samples taken during 17 storm events are presented in Table 57.
Regression analysis of the runoff at four sites was used to find mean
loadings in kilograms per hectare (pounds per acre) of pollutants for 2.54 cm
(1.0 in.) of runoff. The values, 95% confidence limits, and ranking of sites
are shown in Table 58. Several observations can relate the sampling data to
land use.
• Suspended Solids - The most significant solids loading appears to
come from the construction activity upstream from Station P-30.
The Hunting Bayou area shows solids pollution from commercial-
industrial areas and barren urban land. The fully developed
residential Westbury has approximately the same solids
concentration as the forested Station P-10. The conclusion is that
suspended solids pollution from residential areas will be a problem
only during construction.
125
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Table 57.
RUNOFF QUALITY [1]
mg/L
Dry-weather flow
The Woodlands -
The Woodlands -
Storm flows
The Woodlands -
The Woodlands -
The Woodlands -
The Woodlands -
Westbury
Hunting Bayou
No. of -
storms
P-10
P-30
i
P-10 8
P-30 12
Lake B 8
Lake A 8
2
5
Table 58.
Pollutant
Suspended
solids
COD
Soluble
COD
Total
phosphorus
Kjeldahl
nitrogen
N03
t
Note: P-10,
HB -
WB -
lb
acre-
Suspended solids
Range Average
23
81
7-67
109-321
283-2,880 --
24-245
24-70
71-207
Total COD
Range Average
50
51
43-63
40-51
49-123
39-54
39-54
77-179
Total phosphorus
Range Average
0.06
0.14
0.03-0.09 • —
0.13-0.30
0.11-0.53
0.73-1.14
0.73-1.14
0.41-1.28
Kjeldahl nitrogen
Range Average
0.93
1.67
0.10-1.61
0.06-1.41
1.79-4.14
1.48-2.19
1.48-2.19
1.56-3.94
POLLUTANT LOADINGS FROM RUNOFF [1]
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Area
Mean Ib/acre-in.
Confidence limits
Rank (Decreasing
1 2
P-30 HB
43 38
±18 ±5
HB P-10
19 14
±7 ±1
P-10 P-30
10 9
±1 ±1
HB WB
0.28 0.24
±0.12 ±0.66
HB WB
0.95 0.40
±0.10 ±1.10
WB HB
0.088 0.087
±0.032 ±0.013
pollution »-)
3 4
WB P-10
14 8.2
±74 ±2
P-30 WB
13 9.5
±1 ±24
HB WB
4.4 4.1
±2 ±28
P-30 P-10
0.021 0.014
±0.007 ±0.003
P-30 P-10
0.30 0.28
±0.06 ±0.10
P-30 P-10
0.020 0.012
±0.018 ±0.008
P-30 - The Woodlands
Hunting Bayou (Houston)
Westbury (Houston)
Tn. x 2'27 = ha-cm
126
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• Organics (COD, soluble COD) - The data indicate that organic
pollutants should decrease as the forested area is developed. The
exception is the high particulate COD at Hunting Bayou when
compared with all other stations. This pollution is probably due
to oils, grease, and litter dumped in the open channels and will
probably not be as apparent in the more residential woodlands.
• Nutrients - The nutrient samples indicate where the most
significant problems will develop. Both developed areas, Hunting
Bayou and Westbury, show nutrient levels that are much higher than
the stations at The Woodlands. Levels increase as land use evolves
from forested to developing residential, to residential, to mixed
urban.
The runoff sampling data show that nutrients are a problem and if the attempt
to decrease the use of fertilizers at The Woodlands is successful over the
long term, a-major problem with urbanization will be mitigated. However,
considering the long-term construction period at the site, it appears that
planning should also consider interim controls of suspended solids.
Effect of Lake Impoundment
One of the drainage planning concepts was to use the major lakes at the
development to absorb stormwater surges and equalize the concentrations of
pollutants in runoff. An analysis of. flow and pollutants into and out of the
lake system during a 1975 storm is shown in Table 59 and Figure 36. The
results indicate that the lakes are an effective sediment trap, reducing the
suspended solids loading by 80%. However, the results show some enrichment
in the lakes for certain forms of nitrogen and phosphorus. The reason for
the nutrient enrichment was not determined, but is probably due to a
combination of factors including: (1) direct runoff from areas adjacent to
the lake, (2) direct nutrient-enriched rainfall, and (3) high concentrations
of these nutrient forms in the lake before runoff began.
Table 59. WATER QUALITY ANALYSIS
OF THE WOODLANDS LAKE SYSTEM DURING A 1975 STORM [1]
mg/L
Influent
Effluent
Parameter
Average Maximum Average Maximum
Orthophosphate
Total phosphate
Ammonia
Nitrate
Kjeldahl nitrogen
Suspended solids
Total COD
Soluble COD
0,005
0.11-
0.11
0.15
1.86
1273
63.7
32.0
0.013
' 0.36
0.15
2.1
3.1
2660
87.0
45.0
0.015
0.10
0.16
0.28
1.3
245
41.8
26.4
0.048
0.19
0.26
0.32
2.
356
45.0
31.0
:127
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B
at
CO
a
ui
ui
Cu
2800
2400
2000
1600
1200
j 800
e
3
400
0
2800
j 2400
v
30
I
,- 2000
a
j
» 1800
a
j
a
J 1200
9
3
9
j 800
c
9
400
0
LAKE INFLOW
SOLIDS
CONCENTRATION
6 8 10 12 14 16
TIME FROM START OF STORM, h
18 20
LAKE OUTFLOW
DISCHARGE
6 8 10 12 14
TIME FROM START OF STORM, h
16 18
20
Figure 36. Effect of lake impoundment on storm
flowrates and suspended solids concentration [1].
128
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The peak flow out of the lakes was only slightly lower than peak inflow;
however, the storm was quite large, 10 cm (4 in.) of rain in 10 hours. The
first peak of the storm, in which 5 cm (2 in.) fell in 3 hours, was
effectively controlled as the influent peak of 2,800 L/s (100 ftj/s), at
about 2 hours, was reduced to 550 L/s (20 ft3/s) at discharge.
Using lakes for dampening hydraulic and pollutant peaks appears to be
effective, particularly for suspended solids.
Porous Pavement
A parking lot was built at The Woodlands conference center to test porous
pavement. The lot had two sections, one-half conventional pavement and one-
half porous pavement with a sand and gravel underdrain.
Tests indicated that runoff would penetrate the porous pavement and build up
in the underdrain creating a slowly draining reservoir. The result was
similar to a detention pond: runoff was delayed and peaks dampened [2].
A comparison of runoff quality between two sections of the lot showed that
organic and nitrate levels were lower in the runoff from porous pavement,
while ammonia levels were higher. The investigators suggest that anaerobic
decay may be taking place in the underdrain. Lead and zinc concentrations
were much lower in the runoff from porous pavement. Lead averaged 0.05 mg/L,
while zinc was 0.18 mg/L [2].
The safety and driveability of the porous pavement was satisfactory; however,
it was susceptible to clogging and will require periodic vacuum sweeping and
high pressure washing to remain permeable. Maintenance can probably be
decreased by better control of dirt carried onto the lot by construction
vehicles.
IMPACTS
The water resources plan for The Woodlands will succeed only if the effects
of the natural drainage system are acceptable to the residents of the
community. Investigation of the environmental impacts centers on the
multipurpose lakes and whether they can maintain an acceptable water quality
while receiving storm runoff and sewage treatment effluent. The
socioeconomic impact concerns the willingness of the residents to accept the
community as planned with both restricted forest clearing and several high
density townhouse developments needed to provide an overall community
density.
Environmental Impacts
Several studies have been conducted to examine potential problems in the lake
system [3-6]. The investigations included: (1) the eutrophication potential
in the lakes, (2) chlorine and ozone toxicity as applied to the disinfection
of wastewater recycled to the lakes, (3) bacterial characteristics of the
runoff, and (4) organochlorine compounds in the runoff and lakes.
129
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Eutrophication Potential--
Nutrient enrichment in a lake system can speed the natural
eutrophication process and cause unwanted algal blooms. A large algae
population is aesthetically unpleasing, interferes with recreation, and
may cause fish kills. In view of the expected increase of nutrients in
the runoff as The Woodlands develops, a study was made of the effects of
nutrient addition on algae growth at the site.
Samples taken from Panther Branch, the lake system, Hunting Bayou, and
Westbury were spiked with nitrogen or phosphorus or both and the algae
growth was observed. In most cases, the low flow conditions were
phosphorus limited and storm flow conditions were nitrogen limited.
Recommendations from this study include: (1) sewage treatment effluent should
have phosphorus removed before it is recycled to the lakes, (2) there is no
reason to remove phosphorus during storms, and (3) the "first flush" of a
storm should be diverted or treated to prevent nutrients from entering the
major lake systems [3].
Disinfectant Toxicity— -
Recyling of wastewater effluent to a recreational lake will require adequate
disinfection. The residual levels of disinfectant in the lake system may^be
a problem to the fish population. A study was conducted to examine chlorine
and ozone residuals in connection with sewage treatment at The Woodlands.
The results of the study with channel catfish showed that in a 96-hour
bioassay the LC50 for chlorine is 0.07 mg/L and for ozone is 0.03 mg/L.
Therefore, based on the "Aquatic Life Water Quality Criteria" of 1/10 value,
the acceptable level for chlorine is 0.007 mg/L and ozone should be below
detection levels [4].
Bacterial Characteristics—
An important part of the multiuse concept for The Woodlands' lakes is their
recreational value. A test of the suitability of the lakes for both
stormwater control and recreation is the ability to meet current Texas
standards for acceptable bacteria levels. The standard is a 30-day mean
fecal coliform level of less than 200 organisms/100 mL for water contact
recreation and 2,000/100 mL for noncontact recreation. Mean values for a
number of samples taken at different sites and under different conditions are
shown in Table 60. -
The data from P-10, P-30, and Westbury show a trend of deterioration with
urbanization and high levels of bacteria even in the rural stream represented
by Station P-10. It appears doubtful that the lakes would be acceptable for
contact recreation.* Since the standard is based on a mean of five samples in
30 days, it would include both storm and low flow periods, and the lakes
would probably be acceptable for noncontact recreation. An interesting
result is the apparent settling and die-off of coliform in the existing lake
130
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system. During storm periods, coliform levels are an order of magnitude
lower in the lake effluent than in the influent.
Table 60. MEAN VALUES OF FECAL COLIFORMS [5]
Organisms/100 mL
Site
P-10
P-30
Lake B
Lake A
P-10
P-30
Westbury
Lake B
Lake A
Chlorinated sewage
Fecal
Condition coliform/ TOO mL
Low flow
Low flow
Low flow
Low flow
Storm
Storm
Storm
Storm
Storm
—
135
240
95
35
1,000
2,950
24,500
2,040
220"
18-42
A concurrent study of soil and sediment found the following levels of
coliforms:
Stream sediment
Soils (sampling stations)
Lake sediment
Soils (golf courses and swales)
Fecal coliforms/gram
20-20
20-40
20-60
20-280
Leaching and scouring of the soil could have contributed to the levels of
coliform in the runoff.
Finally, disinfection studies were performed to determine concentrations
required to lower fecal coliform levels. Chlorine concentrations of from 8
to 16 mg/L were required to satisfy disinfectant demand in samples from
Station P-30 and 10 mg/L in samples from Lake A. An ozone dose of 32 mg/L
was required to disinfect a Lake A sample [5].
Organochlorine Compounds— , ,
Another project was undertaken to make a preliminary study of the
organochlorine compounds in the water system of the developing community. .
Emphasis was placed on the examination of soil, water and fish for
concentrations of polychlorinated biphenyls (PCBs) and chlorinated
hydrocarbon pesticides.
131
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An unexplained rise in PCB concentrations was observed in the first half of
1974 at most of the sample sites. Peak concentrations reached 341
parts/billion (ppb) in the soil and 8.2 ppb in the water. In August and for
the remainder of the study, the soil samples returned to a base level of from
1 to 2 ppb and the water less than 0.5 ppb. The 6 months of high values were
not repeated the following year. The investigators speculated that since the
high levels of PCBs coincided with a period of cut and earth moving, an
abandoned landfill may have been disturbed [6].
Chlorinated hydrocarbon pesticides found in Panther Branch fish included
trace amounts of DDE both upstream and downstream and trace quantities of
dieldrin upstream, while 2.1 ppb of dieldrin were found in fish at Station P-
30. The Woodlands golf course was also investigated for pesticide residue;
Mirex, a pesticide used for fire ant control, was discovered. The
concentration ranged from 5 to 15 ppb in water, 15 to 30 ppb in soil, and 30
to 55 ppb in mosquito fish. Chloradane was also found in golf course runoff
and again showed amplification in the fish, with 10 to 40 ppb in crayfish,
while only 5 to 20 ppb were found in soil and water.
Environmental Summary—
The studies of the environmental impacts caused by the water management
system bring out several important points for management of the lakes:
1. Preservation of aesthetic values by control of eutrophication can
be most easily accomplished by flushing and dilution with low
phosphorus water during low flow periods. Treated wastewater or
well water will be acceptable.
2. Maintenance of fish in the lakes will require that recycled
wastewater or disinfected stormwater have very low residual
chlorine or ozone concentrations.
3. Swimming and other water contact recreation would require
disinfection of runoff since even upstream water quality is of only
marginal bacterial quality. Boating and noncontact recreation can
be maintained especially in the downstream unit of double lake
systems, such as Harrison Lake (Lakes A and B).
4. Pesticides used at the site will contaminate fish and, if
recreational fishing is allowed, pesticides will have to be
controlled and fish sampled periodically. Lakes constructed
downstream of areas using pesticides (golf courses) may have
restricted fishing.
Socioeconomic Impacts
The socioeconomic questions at The Woodlands are: (1) will homebuyers accept
a natural drainage system with swales and requirements for leaving part of
their lots in a natural condition, and (2) can the quantity of open space,
flood plain reserves, and lakes envisioned be maintained and still allow the
developers to profit.
132
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Acceptance of Natural Drainage and the Forest Setting--
A series of photographs of The Woodlands are shown in Figures 37 and 38. The
drainage system is shown in Figure 37 and various examples of the forest
setting at The Woodlands are shown in Figure 38. In almost all cases, the
individual homesites are being maintained with more natural vegetation than
similar developments in north Houston. Forested landscaping is encouraged by
a monthly award for best natural landscaping and by sales brochures
emphasizing the forest setting and, thus, attracting homebuyers interested in
the concept. The natural setting has become an identifying point for the
community and both homeowners and developers are interested in maintaining
the idea. The only legal control over clearing at the site is a deed
restriction requiring a permit to cut trees that are in excess of 15 cm (6
in.) in diameter.
The natural drainage system itself does not receive the same public attention
given to natural landscaping. Homeowners complain only if their property
floods, which has not been a significant problem. The developers believe
that the use of a natural system has saved a great deal of initial capital
cost and indicate that maintenance has not been a problem [7]. The best
indication of the success of the plan is that the developers intend to follow
the same concepts in the future development at The Woodlands.
Open Space--
The development has maintained a significant amount of open space around the
drainage system and the golf course. Present plans are to continue the
existing drainage concepts and offset the cost of land committed to flood
plains by savings in the initial cost of natural drainage. The next area of
development contains a large recreational lake that will be the focal point
of the area. The original plans for development had included high density
townhouse and apartment areas to offset the open space and provide an overall
density that would be profitable to the developers. In the first years of
the community, the townhouses did not sell well because of a homeowner
preference for single-family housing. However, there has been a recent
upsurge in the demand for townhouses, and another section of high density
development will be built.
Socioeconomic Summary—
In any housing development, the values and opinions of the homebuyer will
eventually prevail. An appealing environment^'11 raise home values, but the
buyer decides what characteristics are appealing. The natural landscaping at
The Woodlands is apparently successful and will be carried into subsequent
development phases. Economics now favor natural drainage, lake systems, and
open space. However, if economics should reverse and favor a denser, more
conventionally sewered site, the environmental issues would probably not
outweigh the economics.
133
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Figure 37. Natural drainage system at The Woodlands:
(a) roadside swale in a residential neighborhood, (b) area drainage
at the conference center, (c) influent to Lake Harrison,
and (d) major drainage stream.
134
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Figure 38. Landscaping with natural vegetation at the Woodlands:
(a) office building and (b) and (c) the front yards of award
winning homes.
135
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SECTION 8
BEST MANAGEMENT PRACTICES
ORANGE COUNTY, FLORIDA
The primary water quality problem in Orange County is the degradation of the
many lakes and waterways; approximately one-tenth of Orange County's 2,590
km2 (1,000 mi2) is surface water. Stormwater runoff is depositing nutrients
and sediments in the nearly 1,100 waterways and lakes, causing an increased
rate of eutrophication. An areawide strategy has been developed to reduce
the Stormwater runoff loadings by implementing BMPs.
The BMPs used in Orange County include nonstructural and low structural
facilities to 'control and/or treat Stormwater runoff near its source. The
BMPs in Orange County have demonstrated the effectiveness of individual unit
controls to reduce the volume and pollutant loadings of Stormwater runoff to
the receiving waters.
The long-term environmental and socioeconomic impacts of the BMPs have not
been fully identified because they have only been implemented recently, but
the proposed program studies have projected the impacts of full program
implementation. The BMPs have effectively reduced flooding hazards and have
provided multiuse facilities for recreation and/or aesthetic enhancement.
PROGRAM DESCRIPTION
The Stormwater runoff problems in Orange County are similar to those in other
cities throughout the country. Orange County emphasizes water quality
problems more than many other cities, however, because of the great number of
lakes and waterways in the region and the area's dependence on tourism as a
major source of income.
Area Characteristics
The important area characteristics to consider when designing and
implementing source controls are topography, climate, rainfall
characteristics, hydrology, land use, and soil types. Orange County is in
east central Florida with Orlando being the largest city in the region, as
shown in Figure 39.
The topography of the Orange County area is generally flat with increasing
elevations from east to west. The eastern portion is predominantly less than
11 m (35 ft); the western highlands, above 32 m (105 ft), are gently rolling
sandy hills; the intermediate area in the center of the county is an
undulating topography varying between 11 m (35 ft) and 32 m (105 ft).
136
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MOUNT
jDORA
o
0 5 10
SCALE MILES
COUNTY
COUHTY"
Figure 39. Orlando and Orange County, Florida [1].
The climate of Orange County is much like that of peninsular Florida, ,a
variation between humid wet and dry seasons creating a semi tropical
environment. The average temperature is about 21°C (70°F).
Over 50% of the annual precipitation occurs from June through September.
These rainstorms are usually afternoon showers or thunderstorms that can
produce between 1.3 and 5.1 cm (0.5 and 2 in.) of precipitation in less, than
1 hour. The frequency of occurrence of these storms can cause flooding.
Soil types and their capacities to infiltrate precipitation are interrelated
with the area's hydrology. The western third of Orange County is designated
as a prime recharge area. The soil capabilities and hydrology in Orange
County are shown in Figures 40 and 41. The land use in specific areas of
soil affects the natural rates of percolation.
Problem Assessment
The environmental impacts from stormwater runoff that could adversely affect
the community and receiving waters are caused by dissolved oxygen depletion,
high pathogen concentrations, and increased nutrient loadings. Socioecohomic
impacts to the community might include aesthetic deterioration and
interference with recreational uses.
137
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LEGEND
q
o
8 4 8 1216
SCALE MILES
MODERATE
AND WELL
DRAINED SOILS
POORLY DRAINED
SOILS
VERY POORLY
DRAINED SOILS
OSCEOLA CO.
Figure 40. Soil capability of Orange County, Florida [2].
LEGEND
AREAS
DOMINATED .
BY SUBSURFACE
AQUIFER RECHARGE
I
AREAS DOMINATED
BY SURFACE ,
DRAINAGE 1
PRINCIPAL ]
DRAINAGE WAYS
l
0 4 8 • 1 2 1
HHE55HH5S
SCALE MILES
OSCEOLA CO.
Figure 41. Hydrology of Orange County, Florida [2].
138
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The problem in Orange County is maintaining water quality and simultaneously
encouraging community growth and development. The extensive waterways and
lakes in the Orange County area offer scenic beauty, recreation, and attract
tourism, but the resulting development and growth can degrade the quality of
the waterways and lakes.
Urban residential, urban commercial/industrial , and agricultural/improved
pasture are the land uses contributing the largest pollutant loads to the
receiving waters in the Orange County area. Other nonpoint sources include
construction activities, hydro!ogic modification, saltwater intrusion, and
silviculture. The nonpoint source loading rates for the major pollutants in
Orange County area are compared in Table 61.
Table 61. POLLUTANT LOADING AND CONCENTRATION COMPARISON
BETWEEN LAND USES AND NATURAL AREAS IN ORANGE COUNTY [3]
Pollutant loading, lb/acre-yr
Pollutant concentration, mg/L
Land use
Residential
Agricultural/
improved pasture
Commerci al
Natural8
Well -drained soils
Fl atwoods
Range
Swamp
Suspended
BODg solids
28.9
15.1
78.9
6.7
0.0
2.3
2.3
12.3
310.3
391.7
884
31.3
0.0
25.1
25.1
24.3
Total
ni trogen
5.9
5.6
12
3.3
0.0
2.1
2.1
4.9
Total
phosphorus
1.6
1.0
2.4
0.15
0.0
0.1
0.1
0.2
Suspended Total Total
BODg solids nitrogen phosphorus
5.0
7.0
9.1
2.7
—
—
__
—
54.4 1.03
180 2.58
101 1.38
8.7 1.35
._
—
__
—
0.28
0.46
0.28
0.06
™
—
~
•--
a. Weighted averages used.
Increases in the loading rates for the three major developed land uses range
from 4 to 42 times the loading from natural undeveloped areas. Total
phosphorus and sediments have been determined to be the pollutants with the
greatest impact on the receiving waters [3]. e? •]
Countermeasure Philosophy , ?:;
'—~~. . .','••''> .:*. •' "-**'.' ." ,\-
The BMP countermeasure strategy in Orange County has been implemented by the
construction of low structural and nonstructural facilities. These controls
are supported by good maintenance practices and are required by ordinance.
Orange County has several programs oriented toward solving water quality
problems resulting from stormwater runoff. Southern Florida Water Management
District requires the detention of the first 2.54 cm (1 in.) of rainfall
runoff and allows for release at specified rates for control of floodwaters
[4]. Orange County has stormwater and subdivision regulations to prevent
further water quality degradation, which have also helped to control erosion,
reduce flooding, and recharge the groundwater.
139
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Orange County now requires all new development to provide the retention of
the first 2.54 cm (1 in.) of rainfall, but with acceptance of the Orlando 208
study, new requirements will be mandated [4, 5]. The new regulations will be
oriented toward recharge capacities, requiring the retention of the first
1.27 cm (0.5 in.) of runoff on soils with high percolation rates and the
first 0.9 cm (0.35 in.) of runoff on soils with low percolation rates [4, 6].
Implemented Controls—
The abatement practices for specific land use categories were selected during
the formation of the counter-measure strategy. Several examples of the
abatement practices have been studied for operation and performance, costs
and resources, and impacts.
Percolation is the predominant method used for treatment of stormwater runoff
in Orange County. Alternative methods of percolation, corresponding to
percolation rate potentials of the areas served, are first flush diversion,
retention, swales, underdrains, detention, and natural treatment. Vacuum
sweeping and fabric bags have also been implemented as abatement practices.
Piversion/Percolation—Because the greatest concentration of pollutants
occurs inthefirst flush of the watershed runoff, facilities have been
designed to divert and capture the first flush and allow the remainder of the
water to bypass the facility to a receiving water. The diverted stormwater
is then percolated in the basin, preventing the majority of the pollutants
from entering the surface water. A schematic of the diversion structure and
percolation pond is shown in Figure 42.
Retention/Percolation—Percolation ponds without diversion structures are
also used in Orange County. A total capture percolation pond built on well-
drained soils is located at the Wimbleton Apartments. The facility, shown in
Figure 43,oServes almost 5.77 ha (14 acres) of watershed and has a capacity
of 1,974 m (1.6 acre-ft). The pond is 100% efficient, but exceeds design
requirements and has maintenance problems because of excessively steep side
slopes [7].
Swale/Percolation--Swa1e/percolation facilities are also an effective method
for reducing the major runoff pollutants in Orange County. A swale/
percolation facility between an apartment complex and a lake is shown in
Figure 44. This facility has an inlet structure from the service area and
overflows to the lake through an overflow structure if the capacity of the
facility is exceeded. Stormwater remains in the swales until percolated.
Underdrains—Underdrains in residential areas have shown relatively high
efficiency in reducing the pollutant loading from stormwater runoff, but also
are the most expensive to implement. Underdrains use percolation as the
method of treatment,and consist of perforated pipe enveloped in gravel to
provide drainage beneath swales, as shown in Figure 45. The effluent is then
discharged into a storm sewer or a receiving water. Design recommendations
for high efficiency include a minimum of 0.3 m (1 ft) of soil between the
surface and the top of the gravel and the use of some permeable layer, such
as straw, between the soil and gravel to prevent soil from washing into the
140
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FLOW
DIVERSION STRUCTURE
Figure 42. Diversion/percolation pond at 8 Days Inn: (a) schematic
of diversion/percolation facility, (b) diversion structure,
showing stop log diversion baffle.
141
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voids. Underdrains not only operate effectively, but provide an aesthetically
pleasing solution with virtually no maintenance requirements. An underdrain
cross-section and a typical surface view are shown in Figure 45.
Figure 43. Total capture percolation facility.
Residential Swales—Percolation of stormwater in swales (grassed depressions)
has provided efficient treatment when located on soils with high percolation
rates. Swales have exhibited low performance when located on poorly drained
soils or soils excessively compacted during construction. Problems
encountered with swales include erosion on steep side slopes and insect
breeding associated with standing water.
Detention/Sedimentation--The sedimentation basin at Prairie Lake provides
relatively good pollutant removal rates for the unit cost per impervious area
serviced. The facility is 5.5 by 11.5 m (18 by 38 ft) and the depth tapers
from 1.8 m (6 ft) at the inlet to 0.15 m (0.5 ft) at the outlet [7]. A
chickenwire fence at the inlet traps large debris, and a baffle extends to a
depth of 0.6 m (2 ft) into the middle of the pond and acts as an energy
dissipator to prevent short-circuiting. The facility has experienced
resuspension of sediments deposited from previous storms.
Natural Treatment—Natural treatment using a cypress stand provides one of the
least expensive treatment methods for stormwater runoff and yet its reduction
of pollutant loadings is among the highest. Cypress stands are characterized
by marsh vegetation, small cypress at the exterior, and mature cypress with
intermittent ponds and sloughs in the interior. Slow-moving flow through the
stand allows vegetation to grow in the ponds and peat to accumulate. The
erosion rates are also low. Stormwater is channeled into the cypress stand
142
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Figure 44. Swale/percolation: (a) dry swales before rain;
(b) swales during rain, (c) facility filling during rain,
with overflow structure at the right.
143
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with the outflow a function of the inflow rate, storage capacity, vegetation
density, and infiltration rate.
TOPSOIL
PERMEABLE LAYER
GRAVEL
PERFORATED PIPE
in. MINIMUM
12 in. MINIMUM
18 in. MINIMUM
Figure 45. Residential underdrain: (a) typical surface view
of swale above underdrain, and (b) schematic of underdrain system [7].
Fabric Bags—Fabric bags have been mounted in inlets to stormdrains to filter
the stormwater. Although this method is the least expensive, it required high
maintenance and had a lower operational efficiency for suspended solids than
did other methods. The effectiveness on organics removal was high, but this
method was not used after the study period.
144
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Vacuum Sweeping—Vacuum sweeping was implemented as an operation and
maintenance measure to reduce the pollutant loading of runoff from a shopping
mall parking lot. The parking lot area required daily vacuum sweeping of
about 2,977 m (9,760 ft) of the internal curb and weekly sweeping of
approximately 2,657 m (8,710 ft) of the external curb [7]. The vacuum
sweeping efficiency approached 80% for suspended solids in this application.
The vacuum sweeping study area is shown in Figure 46.
^ , ;: ,„,,"• l& "V .«.%-*,],^
Figure 46. Vacuum sweeping study area at Altamonte Mall.
Design Criteria--
The design criteria for stormwater runoff controls should use information on
the permeability of the ground surface, the type of vegetation, the area of
drainage basin, the soil characteristics, and the pollutant loading rates.
These factors were used in a manual entitled "Stormwater Management Practices
Manual" that provides guidance in stormwater facility design in Florida [8].
The approach assumes worst case conditions where 100% of the watershed
contributes to runoff.
The SCS soil groupings for runoff potential and runoff curve numbers, for
selected land uses corresponding to the soil groupings are the basis for
evaluating runoff from a drainage area. Soil groupings classify soils from,
1-owest to highest runoff potential: soil Group A, lowest runoff potential;:
B,.moderately low runoff potential; C, moderately high runoff potential; and
D, highest runoff potential. -,"-••-!••> .
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For each of the soil group classifications, runoff curve numbers (CN) are
determined for various land use types and conditions of vegetative cover.
These numbers range from 0 to 100, with 100 representing the most impervious
soil condition producing the highest runoff [8]. Curve numbers for a
specific land use and soil group may be further adjusted to reflect soil
moisture conditions. As soils become saturated, they become more impervious.
These design considerations, which describe the physical conditions of the
site, are used with regulatory design limits prescribing the volume of runoff
to be diverted to a detention/percolation facility (including possible first
flush loadings). As an example of the use of the design guidelines developed
for the Florida area, a comparison is made between a constructed basin and a
basin designed by the methods presented in the manual described previously.
EXAMPLE DESIGN OF A STORMWATER DIVERSION/PERCOLATION FACILITY
Determine the pounds of pollutants removed per year, the size of the facility required, the
capital cost, and the present value for a diversion/percolation facility from the information
given below and by using Figure 47.
Specified Conditions
1. Total area served = 4.6 acres
2. Impervious area served =3.8 acres
3. Runoff to be diverted for treatment = 1.15 in.
4. Type A soils in percolation basin.
Assumptions
1. The soil is saturated and therefore responds as impervious soil (CN « 100).
2. Pollutant loadings in Ib/acre-yr are
Suspended solids = 254
BOD5 = 36.5
Total nitrogen = 7.8
Total phosphorus = 1.1
Solution
1. Determine the pounds of pollutants removed per year:
a. Locate the total area served on the "contributing watershed area" axis.
b. Project a vertical line to the 1 in. diversion line (maximum value of the graph)
c. Read pounds of pollutants removed per year:
Suspended solids = 1,500
BOD5 = 168
Total nitrogen = 50
Total phosphorus =5.5
2. Determine the percolation basin size:
a. From the intersection of the 1 in. diversion line and the vertically constructed
line, draw a horizontal line to the pivot line for type "A" basin soils in the top
left-hand quadrant.
Note that the CN for impervious areas = 100
b.
c.
From the intersection of the type "A" basin soils pivot line, construct a vertical
line to the CN = 100 line in the lower left-hand quadrant.
From this point, draw a horizontal line through the basin volume axis to the
capital and present value graphs in the lower right-hand quadrant.
Reading from the basin volume axis, the required percolation pond size is
approximately 0.65 acre- ft.
146
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NOTE : IMPERVIOUS AREA
CN * 100
LOADINGS (LB /ACRE-VR)
SS 254
BODg 36.5
6000 SS
880 BOD
185 N
26 P
1 .00
FIRST FLUSH
DIVERSION
VOLUME
(INCHES)
0.10
•JO' .'2, KM2
^-5000 SS
730 BOD
155 N
21 P
4000 SS
590 BOD
125 N
16 P
440 BOD
95 N
11 P
£L 3000+2.43
2000 SS
290 BO
65
2000 --1.62 7
M 6000 5000 4000 3000 2000
1 1 1
BASIN VOLUME-FOR COMPOSITE
"CN" VALUES
PRESENT VALUE
(20 YR, 7K)
ACRE-FT
15 20 25- 30 ACRES
CONTRIBUTING WATERSHED AREA
*CN" VALUES
30 45 60 75
CAPITAL AND PRESENT
VALUE ($1000)
90
Figure 47. Size, efficiencies, and cost of
diversion/percolation basins (ENR 3000) [8].
147
-------
3. Determine the capital and present value of the diversion/percolation facility.
From the intercepted points on the capital and present value lines, draw a vertical line
to the base axis of the right-hand quadrant
Capital cost
Present worth
= $14,067
= $20,336
Comment
The use of this graph is limited to Florida as data used regarding soil, watershed, and
meteorology apply specifically to Florida.
A comparison of values from an implemented facility and the results from this example is
shown in Table 62. Although the design graph is only an aid for first-cut design, it shows
good correlation between values of an implemented facility.
Table 62. DESIGN AND IMPLEMENTED FACILITY COMPARISON9
Graph projected Implemented
values, using facility,
Figure 9 8 Days Inn
Area served, acres 4.6 4.6
; ' Facility size, acre-ft 0.65 0.44
Pollutant loading, Ib/acre-yr
BOD5 36.5 46.9
Suspended solids 254 356
Total nitrogen 7.8 10.57
Total phosphorus 1.1 1-92
Pollutant removal, Ib/yr
BOD5 168 214
Suspended solids 1,500 1,622
Total nitrogen 50 48
Total phosphorus 5.5 8.7
Capital cost, $ 14,067 14,380
Present worth (20 yr at 7%), $ 20,336 26,400
a. ENR 3000.
OPERATION AND PERFORMANCE
The operation and performance of Orange County's BMPs have provided data on
unit effectiveness of low structural and nonstructural facilities. The
analysis of the data has identified design criteria, efficiencies, operation
needs, and associated maintenance problems.
Operation
The stormwater facilities in Orange County have low manpower requirements.,
Most facilities have been designed for specific loadings or as self- ;•;
activating units and therefore, require minimal operational control.
Facility operation may involve changing the elevation of baffles in diversion
chambers to capture different amounts of the first flush or controlling the
release of flow from detention/sedimentation ponds. Self-activating
stormwater controls are shown in Figure 48.
148
-------
Figure 48. Self-activating stormwater controls:
(a) regional self-activating flood gate, (b) regional
stormwater percolation/evaporation pond, (c) small
self-activating diversion/percolation pond.
149
-------
Maintenance
Maintenance of low structural or nonstructural stormwater facilities or
practices include:
• Debris removal
• Landscape care
• Vector and insect control
• Aquatic vegetation control
• Removal or scarification of sediment deposits
• Street sweeper maintenance
• Fabric bag maintenance
• Repair of erosion damage
Generally, the facilities serving urban commercial/industrial or residential
areas will contribute larger quantities of debris, solids, pollutants, and
substantially larger quantities of runoff because of the greater areas of
impervious cover.
Multiuse facilities usually provide, in addition to their designated purpose,
aesthetic appeal or recreation-oriented use and require higher maintenance.
Facilities in public areas will most likely require maintenance equal to that
of a multiuse facility. If there is public access to the facility, a higher
degree of landscape manicuring, control of litter, and continual care will be
required.
Stormwater-facilities that have a fill/drain sequence are likely to need more
maintenance than facilities that remain full of water. Detention ponds or
sedimentation ponds, for example, would collect debris and solids that would
have to be removed during dry periods. These facilities may have short
periods of minimal ponding, when insects might breed and odors develop. All
of these problems would increase maintenance requirements.
Orange County requires developers not only to implement stormwater runoff
controls, but to maintain them. In most cases, developers have assumed
maintenance responsibilities for the stormwater facilities, but in the cases
where they do not, Orange County will maintain the facility and levy a fee
against the development for the required work. For regional flood control
and stormwater runoff facilities, maintenance is the responsibility of Orange
County, the City of Orlando or, in the case of facilities built in
conjunction with state highway projects, maintenance is the responsiblity of
the State Department of Transportation [4].
Performance
The unit performance of the Orange County BMPs has shown substantial success
in implementing stormwater runoff controls that will help meet water quality
goals. The aggregate effect of these BMPs has not been determined, although
the cumulative results of several control measures are expected to be
substantial. , rr.
I bU
-------
Suspended solids, BODg, total nitrogen, and total phosphorus have been
identified as the main pollutant loads that should be reduced. Percolation
facilities on soils with high percolation rates have shown exceptional
pollutant removal. The percolation basins at Wimbleton Apartments and the
diversion/percolation basin at 8 Days Inn are nearly 100% effective in
removing major pollutants [7]. Although these facilities serve relatively
small areas with varying amounts of impervious land, the efficiencies
indicate the potential value of implementing these types of facilities for
control of stormwater runoff in entire basins.
Facilities that have performed well, but not with total efficiency, include
natural treatment via cypress stands, underdrains, swales/percolation, and
sedimentation basins. Vacuum sweeping was of considerable value in
eliminating suspended solids from a parking lot, but had less efficiency on
other pollutants. Similarly, the fabric bag was effective for reducing BOD,-
and total nitrogen, but was of marginal value in other pollutant reductions.
The effectiveness of the BMPs used in Orange County is summarized in Table 63.
The reduced volume of stormwater runoff on a basinwide basis has not been
determined. Substantial reduction of runoff in some areas has been noted on
a qualitative basis by maintenance crews [4]. The reduction of runoff flows
and their subsequent percolation into the groundwater may be considered a
secondary benefit of BMP controls in this area.
COST AND RESOURCES
The costs and resources associated with the BMPs implemented in Orange County
include capital and annual operation and maintenance. The construction of
the stormwater facilities is the main expense. The maintenance costs and
resource use are minimal.
Capital Costs
The capital costs of stormwater control facilities for new development are
the responsibility of the developer. Larger regional facilities constructed
by the county are financed by the county general fund or by the State
Department of Transportation. This state agency works with the counties to
provide stormwater facilities with the construction of new highways.
Capital costs for several types of facilities are given in Table 64. Unit
costs are calculated for impervious and total area served by each facility,
excluding the cost of land.
The ratio of impervious land to total land served by each control measure
varies significantly in some cases; therefore, the unit costs may be compared
more easily on an impervious area basis.
Operation and Maintenance Costs
Annual costs to operate and maintain stormwater facilities vary with the type
of practice implemented. BMPs with significantly lower operation and
maintenance costs are maintained by private individuals or homeowners, as in
151
-------
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152
-------
the case of underdrained swales. A comparison of the operation and
maintenance costs for BMPs in Orange County is shown in Table 65.
Table 64. CAPITAL COSTS OF BMPs IN ORANGE COUNTY
a,b
[7]
Stormwater
control measure
Diversion/
percolation
Retention/
percolation
Swale/
percolation
Underdrains
Residential
swale
Detention/
sedimentation
Cypress stand
Fabric bagsc
Vacuum sweeping1'
Total area
serviced, acres
4.56
13.79
4.92
9.0
2.6
4.4
24.1
4.8
43.5
Impervious area
serviced, %
84
26
74
20
48
80
67
49
100
Total
capital
costs, $
14,380
19,654
14,556
23,233
4,949
3,637
3,823
176
56,267
Unit
capital
costs,
$/acre
3,154
1,425
2,959
2,581
1,904
827
159
37
1,292
Impervious
area unit
costs, $/acre
3,754
5,482
3,998
12,908
3,966
1,033
237
75
1,292
a. ENR 3000.
b. Includes 15% for engineering and legal; land costs are not included.
c. Fabric bags must be replaced every 2 years; two fabric bags are installed.
d. Assumes cleaning equipment is used exclusively for the area being swept.
Table 65. BMP OPERATION AND MAINTENANCE
COSTS IN ORANGE COUNTY3
Stormwater
control measure
O&M costs, $/acre-yr
Diversion/percolation 294
Retention/percolation 642
Swale/percolation 459
Underdrains 0
Residential swale 73
Detention/sedimentation 532
Cypress stand 28
Fabric bagsb 239
Vacuum sweeping0 261
a. ENR 3000.
b. Fabric bags must be replaced every
2 years; two fabric hags are installed.
c. Labor and equipment upkeep.
153
-------
Cost Effectiveness
A comparison of the cost effectiveness of the BMPS in Orange County is
presented in Figure 49.
The efficiency of facilities ranges from 0 to 100% removal of major
pollutants. The capital costs vary from $30 to $5,228/impervious hectare
($75 to $12,908/impervious acre). The cost effectiveness of removing
specific pollutants from stormwater by the implemented controls is shown in
Table 66.
The capital costs of percolation basins at various levels of efficiency and
soil percolation rates are shown in Figure 50. Soil percolation rates have a
major influence on the cost of facilities for higher removal efficiencies.
Economies of scale are greatly inflated by a factor of 40, between 70 and 90%
removal efficiencies, for facilities on poorly drained soils.
IMPACTS
Expanding population and urbanization have resulted in general environmental
degradation. The Orlando Metropolitan 208 study has identified its major
problem as general degradation of lakes and waterways because of excessive
nutrient and sediment loadings. Adverse environmental impacts associated
with the poor water quality of the region include [3]:
• Reduction in species habitat, type, and population
• Increase in pest species (i.e., aquatic growth, insects)
• Increase in eutrophication rates
• Adverse effects to potable water supply
• Decrease in aesthetic appeal
The countermeasure strategy is not fully implemented, but projections have
been made as to the expected environmental and socioeconomic impacts
resulting from full implementation.
Environmental Impacts
The abatement strategy goal is to reduce the receiving water pollutant
loading rates to natural (predevelopment) conditions. The impacts on the
climate, topography, soils, and subsurface geology are anticipated to be
smal 1.
The major environmental impacts will be associated with surface and
groundwater hydrology. Positive impacts are anticipated for surface water in
accordance with planned strategy. These positive impacts will be long-term
improvements in surface water quality, predominantly from the reduction of
nutrients, solids, pesticides and herbicides, heavy metals, and increased
dissolved oxygen. The elimination of pollutants from surface waters via
percolation methods can potentially deposit pollutants in the groundwater.
154
-------
12.908
5,482
3,998
3.966
3.754
-v
UNDERDRAINS
PERCOLATION
BASIN
SWALES/
PERCOLATION
SWALES
DIVERSION/
PERCOLATION
1,292
1.033
237
VACUUM
SWEEPING
SEDIMENTATION
40 60
REMOVAL EFFICIENCY,
80
LEGEND
BODg
TOTAL
NITROGEN
100
FA BRI C
BAGS
TOTAL
PHOSPHORUS
SUSPENDED
SOLIDS
Figure 49. Cost effectiveness of BMPs in Orange County.
155
-------
Table 66. COMPARISON OF COSTS FOR REMOVING
BODg AND SUSPENDED SOLIDS IN ORANGE COUNTY
Stormwater
control measure
Cost of pollutant removal,
$/lb-impervious acre-yr
SS
BOD5
Diversion/percolation 2.31 17.57
Retention/percolation 1.80 12.66
Swale/percolation 0.68 19.45
Underdrains 1.14 51.96
Residential swale 1.34 139.30
Detention/sedimentation 0.07 4.07
Cypress stand 0.005 0.83
Fabric bags 0.22 0.21
Vacuum sweeping 0.03 44.73
40
60
I 80 I MOO EFFICIENCY. »
0.10 0.25 0.50 TREATMENT, in.
Figure 50. Cost comparison of percolation
ponds on well drained and poorly drained soils [7].
156
-------
The severity of the problem has not been determined; however, potential
benefits to the groundwater could be derived from increased recharge rates.
It is assumed that an increase in the quality of aquatic and terrestrial
habitats will also occur. Only minimal long-term positive impacts are
anticipated to rare and endangered species.
Environmentally sensitive areas, such as wetlands, marshes, flood plains,
forests, and timber resources, or ecologically unique areas, should not be
affected by the proposed strategies.
Impacts on land uses, both existing and future, are considered to be
negligible, as are impacts to air and noise pollution. The implementation of
the proposed abatement strategies will not affect present land uses, and any
techniques used will be integrated with existing land uses. Although future
developments will be required to implement BMPs, it is not anticipated that
this requirement will be a deterrent to land development or affect land uses.
Socioeconomic Impacts
The effects of the program on population, economic activity, public
acceptance, aesthetics, and future facilities are some of the considerations
that must be made.
The implementation of the proposed strategies are not anticipated to have any
direct effect on the population or the distribution of the population, but
some demographic changes may occur if the lake degradation continues.
Public acceptance and aesthetics of program implementation are interrelated.
The public's unhappiness over poor aesthetics and general water resource
degradation in Orange County was the impetus for major strides in increased
pollution abatement. The general public not only accepted the control of
stormwaters, but supported program implementation. However, not all goals
set in the strategy were totally accepted by some portions of the community;
the limitation of new development in flood prone areas is an example.
Aesthetic appeal has been incorporated in many stormwater facilities.
Examples of multipurpose and aesthetically pleasing facilities are a
percolation basin used as a volleyball court and an apartment clubhouse
modeled after a showboat, located on a detention pond, shown in Figure 51.
Several adverse economic impacts from planned strategies are anticipated in
the long-term analysis. Increased costs to private industry and services for
construction of stormwater control facilities will be passed on in prices to
customers. Other costs to the community that might be considered excessive
include the duplication of taxes associated with overlapping government
authorities that regulate stormwater.
157
-------
Figure 51. Multipurpose stormwater facilities in Orange County:
(a) percolation basin/grassed volleyball court, and
(b) detention pond/apartment complex clubhouse.
158
-------
SECTION 9
IMPROVED STREET CLEANING PRACTICES
SAN JOSE, CALIFORNIA
In 1976, the City of San Jose, California, undertook a comprehensive, 2 year
project to demonstrate pollution abatement through improved street cleaning
practices [1]. The project findings led to a recommended approach for
designing street cleaning programs, which should be applicable to most areas
of the country.
Although street cleaning has long been widely practiced, its major goals were
litter and dirt removal and dust control (street appearance). Its use as a
BMP to control stormwater pollution has not been practiced. Past studies of
the effectiveness of street cleaning as a water quality control measure were
very limited in scope based only on idealized strip test conditions.
The San Jose study is unique because it measured the effectiveness of'street
cleaning over large areas for 1 year, under many "real world" conditions.
The sources of urban runoff pollutants and the wash-off effects of various-
sized storms were identified, and the cost effectiveness and impacts of
several types of street cleaning equipment were also evaluated. More than
20,000 samples were collected and analyzed.
A parallel study of Coyote Creek, which receives stormwater from the studied
areas of San Jose, was also conducted [2]. This study provided much needed
information on the effects of nonpoint urban stormwater discharges in
receiving water.
DEMONSTRATION PROJECT
The San Jose demonstration project was a comprehensive, detailed study that
evaluated the performance and costs of street cleaning equipment; street
surface pollutants, and their movements with wind, traffic, and rain; the
bearing that such information should have on street cleaning equipment
selection and program design; and environmental and some socioeconomic
impacts.
Area Characteristics
The area characteristics that affect the need for, and the methods of, street
cleaning include (in generally decreasing order of importance):
(1) conditions of street surfaces, (2) traffic congestion, (3) construction
projects, (4) presence of litter control programs, (5) climate, (6) type and
condition of curbs, (7) presence and maturity of vegetation, (8) demolition
work, and (9) unpaved driveways, roads, and parking lots [3].
159
-------
Three study areas, considered representative of the variety of conditions
found in many cities, were selected within the City of San Jose. These were
the Tropicana (residential) area, the Keyes Street (residential/commercial)
area, and a downtown (commercial) area. The locations of San Jose and the
three study areas are shown in Figure 52. The areas selected represented
various land uses, economic conditions, and traffic conditions, and met other
criteria necessary for the demonstration project [1].
Figure 52. San Jose and the three study areas.
The Tropicana area covers about 79 ha (195 acres) and is mostly residential.
The area includes a portion of a large shopping center and is adjacent to
three schools. There are few vacant lots, some roadside trees, and no
construction activities. Some streets have heavy traffic, but most carry
light traffic. Stormwater is eventually discharged into Silver Creek, a
tributary of Coyote Creek. The area is normally swept every 5 weeks.
The Keyes Street area covers about 37 ha (92 acres) and its major land use is
residential, with some strip commercial use. The study area is adjacent to
several schools and playing fields. This area has few vacant lots, many
roadside trees, and no construction. Several streets have heavy traffic, but
most have light traffic. The stormwater is discharged directly into Coyote
Creek. This area is also normally swept every 5 weeks.
160
-------
The downtown area covers about 40 ha (100 acres). Its major land uses are
commercial and industrial, with some older single- and multiple-family
residential areas, much roadside vegetation, and many vacant lots (previously
cleared for redevelopment). There is some construction and several streets
have heavy traffic. The stormwater from this area is discharged directly
into the Guadalupe River. The downtown commercial part of the study area is
normally swept daily, and the remainder of the area is normally swept every 5
weeks.
More detailed information about the study areas is provided in Tables 67, 68,
and 69. In all the areas, the curb types are 90° straight-edged concrete,
the topography is flat, and the ambient air conditions and soils are similar.
Table 67. GENERAL CHARACTERISTICS OF THE THREE STUDY AREAS
Characteristics
Drainage
area, acres
Curb length,
miles
Number of
inlets
Inlets/ curb-
mile
Inlets/acre
Acres/curb-
mile
Land use
Vacant lots
in area
Construction
in area
Traffic
density
Notes:
Tropicana
195
12.7
55
4.3
0.28
15
Residential ,
low income,
built 1960,
some commer-
cial, adjacent
to 3 schools
Few
None
Light to
heavy
Good air
sampling sites;
moderate
vegetation,
few large
trees; and
minimal leaf
removal problem
Study area
Keyes
92
5.4
17
3.2
0.18
17
Commercial
and older
residential,
adjacent to
school and
playing fields
Few
None
Light to
heavy
Adjacent to
college stadium
and subject to
heavy traffic
and parking
periodically
Downtown
100
7.0
25
3.6
0.25
14
Commercial ,
industrial ,
older resi-.
dential
Many
Some •
Light to
heavy
Air
quality
sampling
point
161
-------
Table 68. SURFACE AREA AND LAND USE
IN THE StUDY AREAS
Surface area
Rooftops (<3 stories tall)
Rooftops (>3 stories tall)
Lawn/landscaped area
Vacant space
Sidewalks
Street
Land use
Commercial
Residential
Industrial
Other (institutional, vacant
land, etc.)
Percent
Downtown
24.0
2.0
1.0
48.0
4.0
21.0
100.0
33.0
2.0
31.0
34.0
100.0
of study
Keyes
17.0
•o
39.0
21.0
4.0
19.0
100.0
11.0
86.0
0
3.0
100.0
areas
Tropicana
16.5
0
37.0
28.5
4.0
14.0
100.0
0
83.0
(some)
17.0
100.0
Table 69. ESTIMATED DAILY TRAFFIC VOLUMES INI THE TEST SITES
Test sites
Weighted Estimated Estimated
average minimum , maximum
daily traffic9 daily traffic daily traffic0
Tropicana - good asphalt
streets
Keyes - overall
Keyes - good asphalt
streets
Keyes - oil and screens
streets
Downtown - overall
Downtown - good asphalt
streets
Downtown - poor asphalt
streets
2,200
4,600
8,300
200
7,700
10,000
2,800
100
50
200
50
500
1,500
500
18,000
26,000
26,000
1 ,000
25,000
25,000
7,500
a. Estimates based on field measurements. Weighted by representative
street segment lengths.
b. Minimum estimated daily traffic for any one street segment in
test area.
c. Maximum estimated daily traffic for any one street segment in
test area.
162
-------
Preliminary street surface participate sampling suggested that sites with
siqnificantly different street surfaces should be treated separately in the
study. For this reason, the Keyes Street and downtown study areas were each
divided into two test sites. The Tropicana study area was best treated as a
single test site. Thus, a total of five test sites were used in the initial
field activities [1]:
• Tropicana - good asphalt street test site
• Keyes Street - good asphalt street test site
• Keyes Street - oil and screens street test site
• Downtown - good asphalt street test site
• Downtown - poor asphalt street test site
Typical street scenes within the five test sites are shown in Figure 53.
Buffer zones were established around each study area. The buffer zones were
swept at the same time and with the same number of passes as the test sites
to prevent excessive tracking or blowing of street pollutants into the study
areas. The buffer zones around the Keyes Street study area are identified in
Figure 54; the locations of the two different types of street surfaces (test
sites) in this area are also indicated.
Although samples of street surface pollutants were initially collected from
all three study areas, an illegal discharge in the downtown area required
that the area be eliminated from further study. Most of the later studies
were conducted in the Keyes Street and Tropicana study areas.
Climatic features of most interest to street cleaning operations are
rainfall, low temperatures, and snow. San Jose's mean annual precipitation
is 33 cm (13 in.), most of which occurs from November through April.The
normal monthly temperature ranges from 9.6 to 20°C (49 to 68°F); on the
average, there are very few days in which temperatures remain below freezing,
and snow is extremely rare in San Jose.
Problem Assessment
Pollutant characteristics, sources, and accumulation rates were studied in
death to help identify the problem of urban runoff control from street
surfaces. Special, accurate, long-term pollutant monitoring techniques were
developed for the project [1]. These techniques can easily be used by a
public works department to monitor a large area in a relatively short time
with readily available equipment. The sampling equipment in the San Jose
study included a signalized truck with a CB radio and a trailer carrying a 5
kW generator and heavy duty wet-dry vacuum units with accessories and tools.
163
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Figure 53. Typical streets in the five San Jose test sites: (a) Keyes
good asphalt surface, (b) Keyes, oil and screens surface
(c) Tropicana, good asphalt surface, (d) downtown, good asphalt surface,
(e) downtown, poor asphalt surface [1]
164
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SOUTH TWELFTH STREET
-MARTHA ST
SOUTH EIGHTH STREET
LEGEND
i I ASPHALT STREET SURFACE
p::::::::::::j OIL AND SCREENS STREET SURFACE
BUFFER AREA
TEST AREA BOUNDARY
Figure 54. Keyes Street buffer zone and test sites [1],
Pollutant Variations with Particle Size--
Because the chemical composition of different particle sizes on street
surfaces can vary significantly, all San Jose samples were divided into the
following eight particle size ranges before chemical analysis:
Particle size, :
microns (y) ,
<45 • .
45-106
106-250 : • '
250-600 -•••-.
600-850
850-2,000
2,000-6,370 :-
>6,370
The chemical and particle size information was used to determine the
accumulation rates and street cleaning equipment performance for the
different pollutants.
165
-------
Almost all of the pollutants in the test areas showed higher concentrations
with decreasing particle size. Mercury, cadmium, zinc, lead, Kjeldahl
nitrogen, and orthophosphates showed the highest concentrations with the
smaller particle sizes. However, copper and chromium had the lowest
concentrations with the smaller particle sizes.
These data indicate that conventional street cleaning methods, which are most
effective in removing large particle sizes, may be unable to remove enough of
those pollutants found mainly in the smaller quantities of smaller particle
sizes to completely meet pollutant control objectives, unless extra effort is
expended. Street cleaning can remove useful amounts of these pollutants,
however, because they are also found in the bigger quantities of larger
particle sizes.
Pollutant Concentrations in Runoff--
Stormwater runoff from three San Jose storms was sampled and analyzed to
determine pollution concentrations. The results for major pollutants are
summarized in Tables 70 and 71.
Table 70. POLLUTANT CONCENTRATIONS IN STORM RUNOFF
Average pollutant concentration, mg/L
Storm
Solids
Organics
Nutrients
Heavy metals
Study
area
Keyes
Vopicana
Date
3/15-16/77
3/23-24/77
4/30-5/1/77
3/15-16/77
3/23-24/77
4/30-5/1/77
Average
No. of
samples
16
8
la
21
25
10
SS
112
571
75
164
120
220
160
VSS
—
140
—
27
—
32
BOD5
30
22
25
17
28
25
COD
133
350
77
160
260
114
TKN
8.0
3.6
3.1
3.8
15.0
5.2
OP04
3.3
1.7
2.2
0.5
6.0
2.6
Pb
0.27
0.76
0.22
0.20
0.66
0.27
Zn
0.11
0.32
0.10
0.12
0.27
0.12
Cr
0.01
0.03
0.01
0.01
0.02
0.01
a. Partial storm.
b. Average concentrations are flow weighted.
BOD was a key water quality parameter, and 20-day incubation tests
unexpectedly suggested that settled materials could exert a long-term oxygen
demand much larger than the initial effects of BOD discharges.
The recommended water quality criteria for various beneficial uses were
exceeded by the concentrations of a number of pollutants in the urban runoff;
these are summarized in Table 72. Of all the beneficial uses, aquatic Ijfe
may be harmed by the greatest number of pollutants. Street cleaning can
remove portions of these troublesome pollutants from source areas before
rains wash them into the receiving waters.
166
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Table 71. POLLUTANT STRENGTHS IN STORM RUNOFF
Average pollutant
Study
area
Keyes
Tropicana
Storm
Date
3/15-16/77
3/23-24/77
4/30-5/1/77
3/15-16/77
3/23-24/77
4/30-5/1/77
b
Average
Solids Organics
No. of
samples
16
8
la
21
25
10
SS
770
840
480
596
430
580
593
VSS BOD5
204
210 32
—
92
96 61
74
97 94
COD
911
520
—
280
570
680
421
strength, mg/g total solids
Nutrients
TKN
55
5
~
n
14
39
19
OP04
23
~
n
8
2
16
10
Heavy metals
Pb
1.80
1.10
--
0.80
0.71
1.70
1.01
Zn
0.75
0.47
—
0.36
0.42
0.71
0.45
Cr
0.068
0.044
—
0.040
0.033
0.050
0.043
a. Partial storm.
b. Averages are weighted by total solids loads.
Table 72. RUNOFF WATER QUALITY PARAMETERS
EXCEEDING RECOMMENDED BENEFICIAL USE CRITERIA [1]
Beneficial
use
Aquatic life
Marine life
Freshwater for
public supply
Livestock
Recreation
Irrigation
Wildlife
Parameters exceeding
recommended criterion
Cr, Cda, Pba, Hga, BODg, turbidity3, SSa
P04a, Cd, Cu, Zn
Cd, Pba
Pba
P04a
Cd
None
a. Exceeded criteria by more than 10 times.
Pollutant Sources--
On a weight basis, the quantity of the contaminants found on street surfaces
depends on local geologic conditions, with added fractions resulting from
motor vehicle wear and emissions and from inputs from surrounding areas. The
wear of street surfaces normally makes only minor contributions. The more
common street surface pollutants and their sources are listed in Table 73.
167
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Table 73. SOURCES OF COMMON STREET SURFACE POLLUTANTS
Source
Pollutant
Local soil erosion
Local plants and soils (transported by
wind and traffic)
Wear of asphalt street surfaces
Spills and leaks from vehicles
Spills from vehicles (oil additives)
Combustion of leaded fuels
Tire wear
Wear of clutch and brake linings
Wear of vehicle moving metal parts
Deicing compounds (traffic dependent);
possibly also roadway abrasion and
local soils
Particulates (inert)
Nitrogen and phosphorus
Phenolic compounds
Grease, petroleum, n-paraffin,
lead
Phosphorus and zinc
Lead
Lead, zinc, and asbestos
Asbestos, lead, chromium,
copper, and nickel
Copper, nickel, and
chromium
Chlorides
A comparison of pollutant runoff yield and street surface loading
differences during storms revealed notable differences. COD5 Kjeldahl
nitrogen, and orthophosphates occurred in much greater proportions (about 3
to 180 times) in the total runoff than from the street dirt, indicating that
erosion from offstreet areas (such as parking lots, landscaped areas, vacant
land, rooftops) during storms is probably responsible for most of the
organic and nutrient yields. Thus, if organics and nutrients must be
significantly reduced in the runoff, street cleaning by itself may not be
sufficient.
Pollutant Accumulation Rates--
The accumulation rates of street surface contaminants must be known to
understand the magnitude of the problem a street cleaning program must
address and to determine the most effective control methods.
The San Jose study showed that the accumulation rates varied widely in the
different test sites. The deposition and accumulation dynamics of street
contaminants seem to be governed primarily by geographical location, season,
period of time since last swept or the last rain, land use, street surface
type and condition, and conditions in the area, such as the presence of
vacant lots, commercial development, pedestrian and automobile traffic,
parking, and ambient and traffic winds. The specific dynamics are,
therefore, a function of many site conditions that can vary widely. Such
variations should be considered in scheduling street cleaning programs for
different types of areas.
168
-------
Earlier nationwide studies have shown that total solids accumulation rates
vary over a very wide range, 0.8 to 762 kg/curb-km-d (3 to 2,700 Ib/curb-
mi'.d) [4], Local monitoring programs, therefore, are essential to
understand the complex influencing factors before designing a comprehensive
street cleaning program.
In San Jose, the lowest overall pollutant accumulation rates occurred in the
oil and screens test site, which also always retained the highest surface
loadings. The downtown test site had the highest accumulation rates of the
five sites, thought to be due to the poor condition of the asphalt streets
and the characteristics of the area.
Over time, the accumulation rate (deposition rate minus removal rate) varied
in a sawtooth pattern of deposition and removal as shown in Figure 55. The
deposition rate depends on the area characteristics, while removal can occur
by street cleaning or by winds or rain. Without removal, the particulate
loading levels off over time, after 1 to 3 months. This effect is thought
to be caused by wind and automobile-related air turbulence suspending the
particles in the air. These patterns should be identified and considered in
establishing optimum street cleaning frequencies. However, longer periods
between street cleaning may not significantly increase loadings, although
they could increase roadside airborne particulate concentrations.
STREET
CLEANED
STREET
CLEANED
« PERIOD OF
STREET SURFACE
STREET SAMPLING
CLEANED
ACTUAL LOAD
RESIDUAL LOADING-
CLEAN STREET
TIME
Figure 55. Sawtooth pattern of particulate deposition and removal [1].
The pollutant removal capabilities of storms were studied because of their
effect on the loadings remaining on the streets after rain. The monitored
storms had a much smaller removal effect in the oil and screens test site
than in the test sites with asphalt streets, presumably because of the
trapping action of the rough oil and screens surface. In general, smaller
storms left the street surfaces cleaner than larger ones, which probably
washed large amounts of eroded materials onto the streets. The smooth
asphalt surfaces were almost completely cleaned by the less intense storms.
169
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Countermeasure Philosophy
The purpose of street cleaning as a pollution abatement measure is to limit
the buildup of pollutants at the source, i.e., on street surfaces, and
thereby control their migration into receiving waters and the atmosphere. A
street cleaning program that is most cost effective in meeting its
environmental objectives should consider the interrelationships between area
characteristics and management options. The management options with this
control measure are the selection of cleaning equipment and a cleaning
program.
Street Cleaning Equipment--
Different street cleaning equipment and their use are described in Table 74.
The purpose of the demonstration project, however, was not to compare
specific types of street cleaning equipment, but to determine the range and
capabilities of street cleaning equipment in general and under various "real-
world" operating conditions. Several street cleaning programs were evaluated
under various operating conditions and cleaning frequencies, using the
following three types of street cleaning equipment:
• Four-wheel mechanical street cleaner
• State-of-the-art mechanical street cleaner
• Vacuum-assisted street cleaner
Street Cleaning Program--
Before designing a street cleaning program as a BMP, there should be good
evidence that street cleaning is either the most appropriate approach or can
be combined in an integrated control measure approach to combat the specific
pollutant problems. The potential multiuse benefits or tradeoffs must be
considered, i.e., clean streets and water pollution control. This requires
first determining the sources of the problem pollutants.
If the objective of a street cleaning program is to remove the most
pollutants from the runoff, then an appropriate program could simply involve
cleaning the streets with the highest unit removal rates (load removed per
pass) and keeping the number of passes a year for other streets to a minimum.
No service objectives are this simple, however, and more complex program
design techniques are usually necessary.
The following three steps briefly summarize the procedure recommended by the
San Jose demonstration project to design a street cleaning program.
1. Determine an allowable street surface residual loading (load/curb
length) from the city's street cleaning objectives. These
objectives are determined by environmental, aesthetic, safety, and
public relations requirements to meet urban runoff load allocations
and fugitive dust emission allocations, to control debris and oil
accumulation in traffic lanes, and to reduce service area
compl aints.
170
-------
2. Measure or estimate the long-term average participate accumulation
rate on street surfaces. This will vary with type of street
surface and with street cleaning frequencies.
3. Determine the maximum allowable effective days of accumulation
(EDA) from charts, and then determine the needed combinations of
cleaning interval and cleaning efficiency. The use of the charts
and the impact of frequent rains on the procedure are described in
the demonstration project final report [1].
Because of the interdependence of some variables, this procedure should be
checked and repeated periodically.
Table 74. TYPES OF AVAILABLE STREET CLEANING EQUIPMENT
Type
Characteristics
Use
Mechanical street
cleaner
Vacuum-assisted
mechanical street
cleaner
Regenerative
street cleaner
Small, industrial-
type vacuum sweeper
Hand sweeping
Street flusher
Rotating brooms, plus water spray
to control dust. Dirt is trans-
ported to storage hopper on moving
conveyor. May be self-dumping,
3- or 4-wheel.
Vacuum system transports dirt from
rotating brooms to hopper. Trans-
ported dirt is saturated with water.
Recycled air blasts dirt and debris
from road surface into hopper; air
is then regenerated through dust
separation system.
Vacuum is applied directly to
street.
Push cart or motor scooter and
hand tools.
Water tank, pressure supply, and
three or more individually
controlled nozzles.
Used for most street cleaning in
most U.S. communities.
Used in Europe for many years.
Has seen limited use in U.S. for
some time.
Relatively minor use.
Most useful for parking lots, side-
walks, factory floors. Of limited
use on city streets.
To back up machines, and for areas
machines cannot reach, particularly
around parked cars in business
districts.
Mostly for aesthetic purposes.
Generally (and preferably) used to
quickly displace dirt and debris
from traffic lanes to gutter. Up
to 22 ft wide street on one pass.
Has potential problems with trans-
port rates and volumes, and if
pollutants enter storm sewer they
might be flushed into the receiving
water.
171
-------
OPERATION AND PERFORMANCE
The purpose of the San Jose demonstration project was to determine the range
and capabilities of street cleaning equipment in general. Very little data
showed any significant differences in the performance of different types of
street cleaning equipment.
Project Operation
Equipment Operation—
Specific equipment maintenance was not monitored in detail in the San Jose
project, but the maintenance costs were and were included in the street
cleaning cost analysis. A survey of cities nationwide found the following
equipment components to be the most subject to wear, in descending order:
brushes, conveyor and elevator drives, tires, elevators, flights, hydraulic
systems, and transmissions [5].
Although synthetic broom materials last longer (by curb length swept) than
steel or natural fibers, broom life is not the most important factor. The
goal is removal effectiveness, and this has been shown to depend on broom
fiber, brush speed, pattern (strike), and forward speed. Half the cleaning
equipment surveyed was operated with a main broom rotational speed between
1,500 and 2,000 r/min and with a strike of 10 to 15 cm (4 to 6 in.) [6]; all
these determinations must depend on many local conditions.
Cleaning Frequency—
The cleaning frequencies used in the San Jose study ranged from two passes
every day to fewer than one pass every month. Each piece of equipment was
evaluated in the field in two different 7 week periods or phases: once
during the winter and once during the summer (with the exception of the
vacuum-assisted cleaner).
During the first 2 weeks of each phase, cleaning every weekday was tested
with single passes the first week and two passes the next week. During the
last 5 weeks of each phase, weekly cleaning intervals were tested. The
equipment was rotated through the different test sites at the end of each
phase. This schedule allowed the different characteristics and long-term
seasonal differences in the test sites to be evaluated in addition to the
range of equipment effectiveness.
Adjacent buffer zones, up to three times the size of the test sites, were
also cleaned to reduce potential edge effects (tracking of particulates into
the test sites from the adjacent areas).
The long-term and frequent sampling in the test sites measured contaminant
accumulation rates on street surfaces and identified the range of performance
that may be expected from currently available street cleaning equipment.
172
-------
Performance
The street surface condition, initial loading (total loading value and
particle size distribution), and various other environmental factors affect
street cleaning performance. Equipment operation variables that most affect
street cleaning performance include the number of passes and the street
cleaning interval.
The most important measure of cleaning effectiveness is the load per curb
length removed. This removal value and the unit curb length costs are used
to calculate the cost for removing a unit load of pollutant for a specific
street cleaning program. The often-used effectiveness measure, percentage of
the initial loading removed, is very misleading because it does not measure !
the magnitude of the amount of material removed.
It must be stressed that the performance of street cleaning equipment as
measured in the San Jose tests may not adequately indicate how well the
equipment operates under other conditions.
Equipment Effectiveness--
Street cleaning equipment performance results, obtained by comparing San Jose
street surface samples collected just before and just after cleaning, are
presented in Table 75. These results cover 26 different test conditions,
representing different test sites, equipment types, seasons., number of
passes, and cleaning intervals. The two measures of cleaning effectiveness,
load per curb length (preferred) and percentage of initial loading removed
(misleading), are presented.
From statistical tests on the performance data, the characteristics of the
area to be cleaned (street surface conditions and accumulation rates) and, in
most cases, the street cleaning program (number of passes and cleaning
intervals), were found to be more important than the selection of the type of
street cleaning equipment. Other considerations, such as maneuverability,
life-cycle costs, hopper capacity, etc., may be important when selecting
equipment.
Program Effectiveness--
The design of an effective street cleaning program should incorporate local
conditions and operating procedures that may affect the pollutant removal
efficiency. The number of passes and the cleaning interval influence removal
effectiveness, and other considerations, such as particle size distribution,
expected pollutant removal effectiveness, loading distribution, and potential
for redistribution, should be evaluated.
Number of Passes—In most cases, two passes with the same piece of equipment
removed a larger quantity of material from the street than did a single pass,
as expected. Exceptions were found, however, in some of the tests on oil and
screens streets. Here, two passes per day with the state-of-the-art
mechanical four-wheel street cleaner resulted in a higher residual loading on
the street surface than before the tests. This may be due to the extra
173
-------
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174
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erosion caused by excessive mechanical action of the broom on the weaker oil
and screens surface.
Cleaning Interval--The effect of cleaning interval on total annual solids
removal is summarized in Figure 56. The curves, idealized for different
street surfaces in this plot, show decreased removal quantities per equipment
pass as the number of passes per year increases. By increasing from 10 to
100 passes per year, the potential effort and cost can increase by 10 times,
whereas the amount removed only increases by a factor of about 3 to 5.
50.000 r- /
•2 40.000
30.000
20.000
10.000
OIL AND SCREENS /
SURFACED STREETS/
'SMOOTH ASPHALT
STREETS IN
GOOD CONDITION
J
10
100
1.000
PASSES, number/yr
Figure 56. Variation of annual solids removal with
number of equipment passes [1].
Particle Size Removals-The variation of removal efficiency with particle
size is important because pollutant concentrations can vary with particle
size. Particle size removals for the five San Jose street conditions, for
all street cleaning programs combined, are shown in Figure 57.
The effectiveness of street cleaners generally increases with particle size,
although there are quite large variations. This efficiency increase is
confirmed by the decrease in the street surface median particle size after
c?ean™g, and by hopper median particle sizes always being much arger than
the median particle sizes on the street surface before cleaning (Table 75).
Thus, there is a larger fraction of smaller particles on the street after
street cleaning than before.
Pollutant Removal-The pollutant removals from San Jose streets are given in
Table /fa tor each of the five test sites. The figures represent averages for
all street cleaning programs combined. The percentage removals for the
various pollutants were about the same as the total solids percentage
removals. They ranged from 20 to 50% for all pollutants and test sites,
except at the oil and screens test site. The mass removal rates at a given
175
-------
KEYES- OIL
AND SCREENS
-16
TROPICANA- GOOD
ASPHALT
STREET SURFACE
PARTICLE SIZES
PARTICLE SIZES
PARTICLE SIZES
56
48
40
2 24
ce
16
8
DOWNTOWN-GOOD
ASPHALT
STREET SURFACE
DOWNTOWN-POOR
ASPHALT
STREET SURFACE
PARTICLE SIZES
PARTICLE SIZES
Figure 57. Total solids removal by particle size,
from various street surfaces [1].
176
-------
site varied greatly, however, because of the large variation in initial
loadings, for these pollutants.
Table 76. REMOVAL EFFECTIVENESS FOR VARIOUS POLLUTANTS BY TEST
SITE (AVERAGES FOR ALL STREET CLEANING PROGRAMS COMBINED) [1]
Keyes - good Tropicana - good
Keyes - oil and asphalt street asphalt streets
screens test site test site test site
Downtown - poor
asphalt street
test site
Downtown - good
asphalt streets
test site
Pollutant
Amount Amount Amount
removed, Removal, removed, Removal, removed, Removal,
Ib/curb mi % Ib/curb mi % Ib/curb mi %
Amount
removed, Removal,
Ib/curb mi %
Amount
removed, Removal,
Ib/curb mi %
Total
solids
Chemical
oxygen
demand
Kjeldahl
nitrogen
Ortho-
phosphate
Lead
Zinc
Chromium
Copper
Cadmium
170
12
0.14
0.0089
0.15
0.066
0.071
0.13
0.00024
9
9
6
7
5
12
9
13
8
130
16
0.28
0.018
0.81
0.079
0.051
0.081
0.00030
33
33
32
31
30
29
32
34
30
100
9.7
0.21
0.017
0.40
0.049
0.039
0.072
0.00027
43
46
47
44
44
45
50
51
45
540
61
1.3
0.079
1.0
0.27
0.24
0.50
0.0015
40
40
38
38
37
39
42
43
40
83
11
0.16
0.012
0.49
0.072
0.047
0.039
0.0023
34
32
33
32
31
31
36
38
49
Loading Distributions—Cumulative loading distributions across three
different street surfaces are compared in Figure 58. Only about 60% of the
total solids fell within the normal 2.4 m (8 ft) path of a street cleaner on
the oil and screens surface, while over 95% of the total solids loading could
be reached on the asphalt surfaces. These distributions can be further
modified by car parking conditions.
100r
ce.
o
40 -
20 -
Figure 58.
DISTANCE FROM CURB, ft
Cumulative loading distributions across streets
with different surfaces [1].
177
-------
A study of solids distribution across the street by particles size found
about an even distribution in the Tropicana-good asphalt test site, no clear
trends in the Keyes-good asphalt test site, and more large than fine
particles near the curb in the Keyes oil and screens test site.
Loading Redistribution—Street cleaning redistributes surface materials
across the street. Average distributions before and after cleaning at three
of the five San Jose test sites are compared in Figure 59. On the smoother
surfaces (Tropicana-good asphalt and Keyes-good -asphalt), the peak load at
the curb was reduced by about 80%. Some areas show increases in solids
loadings caused by broom action and turbulence partially redistributing the
high curbside loading out into the street. The rougher oil and screens
streets had much higher loadings in the center of the street; as a result,
removals on a load per unit area bas'is were much more uniform across this
surface.
Effects of Parking—Vehicles parked along a street cleaning route reduce the
length of curb that can be cleaned. In addition to the curb length blocked
by a car, front and back clearances also cannot be cleaned because of the
turning radius of the street cleaner. Since most of the street surface
pollutants are close to the curb on smoother streets with little parking,
parked vehicles can drastically reduce the cleaning effectiveness of normal
cleaning programs on these streets. Parking restrictions may be used to
improve cleaning effectiveness.
Parking can also strongly affect the accumulative loading distribution across
a street surface. The effects of different parking conditions on solids
distributions across two different street surfaces are shown in Figure 60.
Extensive parking blocks the migration of particulates toward the curb,
resulting in higher middle-of-the-street loadings than on streets with little
or no parking. Further, this blocking effect is much more noticeable on
smooth asphalt surfaces than on oil and screens surfaces, whose roughness
itself seems to control particle migration.
Parking Restrictipns—Parking restrictions, which allow street cleaners
access to the curb, usually (but not always) improve cleaning effectiveness.
The effects of parking restrictions on removal effectiveness, for various
street surfaces and parking densities, are shown in Figure 61.
In many situations, parking restrictions will about double the removal
effectiveness; in most situations, they will improve it. However, on good
asphalt streets with extensive 24 hour a day onstreet parking (as in high
density residential neighborhoods), most of the solids load will not fall
within the 2.4 m (8 ft) strip next to the curb. If more than about 80% of
the curb length is occupied by parked cars, it would be more effective to
clean around the parked cars (i.e., without parking restrictions), as shown
in Figure 61. Similarly, it should be more effective to clean around the
parked cars if more than about 95% of the curb length is occupied on good
asphalt streets with extensive daytime only parking, or likewise on oil and
screens surfaced streets. Of course, periodic curbside cleaning would still
be needed to remove sidewalk-originating litter.
178
-------
0.025
C-J
r 0.020
0.015
0.010
0.005
(-53%)
(81%)
(-60%)
LEGEND
INITIAL LOADING DISTRIBUTION
RESIDUAL LOADING DISTRIBUTION
VALUES IN PARENTHESES INDICATE V. REMOVAL
TROPICANA- GOOD ASPHALT TEST SITE.
OVERALL REMOVAL EFFECTIVENESS
ABOUT 40%.
(-50%)
5 10 15
DISTANCE FROM CURB,ft
20
0.025 |—i
Z 0.020
2 0.015
-i 0.010
0.005
(-33%)
*b
(58%)
I
I
(79%)i
f.\ I
(53%)
KEYES-GOOD ASPHALT TEST SITE.
OVERALL REMOVAL EFFECTIVENESS
ABOUT 26%.
08%)
0
5 10 15
DISTANCE FROM CURB.ft
20
0.025
0.020
0.015
0.010
0.005
0
(36%)
(-7%)
(16%)
-140%)
KEYES-OIL AND SCREENS TEST SITE.
OVERALL REMOVAL EFFECTIVENESS
ABOUT 12%.
(16%)
5 10 15
DISTANCE FROM CURB,ft
20
Figure 59. Redistribution of total solids from street cleaning
in three different test sites, averaged for all equipment types [1].
179
-------
SMOOTH ASPHALT STREETS
IN GOOD CONDITION
/
l
i
PARKING CONDITIONS
EXTENSIVE-LONG TERM
EXTENSIVE-SHORT TERM
MODERATE
LIGHT OR NONE
TRAFFIC NEXT TO CURB
I
i
8
10
12
14
16
18
20
DISTANCE FROM CURB, ft
l- en
= cc
CO =3
— es
100
go
80
70
60
50
40
30
20
10
0
OIL AND SCREENS
SURFACED STREETS
PARKING CONDITIONS:
EXTENSIVE
LIGHT OR NONE
TRAFFIC NEXT TO CURB
_1_
_L
6 8 10 12 14
DISTANCE FROM CURB, ft
J
18
20
Figure 60. Effects of parking and street conditions
on solids loading distribution [1].
180
-------
MAXIMUM
ASPHALT STREETS IN GOOD CONDITION
20 30 40 50 60 70
CURB LENGTH OCCUPIED BY PARKED CARS,
90 100
20
o
z
Ul
15
10
PARKING RESTRICTIONS (100% EFFECT-/..
OIL AND SCREENS SURFACED STREETS
_L
_L
_L
_L
NO PARKING RESTRICTIONS
10 20 30 40 50 60 70
CURB LENGTH OCCUPIED BY PARKED CARS,
80
90
100
Figure 61. Effects of parking restrictions during street cleaning
on solids removal from two different street surfaces [lj.
181
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COSTS AND EFFECTIVENESS
Costs
Extensive and detailed information on costs and labor requirements for street
cleaning and all support activities in San Jose was collected and evaluated
for the year ending September 30, 1977. These are summarized by specific
item in Table 77.
Table 77. ANNUAL STREET CLEANING COSTS
AND LABOR REQUIREMENTS, SAN JOSE [1]
Costs Labor
Item
Maintenance supplies
Operation supplies0
Debris transfer and
disposal d
Equipment depreciation
Labor6
Equipment operators
Maintenance personnel
Supervisors
Total
Percentage
$/yra of total Man-d/yr
97,000
30,000
67,000
32,000
338,000
183,000
83.000
830,000
12
3
8 780
3
41 3,400
23 1,200
10 650
100% 6,030
Percentage
of total
1.3
—
56
20
11
100%
a. ENR 3000.
b. Includes broom replacements.
c. Tires, fuel, and oil.
d. Front-end loaders removed interim piles from streets, and
dump trucks transported.them (maximum 15 miles) to landfill.
e. Includes administration, warehouse, secretary, and overhead
costs.
Labor accounts for about 75% of the total annual cost, making street cleaning
a labor-intensive urban runoff control measure.
Those categories that might be affected by a significant change in the street
cleaning equipment used (maintenance supplies and labor) make up only 35% of
the total costs. Thus, a major change in equipment type could only slightly
reduce the total costs.
During the same year, 89,720 km (55,761 mi) of street surface was cleaned by
the San Jose Public Works Department, at $9.25/curb-km ($14.88/curb-mi) (ENR
3000). The unit labor requirement was 0.54 man-h/curb-km (0.87 man-h/curb-mi)
182
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While the San Jose unit costs may appear high, most other evaluations do not
include all the costs because few other jurisdictions have all the
information available. Most other street cleaning cost evaluations include
only supplies and operator labor expense (55% of San Jose's total cost), and
assume the curb length cleaned equals the increase in the street cleaner's
odometer reading (actually about half).
A 1973 nationwide survey of 400 city street cleaning program costs yielded
typical unit costs (adjusted to ENR 3000) between $0.026 and $0.035/kg ($24
and $32/ton) removed [5]. This corresponds to about 1% of a typical city's
budget, but above, did not include all the associated costs.
A large portion of the typical street cleaning budget (35% in San Jose) goes
for equipment maintenance. A 1973 survey of 14 cities across the nation
yielded an average total maintenance cost of $1.62/curb-km cleaned
($2.61/curb-mi), ENR 3000 [7]. The greatest portion of this was spent for
brooms and brushes ($0.40/curb-km) and for major repairs ($0.39/curb-km). In
comparison, San Jose's average total maintenance cost was $3.12/curb-km
($5.02/curb-mi).
The cost and labor requirements to remove solids increase with the number of
cleaning passes per year; i.e., the rate of return decreases as the streets
are cleaned more often. Computer analysis of the San Jose data revealed the
relationships shown in Figures 62 and 63; unit costs increased from $0.030 or
$0.039/kg ($0.013 or $0.018/1b, ENR 3000) for two passes a year to $0.57/kg
($0.26/lb) for 150 to 300 passes a year. More frequent cleaning lowers the
solids loadings on street surfaces, and so results in lower removals per
pass; the cost per pass remains about the same.
0. 25
0. 20
0.15
0. 10
0.05
ASPHALT STREETS IN,
GOOD CONDITION,
X OIL AND SCREENS
XX SURFACED STREETS
' OR ASPHALT STREETS
IN POOR CONDITION
J
10
100
1.000
PASSES.NUMBER/yr
Figure 62. Variation of unit cleaning costs
with number of passes [1].
183
-------
o.ois r
o.oio -
ac
tu
a:
0.005 -
100
J
1 ,000
PASSES.NUMBER/yr
Figure 63. Variation of unit labor
requirements with number of passes [1].
Effectiveness
Street cleaning is most effective in controlling heavy metals, and moderately
effective in controlling oil and grease, floating matter, and salts. It is
less effective in controlling bacteria, and poorly effective in controlling
sediment, nutrients, and oxygen-demanding matter. In other words, street
cleaning is most effective in controlling street-originating pollutants. Any
one control measure has limited capabilities, and a combination of measures
is generally needed even to control a single pollutant problem. The San Jose
demonstration project final report describes decision analysis methods that
may be used to help select urban runoff control programs [1].
The average unit costs to remove various pollutants from the five test sites
by street cleaning are presented in Table 78. Overall, the unit costs ranged
from $0.059/kg ($0.027/lb) of total solids removed to $133,000/kg
($60,200/1b) of cadmium (ENR 3000). The average unit labor needs over all
the test sites are also shown, and range from 0.01 h/kg (0.005 h/lb) of total
solids removed to 990 h/kg (450 h/lb) of cadmium.
Comparing unit removal costs between different test areas, it costs more
$0.24 to $0.40/kg, ($0.11 to $0.18/lb) to remove solids from the asphalt
streets in good condition than from both the poorer quality asphalt streets
$0.06/kg ($0.027/lb) and the oil and screens streets $0.18/kg ($0.08/lb).
184
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185
-------
These comparisons are as expected, considering the different initial loadings
on, nature of, and removals from the various street surfaces. Similar
comparisons are generally true for the other pollutants, except in the oil
and screens test site. There, street surface particulates are abundant, but
the pollutant concentrations are relatively low. The major source of tlae
particulates in this test area is street surface wear material, which is
relatively "clean."
The costs of removing pollutants by street cleaning were compared with the
costs of removing them from runoff by various stormwater treatment processes.
When flow equalization (storage) and collection facility costs are excluded,
the average unit treatment costs are significantly less than the unit street
cleaning costs for all pollutants. However, when flow equalization costs are
included, the average unit treatment costs are generally higher than similar
costs for street cleaning; comparisons for heavy metals could not be made
because of lack of data. In making such comparisons, it must be remembered
that treatment measures affect only water quality, whereas street cleaning
can also improve air quality, aesthetic conditions, and public safety.
IMPACTS
Environmental Impacts
Environmental impacts of street cleaning can include impacts on urban
hydrology, receiving water quality, and air pollution.
Hydro! ogy—
Without street cleaning, street litter and particulates accumulate in sewers
and catchbasins when rains are sufficient to transport these materials off
the streets, but not heavy enough to flush out the sewers. Street cleaning
obviously reduces these accumulations, and thus helps reduce sewer plugging
and the need for sewer cleaning.
Water Quality-
Pollutants in urban runoff were compared with their presence in the effluent
from the advanced secondary sewage treatment plant serving the San Jose
study areas. Occurrence ratios for those parameters in urban runoff
exceeding that in sewage treatment plant effluent are presented in Table 79.
These ratios suggest that urban runoff may have more important short-term
impacts on receiving waters than treated sewage effluent. Lead, chromium,
and suspended solids have greater annual yields in the runoff than in the
treated secondary effluent. Such a tabulation helps characterize the nature
of local runoff pollution problems.
The effects of street cleaning on water quality in Coyote Creek, the
receiving water for runoff from the San Jose study areas, can only be
inferred at this time. These effects may be more directly, determined under
proposed future extensions to the Coyote,Creek project [2]. Much is known,
however, on which to base such inferences. The nature of the pollutants in
the urban runoff is known (Tables 70, 71,, 72, and 79), and the extent to
which street cleaning can reduce these pollutants is known (Tables 75 and 76).
186
-------
Table 79. COMPARISON OF URBAN RUNOFF AND
ADVANCED SECONDARY WASTEWATER TREATMENT PLANT
EFFLUENT AT SAN JOSE
Ratio of average Ratio of peak (1 hr)
runoff to STP runoff to STP Ratio of runoff to
Parameter effluent concentration effluent concentration STP annual yields
BOD5
COD
TKN
SS
Cd
Cr
Cu
Pb
Zn
Turbidity
TOC
1.1
5.6
0.28
9.2
5
1.3
0.37
41
2.1
2.5
3.5
1.4
' 10
1.1
32
20
2.5
1.1
150
6.3
6.5
9.7
0.17
0.20
0.005
1.3
0.07
1.6
0.5
28
0.33
—
•
Pollution conditions in Coyote Creek are also known from a separate study,
which involved intensive, manual sampling of the biological and water
quality conditions of the creek from March through June 1978 [2]. The upper
reaches of Coyote Creek pass through an undeveloped area and are not
affected by urban runoff until the creek reaches San Jose. In the urbanized
areas, which include the street cleaning test sites, there were no
discharges other than urban runoff. The small receiving capacity of Coyote
Creek is typical of many western streams.
Variations in sediment quality trends were observed along Coyote Creek as
shown in Figure 64. The pollutant levels rose rapidly as the flow passed
through the urbanized area of San Jose, indicating the potential water
quality impacts of urban runoff can change a stream environment. The median
sediment particle size decreased through the urbanized area, indicating an
increased silt content. Of 12 fish species observed in the creek, only 4
occurred in the urban reaches and 98% of these were mosquito fish (Gambusia
affinis), which are pollution tolerant. Urban samples of organism tissues
contained up to three times the lead and zinc concentrations than did
upstream samples. And the lower creek reaches were dominated by pollution
tolerant species of benthic macroinvertebrates and attached algae.
To determine how street cleaning affects the creek would require a major
change in the city's street cleaning program plus extended monitoring of
creek conditions until they returned to equilibrium under the new urban
runoff loads. Also, the creek pollution comes from many urban sources, and
offstreet urban pollutants may not be controllable by street cleaning.
187
-------
LEGEND
ORTHOPHOSPHATES (MAXIMUM 6.7 mg/kg)
BOD5 (MAXIMUM 1900 mg/kg)
LEAD (MAXIMUM 400 mg/kg)
V///////////////A
DIRECTION OF FLOW
URBANIZED
CREEK MILES
LEGEND
MEDIAN SEDIMENT PA.RTICLE SIZE (MAXIMUM 400 LI)
SPECIFIC CONDUCTANCE (MAXIMUM 1600 ymhos/cm)
TURBIDITY (MAXIMUM 160 NTU)
DIRECTION OF FLOW
20
30
40
URBANIZED
CREEK MILES
Figure 64. The variation of sediment quality along Coyote Creek [2].
188
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Air Pollution--
From vehicle travel statistics and typical emission rates, it has been
estimated that nationwide participate emissions (<20y) from street surfaces
totaled 32 million metric tons (35 million tons) in 1972 [1]. This source is
considered very important, when compared with the estimated 1972 total of 26
million metric tons (29 million tons) of particulate emissions from all point
sources combined (industrial processes, stationary fuel combustion, solid
waste disposal, transportation, etc.).
The accumulation rate of particles on street surfaces is highest shortly
after street cleaning or rain (Figure 55). Dust is also stirred up during
street cleaning, which can immediately settle back on the streets. Over
time, the accumulation rate levels off. The deposition rate is believed to
be constant and approximately equal to the initial high accumulation rate,
and the increasing difference between the deposition rate and the
accumulation rate over time is believed to result from particulate losses to
the air. In this way, dust (fugitive particulates) emissions were estimated
from the accumulation rates.
A number of local factors can affect dust emissions. Those measured at the
San Jose test sites, and the importance of their effects on emission rates,
are listed in Table 80. Since the loading of particulates on the street
surface was found to influence emissions, improved street cleaning could
affect their control.
Table 80. THE IMPORTANCE OF FACTORS
INFLUENCING FUGITIVE PARTICULATE EMISSION RATES
Factor
Importance
Pavement material High
Pavement condition High
Particulate loading High
Traffic density High
Wind speed High
Traffic speed Medium
Particulate size distribution Medium
Wind direction Low
Relative humidity Low
About 80% of the increased number of atmospheric particulates near streets
was found to occur in the 0.5 to 1.0 jj size range, whereas 90% of their
weight was associated with sizes greater than 10 jj.
Emission rates for total solids from three different test sites are presented
in Table 81. The rates increase considerably with time since street
cleaning; therefore, street cleaning frequencies can affect fugitive
particulate emission rates from road surfaces.
189
-------
Table 81. FUGITIVE PARTICULATE EMISSION RATES FOR
LOSSES OF TOTAL SOLIDS FROM STREET SURFACES
Emission rates
Time after
street cleaning
or significant
rain, d
2-4
4-10
10 - 20
20 - 30
30 - 45
45 - 60
60 - 75
Average
Tropicana - good
asphalt
Ib/curb-
mi-d
4
4
5
7
8
9
12
6
g/ vehicle-
mi
1.7
1.7
2.1
2.9
3.3
3.7
5.0
2.5
from test sites
Keyes - good
asphalt
Ib/curb-
mi-d
4
4
5
7
8
9
12
6
g/vehicle-
mi
0.44
0.44
0.55
0.77
0.88
0.98
1.30
0.66
Keyes - oil
and screens
Ib/curb-
mi-d
<1
3
4
5
10
—
—
4
g/vehicle-
mi
<4.5
14
18
23
45
—
—
18
Emission rates from the Keyes-good asphalt and Tropicana-good asphalt test
sites were the same because their accumulation rates were similar. Major
differences in traffic volumes between the three sites resulted in
significantly different emission rates on the basis of vehicle travel
distance. The average particulate losses from the three test sites ranged
from 0.4 to 11 g/vehicle-km (0.66 to 18 g/vehicle-mi); this compares with
typical range of 1.2 to 3.1 g/vehicle-km (2 to 5 g/vehicle-mi) reported by
others [8, 9, 10], The ratios of the emission rates of other pollutants to
total solids are given in Table 82.
Table 82. RELATIVE FUGITIVE PARTICULATE EMISSION RATES
(BY WEIGHT) OF VARIOUS POLLUTANTS FROM THREE TEST SITES9
Parameter
Total solids
Chemical oxygen demand
Kjeldahl nitrogen
Orthophosphates
Lead
Zinc
Chromium
Copper
Cadmium
Keyes - good
asphalt and
Tropicana -
good asphalt
test sites
1
0.12
0.0018
0. 0001 5
0.0043
0.00047
0.00033
0.00052
0.0000017
Keyes - oil
and screens
test sites
1
0.05
0.00077
0.00010
0.0010
0.00020
0.00030
0. 00046
0.00000025
a. Ratio to total solids.
190
-------
Such particulate losses from street surfaces can contribute a large portion
of an area's total particulate emissions. The San Jose data (Table 81)
suggest that cleaning an asphalt street weekly instead of every 2 or 3 months
will reduce particulate emissions to one-third.
Dust levels inside the San Jose street cleaning equipment cabs and around the
equipment while operating were also measured, with and without the water
spray in use. Without the water spray turned on, levels in the cab were 4.6
times ambient (outside the cab), and behind the street cleaner they were 7.3
times ambient. With the water spray, the level inside the cab fell to about
the same as ambient, but behind the street cleaner the level fell only 20%.
Most of the concentration changes caused by the spray occurred in the smaller
particle sizes.
Socioeconomic Impacts
The high labor intensity of street cleaning should be considered a positive
social impact, at least under present labor market conditions.
In the demonstration project no direct information was obtained on how street
cleaning affects land values, taxes, or energy requirements.
Aesthetics and public safety obviously benefit from the litter and dust
control achieved by street cleaning, regardless of its pollution abatement
effectiveness. However, such impacts were not measured in the demonstration
study.
191
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SECTION 10
BEST MANAGEMENT PRACTICES
MIDDLESEX COUNTY, CONNECTICUT
A number of small, onsite stormwater control facilities have been constructed
in Middlesex County to reduce the impact new development has on the quality
and quantity of water. Legislation at the state and local levels is oriented
toward the control of erosion/sedimentation and flooding in the waterways and
wetlands. Implementation of stormwater control regulations are the
responsibility of each town, as there are no county governments.
The types of stormwater controls in Middlesex County are representative of a
small systems approach for privately owned single developments. New
residential and industrial development have used sedimentation ponds,
recharge basins, and dry wells to achieve almost total runoff volume and
pollutant load control. The feasibility of proposed stormwater controls is
evaluated on a case-by-case basis because no exact design criteria are
outlined. This method allows versatility in selecting a unique solution for
each site. Most facilities are conservatively designed, often controlling 10
to 25 year return frequency storms.
The adverse environmental and socioeconomic impacts of new developments are
avoided to a great extent by an environmental review of the proposed
development area before development approval. These studies are made, at no
charge, through the local Soil Conservation District and the Eastern
Connecticut Resource Conservation and Development Area [1].
PROJECT DESCRIPTION
Urban stormwater runoff controls constructed in Middlesex County are
representative of small-scale facilities used for single and often isolated
new developments. The purpose of these controls is to prevent flooding,
erosion, and water quality problems in the major rivers and streams.
Regulatory requirements are enforced at the local level through land use
restrictions and ordinances that require developments to provide facilities
for handling increased runoff from the developed area.
Three typical, private runoff control installations, two industrial sites and
one residential site, are used as examples of the small-scale facilties.
Since most facilities are privately owned, no monitoring of efficiency,
particularly pollutant reduction efficiency, has been conducted. Rather,
these facilities illustrate the type of controls that have been used and
appear to be a most promising solution for controlling runoff.
192
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Area Characteristics
Connecticut is in the southwestern portion of New England and is bordered on
the south by Long Island Sound. Middlesex County is in the south central
portion of the state as shown in Figure 65.
O
MASSACHUSETTS
"T
CONNECTICUT
~\
CONNECTICUT RIVER
* STORHWATER
FACILITIES
Figure 65. Middlesex County, Connecticut.
The topography of Connecticut is predominantly rolling hills, varying in
elevation from 90 to 600 m (300 to 2,000 ft); the highest elevations in
Middlesex County are near 300 m (1,000 ft). The state is bisected by the
Connecticut River and has numerous rivers and streams in four other major
river basins in the state.
The annual precipitation in Middlesex County is about 117 cm (46 in.), which
falls in relatively uniform amounts throughout the seasons. Heavy rainfall
storms occur during the summer and fall, usually resulting from localized
thunderstorms that occur on the average of about 20 to 30 days per year.
During the spring, melting snow, combined with rainfall, produces high runoff
volumes and is one of the principal causes of flooding.
193
-------
Connecticut has five major population centers but is mostly rural. About two-
thirds of the state is forest with the remaining area developed; about 20% of
the developed area is agricultural. Middlesex County, just south of
Hartford, is rural with many small towns that have light industrial
developments.
Problem Assessment
The State of Connecticut has become aware of the erosion, sediment
deposition, and water quality problems associated with residential and
industrial development. The increased degradation of the rivers and streams
by sediment deposition has been determined to be the most critical problem in
Connecticut [1]. Many areas in ,the state have soils with relatively large
percentages of fine-grained particles of clay and silt. These particles
remain in suspension longer because of their lower settling velocities, which
prolong turbidity. Pollutant characteristics of urban runoff have not been
monitored; however, runoff quality is assumed equivalent to representative
nationwide values for similar land uses.
Approach to Stormwater Control
Middlesex County has numerous stormwater control facilities that were
constructed as a result of erosion and sediment control ordinances. The
State of Connecticut passed legislation to provide local jurisdictions with
authority to regulate and enforce stormwater ordinances. The local
ordinances require new developments to control stormwater both during and
after construction.
During construction, basic guidelines of the stormwater ordinances provide
for erosion and sedimentation control and apply to all changes of the land
surface, including clearing, grading, excavation, and filling. The
ordinances state that the development should be suited to the site,
topography, and soils to minimize erosion potential; retain and protect
native vegetation; and minimize land exposure. The development should also
use soil and surface stabilization techniques to protect exposed areas
(temporary vegetation or mulching, or both) and have sediment or desilting
basins to collect runoff [2],
Postdevelopment controls required by the ordinances must accommodate the
increased runoff caused by changes in soil and surface conditions
(imperviousness), erosion, flooding, and sediment damage to the development
property, adjacent property, or receiving waters. No specific design
requirement is stated for the control facilities except for stormwater
conveyance systems. All culverts and other stormwater drainage systems,
except street drainage, are designed for a 25 year return frequency. Street
drainage systems are designed on a 10 year return frequency, or as approved
according to the condition of the site.
The control program prevents alteration of natural water courses, natural
drainage and runoff, or existing drainage and runoff by controlling the
discharges into storm drains, ditches, streams, or rivers with adequate
carrying capacity for the additional runoff volume [3], The ordinance
194
-------
provisions generally allow wide enough latitude in the types and degree of .
control to fit each development site. Final stormwater plans have to be
approved before development is allowed to proceed, and most facilities have
been conservatively designed.
Implemented Facilities
Three facil ities were selected that demonstrate a most promising technology
in controlling*stormwater and are representative of the types of control used
in Middlesex County. Two of the controls serve light industrial developments
and the third, a residential development.
Most of the facilities incorporated both pre- and post-development measures
to control erosion, sedimentation, and runoff volume. Two of the facilities
control all of the onsite runoff. .
Sedimentation Ponds (Industrial Site)--
A light industrial site has been developed in an environmentally sensitive
area in the town of Essex. The dual use of the area for industry and open
space to preserve the surrounding wetlands required substantial mitigating '
measures by the developers.
The Essex industrial site is adjacent to the Mud River on approximately 20 ha
(50 acres) of low lying wetlands. The Mud River, classified as "A" Standard
(the cleanest rating, suitable for water supply) ,.,has' an estimated flow range
from about 0.03 to over 0.6 m3/s (1 to over 20 ft /s), and drains an area of
about 830 ha (2,050 acres) [4]. The river basin is identified as a future
permanent open space with limited development and as a high priority aquifer
area. The Connecticut River Estuary Regional Planning Agency (CRERPA)
recommended in its Regional Plan of Development that the area be protected as
a natural resource [4]. '•' '
The developers of the industrial site had to provide erosion and sedimentation
controls during construction and stormwater controls as permanent facilities.
The final runoff control facilities, constructed in a two phase program, are
shown in Figure 66.
The first phase involved constructing a primary sedimentation pond to treat
runoff from'the area during construction. Some portions of the site'could not
drain into this pond and the runoff from these areas was controlled by a
barrier of hay bales between the construction site and the Mud River. The
second phase involved constructing the facilities to control runoff -
principally from the impervious areas of the development.
Approximately 40% of the developed site, building and parking area, is now
impervious. Stormwater is collected from the roof of the building and the
parking area and is piped to the primary sedimentation pond. It then flows to
the secondary sedimentation pond, which also receives flow from two
intermittent streams. Flows are then conveyed to the marsh control area, then
discharged to the Mud River. Stormwater collected at the northern end of the
building, a new extension, is piped to a separate pond that discharges to the
Mud River. The stormwater facilities and service area are shown in Figure 67.
195
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X
O)
•I—
10
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S-
Z3
-a
O
O
O
O)
s_
o
+J
q-
o
o
•P
ns
I
O
CO
0}
S-
196
-------
Figure 67. Stormwater control facilities at an industrial park, Middlesex
County: (a) primary sedimentation pond, (b) secondary sedimentation pond,
and (c) a portion of the parking area served by the sedimentation ponds.
197
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The facilities were designed to handle a 50 year, 24 hour rainstorm. Runoff
calculations were based on the SCS methodology [5]. The secondary
sedimentation pond incorporated additional storage capacity and a pumping
station for fire-fighting capabilities for the building. The design criteria
for the Stormwater control facilities are presented in Table 83.
Table 83. DESIGN PARAMETERS FOR INDUSTRIAL
STORMWATER FACILITIES IN ESSEX
Parameter
Value
Design storm
Design storm frequency, yr 50
Design storm duration, h 24
Design rainfall, in. 6.4
Contributing area, acre 43
Average watershed slope, % 6.5
Primary sedimentation pond
Pool surface area, ft2 17,500
Length, ft 350
Width, ft 50
Average depth, ft 5
Total volume, ft3 88,000
Permanent pool volume, ft 23,000
3
Stormwater storage volume, ft 65,000
Peak discharge rate, ft /s 22
Detention time at peak discharge rate, h 2.0
Secondary sedimentation pond
Pool surface area, ft2 65,000
Average depth, ft , 5.5
Total volume, ft3 500,000
Permanent pool volume, ft3 350,000
3
Stormwater storage volume, ft 150,000
Peak discharge rate ft /s 34
Detention time at peak discharge rate, h 3.2
Marsh control area
2
Surface area, ft
Peak inflow rate, ft /s
30,000
34
198
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Dry Wells and Drainage Swales (Industrial Site)—
A light industrial site, developed in the town of Durham, is located in an
area of well drained to excessively drained soils with a sandy and gravelly
substratum on rock terraces. The area has slightly rolling hills and is
heavily forested with considerable ground cover. No streams of major
significance are near the industrial site.
The stormwater control facilities serve a total area of 3 ha (7.5 acres), of
which 1,860 m2 (20,000 ft2) is building, 2,830 m2 (30,500 ft2) is parking lot,
0.8 ha (2 acres) is woods, and the remaining area of 1.8 ha (4.35 acres) is
lawn.
The site and the permanent stormwater control facilities are shown in
Figure 68. Two phases of stormwater control were implemented to meet the
Durham requirements for erosion control during construction and
postconstruction runoff control. Erosion was controlled by a temporary
retaining berm on the western property line with release openings every 60 m
(200 ft). The openings had riprap and hay bales to filter the sediment in
runoff. Permanent postconstruction control facilities use the well-drained
soils as an integral feature in operating the controls. A large drainage
swale collects runoff from the lawn area, providing percolation and
evaporation. Percolation dry wells collect and control stormwater runoff from
the parking lot, building roof, and some runoff from the lawn areas.
The design of the facilities was based on the soil's percolation capacity.
Soil borings indicate topsoil depths of about 0.23 m (0.75 ft) on top of sand
and sandy till with average percolation rates of 2.54 cm/8 min (1 fn./8 min).
The dry wells were installed in two groups of three and four in series,
together providing about 36 m3 (1,275 ft3) of storage capacity with nearly 19
m2 (200 ft2) of percolation surface.
Recharge Basin (Residential Site)—
A small residential site, which was a sand and gravel pit, has been developed
adjacent to the Connecticut River in the town of Haddam. A stormwater
recharge basin provides runoff control for the site and adjacent areas by
using the high percolation rates of the soils and storage from previous
excavations. Land reclamation, control of stormwater runoff, and the
resulting recharge of the groundwater are positive environmental impacts of
the project. The recharge basin was selected over the alternative of
providing a drainage system because a drainage system would require easements
through private property to discharge flow into the Connecticut River.
The residential site is on 5.5 ha (13.5 acres) of reclaimed land. The land is
gently rolling and is heavily forested. The developed site is flat and the
soil is generally classified by SCS as group "A" (low runoff potential). The
soil consists of deep, well to excessively drained sands and gravels, and has
a high infiltration rate, better than 2.54 cm/4 min (1 in./4 niin), when
thoroughly wetted.
199
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LEGEND
Q-O DRY WELLS
RUNOFF TO DRY WELLS
RUNOFF TO DRAINAGE SWALE
(ROOFTOP DRAINAGE)
OOOO
ooo
PARKING LOT
Figure 68. Industrial stqrmwater control using percolation, Middlesex County:
(a) depressed parking lot with catchbasin inlet, (b) grassed .drainage swale*
and (c) schematic of the industrial site and the stormwater control faciliities,
200
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The site is divided into 13 parcels of approximately 0.4 ha (1 acre) each, one
of which is the recharge basin. Runoff is collected and transported to the
recharge basin by about 900 m (3,000 ft) of grassed drainage swales, which are
riprapped in those portions subject to erosion. The 0.4 ha (1 acre) recharge
basin has a 1.5 m (5 ft) chain-link fence around its perimeter. It is seeded
with a native grass to prevent erosion. The site and its stormwater
facilities are shown in Figure 69.
The design of the recharge basin assumed that the development site land use
would be single-family residential and would include the runoff contribution
from an additional 9.7 ha (24 acres) of land adjacent to the development site.
.The recharge basin contains and percolates all the flow from the total area
for a 100 year storm. The design criteria for the recharge basin are
presented in Table 84.
Table 84. DESIGN PARAMETERS FOR RESIDENTIAL
STORMWATER FACILITIES IN HADDAM
Parameter
Value
Design storm
Design storm frequency, yr TOO
Design storm duration, h 24
Rainfall, in. 7.1
Contributing area, acre 37.5
Average watershed slope, %
Developed site (13.5 acre) 4
Adjacent contributing area (24 acre) 23
Recharge basin
2
Surface area, ft
Length, ft
Width, ft
Depth, ft
3
Capacity, ft
Side slopes
Percolation rate, in./min
Percolation area, ft
44,400
270
180
17
457,000
3:1
1
44,400
PERFORMANCE
Information on the performance of the three small, onsite stormwater control
facilities is limited. The facilities have operated for about three years,
but because of their rural locations and private ownership, no monitoring has
occurred. Projections of the pollutant loadings and runoff volume reductions
were used to estimate the level of efficiency expected for these facilities.
The projections are based on representative pollutant concentrations for the
various land uses that are assumed consistent with conditions at the
development sites. These concentrations are summarized in Table 85.
201
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o
LEGEND
I I EXISTING HOME
DEVELOPMENT BOUNDARY
DRAINAGE SWALES
Figure 69. Residential development with stormwater control facilities,
Middlesex County: (a) schematic of the recharge basin and service area,
(b) view of reclaimed area and homes under construction,
and (c) the stormwater recharge basin.
202
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Table 85. REPRESENTATIVE POLLUTANT CONCENTRATIONS
IN STORMWATER RUNOFF FOR SEVERAL LAND USES [6-8]
Land use BODg, mg/L Suspended solids, mg/L
Rooftop drainage
Paved parking lot
Low density residential
General forest
10
20
20
20
30
320
280
66
Volume Reduction
The runoff volume has been reduced 100% by the residential recharge basin and
the industrial dry wells, drainage swales, and percolation facilities because
of the high percolation rates of the soils at these particular sites.
Assuming that the dry wells could provide the reported percolation rates
throughout a storm, calculations show a 100% volume reduction for a 10 year,
24 hour storm. During the 3 years of operation, the dry wells have contained
the flow from all storms. The residential recharge basin, designed to
contain the total volume of runoff for a 100 year storm, would be 100%
effective for any storm less than the design storm regardless of the time
required to percolate the stormwater.
The sedimentation ponds at the other industrial site were not designed to
reduce the total volume of runoff, but rather to detain the runoff before it
is discharged to the receiving water. The ponds have a combined stormwater
storage volume capacity of about 6,100 rrr (215,000 ft3) and provide a total
estimated detention time, at peak storm flow, of about 5 hours.
Pollutant Loading Reduction ••
The reduction of pollutant loadings by the stormwater facilities providing
storage and using percolation methods are high and are estimated to have
reduced pollutant loadings, suspended solids and 8005, to the stream by 100%.
The site using sedimentation ponds to discharge stormwater to a marsh area
before it enters the river has an estimated reduction of 95% for suspended
solids. The reduction of BODg in the sedimentation ponds has not been
measured or estimated, but is assumed high. The estimated performance of all
three facilities is summarized in Table 86.
COSTS ,
Most of the stormwater controls in Middlesex County are required of and
provided by private developers who also .bear the construction and the
operation and maintenance costs of the facilities. Most of the costs of the
controls is for construction, over 50% of which is associated with excavation
and grading. Annual operation and maintenance costs are usually minimal and
are generally limited to landscape upkeep.
203
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Table 86. ESTIMATED ANNUAL POLLUTANT LOADING
REDUCTIONS BY STORMWATER CONTROL FACILITIES,
MIDDLESEX COUNTY
Loadings,
lb/acre-yr
Facility
Sedimentation ponds
(Industrial site)
Dry wells
(Industrial site)
Recharge basin
(Residential site)
SS
1,850
2,060
550
BOD5
150
160
70
Pollutant
%
SS
95
100
100
reduction,
BOD5
100
100
No construction costs for the three example facilities were available;
therefore, estimates of the capital costs were based on estimates of the
quantity of excavation, materials, and equipment, using standard construction
cost estimating guides. These costs are assumed representative of small-
scale onsite stormwater controls, usually serving less than 4 ha (10 acres)
of impervious area.
Capital cost estimates for the stormwater controls are shown in Table 87.
The construction costs can vary significantly, depending on the type of
control used and the availability and use of natural land features integrated
in the control design. Unit costs range from about $7,500 to $16,000/ha
($3,000 to $6,600/acre) of impervious area.
Table 87. SUMMARY OF COST ESTIMATES OF
STORMWATER CONTROLS, MIDDLESEX COUNTY3
Facility
Impervious
area served, Capital
acre cost, $
Cost of
pollutant removal
Treatment or $/acre-lb-yrb
Cost/area, volume reduction
$/acreb cost, $/Mgal
SS
BODc
Sedimentation
ponds
Dry wells
Recharge
basine
6.53
1.15
5.2
29,000
7,600
15,700
4,400
6,600
3,000
7,000C
6,900d
420d
0.40
2.80
0.20
—
35.70
1.10
a. ENR 3000.
b. Based on impervious area.
c. Treatment cost for design storm runoff.
d. Volume reduction cost for design storm runoff.
e. Residential area, pollutant loadings based on total area.
204
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The operation and maintenance costs are usually minimal because the
facilities are self-activating and require little care. Several of the
controls are used as recreational areas, and landscape maintenance is
provided as a part of the overall grounds upkeep. After construction of the
Essex industrial complex, the primary sedimentation pond required dredging to
remove the sediment accumulated during construction before it was converted
to a postconstruction control. The cost of dredging was about $500.
IMPACTS
The environmental and socioeconomic impacts of the onsite stormwater control
facilities in Middlesex County are highlighted by the use of small, almost
totally effective control measures that maximize the use of existing physical
conditions of the land. The facilities are integrated into the surrounding
landscape and protect the environmentally sensitive areas.
Environmental Impacts
The stormv\tate,r controls used in Middlesex County protect the surrounding
environment from land use changes, increased impervious areas, and land
disturbances caused by development. Several of the industrial developments
were constructed in wetlands areas and would not have been allowed unless
they adequately controlled runoff.
Each proposed development must provide runoff control plans and receive an
environmental, review assessment to ensure that it protects and is compatible
with potential environmentally sensitive areas. The assessment evaluates the
topography, geology, soils, natural drainage, aquifers, and groundwater
conditions. Natural vegetation and ground cover and their ecological
function are also assessed. Developments in wetlands areas are designed to
be compatible with and to maintain the condition and function of the
wetlands. Other impacts, such as air pollution, erosion and sedimentation, *
and flooding, are also assessed.
The three,example stormwater control facilities were conservatively designed
and provide almost total runoff pollutant and volume control. Collectively*
they prevent an estimated 18,200 kg/yr (40,000 Ib/yr) of suspended sol-ids andu
about 1,700 kg/yr (3,700 Ib/yr) of BOD from entering receiving waters. The
sites control a total development area of about 18 ha (45 acres) and, on the
average, reduce suspended solids by about 1,000 kg/ha-yr (890 lb/acre-yr).
These unit removals represent a significant pollutant load potential that
could significantly affect wildlife habitats and supporting vegetation and
the characteristics and ecology of the receiving streams.
Preservation of the natural environment is one of the principal considerations
in the design of new developments. The Essex industrial development in a
wetlands area provides a wildlife area around the sedimentation ponds. The
area was planted with a mix of trees, grasses, small grains^ shrubs, and vines
as shown in Figure 70. • • ••.
205
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Figure 70. Vegetation around sedimentation pond supporting
wildlife displaced by the adjacent industrial site, Middlesex County.
Socioeconomic Impacts
The socioeconomic impacts of the stormwater controls have been minor,
principally because the controls are small onsite facilities constructed on
private developments. The developer is responsible for the cost of
construction; therefore, the general public is not burdened with costs, either
through assessments or from local city budgets.
Several developments have incorporated stormwater controls into the overall
landscaping as shown in Figure 71. The sedimentation ponds have permanent
pools, and the adjacent landscaped areas are used for recreation and picnics
and improve aesthetics. The sedimentation pond also has a multipurpose use
with extra pool capacity and a pumping station for fire-fighting capability.
206
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(a)
(b)
Figure 71. Landscaped multiuse stormwater detention pond, Middlesex County:
(a) picnic facilities for employees, and (b) fire-fighting water
source and pumping station for the industrial complex.
207
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SECTION 11
STORMWATER RUNOFF CONTROL
BOULDER, COLORADO
The primary stormwater concern in Boulder, Colorado, is controlling the
flowrate of stormwater and snowmelt runoff. The city is located in the
foothills of the Rocky Mountains and is susceptible to flooding. Land
development near the natural water courses that flow through the city and on
the steeper slopes of the foothills has increased the potential for runoff,
flooding, and erosion/sediment deposition. The effect of stormwater and
snowmelt runoff on the long-term water quality of the receiving streams in
Boulder is currently unknown. •
Boulder has established a program to control stormwater runoff. The
principal BMPs are the construction or enlargement of drainageways to
accommodate the flow from a 100 year storm; onsite source detention; and land
use planning. Instream detention ponds are used with new or enlarged
drainageways, and sedimentation ponds are built downstream of residential
developments on steep hillsides to reduce suspended solids in runoff.
PROJECT DESCRIPTION '
The stormwater and showmelt runoff problems in Boulder are representative' of
the problems in semiarid, mountainous areas where the quality of the
receiving streams near the smaller population centers is high. As t: '
development increases, however, the effect of increased runoff has:-created
flooding and the potential for water quality problems. In Boulder, water
quality is currently considered a secondary problem to flooding, but limited
monitoring shows that the effects of stormwater pollutants may become a more
important issue. " - * :
Area Characteristics • . • ;,
Boulder is northwest of Denver in the foothills of the Rocky Mountains. The
city is about 32 km (20 mi) east'of the Continental Divide. ; The elevations
in Boulder vary from about 1,600 to 1,900 m (5,300 to 6,200 ft) and generally
slope from west to east [1]. .The location of Boulder is shown in Figur'e 72'.
Boulder's annual temperatures range between -9° and 30.5°C (16° and 87°F) ':<;j
with an annual precipitation of about 47 cm (18.5 in.). Precipitation is
light in the winter, 16% of the annual precipitation. About 52% of the "
annual precipitation occurs between March and June, creating large spring •-'
runoff volumes [2].
208
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ROCKY MOUNTAIN
NATIONAL PARK
Figure 72. Boulder, Colorado.
The population of Boulder and the surrounding unincorporated communities has
increased from 21,500 in 1950 to about 95,000 in 1979 [3]. This represents
an increase of 442% in population over three decades, and much of the
increase occurred in Boulder.
Problem Assessment
Boulder's stormwater problem is the result of increasing development on
natural streams and drainageways in Boulder Creek basin. The basin is
drained by the Boulder and South Boulder creeks and is nearly 950 km^ (370
mi2), of which about 640 km^ (250 mi2) is mountainous terrain. The two
creeks have discharges that vary from about 0.14 irr/s (5 ft3/s) to over 8.5
m3/s (300 ft3/s) [4]. The Boulder Creek basin is shown in Figure 73.,
The development density near the natural drainageways has increased
significantly and has resulted in higher runoff rates. New development,
especially on steeply inclined areas, has resulted in higher runoff rates
causing erosion and sediment deposition problems. Portions of the developed
area in Boulder are located within the 100 year flood plain and are
susceptible to flooding, as shown in Figure 74.
209
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BOULDER CREEK BASIN BOUNDARY
Figure 73. Boulder Creek basin.
DENVER
BOULDER
TURNPIKE
CORPORATE |
LIMITS
Figure 74. Potential flood hazard areas affecting
a portion of the developed area in Boulder.
210
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Limited sampling of stormwater and snowmelt runoff indicates that runoff
pollutants can have a short-term influence on the streams flowing through
developed areas. A sampling program conducted on a 50 ha (125 acre) drainage
area with 89% residential land use was used to characterize runoff and base
streamflow [1]. The runoff from the area enters the stream through a storm
drain. Only one storm was sampled for rainfall runoff and up to five
snowmelt runoff events were sampled to establish concentration ranges and
flow weighted means. The results are summarized in Table 88.
Table 88. CHARACTERIZATION OF BASE STREAMFLOW,
STORMWATER, AND SNOWMELT RUNOFF, BOULDER [1]
Pollutant concentration, mg/L
Base flow
Parameter
Total solids
Suspended solids
BOD
COD
N03-N
Total phosphorus
Oil and grease
Stormwater3
Snowmelt
Flow-weighted Flow-weighted Flow-weighted
Range mean Range mean Range mean
240
1
0.8
0
4.4
0.004
- 360
- 12
- 1.4
- 11
- 5.8
- 0.04
2
310
9
1.1
6
4.9
0.01
2
100
12
1
3
1
0.02
2
- 680
- 460
- 48
- 300
- 4
- 0.14
- 19
240
180
18
120
1
0.12
8
110
'o
6
7
0.1
0.001
3
- 4,600
- 630
- 30a '
- 730
- 6
- 0.35
- 70b
1,300
200
10
216
3
0.05
25
a. Results of one monitored storm.
b. Results of three monitored storms.
Receiving stream impacts of snowmelt runoff for one monitored event show an
increase in peak concentration of 2 to 4 times for most pollutants, below the
storm sewer discharge point, as shown in Table 89. Peak COD concentrations
showed over a 40-fold increase from above to below the storm sewer [2].
Countermeasure Philosophy
BMPs are being implemented in Boulder to attenuate peak flows of stormwater
runoff by source detention and construction or enlargement of drainageways.
Ordinances were adopted to establish (1) a storm drainage and flood control
utility, (2) land use planning criteria, and (3) criteria identifying the
magnitude of stormwater control required.
The storm drainage and flood control utility is not only a regulatory agency,
but also is responsible for the financing, construction, and operation and
maintenance of public stormwater facilities.
Land use planning has effectively eliminated future development from areas
consistently affected by flood waters and ensures the availability of
drainageway easements.
211
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Table 89.
CONCENTRATIONS ON BOULDER CREEK
EFFECT OF PEAK SNOWMELT RUNOFF
3 [2]
Peak pollutant
concentrations, mg/L
Parameter
Total solids
Total volatile solids
Suspended solids
Volatile suspended solids
COD
Total phosphorus
TKN
NOa-N
Oil and grease
Lead
Chlorides
Storm sewer
6,490
425
1,230
330
935
3.3
5.9
4.7
155
2.3
3,185
Boulder
Above
347 1
44
58
25
4
0.2
0.4
0.2
7
0.4
100
Creek
Below
,032
102
214
84
165
0.2
1.6
0.5
26
0.8
398
Ratio of stream
concentrations
above and below
the storm sewer
outfall
3.0
2.3
3.7
3.4
41.3
1.0'
4.0
2.5
3.7
2.0
4.0
a. Values represent one snowraelt event.
Controlling stormwater and flood waters has been designated, by the city, as
the responsibility of owners of property greater than 0.4 ha (1 acre). These
landowners must limit runoff to the natural runoff rates existing before
development and are assessed a fee based on the amount of runoff in excess of
the natural runoff rate. They must also control runoff traversing their
property, even though the runoff may come from other property [5].
Implemented Controls—
The controls used to attenuate peak stormwater and snowmelt flows include
improving drainageways and constructing detention facilities to reduce
flooding. The construction of stormwater control facilities is only about
20% complete. Although these facilities were planned principally for flood
protection, a number of the facilities also control water quality. Sediment
ponds at new construction sites and stormwater detention ponds trap eroded
sediment and reduce pollutants in the stormwater. A portion of the planned
drainageway improvements and detention facilities to reduce peak stormflows
and flooding are shown in Figure 75.
Natural Drainageway Improvement--Control of runoff by channelizing the •
drainageways to improve their hydraulics has been a popular method in this
country. Boulder has widened and improved several stream channels so that
the flow from a 100 year storm can pass through the city without causing
excessive damage. The improvement of these drainageways complements the
surrounding landscape by using a 4:1 slope on the embankment when possible,
as shown in Figure 76(a).
212
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DRAINAGE CHANNEL
IMPROVEMENT
K./'
DETENTION
PONDS
Figure'75. A portion of planned stormwater control facilities, Boulder.
•":;• ' '• I .. t :•'.., •' ; '..•"..' ,; . .. ;
Drainageway.Construction--The increased development near streams has made
natural drainageway improvements difficult in some portions of the city.
Construction of new drainageways through these,developed areas, where
easements can be obtained, is an alternative to provide sufficient
runoff control. When possible, these are constructed in a natural
configuration for aesthetic enhancement, as shown in Figure 76(b).
Deten ti. on. Ponds--1 n s trearn arid off stream-detention ponds are also extensively
used to control stormwater. When land,is unavailable-for new or improved
drainageways, instream and offstream, detention ponds are constructed at
- locations, upstream of confined areas. -
•Detention ponds provide storage -and attenuate peak flows; this reduces the
r-size requirements of the downstream drainageway. Boulder has constructed a
wide variety of instream detention facilities, including detention ponds in
series with restricted pipe outlets and small check dams in stream beds.
Instream Detention facilities are shown in Figure :7;7. A few developments
have- offstream, ponds that remain partially full during honstpnrj periods.
Onsite Source Detention--The m.ost widespread method of stormwater control for
private landowners is onsite source detention, asi;ng rooftop and parking lot
.storage methods. Each method ;uses the existing features of the development
to store stormwater runoff with the release rate determined by the drain
capacities. Examples of typical rooftop and parking lot storage are shown in
Figure 78.
213
-------
Figure 76. Drainageways in Boulder with 4:1 slopes: (a) natural
drainageway improvement, and (b) newly constructed drainageway.
214
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Figure 77. Stormwater detention facilities, Boulder: (a) a series of
three detention ponds with restricted outlets used and maintained as
pasture by local residents, (b) check dams creating storage along a natural
drainageway, and (c) instream detention facility adjacent to a highway.
215
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Figure 78. Onsite detention facilities: (a) rooftop storage uses
potential storage volume created by parapet wall and waterproofed
flashing, (b) stored volume is released through a series of drains,
(c) drain has control orifice and debris trap, and
(d) parking lot storage uses depression in lot to store runoff.
216
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Sedimentation Ponds—Boulder also uses sedimentation ponds in new
developments on relatively steep slopes to control erosion from road
construction and home foundations. A sedimentation -pond containing sediment
from a new development on steep slopes with erodible soils is shown in Figure
79. Sediment is removed from the pond and used for fill and construction
material as needed.
Percolation Ponds—The use of percolation ponds to control stormwater runoff
is limited in Boulder. One of the percolation facilities shown in Figure 80
collects and percolates runoff from a shopping mall in a grass-covered swale
partially covered with No. 2 stone. A residential area also uses a
percolation pond consisting of a grass-covered depression to contain and
infiltrate the stormwater.
Land Use Planning--The city has implemented land use planning to complement
the facilities that control stormwater runoff. Land use planning regulates
development in the 100 year flood plain and has divided it into developable
and nondevelopable areas. Development is excluded from that portion of the
flood plain that conveys the main portion of a 100 year flood at velocities
and depths that would cause significant damage. Development, if properly
protected, is allowed within the 100 year flood plain in the areas that would
transport slowly moving flood waters.
Design Criteria—
The design of drainageways and detention ponds to control stormwater runoff
is approached on a case-by-case basis and depends on the site conditions and
stormwater problems. The stormwater problems are usually flooding and
conveyance of peak storm flows. The criteria for improving or constructing
drainageways are keyed to their peak flow carrying capacity rather than
quality or storage/detention time criteria. The design of iristream or
offstream detention facilities uses flow or hydrograph routing procedures to
provide adequate volume for containing peak flows at the source or instream
where adequate volume is available.
Natural Drainageway Improvement—Soulder's program of stormwater runoff
control emphasizes the use of natural drainageways when possible. The
improvement of the drainageways to transport runoff from the 100 year flood
includes the following design criteria [6]:
• Adequate capacity to contain 100 year runoff volume
• Critical velocity limited to 3 m/s (10 ft/s)
• Use of roughness factors representative of unmaintained drainageway
conditions
• Control of water surface profile by construction of controlled
drops or check dams
• Prevention or planning for future intrusion in the flood^plain that
may affect the water surface profile
217
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Figure 79. Sediment pond serving new.residential subdivision developed
on a steep erodible hillside, Boulder: (a) portion of the new
development creating erosion and sediment problems, (b) sediment pond
with retained sediment in the bottom, and (c) entrance channel to
sediment pond protected by gabion retaining walls.
218
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Figure 80. Percolation pond with grass-covered swale and stone-covered
bottom receives runoff from a commercial area in Boulder.
Drai nageway Constructi on--New drainageways have been necessary because of the
intrusive development into the 100 year flood plain. The suggested design
criteria for major grass lined channels are shown in Table 90.
Table 90. SUGGESTED DESIGN CRITERIA
FOR MAJOR GRASS-LINED DRAINAGE CHANNELS [6]a
Parameter
Runnoff from
100 yr storm
Velocity, V, ft/s ,2.0 * V ^ 7.5
Depth, D, ft 1.0 > D < 4.0
Side slope <3:1
Horizontal curvature, ftb R = 2W, R>100
Freeboard, ft 1.0 to 2.0
Roughness coefficient0 0.030 to 0.035
a. Assume that major channel will be
discharging to a river or large
drainage area.
b. R = radius or curvature,
W = width of channel top
c. May be increased if heavy in channel
vegetation is anticipated in the future.
219
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Detention Ponds—The detention ponds are located instream and offstream,
mainly in upstream areas. The design of facilities is based on the Rational
Method or the Colorado Urban Hydrograph Procedure (CUHP). The Rational
Method (Q = CIA), presented in numerous textbooks, is a direct relationship
between rainfall and runoff for small watersheds.
The preferred design criteria follow the CUHP because of its greater
dependability and detailed analysis of the storm hydrograph. The CUHP is
similar to the unit hydrograph method, except that the coefficients are
determined by actual field data collection and studies in Colorado.
The design of instream and offstream detention ponds has one of two goals:
• Reducing runoff rate
• Providing instantaneous storage
Detention ponds that reduce runoff rates are generally in upstream areas and
use an undersized drainpipe of no less than 30.5 cm (12 in.) in diameter to
regulate the flow release rate [6]. Drainpipes smaller than this tend to
plug up with debris.
Detention ponds are more often used to provide instantaneous storage to
attenuate the peak flow before it enters constricted areas, where flow cannot
be transported within the drainageways. Design of these facilities has been
limited to the size of the easement available. The release rate of these
facilities is uncontrolled.
Onsite Source Detention—Boulder requires onsite detention for all
developments larger thTn 0.4 ha (1 acre) that are not part of a larger
development. The onsite detention facilities must meet standards established
by the Urban Storm Drainage Criteria Manual (USDCM) [6] or general urban
hydrology practice [5]. Rooftop and parking lot storage design parameters
suggest a maximum stormwater storage depth of 7.6 cm (3 in.), and the rooftop
drainage outlet should be large enough to release about 1.3 cm (0.5 in.) of
ponded depth per hour [6].
Sedimentation and Percolation Ponds—Boulder has no specific design criteria
for sedimentation or percolation ponds. Stormwater facility design usually
follows suggested parameters of the USDCM.
OPERATION AND PERFORMANCE
The operation and performance data from Boulder's program to control
stormwater runoff rates are limited because the implementation of the control
program is approximately 20% complete. The preliminary results indicate a
reduction of flooding in the historic problem areas of the city from rainfalls
of the 2 and 5 year frequency [7],
220
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Operation and Maintenance
The operation of stormwater facilities is divided between the city and the
private sector. The city operates the facilities that transport ,100 year
storm flows and the instream and offstream detention facilities. The private
sector is responsible for onsite detention facilities, such as rooftop or
parking lot storage. Instream detention ponds may have adjustable stop logs
that can regulate the water elevation of the pond.
The maintenance of stormwater runoff controls in Boulder includes practices
such as:
• Debris removal in drainageways
• Landscape care
• Repair of erosion damage
• Removal of deposits in sedimentation ponds
• Rooftop storage drain maintenance
• Parking lot storage drain maintenance
The city is responsible for maintaining the drainageways, natural or man-
made; the associated instream and offstream detention ponds; and the
drainageway easements on private property. All other facilities on private
property are maintained by the owners and usually require landscape care and
debris removal.
Special attention must be taken to avoid flooding of rooftop and parking lot
storage facilities because leaves or debris can plug drains. Damage to
buildings may result from plugged rooftop drains. Facilities, such as those
located at apartment complexes and commercial centers, also require a high
degree of maintenance and landscape care to achieve an acceptable level of
, aesthetics. Boulder lowers its maintenance costs by allowing residents near
some stormwater facilities to use the land in exchange for upkeep. An
example is a detention pond where a resident maintains the facility in
exchange for grazing rights for his horses.
Performance
The Boulder control program, although only 20% completed, has reduced
flooding in certain sections of the city [7]. A sedimentation pond located
on Two Mile Canyon Creek has reduced the sediment loading from hillside
development upstream of the facility. The large quantity of solids that must
be removed from the pond annually indicates that the facility is effective.
However, the annual quantity of sediment settling in the pond has not been
monitored, and estimate^ of annual flooding are further complicated because
several city departments use the sediment for construction and repair.
Although Boulder's principal goal is flow control, a dual benefit of storage
is water quality control. Using the performance data on source detention and
treatment efficiencies presented in previous case histories (Montgomery
221
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County, Middlesex County, and Orange County), it is estimated that
significant water quality benefits can be achieved by the facilities in
Boulder. Trap efficiencies in permanent pool detention basins for most
stormwater pollutants can range from 50 to 90%. Percolation facilities can
reduce pollutant load to receiving waters by 100% but may contribute to
groundwater pollution.
COSTS
The construction costs of several stormwater controls were analyzed and are
presented in Table 91. The cost data are limited. The costs of the
sedimentation and detention ponds may not reflect the true cost since both
facilities were created by the excavation of soil required as fill on other
projects, and the cost for the work is not reflected in the table.
Table 91. CAPITAL COSTS OF STORMWATER
CONTROL FACILITIES, BOULDER3
Storm Area Storage Cost/acre Storage
design, served, capacity, Construction served, cost.
Facility Ownership yr acre ft3 cost, $ $/acre $/ft3
Sedimentation
pond
Detention
pond
Drainageway
improvement
Public
Public
Public
~b 977
100 347
100 900
102,000 20,300
436,000 90,200
—c 829,500
21
260
840
0.20
0.21
--C
a. ENR 3000.
b. Temporary solution, no design.
c. Drainageway improvements for 7,490 ft of channel at about $110/1inear ft.
Information about the annual cost of operating and maintaining the facilities
is also limited. The cost of maintaining the percolation pond that serves an
area of 13 ha (32 acres) is about $250 annually.
IMPACTS
The impacts of the Boulder program at the present stage of implementation
appear minimal. Anticipated socioeconomic impacts, such as Boulder's minimal
growth policy and land use planning, may be more significant than
environmental impacts.
Land use controls will be the most significant results of the program. The
project effectively eliminates new development in dangerous areas of the
flood plain. However, this land is used as open space and provides more area
for wildlife. Flood plain ordinances do allow some specialized industry,
such as material-loading facilities and agriculture, to be located in the
flood plain, although recreational facilities and wildlife refuges are
preferred [7].
222
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Socioeconomic impacts associated with the program are mostly positive. The
stormwater controls have reduced property damage caused by floods and created
more open space and recreational areas, as shown in Figure 81. Although
relocation of homes and businesses within the plain is a significant problem,
public acceptance of the program is high. The completion of the program is
not anticipated to substantially affect demography or economic activity in
the area; however, more public funding to finish the program is required.
Figure 81. Multiuse stormwater facilities, Boulder: (a) park/detention
pond, and (b) grassed, residential percolation pond.
223
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PART 2
COMBINED SEWER OVERFLOW CONTROLS
l
SECTION 12
INLINE STORAGE CONTROL
SEATTLE, WASHINGTON
Inline storage uses existing excess storage capacity in combined sewer
interceptors and trunk sewers. Given that storage (and/or treatment) of
combined sewer overflows is generally a far less costly control measure than
equivalent sewer separation in large metropolitan areas, inline storage
control is a particularly attractive measure where excess capacity exists and
is readily adaptable to control. It has the potential for low capital
investments and multiuse capabilities.
The inline storage control system in Seattle, Washington, is an example of
this most promising technology. The components of the Seattle facilities
include a comprehensive integrated system of collection, treatment, and
disposal functions, which can be controlled by a computer. Support functions
include system data qollection and logging. While necessary to control flow
during wet weather, these support functions also provide total system
monitoring capability during dry-weather periods.
The Seattle system has demonstrated that inline storage is effective in
controlling overflows within the limits of (.1) the available excess storage
capacity, (2) the existing system, and (3) the local characteristics. The
high level of operational flexibility and reliability of system components
also contributes to the effective performance of the system.
The costs of the Seattle system are competitive with the least expensive
alternative offline storage/treatment systems. Also, the multiuse
capabilities of the inline storage offer greater potential benefits.
Assessment of the environmental and socioeconomic impacts of the inline
storage control program indicates positive improvements in the receiving water
quality with minimum adverse impacts on the community. The system can also
serve as the foundation for future expansion of the combined sewer program, if
water quality objectives become increasingly stringent.
CONTROL OF COMBINED SEWER OVERFLOW
The combined sewer overflow problem in Seattle is similar to that experienced
in other major cities in the country with combined sewer systems. However,
local characteristics are of major importance and necessarily dictate system
configuration and the approach used to meet water quality, goals.
224
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Local Characteristics
Important local characteristics to consider when selecting a method to reduce
combined sewer overflows include: topography, rainfall, and land use in the
combined sewered area. Seattle, a major west coast city, is bounded by
freshwater lakes and marine waters consisting of bays, estuaries, and rivers
of Puget Sound, as shown in Figure 82.
Figure 82. Seattle and surrounding receiving waters.
The Seattle topography is characterized by rolling hills of approximately
120 m (400 ft) in height. Much of the highly impervious metropolitan area of
the city is located on these hills with some areas on extremely steep slopes.
These characteristics are unlike those found in most cities with combined
sewers in the midwest and the east.
.Seattle's climate is predominately maritime, with rainfall ranging from 50 cm
(20:in.) to 100 cm (40 in.) annually. Most of the rainfall occurs during the
winter, October through March, and is characterized by long duration,
moderate intensity regional storms. Summer storms are more localized and
have shorter durations.
225
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The service area of Seattle encompasses nearly 21,900 ha (54,000 acres), of
which 6,080 ha (15,000 acres) is served by combined sewers, 9,320 ha (23,000
acres) by partially separated sewers, and 6,480 ha (16,000 acres) by sanitary
sewers [1]. The total area served by combined sewers and partially separated
sewers is 15,390 ha (38,000 acres).
The City of Seattle started a program of combined sewer separation in 1970 to
eliminate local flooding in specific problem areas in the city [2]. The
separated storm sewers either discharged directly to the receiving water or
were reconnected to the combined sewer downstream of the problem area. The
combined and partially separated sewer areas are shown in Figure 83.
Land use is an important factor in determining the volume of stormwater flow
entering the combined sewer system. Impervious areas that have steep
topography can add significant amounts of wet-weather flows to a combined
sewer system. In Seattle, a large portion of the total area, 27%, is covered
by streets and parking lots. Land use characteristics of Seattle are shown
in Table 92.
Table 92. LAND USE CHARACTERISTICS, SEATTLE [3]
Acres
Land use
1961 1970
Change Percent
1961-1970, of 1970
acres total
Total land
Total developed land
Vacant
Water
Total residential
Single family
Multifamily
Commerci al a
Manufacturing
Public services0
Parks/open space
Streets, parking,
54,098
44,776
7,098
401
19,911
17,998
1,913
2,137
3,235
2,507
2,703
54,098
46,149
6,184
690
20,307
18,161
2,146
2,148
3,666
2,896
2,327
—
1,373
-914
289
397
163
233
11
431
389
-376
--
85.3
11.4
1.3
37.5
33.6
4.0
4.0
6.8
5.4
4.3
and miscellaneous
14,284 14,805
521
27.4
a.
Includes retail, finance, insurance, real estates,
and service categories.
Includes manufacturing, wholesale trade, trans-
portation, communications, and utilities categories.
c. Includes government and education categories.
b.
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kxT^x'&a&i
mmm UASHIHGTON
LEGEND
[=| COMBINED SEWERS
[j:|| PARTIALLY SEPARATED SEWERS
RI1 SANITARY SEWERS
-70-i'^^r
*>-'-:'
Figure 83. Combined and partially separated
service areas, Seattle [4]
227
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Combined Sewer Overflow Problems
Combined sewer overflows, the discharge of untreated mixtures of sanitary
sewage and stormwater runoff to receiving waters, decrease the quality of
receiving waters and can adversely affect the environment and community,
especially restricting the beneficial use of local community facilities.
Impacts on Receiving waters--
Adverse impacts on receiving waters are characterized by the pollutant
concentration in the overflow and in the receiving water after an overflow
event. A wide range of pollutant concentration values was obtained from
analysis of Seattle's overflows, with the average values considered
representative for the entire combined sewered area [1]. These values are
shown in Table 93. The presence of these pollutants has contributed to the
decline of water quality of the receiving waters surrounding Seattle.
Table 93. COMBINED SEWER OVERFLOW POLLUTANT
CONCENTRATIONS, SEATTLE [1]
mg/L Except as Noted
Parameter
Minimum Maximum Average
BOD
COD
Suspended solids
Ammom"a-N
Potassium
Copper
Lead
Mercury
Chromi urn
Cadmi urn
Zinc
Total col i forms, No. /1 00 ml
15
100
141
0.5
1.2
0.1
0.5
0.01
0.02
0.01
0.2
1-8x103
82
330
296
1.5
1.7
0.3
0.9
0.01
0.20
0.02
0.5
7,000x103
60
236
217
0.9
1.4
0.2
0.6
0.01
0.10
0.01
0.4
200x103
Fecal coliforms, No./100 ml 3.6xl03 780x103 250x103
A study is currently being conducted on Lake Washington to show the relative
impacts of stormwater runoff and combined sewer overflows on benthic sediment.
Results of the samples collected from the bottom sediments in areas subject to
the storm influences are shown in Figure 84. Composited information includes
three categories of pollutants: (1) organics (total organic carbon plus-oils
and grease), and total phosphorus; (2) heavy metals (copper, lead, zinc,''and
mercury); and (3) total chlorinated hydrocarbons (TCH) and polychlorinated
biphenyls (PCBs). The length of. each histogram bar, representing these '
categories, is relative to the lowest value found in the lake for each '
constituent [5]. The average strength of the various pollutants found-ih:
228
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the sediment near the outfalls and the control areas is shown in Table 94.
LEGEND
Q CONTROL SITE
^ STORM DRAIN
fjl-j COMBINED SEWER OVERFLOW
=1
TCH'S & METALS OR6ANICS &
PCB'S
70 60 50 40 30 20 10 0
RELATIVE IMPACT SCALE
Figure 84. Comparison of the relative impacts
on benthic sediments from combined sewer overflows
and stormwater runoff in Lake Washington [5].
Elliott Bay and Duwamish River received most of the combined sewer overflow
load from the metropolitan area of Seattle. The effects of these discharges
were responsible for bacterial, floatable, sediment, and organic problems in
the nearshore areas of Elliott Bay. Serious dissolved oxygen deficiencies in
the Duwamish River have also been documented [6].
Impacts on Local Community Facilities—
Loca] problems including surcharged sewers and local flooding often developed
from inadequate carrying capacity of the sewers and interceptors. In certain
areas of Seattle, these problems caused basement flooding in homes and
businesses resulting in health hazards and flood damage [7,8],, Adverse
impacts, caused by combined sewer overflow, have also been suspected in the
commercial fisheries and spawning waters in the area [1]. A summary of
combined sewer overflow impacts on community and beneficial local uses is
presented in Table 95.
229
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Table 94. AVERAGE SEDIMENT POLLUTANT STRENGTHS
IN LAKE WASHINGTON [5].
Pollutant0
Combined sewer Stormdrain Control
outfall areas outfall areas areas
Orgam'cs
TOC, % 1.2 1.2 0.4
Oil and grease, mg/kg 1,385 1,880 187
Total phosphorus, mg/kg 750 338 375
Heavy metals, mg/kg
Copper 178 34 14
Lead 210 320 55
Zinc 250 140 75
Mercury 0.25 0.14 0.09
TCH and PCBs, ug/kg 60 97 9
a.
All units are expressed on a dry weight basis.
Table 95. COMBINED SEWER OVERFLOW IMPACTS ON LOCAL
COMMUNITY AND BENEFICIAL USES, SEATTLE [1].
Use
Combined sewer
overflow impacts
Residential
Swimming
Shellfish
Fish spawning/rearing
Juvenile fish migration
Recreational boating
Shoreline parks
Commerce
Industry
Coli forms/f1oatables
Col iforms/fl eatables
Coliforms/virus
Toxicity/suspended solids
Toxicity
Floa tables
Floatables
Minimal
Negligible
Inline Storage Methodology
Inline storage control was implemented by the Municipality of Metropolitan
Seattle (Metro) to mitigate the adverse combined sewer overflow impacts to
the receiving waters surrounding Seattle. Advantages of this type of control
include (1) low capital investments by using existing facilities; (2) inte-
gration with dry-weather collection, treatment, and disposal functions; and
(3) adaptability of the system as the foundation for future expansion.
230
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The inline storage also controls the existing dry-weather transport system
(combined interceptors and trunk sewers) by using regulators and pumping
stations to maximize the use of the excess capacity. By using and controlling
this excess capacity, storm flow peaks are retained in the interceptors until
downstream treatment capacity is available following a storm.
The principal goals of the inline storage program are interrelated and include
the following:
• Reduction of overflow frequency
• Reduction of loading to receiving waters
• Reduction of overflow volume
The sewer separation program conducted by the City of Seattle to correct local
flooding problems provides an additional benefit to the inline storage system.
The separationoproject has reduced annual overflow volumes by an estimated
1.14 million m (0.3 billion gal) [2], thus reducing the flow to be controlled
by the inline storage system.
Facilities Description—-
The inline storage control system was developed from a portion of the regional
interceptor collection system and serves areas primarily affected by combined
sewers. The inline storage system uses 19 regulator stations and four pumping
stations to provide an estimated 86,260 m3 (22.79 Mgal) of storage [3,9].
Regulators and pumping stations, key components of the inline facilities,
control the unused storage capacity. Regulators are usually located at the
intersection of interceptor and trunk sewers; they provide storage in the
trunk sewers and control flows from the trunk sewer either into the
interceptor or to a receiving water if the interceptor is full. A summary of
the system storage capacities upstream from regulator and pumping stations is
shown in Table 96.
Seattle's regulator stations are mechanical systems with motor or
hydraulically operated gates and can be controlled statically or dynamically
or by centralized remote dynamic control. In the local static control mode,
the regulator gates are set at a single preset elevation to control flows.
Local dynamic control, the multipositioning of the regulator gates by an
operator, achieves the desired flow control in response to varying flow
conditions as monitored by the central control facility. Remote dynamic
control of the regulators is achieved by supervisory or automatic program
control from the centralized computer facility.
Pumping stations are also located along the interceptor system to assist in
transporting wastewater flows. These stations, in addition to the regulators,
provide control of the inline system and are controlled from the central
computer facilities. Regulator and pumping station facilities are shown in
Figure 85.
System Monitoring—System monitoring is required for remote dynamic control of
pumping stations and regulators. The regulator and pumping stations each have
231
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over 20 monitoring points to supply information for control logic and
equipment operation. In addition, water quality data and data from a network
of 11 rainfall gages are collected and used as input to the control function.
Table 96. INLINE STORAGE
POTENTIAL, SEATTLE [3,9]
Type of storage
Maximum achievable
storage, Mgal
Trunk storage
Hanford No. 1
Hanford No. 2
Denny Way
West Michigan
Chelan
Eighth Avenue
Harbor
Lake City
Lander
Connecticut
Dexter
King
North Michigan
Norfol k
Brandon
University
Monti ake
Ballard
Interceptor storage
upstream from
pumping stations
East Marginal
Duwamish
Interbay
Matthews Park
0
3.50
0.60
0.01
0.40
0.30
0.02
2.60
0.80
0.50
0.30
0.05
0.60
0.40
0.30
1.90
0.70
0.60
0.10
2.00
5.80
1.40
Total
22.79
Computer Control—Seattle's computer-controlled inline storage is currently
the most sophisticated system of inline storage. The computer systemy' CATAD
(Computer Augmented Treatment and Disposal), is a reactive model, rather than
a predictive model, and uses the monitoring information received toijo'perate
the system. CATAD monitors pumping stations, regulator stations, se'wage
treatment plants, and rainfall gages and uses programmed informationjsuch as
storage capacity of sewers, related overflow points (with the least;.d'airjaging
effects), and treatment plant flow capacity to provide the control operation.
The central computer console can monitor up to seven regulators, pumping
stations, and rain gages simultaneously. CATAD's central terminal facilities
include a computer, supporting hardware, control console, interceptor and data
display map, data loggers, and event recorders. • •
CATAD also has two satellite control terminals, one at the West Point .
treatment plant and one at the Renton treatment plant. These terminals;'are a
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Figure 85. Regulator and pumping station facilities, Seattle: (a) exterior
of regulator station showing motorized regulator gates, (b) typical and auto-
matic sampler at regulator stations, (c) Denny Way regulator outfall,
(d) typical pumping station, and (e) interior of regulator station.
233
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smaller version of the central control console and can monitor and control.
only one regulator or pumping station at a time. The terminal at the West
Point treatment plant uses the CAT AD computer; the terminal at the Renton
treatment plant operates from its own minicomputer.
facilities are shown in Figure 86.
Implementation and Design Considerations
Seattle's computer
The storage potential of the interceptor and trunk sewer is the basis' for
implementing an inline storage program. With limits on the amount of,
available storage in the system, only a given level of control is possible.
If higher levels of overflow control are needed, several options are
available for increasing the use of the storage capacity or providing
additional storage.
Increasing the density and type of system monitors, such as rain gages/and. ,
level sensors coupled with sensitivity analyses of the response of various
segments of the inline system, can be used to maximize the existing capacity.
Increases in control sophistication achieve similar results: local control
to remote or automatic control, or reactive. control to predictive control.
Using the existing inline storage as the basic system, implementation
considerations for additional storage could include:
• Complete or partial sewer separation
• New inline storage facilities
• Offline storage
e Source storage or BMPs •
OPERATION AND PERFORMANCE
The operation and performance of Seattle's inline storage system have*
provided a level of control consistent with planned goals to reduce the
impacts of combined sewer overflows. Systemwtde operation has evolved
through increasingly sophisticated operational modes to its present .mode of
combined supervisory/automatic computer control.. Optimization of.the •
system's performance is keyed to knowledge and experience of the system's
characteristics and response to rainfall events. , ' '
System Operation ,'• '..••. ,;;;- .
-;' -'; i .'• ',
The present mode of operation (supervisory/automatic computer control) has;
evolved from local static control of the regulators and pumping stations to
fully automatic centralized computer control. Under supervisory/automatic,
control, the CATAD system is operated continuously by supervisory per:spnnel
during the normal 40-hour workweek and is returned to automatic computer,
control the rest of the time, irrespective of storm conditions.
The system routes, optimally stores, or permits overflows, recognizing, each
segment or portion of the inline system responds differently to rainfall
(different slopes, capacities, and configurations). Depending on the size
and type of storm, or frequency of storm recurrents, the system can create
234'
-------
Figure:86. Seattle's computer facilities, (a) CATAD computer hardware,
(b) CATAD central control console and display map, (c) CATAD satellite
control[terminal at West Point sewage treatment plant.
235
-------
storage capacity in certain critical areas to relieve overburdened portions
of the system when ful1.
Preferential use of outfalls is also possible when overflows occur during
large storms. A priority system of allowing overflows in selected locations
is based on potential impacts and capacity of the receiving water to handle
the discharge (such as tidal flushing capacity).
The system can respond to various levels of rainfall. Small storms that can
be totally stored in the system are retained and pumped to the West Point
treatment plant as treatment capacity is available. For larger storms, only
a portion of the volume is retained. For these storms, first flush materials
containing heavy pollutant loads may be diverted to the interceptor for
treatment. Storms of long duration may be totally stored or have a large
portion stored, depending on downstream treatment capacity and frequency of
rainfall. System operation and performance, however, show reduced
effectiveness when large areawide storms hit the system when full from a
previous storm.
System operation during dry-weather periods provides valuable surveillance
and monitoring information in addition to the dry-weather flow control
capability.
System Performance
The inline storage system performs better on storms with small rainfall
volumes and is less effective on storms with large rainfall volumes that
occur less frequently. Both the inline storage system and the sewer
separation program have reduced combined sewer overflow volumes and frequency.
Overflow Volume and Loading Reduction—
The annual volume of combined sewage retained in the system and sent to
treatment and stormwater removed from the system through the separation
project has totaled 3,400,000 m3 (0.9 billion gal). However, the pollutant
loads contained in the stormwater from the separate sewers are still
discharged to the receiving waters. Inline storage was responsible for an
estimated reduction of 2,270,000 m3 (0.6 billion gal), or 32% of the total
annual precontrol overflow volume, as shown in Figure 87 [2]. Implementation
of additional storage or controls to integrate with the inline system are
planned for further reduction of the overflow volume.
Using the annual volume reduction (32% for inline storage) together with the
average combined sewer overflow pollutant characteristics (Table 93), an
estimated 493,000 kg (1,086,000 Ib) of suspended solids and 136,000 kg
(300,000 Ib) of BOD were prevented from entering the receiving waters and were
sent to treatment. Estimated average loading decreases of 58% ammonia and up
to 76% COD have previously been reported for the inline storage system [10].
By April 1967, program implementation had reduced the amount of raw sewage
entering Lake Washington by 98% [6]. Removal of all sewage nutrients from
the receiving waters surrounding Seattle is not possible because of the
236
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limited storage capacity of the interceptors and trunk sewers. The largest
nutrient and bacterial loadings occur during the winter months. Winter storms
may result in more than 50% of the sanitary flows being discharged at overflow
points before reaching the West Point treatment plant [11].
0= —
UJ -_
CONSTRUCTION OF
16 CATAD REGULATOR
STATIONS - METRO
PARTIAL SEPARATION OF
19,000 ACRES COMBINED SEWERS -
CITY OF SEATTLE
CONSTRUCTION OF BALLARD
UNIVERSITY & MONTLAKE
REGULATOR STATIONS - METRO
•Q-
V950
IMPLEMENTATION
'201' FACILITY
CSO CONTROLS
I
OF
PLAN
1960
1970
1980
YEAR
1990
2000
Figure 87. Reduction of combined sewer overflow
volume, Seattle [2].
Overflow;Frequency Reduction—
The frequency of overflows has been significantly reduced to approximately 40
overflows per year [3,12]. The occurrence of overflows during the summer
recreation period have been limited to five or six. The monitoring of
overflows did not begin until 1970 with the installation of CATAD components
at regulator stations. Before 1970, the exact number of overflows was
unknown.
Impact ;of Operational Mode on System Performance—
The mode of operation has a significant impact on the efficiency and
performance of inline storage. The system performance was analyzed for three
modes of operation: local dynamic, supervisory, and combined supervisory/
automatic computer control. A performance comparison, using the total storm
rainfall and the total system overflow volume per event from records collected
from the CATAD monitoring system for the period 1970 to 1976, is shown in
Figure 88. Over 5,000 individual data points were analyzed to develop the
performance relationships. The supervisory performance regression line
237
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indicates comparable efficiency to that of supervisory/automatic computer
control. This is attributed to operator knowledge, experience, and ability to
predict system response, providing a high level of control approximately
matching that of the control provided by the conservative model used in the
automatic mode.
100 -
go
so
70
"5. 60 —
50
40
30
20
10
DYNAMIC LOCAL CONTROL
I
I
COMBINED
SUPERVISORY
AND COMPUTER
CONTROL
J (
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
RAINFALL, in.
Figure 88. Comparison of inline system efficiency
under three modes of operation, Seattle.
Operation and Maintenance
System Reliability-
Seattle's inline storage system has demonstrated a high degree of
reliability in system operation with computer downtime estimated at 1 to 2%.
The mechanical system reliability exhibits the commitment to preventive
maintenance to ensure that all system components are in operation condition.
Regulators, treatment facilities, and pumping stations are monitored and have
fail-safe controls installed. These safeguards include duplication of data
sent to the computer to ensure correct information transfer and automatic
monitoring of several integral parts of each station to detect an emergency
situation or an incompatible response for the operating mode. When computer
control is lost at regulator stations, the station automatically reverts to
local control rather than cease operation altogether. Similar backup systems
are available for loss of power to a station and include auxiliary engine-
generator sets and manual overrides in the motor-operated gates.
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System Maintenance— :
System maintenance is usually one of the first tasks cut'back when there is a
lack of time or budgeted funds; especially when the facilities are used only
to control wet-weather flows a part of the time.
The Seattle system, however, is a relatively low maintenance facility because
inline storage uses existing sewer facilities. The portions of the system
that require regular maintenance include the regulator and pumping stations
and the associated computer hardware. Because of the complexity of the
computer maintenance required, Metro contracted with a private company to
maintain the computer but the supporting facilities and. telemetry
appurtenances are maintained by Metro personnel. Metro has developed
specialized teams of maintenance technicians trained in the fields of
instrumentation, engine maintenance and repair, electricity, mechanics, and
general maintenance. Maintenance responsibilities are divided into two
nonoverlapping main categories of (1) telemetry control units and equipment
contained within the unit, and (2) all .other station equipment.
Operation and Maintenance Problems--
The preventive maintenance program and CATAD's monitoring system have limited
major operation and maintenance problems by identifying and isolating
defective equipment or control losses. Metro's specialized technicians
usually solved the operation and maintenance, problems with the exception of
telemetry loss due to telephone circuit difficulties or computer-related
problems [9].
Other problems specifically relating to design or installation of the system
that can reduce the performance of the facilities or cause a loss of control
include:
• Lack of accuracy of, or improper control ranges of,
instrumentation equipment
• Improper installation or calibration of sensor or mechanical
hardware
9 Corrosion problems with metal instruments and mechanical
devices
9 Odor problems resulting from trapped stagnant water in the
system, such as the conduits from the regulators to the
overflow structures
The West Point treatment plant experiences solids handling problems during
the rainy season. During storms, the plant can receive up to a 100% increase
in suspended solids load over the average daily rainy season load of about
90,800 kg/d (200,000 Ib/d). The estimated increase in load resulting from
the captured storm flows in the inline system is between 9,100 to 13,000 kg/d
(20,000 to 30,000 Ib/d). These increased loads can stress the efficiencies
of the plant sedimentation tanks and can increase maintenance problems, such
as grit removal. Grit removal averages 4,090 kg/d (9,000 Ib/d), but during
239
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storms it can increase by up to 4 times resulting in an additional 5 nrVd
(7 yd^/d) of grit that must be handled and disposed of.
ECONOMIC AND ENVIRONMENTAL IMPACTS
Inline storage offers a most promising, cost-effective alternative for
combined sewer overflow control. Capital costs of inline storage control are
lowj and intangible cost benefits of the system from multiuse capabilities,
although not as easily identified, are also a positive consideration.
Dwindling resources, increasing costs, and greater public awareness of
environmental issues require facilities to be cost effective and have
minimum adverse impacts on the community and the environment.
Costs of Inline Storage
System implementation costs include capital and annual operation and main-
tenance. Modification of the existing system to provide areawide control
also includes combined sewer separation work by the City of Seattle,
affecting approximately 43% of the Seattle metropolitan area. Major
resource use is limited to energy consumption to operate the system and to
those resources used to construct the system.
Capital Costs--
Capital costs of the inline system included; (1) modification or construction
of the regulator and pumping stations; and (2) installation of the computer
and peripheral control equipment, telemetry and control interfacing, and
systemwide surveillance instrumentation.
Modification and construction costs of Seattle's regulator stations ranged
from approximately $200,000 to $1,200,000 [9]. A summary of the regulator
costs, including engineering, is presented in Table 97.
Total capital costs for the inline storage system and the sewer separation
project are presented in Table 98. Inline system capital costs are broken
down into costs for regulators, pumping stations, and computer facilities
(including software). Of the $924,000 (ENR 3000) engineering costs for the
computer system, approximately 20% represents computer program
development [9]. Costs of installing only monitoring instrumentation and
associated telemetry for pumping stations are estimated at $14,000 to
$19,000 per station.
Operation and Maintenance Costs--
Operation and maintenance costs of Seattle's computer-controlled inline storage
system were estimated to be approximately $440,000 per year, and include parts,
material, labor, and utilities. A major portion of this cost is utility
expenses. Typical utility charges for electricity have approached 50% of the
annual operation and maintenance cost for pumping stations. An estimate of the
operation and maintenance costs for the remote control stations and the central
computer facility, adjusted to ENR 3000, is presented in Table 99 [9].
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Table 97. SUMMARY OF REGULATOR STATION .
MODIFICATION AND CONSTRUCTION COSTS, SEATTLE0
Construction
or modifi-
Regulator station cation costsb
Hanford No. 1 $ 276,500
Hanford No. 2 701 ,700
Denny Wayc 1,132,400
West Michigan 161,700
Chelan 196,700
Eighth Avenue 177,600
Harbor 210,300
Lake City 324,200
Lander 623,900
Connecticut 390,800
Dexter 928,600
King 233,200
North Michigan 256,900
Norfolk 240,000
Brandon 207,800
University 1,036,800
Monti ake 709,600
Ballard 669,600
Total $8,478,300
a. ENR 3000.
b. Including taxes.
c. Includes two regulators in one
Engineering
costs Total costs
$ 41 ,400 $
109,100
128,000 1,
24,200
29,500
26,600
31,600
94,400
95,500
60,400
139,300 1,
37,000
38,500
36,000
31 ,000
155,500 1,
106,400
100,400
$1,284,800 $9,
station.
Table 98. TOTAL SYSTEM CAPITAL COSTS FOR
STORAGE AND SEWER SEPARATION, SEATTLE9
System
Inline storage
Regulator stations
Pumping stations modifications
Computer facilities and
interfacing
Total
Sewer separation project
a. ENR 3000.
b. Average for 19 regulators.
c. Average for 4 pumping stations.
317,900
810,800
260,400
185,900
226,200
204,200
241 ,900
418,600
719,400
451 ,200
067,900
270,200
295,400
276,000
238,000
192,300
816,000
770,000
762,300
INLINE
Average cost
per station Total cost
$513,800b $ 9
68,500C
5
$ 15
$147
,762,000
274,000
,717,000
,753,000
,810,000
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Table 99. ESTIMATE OF INLINE STORAGE
OPERATION AND MAINTENANCE COSTS, SEATTLE9 [9]
Remote stations
Operation
Maintenance
Subtotal
Labor and
fringes
$143,900
58,700
$202,700
Utilities
$101,600
$101,600
Other '
$ 2,300
24,300
$26,500
' Total
$247,700-
83,100
$330,800
Central computer
facilities
Operation
Maintenance
Subtotal
Total
$ 60,600
31 ,400
$ 92,000
$294,700
$ 6,300
$ 6,300
$107,900
$12,000b
$12.000
$38,500
$ 78,900
31 ,400
$110,000
$441,100
a. ENR 3000.
b. Approximately 75% of this cost is building rental.
Operation and maintenance costs for the first 5 months of 1978 were 'used
to estimate annual costs for various sizes of regulators, pumping stations,
and CATAD telemetry control components, as shown in Table 100. The, sizes
of these units are based on dry-weather capacities.
Table 100. ESTIMATES OF ANNUAL OPERATION AND MAINTENANCE
COSTS OF INLINE STORAGE FACILITIES BASED ON
ACTUAL 1978 FIGURES, SEATTLE9 . .. .
System component
Projected annual operation .•;•
Capacity, Mgal/db and maintenance cost, $ ; j
Regulators 2-8
Pumping stations
Small 12-15
Large 45-50
Computer systemsc
CATAD telemetry control
units and modules
Telemetry and circuits
3,100-4,000 ""t
8,200-13,400 ' •-•'-
100,000
18,000
12,000 •'• ''•'
a. ENR 3000.
b. Dry-weather flow capacity.
c. Contracted maintenance for 42 stations.
242
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(
Since these facilities also function during dry-weather periods, operation
and maintenance costs may be shared or apportioned between the dry-weather
function and the wet-weather function.
Cost Effectiveness of Inline Storage
A comparison of the cost effectiveness of Seattle's inline storage and sewer
separation project is presented in Table 101. For equivalent results of
combined sewer overflow volume reduction, inline storage is over 18 times
more cost effective than sewer separation. On a per acre basis, sewer
separation is over 6 times as expensive as inline storage. However, both
the inline system and the separation project must be viewed as a combined
areawide effort that was made to solve two different problems: overflow
control and local flooding. Each countermeasure was unique to each problem
and both resulted in overflow reduction—inline storage would have little
effect on reducing local flooding.
Table 101. COST EFFECTIVENESS OF COMBINED
SEWER OVERFLOW COUNTERMEASURES, SEATTLE^
Annual
combined
overflow Excess Overflow
volume storage volume Storage
Total capital Contributing reduction, capacity, Cost, reduction cost,
, Control measure costs, $ area, acres Hgal Mgal $/acre cost, $/Mgal $/gal
Inline 'storage
'Sewer separation
Combined inline
and separation
., projects
15,753,000
147,810,000
163,563,000
15,000
23,000
38,000
600
300b
900
22.79 1,050
1 — 6,430
4,300
26,260
492,700
131,740
0.69
a. ENR 3000.
b. Still allows stornwater volumes and pollutants to be discharged.
Unit costs,of inline storage, on an area basis, can range from Seattle's
system costs of $2,590/ha ($1,050/acre) for a highly sophisticated system,
down to as low as $170/ha ($70/acre) for simple, locally controlled dam
devices [10]. The characteristics and capacity of the existing system
would have significant impacts on the unit costs of inline storage.
Multiuse Benefits
Seattle's CATAD system has many multiuse benefits that have not been assigned
a dollar value; these include systemwide monitoring and surveillance, dry-
weather flow control, and system maintenance.
Seattle's CATAD system is operated continuously during both dry- and wet-
weather periods. Dry-weather operation of the system has provided Metro
a total system monitoring capability, and combined with surveillance of the
system facilities, an awareness of total system operation. System
monitoring also provides instant monitoring of equipment failure. The
immediate identification of an equipment failure allows repair crews to
243
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correct the problem before a station is severely damaged. The ability to
watch and control the dry-weather system, plus the potential for data base
compilation on a continuous basis, may provide benefits equal to those of
wet-weather control.
The integration of computerized inline storage with dry-weather facilities
has the potential to alter the operation and performance of the dry-weather
treatment plant. Inline storage could control flows, reducing the influence
of diurnal flows on the sewage treatment plant, and could contain emergency
spills in the system before they reach the plant or enter the receiving water.
The reduction of peak flows-could increase treatment efficiency and reduce
the effect of slug loadings to the treatment plant. This integration of
controls may also reduce the need for treatment plant expansion.
System maintenance and repairs using CATAD for flow control is indicative of
the multiuse benefits. Total system flow to a sewage treatment plant can
be significantly reduced to allow for maintenance and repair of the inter-
ceptors and the treatment plant. Maintenance and repairs of long duration
at a treatment plant can be implemented by reducing peak flows, therefore
allowing total units to be taken out of service. Repairs requiring a short
time and low flows can be accomplished by accentuating the diurnal flow
pattern.
Metro Seattle has used flow control extensively for maintenance and repair
in the sewers. Reducing the flow and virtually isolating sections of
sewer have enabled repair crews to enter the isolated sections for TV
inspection. This extensive program of flow control for TV inspection and
repair of sewers was started in February 1977 and is ongoing at the time of
this report.
Environmental Impacts
Improvement of receiving water quality has been the main objective of the
efforts by Metro and the City of Seattle. The effects of Metro's inline
storage and Seattle's sewer separation have been substantial, with
approximately 98% of the municipal sewage previously entering Lake Washington
now being intercepted, thus reducing the nutrient loading which was a main
cause of euthrophication [6]. A majority of the raw sewage previously
entering Puget Sound has also been intercepted and regulated. Coliform
levels have been reduced in Elliott Bay by 63% to 98% [13]. Impacts on the
receiving waters surrounding Seattle have been [9]:
• A 78% reduction of peak loading of nitrite-nitrate nitrogen
• A 81% reduction of peak loading of phosphate
• A 80% reduction of COD loading for total storm rainfalls
between 0.025 cm (0.01 in.) and 6.9 cm (2.72 in.)
• A 85% reduction of peak solids load
• An increase from 2.5 mg/L to 4.5 mg/L of dissolved oxygen in
the Duwamish River [13]
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A total reduction of loadings to receiving waters has resulted in a rapid
increase in transparency measurements in Elliott Bay and Lake Washington.
Transparency readings during 1970, the year Seattle's'inline storage was
activated, indicate an average increase from about 1.8 m (6 ft) to 5.5 m
(18 ft) [13].
During the period of implementation of both the inline storage system and
sewer separation, and the dry-weather collection and treatment facilities,
fish catches from surrounding waters have increased. This indicates a
general water quality improvement that is attributable, in part, to the
overall wastewater control program in Seattle [13]. Trawl catches of the
English sole population and adult salmon returns are shown in Table .102..
Table 102. SUMMARY OF FISH INCREASES
FROM. 1967 THROUGH 1970 [13]
Adult salmon returns
(Green River hatchery)
Trawl catches of English sole3
Year Chinook salmon Coho salmon 1st Ave. So. 16th Ave. So. Station KW
1967
1968
1969
1970
5,030
8,114
6,650
9,000
12,736
50,856
36,000
70,868
8.7
15.5
10.9
32.6
7.0
2.4
9.2
50.2
0.4
0.4
3.9 -
11.7
a. Average catch per trawl at sampling locations.
Socioeconomic Impacts
Public Acceptance--
Concern for aesthetics and public use has been a major force behind the
public acceptance of major projects to correct water pollution problems.
Because:;the receiving waters in the Seattle metropolitan area are used for
commercial and recreational fishing, swimming, boating, bathing beaches, and
serve commercial marinas and seaplane businesses, the deterioration of water
quality was quite noticeable. Before correction of these problems, raw
sewage and combined sewer discharges were responsible for high turbidity,
odors, unsightly fleatables, and rapid eutrophication. The public was not
only offended by these unpleasant conditions, but public beaches and swimming
areas were consistently closed for public safety reasons due to high coliform
counts.
Aesthetics and public use have also been major considerations in the
correction of the problem. Treatment plants are landscaped and have shrubs,
trees, and flower gardens to enhance their appearance, and regulator stations
in heavily populated areas have received special architectural design. The
Dexter Avenue regulator station serves as a bus stop shelter, and the Denny
Way regulator station is incorporated into a landscaped waterfront public
park, as shown in Figure 89.
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'71
Figure 89. Denny Way regulator station/landscaped park.
Impacts on Local Community-
Impacts on the local community resulting from the areawide combined sewer
overflow controls have never been quantified and ;i-n some areas are subjective.
Before the sewer separation program, many private homes and commercial
buildings suffered from periodic surcharging of sewers; This problem has not
been eliminated, but has been reduced substantially. The correction of the
situation has eliminated financial loss by the property owners and financial
loss by the city and Metro because of a reduced number of lawsuits.
V/aterfront businesses and recreation areas were affected by the deterioration
of the surrounding receiving waters, but the impacts on land values or tax
assessments have not been quantified. Considering that 80% of the city border
is water [14] and 60% of the shoreline is public parks [1], the adverse
impacts on the local community were significant enough to attack the combined
sewer overflow problem.
Displacement or inconvenience of people or facilities was limited primarily to
the construction of regulator stations and associated facilities. The
temporary inconveniences to the public would be those associated with
construction, including noise, increased air pollution, and traffic
disruption. However, the inline storage solution resulted in minimal
disruptions when compared with the potential inconvenience of sewer separation
construction in high density commercial or business areas.
Impacts on Future Facilities
Inline storage, because of its flexibility of operation, can provide a
foundation for future system expansion or addition of facilities when a higher
246
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level of combined sewer overflow control is needed. Using or expanding inline
storage offers one of the most cost-effective control options available.
Costs of offline storage, for example, can range from $0.16/L ($0.62/gal) of
storage capacity for simple open earth structures to over $1.98/L ($7.50/gal)
for complex concrete storage/sedimentation facilities [10], when compared with
less than $0.18/L ($0.69/gal) for inline storage.
The ability to modify the operation of the system is also valuable to any
facility because future needs are not always predictable. Seattle's computer
augmented inline storage has used some aspects of its potential modified
operations to allow extensive inline repair work to be completed with minimal
system disruption. Long-term modifications can be made at minimal expense by
reprogramming the computer for permanent changes in the system. For example,
a large increase in population could require reprogramming of the computer to
reflect decreased detention times due to increased flow.
The Seattle 201 study includes plans to increase storage capacity by using
offline storage to meet the desired reduction of overflows to 10 per year [15,
16]. The offline storage units would be adaptable to the present control
system, and CATAD can be modified to control offline storage as -well as inline
storage. Projected costs and annual overflow volume reductions for future
system expansions are shown in Figure 90.
400
300 —
200 -
CO
o
100 -
220
25 50 75
ANNUAL VOLUME REDUCTION
25 Mgal
100 %
Figure 90. Estimated costs and overflow volume
reduction for future system expansion, Seattle [16],
247
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Section 13
CONTROL OF COMBINED SEWER OVERFLOWS USING
STORAGE/SEDIMENTATION - SAGINAW, MICHIGAN
Controlling combined sewer overflows from metropolitan areas can be achieved
by a combination of storage and treatment processes that take advantage of
the existing system characteristics and capacities and require relatively
little land area for construction of the treatment system. Storage and
treatment by sedimentation both reduce the volume of overflow to receiving
waters and provide treatment approaching primary levels for all overflows.
Saginaw's Hancock Street facilities are an example of the use of inline
storage and offline storage/treatment as a part of a citywide plan to
eliminate uncontrolled combined sewer overflows to the Saginaw River.
The Hancock Street facilities consist of an integrated system of inline
storage, using existing interceptor capacity controlled by modified regulator
stations; a flood protection pumping station; and a storage/treatment basin
capable of treating and disinfecting all overflows. The basin exhibits a
high suspended solids concentration removal efficiency, over 70%, on the
treated overflows. Removals of BOD and heavy metals are also high, ranging
between 40 and 60%. The storage/treatment basin design is similar to that of
New York's Spring Creek [1] and Boston's Cottage Farm [2] facilities.
The storage/treatment basin was designed for multiuse, with the construction
of a two-level parking garage over the basin. This resulted in no loss of
land area for community use and can be considered a socioeconomic benefit to
the older central commercial district adjacent to the facilities.
A limited monitoring program was conducted at the storage/treatment
facilities as a part of this case history assessment. The program afforded
an opportunity to collect much needed data on the operation of this type of
control system. The results were used to characterize the combined flow
entering the basin, to evaluate the effectiveness of the basin in terms of
hydraulic and pollutant removal performance, and to estimate the effect of
the basin on loads discharged to the Saginaw River.
CONTROL SYSTEM DEVELOPMENT AND DESCRIPTION
Saginaw is typical of most medium-sized cities with combined sewer systems
that experience periodic discharge of combined sewage to nearby receiving
waters. Concern over water quality in the Saginaw River has prompted the
development of a control plan to reduce or eliminate the combined sewage
loads originating from the developed urban areas. The plan includes an
248
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integrated combination of the use of existing facilities and structural
solutions to achieve an optimized control system.
Area Characteristics
Saginaw is in the eastern part of central Michigan north of Detroit, as shown
in Figure 91. The Saginaw River is the principal receiving water flowing
through the middle of the urban area to Lake Huron. The topography of the
area is flat, and the elevation is approximately 180 m (600 ft).
MILWAUKEE
CH ICAGO
ILLINOIS \ IND I ANA\
1 \
OHIO
Figure 91. Saginaw, Michigan.
The area, which is surrounded by the Great Lakes, has a quasi-marine
environment. Temperatures stay warmer later in the year and cooler during
the spring because of the influence of the large water masses.
Saginaw1 s normal annual precipitation is about 70 cm (28.5 fn.), over 50% of
which falls during the summer between May and September. The monthly
distribution of rainfall is shown in Figure 92. The summer storms occur as
showers and thunderstorms. Most of the winter precipitation is snow, which
accumulates until the spring thaw. Beginning in March or April, melting snow
combined with rainfall can produce high overflow volumes and increased
discharge of pollutant loads to the river.
Most of the storms are of low average intensity. Over 90% of the time, the
average intensity of rainfall that produces runoff is less than 0.25 cm/h
(0.10 in./h), as shown in Figure 93. Approximately 525 hours of rainfall
producing runoff occurs annually [3].
249
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3 r
Ul
ce
o
3E
SUMMER PERIOD WITH OVER
50% OF THE PRECIPITATION
H J j A S
MONTH
N 0
Figure 92. Normal distribution of annual precipitation, Saginaw.
1.0
UJ
t—
z
0.10
0.01
7
7
10 20 30 4050 60 70 80 90
gg gg.g 99.99
PERCENT OF RAINFALL TIME THAT RAINFALL IS EQUAL
TO OR LESS THAN STATED INTENSITY
Figure 93. Probability of occurrence of rainfall producing runoff [3],
250
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The area In Saglnaw tributary to the combined sewer system is approximately
4,100 ha (10,200 acres), most of which is fully developed commercial and
residential land uses. The average imperviousness of the area is between 45
and 50% [4].
Problem Assessment
During periods of rainfall, combined sewage overflowed the system at 34
regulator overflow points along the east and west banks of the Saginaw River.
Overflows occurred when the flow in the combined collection system was
greater than two to three times the dry-weather flowrate. It was estimated
that approximately 60 overflows occurred per year [3].
Impacts on the Saginaw River--
Impacts on the Saginaw River from combined sewer overflows included dissolved
oxygen depletion in the downstream segments of the river and increased
bacteria levels.
The Stormwater Management Model (SWMM) was used as a design tool to project
the expected suspended solids and BOD5 loads to the river resulting from a
storm of 4.8 cm (1.9 in.) of rainfall. This storm represents a 1 year storm
frequency. The model was calibrated on a small, 243 ha (600 acre), subarea
of the city and was then used to project the loading from a larger subarea.
Loadings from the city's 4,100 ha (10,200 acre) total area were interpolated
from these results. The citywide 1 year storm loadings were estimated at
18,200 kg (40,000 Ib) of BOD,- and 182,000 kg (400,000 Ib) of suspended solids
[4]. 5
The estimated impact of these loads, together with the continuous point
discharge of treated sewage effluent, could result in a dissolved oxygen
level in the.,river of less than 4 mg/L at a minimum daily river flow of 17.4
m3/s (615 ft /s). At the minimum monthly river flow of 24.8 m3/s (874
ft3/s), the resulting river dissolved oxygen was estimated at between 5 and 6
mg/L [4].
Public health impacts to the river could also be significant because of the
high level of microorganisms associated with combined sewer overflows.
Suspended solids and floatable material also create visual problems and
accentuate public awareness of the health of the river.
Characterization of Combined Sewage--
The results of the limited monitoring program conducted at the Hancock Street
storage/treatment facilities were used to characterize the influent combined
sewage. It was assumed that this influent combined flow was representative
of the overflow quality.
The average suspended solids concentration of the combined flow during the
summer of 1978 (May through September) was about 400 mg/L, arid about 110 mg/L
for BOD,-. More important, however, was the change in pollutant concentration
3
251
-------
as a function of time since the beginning of an overflow. Transient
suspended solids and BOD concentrations of combined flow are shown in Figures
94 and 95. The plotted values represent discrete sample data. Confidence
limits of 10 and 90% were determined to show the range of variability and to
show the trend toward decreasing concentration as the overflow progresses.
Suspended solids concentration extremes ranged from a 10% value of 175 mg/L
to 1,050 mg/L at 90%' during the first 0.5 hour of flow. The similar range
for BODg was 40 mg/L and 355 mg/L.
2000 i—
1750
1500
1250
in
a.
ta
1000
750
500
250
O
O
-8-
DISCRETE VALUES WITHIN
TIME PERIOD
O
O
n
O
©
•90 PERCENTILE
GEOMETRIC MEAN VALUES BY TIME PERIOD
AVERAGE OF COMPOSITE
SAMPLES
10 PERCENTILE
0.5 1.0 1.5
TIME SINCE START OF OVERFLOW, h
2.0
Figure 94.
Characterization of combined overflow suspended solids,
Hancock Street storage/treatment facilities.
252
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in
a
o
ca
500 I—
450 —
400
350
300
250
200
150
100
50
8
o
o
o
o
o
I- o
o
o
©
©
DISCRETE VALUES WITHIN
TIME PERIOD
90 PERCENTILE
GEOMETRIC MEAN VALUES BY TIME PERIOD
AVERAGE OF COMPOSITE
SAMPLES
10 PERCENTILE
0.5 1.0 1.5
TIME SINCE START OF OVERFLOW, h
Figure 95. Characterization of combined overflow BODg,
Hancock Street storage/treatment facilities.
2.0
253
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Other characteristics of the combined sewage are presented in Table 103 for
nutrients and heavy metals. These values represent composite samples of the
inflow to the storage basin. Approximately 53% of the.suspended solids were
inorganic. Fecal coliform values ranged from 1.6 x 10/100 ml to over 6.0 x
10//100 ml. The average pH of the flow was 7.2 and ranged from 6.7 to 7.7.
Table 103. QUALITY CHARACTERISTICS OF COMBINED
SEWER FLOWS ENTERING THE HANCOCK
STORAGE/TREATMENT FACILITIES
Constituent
Total Total Total
Parameter
Average3
90 Percent! le
10 Percent!" le
mg/L
392
1,028
149
mg/L
5.9
9.9
3.5
mg/L
2.8
5.1
1.5
pg/L
118
206
67
pg/L
32
67
15
pg/L
602
1,595
227
a. Geometric average of 7 to 12 samples, each.
Pollutant Loadings--
Because of the configuration of the regulators and the available inline
storage of the collection system, an estimated 50% of the combined sewer load
was treated at the dry-weather treatment facilities. The other half was
discharged to the river during overflow events. Suspended solids loading to
the river has been estimated at about 1.8 to 2.3 million kg (4 to 5 million
Ib). The BOD5 loading was estimated at approximately one-fourth of the
suspended solids load.
During the monitoring program, an estimated 200,000 m3 (52 Mgal) of combined
flow was pumped to the storage facility. Assuming this volume represents
that which would have overflowed, a suspended solids load of 82,000 kg
(180,000 Ib) and a BOD5 load of 23,000 kg (50,000 Ib) would have been
discharged to the river. These loads correspond to a measured total
rainfall, for the storms monitored, of 15.8 cm (6.2 in.).
Recommended Plan to Control Combined Sewer Overflows
A plan to control combined sewer overflows was developed with the following
objectives: maximizing the use of the existing system, protecting receiving
water quality, treating and disinfecting all overflows, and preserving the
function of the existing flood protection system. The plan included modifying
the existing regulator structures to optimize the use of inline storage
capacity and constructing seven storage/treatment basins.
254
-------
The plan was developed from modeling, using the Stormwater Management Model
(SWMM), and statistical evaluation of rainfall/frequency data to evaluate the
most cost-effective combination of storage and treatment. The sizing of
storage to contain the first 1.3 cm (0.5 in.) of runoff was determined to be
sufficient for achieving the following goals of the planned system:
• Reduce the 60 raw overflows to 15 treated overflows.
• Provide the equivalent of primary treatment for the 1 year storm
(design event) in the storage/sedimentation basins.
« Provide the equivalent of secondary treatment on an annual basis by
capturing and treating the combined sewage overflows, i.e., achieve
approximately an 80% suspended solids and BOD mass removal.
3
The total system storage to be provided was estimated at about 140,000 m (37
Mgal) and was split about equally between inline and offline storage. The
system was to operate automatically during a storm event, with no major
mechanical equipment in the basins.
As a part of the overall plan, the Hancock Street storage/treatment facilities
were constructed. None of the remaining six basins have been constructed at
present. A schematic of the existing facilities (34 regulators and 5 flood
pumping stations), the proposed combined sewer treatment facilities, and the
Hancock Street facilities are shown in Figure 96. Based on the large
percentage of small volume rainfall/runoff events, maximum use of the inline
system was considered an important criterion to reduce the offline treatment
capacity required downstream.
Hancock Street Storage/Treatment Facilities
The Hancock Street facilities serve about 650 ha (1,600 acres) of the combined
sewered area. The control facilities are shown in Figure 97.
Inline Storage--
The inline storage potential of the combined sewers was increased by modifying
the existing float-operated regulators in the system. Before modification,
the combined sewers could store a significant amount of excess flows, but due
to the arrangement of the floats, the regulators were imbalanced.
Modifications were made to more accurately proportion the maximum dry-weather
flow capacity, which also improved the use ,of inline storage capacity for
storing combined sewage.
Modifications to the regulators included replacing the tide gates at the
outfall with motor-operated sluice gates controlled by an ultrasonic level
sensing device. A regulator is shown in Figure 98. Before modification, only
dry-weather flows and some wet-weather flows were diverted to the interceptor
during normal river stages; now, all flows are detained and diverted to the
interceptor. The estimated total developable inline storage volume in the
Hancock Street system is about 16,100 m3 (4.26 Mgal); however, only about 25%
of about 4,000 m3 (1-07 Mgal) has been developed [5]. Most of this is
attributed to the modified regulators.
255
-------
WASTEWATER
TREATMENT
PLANT
CONTRIBUTING DRAINAGE
AREA OUTSIDE OF
CITY LIMITS
CONTROL
VALVE
WEST SIDE
INTERCEPTOR
CITY LIMITS
HANCOCK ST.
STORAGE/
'TREATMENT'
' ffASIN-
EAST SIDE
INTERCEPTOR
COMBINED SEWER
AND OVERFLOW
(TYPICAL)
HANCOCK STREET
D.RAIMAGE AREA
EXISTING FLOOD CONTROL
PUMPING STATION
A EXISTING REGULATOR
PROPOSED STORAGE/
TREATMENT FACILITIES
Figure 96. Schematic of the proposed combined sewer overflow
system and the Hancock Street facilities, Saginaw.
256
-------
HANCOCK STREET
.STORAGE/TREATMENT
BASIN
REGULATORS
(It REGULATORS IN
THE HANCOCK SYSTEM)
INTERCEPTOR
Figure 97. Components of the Hancock Street combined
sewer overflow control facilities.
,;.*
* +} w <. * XV.,
Figure 98. Regulator station with
motor-operated sluice gate.
257
-------
Flood Pumping Station--
During storms, combined flows diverted to the interceptor by the regulators
activate the pumping station as the interceptor level increases. Sluice gates
between the interceptor and the pumping station wet well open automatically
and pumping is sequentially controlled by sensing the water elevations in the
wet well. All flows to the storage/treatment basin are pumped. The pumping
station facilities are shown in Figure 99.
Figure 99. Hancock Street flood control
combined sewage pumping station.
258
and
-------
During extreme river flooding, the system will revert back to operation as a
flood control pumping station, with flows to the basin being diverted to the
river once the system has been filled to capacity.
Disinfection facilities, flushing water pumps, sampling equipment, a new
control system, and personnel facilities were added to the existing flood
pumping station when it was integrated into the combined sewage treatment
system.
Storage/Treatment Facilities--
Combined flows pumped to the storage/treatment facilities sequentially fill a
series of paired basins. The facilities operate under two types of storm flow
conditions:
1. For certain low volume storms, the flow will be captured and totally
stored, with subsequent release to the interceptor as capacity
becomes available.
2. For storms producing flow that exceeds the basin storage capacity,
the system will store, treat the combined flow by sedimentation, and
disinfect the overflow before discharge to the river.
A flow schematic of the storage/treatment basin operation is shown in Figure
100. Flow is pumped to the basins through an influent conduit and is first
distributed to Bay 2. When Bay 2 is approximately 60% full, Bay 1 will start
filling. Sequential filling and isolation of Bay 2 allows the first flush
loads to be captured in one bay and reduces the potential maintenance and
cleanup operations for small storms. Floating oil and scum baffles in Bays 1
and 2 are provided to trap floatable material.
Depending on the magnitude of the storm volume, flow enters a transfer channel
for distribution to the last two bays, which fill sequentially. When the bays
are full, flow will pass through the entire basin system, and the basin will
act as a settling tank. Before discharge to the effluent channel, the flows
pass up through No. 3 mesh screens located between the effluent weirs and the
effluent scum baffle to capture any floatable and suspended material not
removed in the settling bays. The Hancock Street storage/treatment facilities
are shown in Figure 101.
Disinfection Facilities--
The combined flows entering the basin are disinfected at the pumping station
using sodium hypochlorite. The hypochlorite feed system is activated when
the main pumps are activated and is paced by the flow into the basin,
measured by the number of pumps in operation at the time. The
storage/treatment basin provides the chlorine contact time before discharge
of the overflow to the river.
259
-------
FLOOD
PROTECTION
BY-PASS
MESH
FINAL
SCREENS
BAY 4
BAY 3
BAY 2
(FIRST FLUSH
RETENTION)
BAY 1
• FLOATING OIL
AND SCUM BOOM
TRANSFER
CHANNEL
Figure 100. Flow schematic of the Hancock Street
storage/treatment facilities.
The system takes dilution water from the river and injects it into the
influent conduit to the basin. The hypochlorite feed pumps discharge into the
dilution water pump discharge line. The system also can add hypochlorite
directly to the storage basin through the flushing system headers.
The hypochlorite is purchased commercially, diluted to about 5% solution, and
stored in two 60,000 L (16,000 gal) storage tanks. Dilution of the
hypochlorite to 5% prolongs the effective life of the solution. The
disinfection facilities are shown in Figure 102.
Dewatering and Cleaning--
After a storm, the basin is dewatered back to the interceptor, as capacity at
the dry-weather treatment plant becomes available. The bays are dewatered one
at a time, through a separate drain system.
The bottoms of the bays have sloped floors, 1:12, to a central flushing
channel, which is sloped the length of the bays. A flushing system of high-
pressure, high-volume flushing nozzles, is located on the walls of the bays.
In addition, four high-pressure, manually operated monitor nozzles are mounted
260
-------
Figure 101. Hancock Street storage/treatment facilities: (a) influent
orifice in Bay 1; (b) storage/treatment basin, Bay 3; (c) final
effluent screens; and (d) effluent screens and effluent channel control gates.
261
-------
Figure 102. Hypochlorite feed system: (a) two 16,000 gal
hypochlorite tanks; (b) and (c) hypochl ori te pumping
facilities using positive displacement pumps.
262
-------
at points along each bay for further washing of the walls and cleanup of the
bottoms. Flushing water is drawn from the river and pumped to the basin.
The basin, after storing several storms, was exceptionally clean, indicating
that the flushing system works effectively. The flushing system components
are shown in Figure 103.
Design Criteria
The design parameters of the Hancock Street system are presented in Table 104.
Parameters include design flows, storage and treatment system sizes and rates,
and disinfection system criteria.
Table 104. HANCOCK STREET STORAGE/TREATMENT DESIGN PARAMETERS [5]
Parameter
Value
Design flows
Design storm frequency, yr
Total accumulated rainfall, in.
Peak design flow, Mgal/d
Peak hydraulic capacity (eight 65 Mgal/d pumps), Mgal/d
Storage/treatment system
Inline storage
Volume, Mgal
Median volumetric displacement time, min
Storage/sedimentation basin (4 bays)
Volume, Mgala
Length (per bay), ft
width (per bay), ft.
Depth (average), ft
Floor side slope, in./ft
Central drain slope, in./ft ?
Surface area (total of all 4 bays), ft
Detention time at peak design flow, min
Median volumetric displacement time, min „
Peak hydraulic loading rate at peak design flow, gal/ft -d
Disinfection
Chemical
Storage tank volume (2'at 16,000), gal
Dosage rate, available chlorine at 4.5%
Design peak flow, mg/L
Range, mg/L
Design feed capacity, Ib Cl/h
1
1.9
323
520
4.3
30
3.5
206.3
51.8
10
1.0
0.2
42,700
15
23
8,100
Liquid sodium hypochlorite at 5%
32,000
12
6-25
1,200
a. Includes influent and effluent conduit storage volume.
b. At high river flows, depth in basin may increase, therefore the storage volume would increase.
c. Projected hypochlorite strength after 90 days of storage.
263
-------
Figure 103. Hancock Street flushing system: (a) high-pressure flushing
water pumps; (b) control valve on pump discharge; (c) and (d) flushing
water manifold and piping; and (e) manual monitor nozzle station in the basin.
264
-------
The Hancock Street basins are covered and special design considerations were
used. These included basin access, placement of equipment in the basin, and
ventilation requirements.
Personnel access is an important safety feature in basins that are covered,
buried, or have enclosed areas subject to potential flooding. In the Hancock
Street basin, the maximum distance between any two access points is 37 m (120
ft). In addition, access hatches are provided for the removal of equipment,
such as motor-operated gates and screens. A system of walkways is provided
for routine maintenance and inspection and access is also provided for the
removal of grit accumulations that cannot be flushed out with the flushing
system.
Mechanical and electrical equipment are placed at levels high enough so that
during surcharged or flood conditions the equipment will not be damaged by
water.
A ventilation system capable of 12 complete air changes per hour on a
continuous basis is also provided. Charcoal filters deodorize the exhaust
air from the basin.
PERFORMANCE AND OPERATION
The performance of the Hancock Street storage/treatment facilities is
characterized using a limited number of collected data from the monitoring
program. Although this information is not considered statistically
significant, it does indicate the level of effectiveness of both the
storage/treatment facilities and the integration of, these facilities with dry-
weather treatment.
The pumping and the disinfection systems are key mechanical facilities in the
overall operation of the basin. Initial startup problems and operational
problems with these systems have been experienced.
Storage/Treatment Performance
The evaluation of the Hancock Street storage/treatment facilities'
performance includes analysis of:
• Overflow frequency and volume reduction
• Pollutant concentration reduction
• Pollutant load reduction
These performance evaluations should be weighed against the projected
citywide combined sewage control system performance and the expected
effectiveness of the Hancock Street facilities.
A 75% overflow frequency reduction was projected for the citywide system of
storage/treatment facilities, storing an estimated 1.27 cm (0.5 in.) of
runoff. The overall pollutant load removal by these facilities has been
estimated at 92% for suspended solids and 90% for BOD [4].
265
-------
The projected performance of the Hancock Street treatment efficiency at the
design storm of 4.83 cm (1.9 in.) of rainfall was 49% for suspended solids
and 32% for BOD. From analysis of rainfall and storm occurrences, an
overflow frequency reduction of about 70% would be expected during the summer
storm period [3].
Overflow Frequency and Volume Reduction—
During the summer monitoring period, 3 of the 11 storm events exceeded the
Hancock Street storage capacity and resulted in overflow. The overflow
frequency reduction effectiveness over this period was 73%.
The estimated total volume pumped to the storage/treatment basin during the
11 storm events was about 200 ML (52 Mgal), with about 79 ML (21 Mgal) of the
total being treated and discharged to the river. Most of the overflow
volume, 50 ML (13 Mgal), occurred during one storm event with a total of 3.43
cm (1.35 in.) of rainfall. The other two storm events causing overflow were
less severe, with overflows of 6 and 23 ML (1.6 and 6.1 Mgal), as shown in
Table 105.
Table 105. SUMMARY OF STORM AND BASIN ACTIVATION EVENTS
DURING THE HANCOCK STREET MONITORING PERIOD
Date
Average Average
precipitation, intensity,
in.c
in./hb
Volume Overflow volume
pumped to treated and
basin, Mgalc discharged, Mgal
5/12/78 0.21
5/30/78 0.68
6/12/78 0.44
7/21/78 0.42
8/16/78 0.50
8/19/78 0.52
9/12/78 0.48
9/13/78 1.35
9/17/78 0.32
9/20/78 0.90
9/27/78 0.39
Total 6.21
a. Average of two rain
0.11 2.62
0.14 4.19
0.19 3.60
0.15 1.08
0.29 3.70
0.15 5.12
0.11 3.68
0.17 16.79
0.13 0.90
0.28 9.66
0.11 0.89
52.23
gage measurements.
0
0
0
0
0
1.60
0
13.27
0
6.14
0
21.01
b. Average rainfall divided by average duration.
c. Starting on 8/19/78
logs. All previous
observations.
, volumes determined from
values were estimated by
pump operation
water level
266
-------
Assuming that without the storage facilties all pumped flows to the basin
would have overflowed, the effectiveness of the basin in reducing overflow
volume is 60%. This overflow reduction is for the basin only and does not
include the inline storage effectiveness. Using an assumed inline system
volume reduction effectiveness of 50%, the effectiveness of the overall
system, inline and offline storage/treatment, may approach 80%.
Pollutant Concentration Reduction--
When the capacity of the storage/treatment basin is exceeded, the system
treats the overflows by sedimentation before discharge to the river. During
the three overflow events, the basin effectiveness averaged 73% for suspended
solids and 54% for BOD. The flows to the basin produced a large range of
average hydraulic overflow rates~in?the bays; fronurates approaching a3 2
secondary clarifier rate, 1.65 m /m -h (970 gal/ft -d), to over 3.85 nr/m -h
(2,270 gal/ft2-d). The influent concentrations and individual storm treatment
efficiencies also varied, as summarized in Table 106.
Table 106. PERFORMANCE OF THE HANCOCK STREET
SEDIMENTATION BASIN
Pollutant removal3
Suspended solids
BOD
Avg nyaraunc
Storm overflow rate,
date
8/19/78
9/13/78
9/20/78
gal/ft2-d
970
1,235
2,270
peaK nyaraunc
overflow rate,
gal/ft2-d
1,500
6,500
6,300
Influent,
mg/L
896
149
420
Effluent,
mg/L
62
27
232
Removal
%
93
82
45
Influent,
mg/L
126
62
42
Effluent,
mg/L
40
20
31
Removal ,
%
68
68
26
Average
73
54
a. From flow-weighted composite samples of influent and effluent.
Heavy metals (including lead, chromium, and zinc), COD, volatile suspended
solids, total phosphorus, and total nitrogen were also analyzed for both the
influent and effluent flows from the basin. The removal efficiency of the
basin for these pollutants is shown in Table 107.
Table 107. SUMMARY OF HEAVY METAL AND OTHER
POLLUTANT REMOVALS FROM THE
HANCOCK STREET SEDIMENTATION BASIN
Removal, %
Storm —• ' ~
date Lead Chromium Zinc COO Volatile SS Total phosphorus Total nitrogen
8/19/78
9/13/78
9/20/78
Average .
75
70
7
51
80
70
56
69
83
40
10
44
90
47
22
53
91
79
48
73
50
14
41
35
—
0
33
--
267
-------
Pollutant Load Reduction--
Estimates of the pollutant load reduction by the Hancock Street storage/
treatment basin alone, and more importantly, the integrated Hancock Street dry-
weather treatment facilities were made using the pumped flow and average
characterization from the 11 storm events. The combined sewage
storage/treatment step is an important part of the treatment of combined
flows; however, the dry-weather treatment process must also be considered.
Both the stored volumes from small storms and the stored volumes plus solids
from the treated overflow from large storms are drained back to the dry-
weather plant as capacity becomes available.
Total loads to the Hancock Street storage/treatment basin of 81,250 kg
(179,000 Ib) suspended solids and 21,800 kg (48,000 lb) BOD were estimated
from (1) the total pumped volume to the basin, and (2) the average flow-
weighted quality of the influent combined flows (410 mg/L for suspended solids
and 110 mg/L for BOD).
The overflow loads discharged to the river from the storage/sedimentation
basin were computed using (1) the influent combined flow quality; (2) the
average treatment effectiveness of the sedimentation basins (73% for suspended
solids and 54% for BOD); and (3) the volume of overflow exceeding the basin
storage capacity.
The efficiency of the dry-weather treatment plant was assumed to be 90% for
both suspended solids and BOD.
The effectiveness of the Hancock Street facilities alone was 89% suspended
solids load reduction and 81% BOD load reduction from the influent flows.
However, with the transport of the solids to the dry-weather treatment plant
and the assumed treatment rate, the effectiveness of the total integrated
system drops to 80% for suspended solids and 73% for BOD, as shown in Figure
104. Total loads to the river from the storm events are 16,050 kg (35,360
lb) suspended solids and 5,860 kg (12,900 Ib) BOD.
Operation
Although the Hancock Street system is relatively free of mechanical equipment
and is a self-activating facility requiring little operator attention, the
mechanical systems used (pumping and disinfection) are critical to the
proper operation of the facility. Several operational problems have
been identified as a reference for future design applications.
Pumping—
Initial startup problems were encountered with the main pumping controls and
with the inlet sluice gates to the pumping station, and the facilities failed
to properly come online for the first four or five storms. The pumping
station inlet sluice gates were designed to open when the interceptor level
rose to a predetermined level and to close at a lower level. Interceptor
level was sensed by a sonic level sensor, which did not provide reliable
level control and caused erratic opening and closing of the inlet sluice
gates. To correct these problems, the controls were modified to provide
268
-------
automatic opening of the inlet sluice gates only. The inlet gates are
manually closed after a storm event. Although a series of minor
modifications were made to the sonic level sensor to improve performance,
confidence in these particular devices is low.
179.000 Ib
143,640 Ib
48.000 Ib
35,100 Ib
i
HANCOCK
STREET
STORAGE/
TREATMENT
>
159.600 Ib /«
" \TREA1
OVERFLOW:
19.400 Ib
r >
IY- \
rHER '
fHENT/
EFFLU
15,96
r
SAGtNAW Rl VER
HANCOCK
STREET
STORAGE/
TREATMENT
EFFLUENT:
3,900 Ib
SAG'INAW Rl VER
HANCOCK STREET EFF I CIENCY= 89«
INTEGRATED SYSTEM EFFICIENCY = 80«
A. SUSPENDED SOLIDS MASS BALANCE
HANCOCK STREET EFFICIENCY = 81 %
INTEGRATED SYSTEM EFFICIENCY=73%
B. BOD MASS BALANCE
Figure 104. Schematic of pollutant load reductions and
process elements of the Hancock Street wet- and
dry-weather integrated systems.
A sequencing pumping control system was selected because the existing pumping
station has no appreciable storage volume, and the existing axial flow pumps
were not suitable for variable speed control. On a rising wet well level, a
pressure switch would start a pump and at the same time start a timed cycle.
If the wet well level did not drop before the timed cycle ended, a second
pump would start and the timed cycle would restart. On falling level, a
pressure switch would stop the first pump that had started and also start a
timed cycle that would stop the next pump if the liquid level remained at or
below the low level. In addition, if the wet well level continued to fall
after the first pump stopped, a second low low level pressure switch would
immediately stop the next pump, without waiting for the first timed cycle to
end, and start a timed cycle of its own to shut down succeeding pumps if the
liquid level remained at or below the low low level. An emergency shutdown
of the pumps was also provided to stop all operating pumps when there was no
longer water in the wet well to be pumped.
When the station was placed in service, it was found that the wrong type of
time delay relay had been furnished. Instead of starting (or stopping) a
pump and starting a timed cycle, the controls would start the timed cycle and
269
-------
then start (or stop) the pump. This proved satisfactory whenever only one
pump was required for operation. However, serious problems occurred when
several pumps were required because all operating pumps would be shut down on
the emergency low level shutdown pressure switch instead of the normal
shutdown controls. This method of operation caused problems with the power
supply system and resulted in short cycling of the pump. During a 16 hour
period of pumping station operation, 46 pump start/stop cycles were recorded.
Design of pumping facilities, where rapidly changing flow and water levels
are unavoidable, should consider methods for achieving continuous pumping.
This may be accomplished by using one variable speed/flow pump in sequence
with constant flow pumps or by providing pump discharge control
instrumentation.
Disinfection--
Several problems were encountered with the hypochlorite feed system.
Progressive cavity pumps with hydraulic variable speed drives were used to
pump the hypochlorite. Problems experienced with these pumps included:
• Gravity leakage through the pumps when the pumps were not
operating.
• Extremely high starting torques.
When the disinfection system was placed in service, it was found there was a
gravity flow of 3.8 to 7.6 L/min (1 to 2 gal/min) when the pumps were off,
even though the manufacturer indicated that there should be none. A motor-
operated valve was placed on the feedline to prevent gravity flow.
Because of the infrequent use of these pumps, it was found that a "set" would
develop between the pump stator and rotor, resulting in extremely high
starting torques. Also, the manufacturer felt that hypochlorite solution
caused the stator material to slightly swell, aggravating the set between
stator and rotor. This extreme starting torque would trip a torque overload
switch, preventing operation of the pump. The torque overload clutches were
locked-in to permit operation of the hypochlorite pumps.
Special efforts should be taken to ensure that operation of critical
treatment systems is accounted for and, where necessary, alternative systems
be provided.
Sluice Gates—
Many problems were experienced with the automatic operation of sluice gates.
Many of these problems can be attributed to the erratic and premature
operation of the ultrasonic level sensors and to faulty manufacture and
installation. Several of the gate operator stem nuts stripped during
operation and faulty machining was found on many of the stems.
Because of existing conditions, several of the sluice gates had nonrising
stems with the stem threads submerged in the stormwater. Risinq stem sluice
270
-------
gates should be used, if possible, so that the stem threads will not be
submerged.
ECONOMIC AND ENVIRONMENTAL IMPACTS
Assessment of the impacts of the Hancock Street storage/treatment system
shows that the combination of inline storage and offline storage/treatment is
a cost-effective solution to reduce the frequency, volume, and the pollutant
load of combined sewer overflows. Comparing the unit costs of inline storage
and the storage/treatment basin shows that use of the excess sewer system
capacity is a most attractive and economical component of the control system.
However, the usable volume is limited by the physical characteristics of the
sewer system.
The use of the dry-weather treatment facilities as an integrated process
component of the wet-weather system for treatment of the stored flows must
also be considered a part of the implemented control strategy. At Saginaw,
additional capacity at the dry-weather plant has been provided to handle the
peak loads from the wet-weather system. Additional capacity in the sludge
dewatering system has also been provided in anticipation of the solids from
combined sewer overflow events. In designing a systems approach using a dry-
weather treatment plant, potential operating problems can be avoided and
designed for by planning and evaluating the expected increased loads.
Economic Impacts
Costs of the inline storage and the storage/treatment basin are evaluated
individually, on a unit process basis, and together as a systems approach for
controlling combined sewer overflows. Annual operation and maintenance costs
are estimated for the combined sewer overflow facilities and the apportioned
cost of the dry-weather plant operations used to treat the stored combined
sewage volumes.
Construction Costs--
The total construction cost of the Hancock Street storage/treatment system is
$7,280,000 (ENR 3000). This cost was developed from the actual construction
cost and includes both the inline storage system modifications (regulator
modifications) and the storage/treatment facilities. A two-story parking
garage was included as a part of the basin construction at an additional cost
of $490,000.
The estimated cost of the storage/treatment facilities is $6,910,000, and
includes modifications to the existing flood pumping station, the storage
basin, and appurtenant facilities. Based on a peak treatment flowrate of 14
m3/s (323 Mgal/d), the treatment cost is approximately $490,000/m3-d
($21,400/Mgal-d).
Costs based on the storage capacity of the system are shown in Table 108.
The inline storage system costs (about $370,000) were for modification of the
existing regulators to increase the usable storage capacity of the combined
271
-------
sewers. The storage capacity attributable to the regulator modification cost
is about 4,000 m3 (1.07 Mgal).
Table 108. SUMMARY OF COSTS OF THE HANCOCK STREET
STORAGE/TREATMENT SYSTEM3
Storage Area Storage Cost per
capacity, served, Construction cost, acre served,
Component Mgal acres cost, $ $/gal $/acre
Storage/treatment
facilities
Inline storage
Total system
3.52
1.07
4.59
1,600
1,600
1,600
6,910,000
370,000
7,280,000
1.96
0.35
1.58
4,300
230
4,530
a. ENR 3000.
The unit cost of inline storage is about six times less than the unit cost of
constructing the storage/treatment basin. Static inline control, however,
can only be developed to "safe" maximum limits of available storage capacity.
Further control of and use of the available storage capacity may be achieved
through automatic or dynamic control of the regulators, to use the storage
more efficiently, but this increases costs.
Use of inline and offline storage together can achieve higher levels of
control at lower costs than offline storage alone. A comparison of the total
Hancock Street system unit costs to those of storage/treatment alone
indicates about a 20% cost saving.
Annual Operation and Maintenance Costs--
The estimated annual operation and maintenance cost for the storage/treatment
system is about $50,000/yr. About $15,000 of the annual cost is attributed
to the dry-weather treatment plant operations for pumping and solids
dewatering and disposal.
Assuming that the Hancock Street facilities alone remove 363,000 kg/yr
(800,000 Ib/yr) suspended solids, the annual treatment cost is approximately
$O.H/kg ($0.06/lb) removed, or about $0.05/m3 ($185/Mgal) stored and/or
treated.
Environmental Impacts
Due to the short period of operation and limited monitoring data on the
facilities, receiving water impacts or benefits can only be projected. These
projections are based on the performance of the facilities and the expected
load reductions from the river.
The overall load reductions to the Saginaw River, on an annual basis from the
Hancock Street area, were estimated using the resulting efficiency of the
storage/treatment system and the dry-weather plant performance (80% suspended
272
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solids and 73% BOD removal). Assuming influent loads of 409,000 kg (900,000
Ib) suspended solids and 114,000 kg (250,000 Ib) BOD per year from the
Hancock Street area, the system, including dry-weather treatment, should
remove about 327,000 kg (720,000 Ib) suspended solids and 82,900 kg (182,000
Ib) BOD.
These removed solids, however, will be collected, treated, and disposed of at
the dry-weather treatment plant. The estimated increase in the total solids
load to the dry-weather plant contributed by the Hancock Street facilities is
about 4% annually. For citywide combined sewage control, the increase would
be about 26%, but no operating problems are foreseen.
Reduction of the overflow volume, plus disinfection of the treated overflow,
should greatly improve the bacterial quality of the river during storm
events. Although the river is principally used for shipping, the public
health aspects of the river water quality are important if the river is to be
used for recreation. During extreme storm events, the probability of
contamination would be higher; but on an annual basis, the number of days of
bacterial quality violation should be reduced in proportion to the overflow
events being totally contained, about 75%.
Aside from the specific pollutant impacts, improvement in the health and
appearance of the river is considered an environmental improvement as well as
a socioeconomic benefit.
Socioeconomic Impacts
The Hancock Street storage/treatment system has a 296-stall, two-level
parking garage over the storage basin, thus providing multiuse of the land
area of the storage/treatment facilities. Other impacts include improved
aesthetics and the potential for contributing to the reversal of the economic
decline of the older business district it serves.
The storage basin/parking garage facilities, shown in Figure 105, displaced
about 0.5 ha (1.2 acres) of run-down warehouses with a new aesthetically
pleasing, low profile structure providing about 1.0 ha (2.4 acres) of
offstreet parking. The people displaced by the facility have relocated
nearby, with little apparent disruption.
The local community favored the garage construction and actively campaigned
for its inclusion in the facilities plan. The parking capacity of the
facilities, however, has not been used to its fullest to date, and as a
result, parking meter receipts have been lower than anticipated. Plans exist
to lease the lower section of the garage, thereby increasing utilization.
Land values and rentals in the business district area have been increasing,
and the facilities may have contributed to that increase.
No complaints of odors from the facilities have been received and most of the
people using the parking are unaware of the treatment facilities below.
273
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Figure 105. Two-story parking garage over the
storage/treatment basin.
274
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SECTION 14
MULTIUSE COMBINED SEWER OVERFLOW FACILITIES
MOUNT CLEMENS, MICHIGAN
Combined sewer overflows have been a problem in Mount Clemens, and have
contributed to the degradation of the Clinton River. Approximately 87% of
the city's 950 ha (2,350 acres) was serviced by combined sewers, with 24
overflow points to the river [1, 2].
Mount Clemens developed and implemented a citywide combined sewer overflow
control program that included partial sewer separation, interception of most
of the combined sewer overflow points with a storage/transport tunnel, and
construction of combined sewage treatment facilities with a multiuse
recreational park and lakelet system to polish the treated flows before
discharge. The facilities are not yet operational, and it is expected that
the use of the existing dry-weather treatment plant as a part of the wet-
weather treatment system will not occur in the immediate future. The
facilities to transport dry-weather flow to the Detroit regional treatment
plant have not yet been constructed.
An EPA demonstration project was conducted in Mount Clemens to evaluate the
feasibility of using physical and biological processes to treat combined
sewer overflows [3]. Results of the demonstration facilities were
incorporated in the design of the citywide solution.
The cost of the entire city program, including the sewer separation, is about
$21.5 million, or $26,000/ha ($10,500/acre). The treatment effectiveness of
the combined sewage treatment facilities is estimated at about 95% for
suspended solids and BOD.
The projected impacts of these facilities include the complete elimination of
untreated combined sewer overflows entering the river, although urban runoff
from the separated areas will still enter the river untreated.
PROJECT DESCRIPTION
Mount Clemens, like most cities in the Great Lakes region, originally
constructed combined sewers to convey sanitary and storm flows. However, as
the cities grew, the capacities of these systems were often exceeded,
resulting in the raw discharge of pollutants during overflows. Mount Clemens
implemented a citywide control program of storage and treatment to eliminate
these overflows. In addition, certain sewers in the city were separated
because it was potentially less expensive than intercepting the combined
275
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overflow points, and the treatment system uses a combination of storage and
pumping to control treatment rates at the wet-weather treatment facilities.
Area Characteristics
Mount Clemens is in southeastern Michigan about 44 km (22 mi) northeast of
Detroit, as shown in Figure 106. The city has a population of about 21,000
and is located on the Clinton River about 8 km (5 mi) from Lake St. Clair [4],
Figure 106. Mount Clemens, Michigan.
The topography is generally flat or gently rolling hills, with elevations
varying between 180 m (590 ft) and 190 m (620 ft). Because of the moderating
influence of the Great Lakes, the climate has gentle fluctuations with a mean
annual temperature of 10°C (SOT) [3, 4]. The average annual precipitation
is about 76 cm (30 in.), of which 55 to 60% occurs from April to September
C5],
The land use in Mount Clemens is predominantly residential (51%). The
remaining 49% consists of commercial, industrial, public and semipublic* and
transportation land uses ranging from 9 to 15% each, and about 1% vacant land
and water surface [2, 4]. The total area of the city served by sewers is 950
ha (2,350 acres), of which 460 ha (1,150 acres) is combined and 490 ha (1,200
acres) is separate. ,
276
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Problem Assessment
The State of Michigan identified the Clinton River as a major polluted river
in the Lake Erie basin. The pollution was primarily a result of effluent
discharges or combined sewer overflows or both from more than two dozen
municipalities and private dischargers.
Mount Clemens, in particular, contributed significant amounts of combined
sewage to the river. About 93 km (58 mi) out of 105 km (65 mi) of the city's
sewers was combined [1, 3]. A storm with an intensity greater than 0.254
cm/h (0.10 in./h) would cause an overflow. An estimated 40 overflows per
year discharged about 2.8 million m3 (760 Mgal) of combined sewage to the
Clinton River annually [1, 3, 6].
The average concentrations of BOD5 and suspended solids in combined sewage in
Mount Clemens were estimated to be 140 mg/L and 350 mg/L, respectively [3].
Using these concentrations and the estimated volume of combined sewage
overflow, the annual combined sewage pollutant loading to the,-river was
estimated at 4.0 x 105 kg (8.9 x 105 lb) of BOD5 and 1.0 x 10° kg (2.2 x 10°
Ib) of suspended solids, or about 480 kg/ha (430 Ib/acre) BODK
kg/ha (1,080 Ib/acre) suspended solids. b
and 1,210
Pollutant concentrations and overflow characteristics for 13 overflow events
in two drainage areas in Mount Clemens are summarized in Table 109. Initial
and average concentration values for suspended solids and 6005 were sampled
and the data show no distinct first flush effect. These average values are
consistent with the estimated citywide values.
Implemented Countermeasure
Mount Clemens received a Demonstration Grant from the EPA to demonstrate the
feasibility of treating combined sewer overflows from 86 ha (212 acres) of the
city in a multipurpose physical and biological treatment facility [3]. The
treatment consisted of aerated lakelets, microscreening, chlorination, and
pressure sand filtration.
The demonstration project showed that the aerated lakelet system approached a
90% removal efficiency for suspended solids and BOD. About 70% of the
removal occurred in the first lakelet. Intermediate algae control using
microscreens had little effect on the overall performance of the system. The
citywide program to eliminate combined overflows and to treat the stored
combined volume was developed around the demonstration project lakelet
system.
Elimination of Overflows--
The overflows from the combined sewers in the city were eliminated by sewer
separation in some areas of the city and by construction of a
storage/transport tunnel to intercept the combined sewage overflow points
along the Clinton River. The combined and separate sewer areas of Mount
Clemens are shown on Figure 107.
277
-------
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o
LEGEND
OVERFLOW POINTS
STORAGE TRANSPORT TUNNEL
COMBINED SEWAGE INTERCEPTOR
SEPARATE SEWER AREA
COMBINED SEWER AREA
CITY BOUNDARY
TREATMENT
FACILITY
SITE
Figure 107. Mount Clemens, combined and separated
sewer areas, overflow points, and control facilities.
279
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Sewer Separation—In some areas of the city, it costs less to separate
combined sewers rather than to build an interceptor for collecting combined
sewage at the overflow points. Sewer separation eliminated combined sewer
overflows; however, stormwater overflows still remain, and untreated urban
runoff is allowed to enter the river. Suspended solids, nutrient, and toxic
material problems may still persist in the river because of these flows, even
though the combined flows are eliminated. The city now has approximately 490
ha (1,200 acres) of separated sewers and 460 ha (1,150 acres) of combined
sewers with future plans to separate another 17 ha (41 acres) in conjunction
with road repaving projects.
Overflow Interception--The storage/transport tunnel used to intercept
combined sewer overflow points along the river is a major portion of the
solution to eliminate discharges. The concrete tunnel stores and transports
combined sewage to the treatment facility. The tunnel is 2.75 m (9 ft)
square and has a storage capacity of about 12.30 m3 (3.25 Mgal).
Combined Sewage Treatment—
The combined sewage treatment facility includes storage and physical and
biological treatment. The facility is located at two sites separated by the
Clinton River because of land availability and system configurations.
Provisions for use of the existing 11,600 m3/d (3 Mgal/d) sewage treatment
plant are incorporated into the design of the combined sewage treatment
system. A schematic of the treatment facility components and flow diagram is
shown in Figure 108. An alternative flow path bypassing the dry-weather
facility is provided until sanitary flows can be sent to the regional
treatment facility in Detroit and the dry-weather plant can be converted to
wet-weather operation.
Retention Basin Site—The combined wastewater flows collected in the
storage/tranport tunnel are pumped to sedimentation tanks to remove the grit
and the heavier fraction of suspended solids. The flows from the
sedimentation tanks are stored in an aerated retention basin and pumped
across the river for final treatment.
A chlorination basin is incorporated in the retention basin to disinfect
emergency bypass flows if the total storage capacity of the retention basin
is exceeded. The chlorination basin can also store excess flows from the
retention basin. Chlorination occurs only during emergency bypassing of
flows, otherwise unchlorinated flows stored in the contact basin are pumped
across the river to the park treatment site. Components of the retention
basin site are shown in Figure 109.
Park-Treatment Site—The treatment park facility "includes the existing
sanitary sewage treatment plant, aerated lakelets, and sand filters.
Modification of the existing sewage treatment plant will provide grit
removal, clarification, and chlorination for the stored flows pumped from the
retention basin site. The effluent from the modified sewage treatment plant
undergoes biological treatment in the lakelet system. The series of three
lakelets (the first two are aerated) are followed by sand filtration. A
portion of the aerated lakelets and one of the pressure sand filters are
shown in Figure 110.
280
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CLARIFIER
CHLORINE
CONTACT
CHAMBER
NOT TO SCALE
COMMINUTDR
GRIT CHAMBER
PRESSURE
SAND
FILTERS
LAKELET NO. 3
LAKELET NO. 2
PARK-TREATMENT
SITE
CHLORINATION
BASIN
RETENTION BASIN
(AERATED)
SEDI MEN TAT ION-RESUSPENS I ON
CHAMBERS
RETENTION
BASIN
SITE
COMBINED SEWAGE INFLUENT
PROPOSED FLOW PLAN
> ALTERNATIVE
FLOW PLAN
Figure 108. Schematic of Mount Clemens combined
sewage treatment facilities.
281
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Figure 109. Mount Clemens retention basin site components:
(a) sedimentation-resuspension chambers (SRC), (b) aerated
retention basin, and (c) chlorination basin with overflow outlet.
282
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Figure 110. Park treatment facility for combined sewage:
(a) a portion of the aerated lakelets, and (b) pressure sand filter.
Design
The stormwater facilities are designed to control the projected peak flow
from a 5 year storm and to meet strict effluent standards in the city's
discharge permit. The design incorporates a combination of storage and
pumping to control flow and treatment rates through the various processes of
the treatment system. The maximum pumping rate of combined sewage into the
retention basin is 950,000 m3/d (250 Mgal/d). The average dewatering rate to
the park treatment facility is 15,000 m3/d (4 Mgal/d). ,The criteria used for
the design of the individual units of the storage and treatment processes are
shown in Table 110.
283
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Table 110. COMBINED SEWER OVERFLOW TREATMENT FACILITY
DESIGN PARAMETERS, MOUNT CLEMENS [3]
Parameters
Value
Storage/transport tunnel
Size, ft
Capacity, ft3
Retention basin site
Peak inflow rate (5 pumps at 50 Mgal/d each), Mgal/d
Sedimentation resuspension chamber (3 bays)
Length per bay, ft
Width per bay, ft
Surface area, ft?
Peak hydraulic loading rate, gal/ft2>d
Minimum hydraulic loading rate, gal/ft2-d
Detention time at peak flow, min
Retention basin (includes chlorination basin)
Surface area, ft2
Volume, ft3
Maximum dewatering rate, Mgal/d
Chlorination
Detention time at peak flow (200 Mgal/d), min
Dosage rate, mg/L
Park-treatment site
Average flowrate, Mgal/d
Clarifier
Length, ft
Width, ft
Depth, ft
Surface area, ft2
Hydraulic loading rate at 4 Mgal/d, gal/ft2-d
Detention time, hr
Chlorination
Detention time at peak flow (5 Mgal/d), min
Maximum dosage rate, mg/L
Lakelet No. 1
Surface area, ft2
Volume, ft3
Detention time, d
Lakelet No. 2
Surface area, ft2
Volume, ft3
Detention time, d
Lakelet No. 3
Surface area* ft2
Volume, ft3
Detention time, d
Sand filtration
Number of filters
Surface area, ft2 „
Hydraulic loading rate, gal/ft -min
9x9
434,000
250
360
64
23,000
3,600
730
30
360,000
3,625,000
5
20
5
118
84
20
10,000
400
3
15
10.0
52,000
500,000
1
122,000
1,100,000
2
100,000
920,000
1.8
4
50
15
OPERATION AND PERFORMANCE
The operation and performance of the wet-weather treatment facilites are
based on the results of the demonstration project and estimates of the
expected efficiency of those process elements not evaluated in the
demonstration project [3]. The facilities are expected to become
operational by mid-1979.
284
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Operation
The combined sewage treatment system can operate either (1) using modified
components of the existing dry-weather treatment facilities (planned system
operation) or (2) bypassing the dry-weather plant with flows being pumped
directly to the lakelet system (alternate system operation). Until
facilities are available to transport the city's sanitary flows to the
regional treatment plant, Mount Clemens will continue to process sanitary
flows through the dry-weather facilities and use the alternate system
operation to handle wet-weather flows.
Planned System Operation—
Under the planned system operation, the dry-weather facilities would be
modified to treat the retention basin flow before it enters the lakelet
system. Modifications include disconnecting the trickling filters and
converting the primary and secondary sedimentation tanks into a wet-weather
clarifier. The chlorine contact chamber following the clarifier would be
used to control bacteria in the lakelet system. Solids handling operations
and plant personnel requirements were established for this mode of
operation.
Solids Handling—Solids handling operations include the collection and
disposal of the solids removed in the wet-weather processes. Following a
storm, the settled solids and grit in the sedimentation tanks are
resuspended in a slurry and pumped to the modified dry-weather treatment
plant. After the resuspended slurry is pumped, the stored combined flows
from the retention basin are pumped to the modified dry-weather plant, then
to the aerated lakelet system. Solids removed in the clarifier are mixed
with the sanitary flow being sent to the regional plant.
The resuspension system in the SRC uses air and water injected through
headers mounted on the walls of the tank. The solids slurry is removed
through a sloped central channel in each tank and pumped across the river.
Solids collected in the retention basin are removed mechanically by front-
end loaders after the basin has been dewatered. Final cleanup operations
are aided by fire hoses mounted at intervals along the retention basin
walls. An access ramp is provided to allow equipment into the basin.
Personnel Requirements—Based on an expected wet-weather operation of about
150 to 180 days per year, labor requirements of about 7.5 man-years will be
required to operate the wet-weather facilities annually. The lakelet
portion of the facilities will be operated year-round to maintain a
consistent water quality so that recreational water quality standards are
met. Approximately 35% of the labor hours will be required for actual
facility operation, while the largest expenditure will be for maintenance,
48% [7, 83.
285
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Alternate System Operation—
Until the dry-weather treatment facilities are modified for wet-weather use,
operational steps for processing the combined flows from the retention basin
include (1) pumping the resuspended solids and grit from the SRC to the dry-
weather treatment facilities and mixing the solids with the city's sanitary
flow for treatment and disposal, and (2) pumping the retention basin flows
directly to the aerated lakelet system for biological treatment followed by
sand filtration before discharge to the river.
Operational Problems—
A number of operational problems were identified during the operation of the
aerated lakelet system in the demonstration project [3]:
• Icing of the surface aerators in the lakelets causing instability
and possible capsizing.
• Algae growth, turbidity, and floating oil in the lakelets.
• Buildup of sludge deposits in the first lakelet.
Design changes in the treatment facilities provided in the citywide program
are expected to reduce or eliminate these problems.
Winter conditions created significant ice buildup on surface aerators,
causing instability and possible capsizing. This problem was corrected in
the lakelets by replacing the surface aerators with a subsurface aeration
system. Another possible solution to prevent icing would be to install
heaters on the surface aerators.
Excessive quantities of algae were encountered in the operation of the
lakelet portion of the demonstration project. In the recently constructed
facilities, phosphorus removal in the treatment process can be provided if
required to limit the growth of algae.
Turbidity in the lakelet system was encountered during the demonstration
project, but lining the lakelets with stone should prevent the problem.
Oil in the combined sewage presented a problem during the operation of the
lakelet system during the demonstration project; noticeable amounts of oil
were present in the lakelets. The new facility, as designed, has a clarifier
with a scum baffle, which should reduce the problem.
Sludge deposits were also a problem in the demonstration project with 23 cm
(9 in.) or an estimated 800 m3 (28,000 ft3) occurring in the first lakelet.
The large deposits were not anticipated in the demonstration project,
indicating the problem of sampling and identifying the heavy solids occurring
in combined sewage overflows. Taking representative samples is the key to
accurate solids characterization. The use of the sedimentation-retention
system and clarifiers at the modified dry-weather plant in the citywide
facilities should reduce this problem. Odor problems in the retention basin
should be minimized by the flushing system and the solids removal program.
286
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Performance
The total project implementation in Mount Clemens should reduce the volume of
combined sewer overflows and pollutant loading to the Clinton River. The
facilities are expected to meet effluent water quality criteria of 10 mg/L
BOD,- and 15 mg/L suspended solids, and emergency bypasses from the wet-
weather treatment facilities that are projected to occur about 4 times a year
would be disinfected. In addition to the performance of the combined sewage
treatment processes, the effectiveness of the total system (including the
impact of the sewer separation program) is evaluated in terms of load
reduction to the river.
Overflow Volume and Pollutant Reductions--
All of the volume of raw combined sewage will be eliminated from the Clinton
River by full project implementation. The construction of the
storage/transport tunnel and associated sewers to intercept the combined
sewer overflow outfalls will collect the flows from 19 out of 24 overflow
points [1, 7]. However, sewer separation will result in the discharge of
stormwater at the 5 remaining overflow points.
About 95% of the suspended solids and BODs in the combined sewage are
projected to be removed by the wet-weather treatment facility. These
projections, shown in Figure 111, are developed from the demonstration
project data and the estimated treatment efficiencies for the physical
processes (at conservative loading rates and operating conditions). The
projected efficiencies of the treatment facility, either using the modified
dry-weather plant or pumping directly to the lakelets, are about equal.
*s too
250
PEAK FLO* FOR SEQUENTIAL UNIT PROCESSES Mgal/d
Figure 111. Projected pollutant removal efficiencies of
Mount Clemens combined sewage treatment facility.
287
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System Efficiency— ,
The projected pollutant loadings before and after full project implementation
are shown in Figure 112. Annual loads were estimated for both combined
sewage and stormwater runoff from the separated areas.
ESTIMATED LOADINGS
PROJECT
300 ACRE
SEPARATE
SEWERS
in
a
o
ca
o
in
CO
*•"
'
1
V)
CO
.a
0
0
o
in
CO
i
BEFORE
IMPLEMENTATION
2050 ACRE
COMBINED
SEWERS
m
ca
0
JS
0
a
o
CO
CO
'
ESTIMATED LOADINGS
AFTER
PROJECT IMPLEMENTATION
1200 ACRE 1150 A.CRE
SEPARATE COMBINED
SEWERS SEWERS
1
1
1
8| f
:i
o^ I ^
s 1 s
CM" 1
1
1
1
*
I ^^u?
1 a
1 s
1 i
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o
" \
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0 | ^ ^|
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ss : 40%
Figure 112. Comparison of estimated pollutant loads
to the Clinton River before and after project implementation.
The stormwater runoff loads were developed assuming the pollutant
concentrations of raw stormwater in Mount Clemens are comparable to the
representative values of 20 mg/L for BOD5 and 415 mg/L for suspended solids
[9J. The assumptions also include the projected wet-weather facility
treatment efficiency of 95% and do not assess the effect of emergency
bypasses from the treatment facility.
A^comparison of the annual load reduction achieved by project implementation
with the preproject load estimates yields about a 90% BOD5 and a 40%
suspended solids reduction. Even though the combined sewage treatment -
efficiency is about 95% for both constituents, the stormwater loads from the
separated areas are assumed to enter the river untreated. If suspended
solids or associated heavy metals are a major criterion for maintaining the
water quality of the receiving water, then the additional solids loading of
stormwater discharges should be considered when separating sewers. The
288
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impacts of BOD loads in stormwater appear less significant than the solids
load; however, oxygen demand may still be affected by other pollutants such
as COD, or other organics.
Using just the loading values after project implementation from the separate
and combined areas, the net annual load reduction is estimated at about 83%
for BODg and 43% for suspended solids.
COSTS
Collection, storage, and treatment of combined sewage by physical and
biological processes can be a most promising, cost-effective alternative for
combined sewer overflow control where high degrees of pollutant removals are
required.
Capital Costs
The total cost to eliminate and treat combined sewer overflows was about
$21.5 million for a service area of 830 ha (2,050 acres), or about $26,000/ha
($10,500/acre). Estimated treatment capital costs, based on pollutant
removal, are about $38/kg ($17/lb) BOD,- and $15/kg ($7/lb) suspended solids
or about $0.005/L ($0.02/gal) treated.
Capital costs of the components of the Mount Clemens project included:
(!) sewer separation, (2) construction of storage/transport system, and
(3) construction of the combined sewer overflow treatment facilities
(retention basin site and park treatment site). The cost of the park
treatment site includes minor modification of the existing sanitary sewage
treatment plant to treat combined sewage and development of the park system
around the lakelets.
The capital costs of the system components and unit costs are summarized in
Tab!e 111.
Table 111. CAPITAL COSTS AND UNIT COSTS OF THE
COMBINED SEWER OVERFLOW FACILITIES, MOUNT CLEMENS3
Control measure
Sewer separation
Interception
Treatment
Retention basin site
Park treatment site
Total capital
costs, $
4,019,000
8,916,000
—
6,934,000
1,653,000
Serviced
area, acres
900
1,150
1J50
--
.
Annual
combined
overflow
volume
reduction ,
Mgal
333b
426
426
-- -
—
Cost,
$/acre
4,466
7,753
--
6,030
1,437
Overflow
volume
reduction
cost, $/Mgal
12,070
21,000
--
Treatment
cost,
$/gai
—
—
--
.016
.002
a. ENR 3000.
b. !.Still allows stormwater volumes and pollutants to be discharged.
289
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The Mount Clemens separation program was estimated to cost about $1,800/ha
($4,500/acre); about 42% less than the estimated regional average cost for
the midwest of $4,300/ha ($10,700/acre) [10]. The net area separated in
Mount Clemens, about 360 ha (900 acres), was achieved with a limited amount
of new sewer construction because of the configuration and locations of the
existing sewers. The areas that were separated were also of lower land use
intensity than in the major cities for which separate costs were evaluated.
The capital costs of the retention basin and the3storage/transport
tunnel expressed as unit storage costs are $68/m ($0.26/gal) for the
retention basin and $730/m3 ($2.75/gal) for the tunnel.
Annual Costs
q
Annual operation and maintenance cost estimates of $0.17/m (($650/Mgal)
of combined sewage treated are based on year-round operation of the
lakelet system, operation of the retention basin when storm activated,
and maintenance of the recreational area. The energy requirements for
the treatment facilities are minimal in comparison to the total
operation and maintenance budget. The operation and maintenance cost
components are presented in Table 112.
Table 112. ESTIMATED OPERATION AND MAINTENANCE COSTS
MOUNT CLEMENS3 [1, 3]
Cost components
Estimated cost, $/yr
Personnel 165,000
Supplies and equipment 40,000
Chemicals 10,000
Power 31,000
Sludge disposal0 18,000
Other 12,000
Total 276,000
a. ENR 3000.
b.
c.
Includes parts, materials, maintenance
of vehicles, etc.
Sludge disposal to Detroit regional
treatment facility.
IMPACTS
The environmental and socioeconomic impacts of this project include the
potential pollutant load reduction to the Clinton River and the benefits
the public may derive from the multiuse park-treatment facility.
290
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Environmental Impacts
The reduction of pollutant loadings, through elimination of combined sewer
overflows, is a necessary step in the overall revitalization of the Clinton
River. The combined sewage treatment facility will remove an estimated
215,000 kg (472,00 Ib) of BOD5 and 531,000 kg (1,170,000 1'b) of suspended
solids annually from the river.
However, the estimated suspended solids discharged to the river from the
separated areas is about 1.2 times the estimated solids load being treated.
Although sewer separation effectively eliminates combined sewer overflows,
the net receiving water benefit from sewer separation, in terms of solids
loads, may be questionable. The potential adverse impacts of BOD from the
separated area is substantially less than in combined sewer overflows. If
the principal receiving water concern is dissolved oxygen depletion, then
partial sewer separation in combination with the treatment of combined
sewage may produce an environmental benefit.
The background receiving water quality in the Clinton River, before the
citywide control facilities were constructed, was monitored for about 2-1/2
years [3]. BOD impacts to the river from all sources are considered minor.
The dissolved oxygen level averaged about 7 mg/L. During the 2-1/2 years of
monitoring, only about 8 out of the 79 days dissolved oxygen samples had
values below 4 mg/L, or about 10% of the time. Assuming the sampling is
representative, dissolved oxygen levels of below 4 mg/L would have occurred
about 37 d/yr. Implementation of the combined sewage control program,
including partial sewer separation, with the resulting high BOD load
removal, should substantially reduce the number of days approaching
dissolved oxygen violation limits.
The suspended solids concentration in the river averaged about 30 mg/L and
ranged from less than 5 to about 200 mg/L. BOD averaged about 9 mg/L and
ranged from 1 to 50 mg/L. Influences from storms can be seen in the data;
however, the overall quality of the river is high. Combined sewage
treatment as a part of the water pollution abatement program instituted by
the state will hopefully return the river to an environment that will
support game fish.
Socioeconomic Impacts
Land use has been affected by the creation of the multiuse treatment
facility. Even though Mount Clemens is nearly 100% developed, a portion of
land was still unused prior to project implementation. The site of the park-
treatment facility was a city landfill, bordered by land considered
undesirable for development because of its proximity to the landfill.
The conversion of the landfill into a multiuse treatment facility, serving
the public as a park-recreation area with a lake system, is a positive land
use change, even though it involved a small percentage of the total city
area. This change also made the surrounding undeveloped area more desirable
property.
291
-------
Although not completely landscaped, the lake system and recent development
adjacent to the park site are shown in Figure 113, along with an artist's
concept of the completed project.
High public acceptance of the project is due in part to the environmental
awareness of the general public, but more as a result of providing a badly
needed park-recreation area with small lakes for water sport. The
elimination of the landfill as an undesirable area and improved aesthetics
have increased the development value of the surrounding land.
292
-------
Figure 113. Present state of the Mount Clemens project:
(a) a portion of the lakelet system, (b) lakelet and land
adjacent to the project area, and (c) an artist's
concept of completed park-treatment area.
293
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SECTION 15
SWIRL REGULATOR/CONCENTRATOR DEMONSTRATION PROJECT
LANCASTER, PENNSYLVANIA
A 7.3 m (24 ft) diameter dual-functioning swirl regulator/concentrator has
been constructed in Lancaster, Pennsylvania, under a demonstration grant from
the U.S. EPA Storm and Combined Sewer Section [1]. The swirl controls
combined sewage overflows at one of the city's overflow points to the
Conestoga River.
The original demonstration project was to include a storage silo as a part of
the facilities, but the silo concept was dropped in 1974 because of the high
construction bid cost of the project. The swirl project was continued,
however; it was felt that adequate control could be achieved by the swirl
alone. The swirl concentrator and a 2.4 m (8 ft) diameter swirl degritter
were constructed and placed in service in early 1979.
Limited performance data have been collected on the swirl facilities because
of sampling and flow monitoring difficulties. Estimates of performance, based
on several storms that were monitored, indicate suspended solids mass removals
ranging from 17 to 80%, depending on the size "of the storm. This range
includes the removals obtained from the hydraulic flow split in the swirl.
The swirl concentrator is one of the least expensive solutions to combined
sewage overflow control. The cost of the Lancaster swirl is estimated at
$148,000/111 s ($6,500/Mgal d) of design capacity, or approximately $3,360/ha
($l,340/acre) of combined sewer service area. The representative construction
cost for the swirl concentrator and appurtenant facilities at Lancaster was
approximately $13,900/ha ($5,500/acre) of combined sewer service area.
The only other full-scale prototype swirl application on combined sewage in
the United States is in Syracuse, New York. Mass suspended solids removal for
this facility averaged 52% for 11 storms sampled [2, 3, 4].
PROJECT DESCRIPTION
The demonstration project was developed to evaluate the feasibility of the
swirl regulator/concentrator as a low cost, nonmechanical combined sewer
overflow control device. The City of Lancaster is evaluating its appli-
cability as a potential alternative to control combined sewer overflows to the
Conestoga River. The principal features of the swirl are its compact space
requirements, ability to operate as an overflow regulator and solids separator
simultaneously, capability to operate at variable inflow rates, and low
294
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operation and maintenance requirements. Hydraulic modeling studies conducted
during the development of the prototype design criteria indicate the swirl
concentrator may remove over 90% of heavy solids (0.2 mm diameter at specific
gravities of 2.65) [5].
Area Characteristics
Lancaster is on the Conestoga River, a tributary of the Susquehanna River, in
southeastern Pennsylvania, as shown in Figure 114. The area has gently
rolling hills varying in elevation from 30 to 150 m (TOO to 500 ft), and has
well-drained soils. The city is about 100 km (60 mi) west of Philadelphia.
CONESTOGA RIVER-
Figure 114. Lancaster, Pennsylvania.
The average annual precipitation is 110 cm (43 in.), which is about evenly
distributed throughout the year. Slightly more precipitation occurs during
the summer months (May through August) from thunderstorms. About 30 to 35
thunderstorms occur per year.
Swirl Demonstration Project
Six combined sewage overflow points in Lancaster discharge untreated overflows
to the Conestoga River during storms. During storms, the combined sewage
discharge rates from these overflow points are estimated to range from 22,700
to 94,600 m^/d (6 to 25 Mgal/d). These untreated flows can add substantial
295
-------
pollutant loads to the river. The river has a relatively high water quality
during nonstorm periods. During the summer, BOD as low as 2 mg/L and total
suspended solids as low as 70 mg/L have been recorded [6].
The swirl demonstration project was constructed at one of these overflow
points to evaluate its effectiveness in controlling stormwater loads to the
river.
Demonstration Area--
The 100 ha (250 acre) demonstration area is adjacent to the Conestoga River in
the southeastern portion of the city, as shown in Figure 115. The developed
portion of the area, about 50 ha (125 acres), is residential. The remaining
area is parklands or undeveloped fields.
O
DEMONSTRATION
DRAINAGE BASIN
SWIRL CONCENTRATOR
CONESTOGA RIVER
Figure 115. Swirl concentrator demonstration
drainage basin in Lancaster [7].
The developed area has an estimated runoff coefficient of 0.59. The
undeveloped half of the drainage area is assumed to contribute runoff directly
to the river [7].
296
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Swirl Facilities-- , ..:
A 7.3 m (24 ft) diameter swirl concentrator was constructed to replace an .
existing combined sewer .overflow regulator oh a 1.5 m (60 in.) reinforced-
concrete sewer. The swirl was designed to operate at 1.1 m3/s (40 ft3/s) with
the capacity to be surcharged to 2.6 m3/s (90 ft3/s). An emergency bypass was
constructed in the wall of the swirl to pass flows up to 3.5 m3/s (125 ftj/s)
to the river without flooding the swirl unit.
A swirl degritter and grit handling system was constructed to remove grit from
the foul underflow of the swirl concentrator before the underflow is pumped to
the municipal sewage treatment plant. The grit is conveyed to a storage bin
and disposed of at a landfill. Disinfection of the swirl concentrator clear,
overflow is provided. Disinfection of raw overflows can also be accomplished
at the diversion chamber ahead of the swirl facilities. A schematic of the
facilities during wet- and dry-weather operation is shown in Figure 116.
During dry weather, sewage enters the swirl concentrator and is directed along
the floor gutters to the foul underflow outlet and then directly to the
pumping station.
Flows from the combined sewer enter a diversion chamber and bar screen before
entering the swirl. The bar screen protects downstream facility components -
from large pieces of debris. The diversion chamber has an emergency bypass to
prevent the swirl facilities from being flooded. The bypass is activated by a
sonic level sensor that opens a bypass gate when flows into the chamber exceed
a preset elevation. All bypassed flows are chlorinated.
The swirl facilities are automatically activated when storm flows enter the
swirl. A sonic level sensor activates a sequence of valves to change the mode
from dry- to wet-weather operation. During wet-weather operation, the
combined sewage is split into a clear overflow stream, which is chl orinated
and discharged to the river and the foul underflow stream. The swirl
facilities are shown in Figure 117.
The foul underflow to the swirl degritter was initially controlled by a pinch
valve linked to a magnetic flowmeter. Because of the slow response time
between the flowmeter and the control valve, flows above the hydraulic design
capacity of the swirl degritter could occur instantaneously as head built up
in the swirl concentrator. A Hydrobrake was installed in the foul underflow
line to correct this problem. The Hydrobrake, with a 17.8 cm (7 in.) opening
at its constriction, is a static device that controls flow by increasing
headloss through the unit. The headless is created by vortex fluid motion.
As the head builds up above the Hydrobrake, the fluid rotation increases,
thereby increasing the headloss through the orifice and decreasing the flow.
The unit was designed to pass a maximum flow of 0.06 m3/s (2 ft-Vs). The
anticipated performance of the Hydrobrake compared with a conventional orifice
is shown in Figure 118. A schematic of the Hydrobrake operation is shown in
Figure 119. ,...-.-.
297
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DISINFECTANT
DIVERSION CHAMBER
-BAR SCREEN
SWIRL CONCENTRATOR /— CLEAR OVERFLOW
SWIRL
DEGRITTER
SOLIDS REMOVAL
(a) WET-WEATHER OPERATION
TO ,
SEWAGE
TREATMENT
PLANT
SWIflL CONCENTRATOR /-CLEAR OVERFLOW
DIVERSION CHAMBER
-BAR SCREEN
SWIRL
DEGRITTER
(b) DRY-WEATHER OPERATION
TO
SEWAGE
TREATMENT
PLANT
Figure 116. Schematic of the Lancaster swirl regulator/concentrator facilities,
298
-------
1 J i * L: s. t* »
Figure 117. Lancaster swirl facilities: (a) 24 ft swirl regulator/concentrator
and control building, (b) swirl regulator/concentrator, (c) automatic
samplers, and (d) 8 ft swirl degritter.
299
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CONVENTIONAL
ORIFICE
A MAXIMUM DESIGN
FLOWRATE
FLOWRATE TO SWIRL DEGRITTER, ftVs
Figure 118. Conceptual performance comparison of the
Hydrobrake and a conventional orifice.
VORTEX MOTION
FLOW LINES
EFFLUENT
'INFLUENT
Figure 119. Schematic of Hydrobrake operation showing
principal of vortex fluid motion.
300
-------
The demonstration facilities also have a 9.5 L/s (150 gal/min) discostrainer
that is being evaluated on the swirl foul underflow. The facilities'
principal sampling system has four automatic samplers that can pull samples
from 12 different locations in the system.
Swirl Regulator/Concentrator Design
The' swirl regulator/concentrator design criteria were developed through
mathematical and hydraulic model simulation [1, 8]. Synthetic settleable
solids, grit, and floatables were used in the hydraulic testing to simulate
combined sewage solids. The specific characteristics of these solids are
summarized in Table 113.
Table 113. CHARACTERISTICS OF THE SOLIDS USED
IN DEVELOPING SWIRL DESIGN RELATIONSHIPS
THROUGH MODEL SIMULATION [8]
Particle distribution
Specific Concentration,
Material gravity mg/L
Settleable solids 1.05-1.2 200-1,550
(excluding grit)
Grit 2.65 20-360
Floatable solids 0.9-0.998 10-80
size,
mm
0.2
0.5
1.0
2.5
5.0
0.2
0.5
1.0
1.5
2.0
5
10
15
20
25
% by weight
10
10
15
25
40
10
10
15
25
40
10
10
20
20
40
•3
The model was tested at flows simulating the range from 0.4 to 4.7 m /s (15 to
165 ft3/s), which were representative of the flows expected at the Lancaster
facility [8, 9].
The design relationships developed in the modeling studies were used to design
the 7.3 m (24 ft) diameter swirl concentrator [5]. The depth of the swirl,
base to weir crest, is 1.8 m (6 ft). A plan and section view of the swirl are
shown in Figure 120.
301
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EMERGENCY OVERFLOW WEIR
SCUM RING
OVERFLOW WEIR
CLEAR OVERFLOW
OUTLET
WEIfl BASE PLATE
CHLORINE PIPE
FLOATABLES TRAP
FLOATABLES DEFLECTOR
SPOILER
DEFLECTOR WALL
TOP VIEW
EMERGENCY OVERFLOW
OVERFLOW WEIR
FLOATABLE DEFLECTOR
SECTION VIEW
Figure 120. Details of the Lancaster swirl regulator/concentrator.
302
-------
The swirl is operated as a gravity flow system that acts as a flow regulator,
provides pollutant removal, and has no moving parts. The major components of
the swirl include the:
o Flow deflector and spoilers
» Clear overflow and foul underflow outlets
• Scum ring and fleatables collector and floor gutters
Flow Deflector and Spoilers--
The flow deflector and spoilers maintain the desired flow regime in the swirl
tank—circular flow patterns that set up the secondary fluid motion. The flow
deflector helps prevent mixing or flow disturbance between the influent flow
and the volume already in the circular flow pattern. It also deflects •tu~
flow toward the center of the swirl, increasing the swirl velocity.
the
The spoilers, which are a part of the clear overflow weir, prevent the
formation of a free surface vortex at the outlet by creating perpendicular
resistance to the circular flows at the surface. The spoilers also create a
relatively quiescent area near the overflow weir so that a uniform flow over
the weir is achieved.
Clear Overflow and Foul Underflow Outlets--
The clear overflow and the foul underflow outlets control the flow split in
the swirl. The Lancaster swirl has a 4m (13.3 ft) diameter clear overflow
weir ring with 13 m (42 ft) of weir. The clear overflow is discharged through
the effluent outfall to the river through a 0.9 m (3 ft) diameter drop shaft
in the center of the circular weir baseplate.
The foul underflow outlet is a 0.3 m (1 ft) diameter pipe connected to the
base of the swirl. During dry weather, municipal sewage is routed through the
swirl to the foul outlet and on to the sewage treatment plant. During wet-
weather operation, approximately 5% of the influent flow, containing the
concentrated solids, is removed through the foul outlet and sent to the swirl
degritter.
Scum Ring, Fleatables Collector, and Floor Gutters--
The scum ring and the fleatables collector are attached to the clear overflow
weir assembly and prevent floatables from overflowing with the clear effluent.
The scum ring acts as a baffle, keeping the floatables outside the clear
overflow weir.
The floatables are then directed by the floatable deflector, a barrier across
the surface of0the water, to a floatables trap. The trap is connected to a
5 7 m3 (200 ft3) floatable storage area under the clear overflow weir plate.
The storage area is formed by a trap ring that is an extension of the clear
overflow weir, 0.5 m (1.5 ft) below the weir plate. After a storm, the
trapped floatables enter the foul underflow outlet as the water level falls
and are transported to the sewage treatment plant.
303
-------
The floor gutters form a contracting spiral from the swirl influent entrance
•on the periphery of the concentrator base to the foul outlet near the center
of the concentrator. The gutters direct dry-weather flow through the swirl
during nonstorm periods and guide the concentrated combined solids to the
center of the swirl during wet-weather operation.
Other Features--
The Lancaster swirl also has disinfection facilities, a flushing system, and
an emergency overflow weir. All of thesclear overflow effluent is chlorinated
before discharge to the river. A chlorine pipe extends over the clear
overflow weir assembly and doses the effluent just before it is discharged.
Flushing lines are located on the sidewalls above the water!ine and under the
fleatables trap area to wash deposits from the swirl after each storm. The '
deposits are washed into the floor gutters and are sent to the treatment
plant.
The emergency overflow weir covers one-quarter of the sidewall of the swirl.
The weir is 15.2 cm (6 in.) higher than the clear overflow weir and provides
an additional 6 m (19 ft) of weir length for flows that exceed the hydraulic
capacity of the swirl. Details of the swirl concentrator components are shown
in Figure 121.
OPERATION AND PERFORMANCE .
During the project evaluation phase of the demonstration project, several
operational problems were encountered that resulted in a limited amount of
useful data being collected. Most of the problems were with the monitoring
and sample collection. These problems are discussed in the following and
offer guidance for future design of monitoring systems.
Operational Problems
The major problems during the evaluation of the swirl were:
• Malfunctioning flowmeters
• Sampling problems
• Clear overflow weir level fluctuations
Fl owmeters—
Malfunctioning flowmeters have been a continuous problem during the first year
of operation. Two types of flowmeters were used in the facilities: a sonic
flowmeter to measure the swirl influent flow and a magnetic flowmeter coupled
with a control valve to throttle the foul underflow to the swirl degritter.
The sonic flowmeter in the 0.9 m (3 ft) swirl influent line would "zero out"
whenever the pipe was surcharged. Under this condition, measurement of
influent pollutant loads is reduced to guesswork. This significantly affects
the facility performance evaluation, i.e., the accuracy of the mass balance is
304
-------
Figure 121. Lancaster swirl concentrator components: (a) sonic level sensor
suspended from catwalk and flushing water!ines; (b) clear overflow weir
assembly showing spoilers, floatables trap, scum baffle, and chlorine line;
(c) floor gutters and foul underflow outlet; and (d) clear overflow weir and
emergency overflow weir and orifice along sidewall.
305
-------
constrained by the accuracy of the flow measurement. Physical methods of
flow measurement, such as depth of flow over a weir or other control surface,
may result in more reliable data, or at least provide a check on the passive
flow measurement devices.
The problem of exceeding the design flow capacity of the swirl degritter
caused by the response delay between the magnetic flowmeter and the control
valve was corrected by the installation of a Hydrobrake. A limit switch was
placed on the control valve to prevent wide-open operation.
Automatic Samplers--
Automatic sampler problems have been identified as the probable cause of
inconsistent and unrepresentative settleable solids characterization. The
swirl is basically a treatment device for the heavier solids fraction and
grit and conventional sampling practices are not suited for the accurate
characterization of these solids without several design considerations to
account for better representation of heavier solids. These considerations
include multiple sampler intake locations, higher sampler intake velocities,
shorter sampler pipe runs, and higher sampler pipe velocities.
Several sample piping runs are over 15 m (50 ft) long and have numerous
fittings and changes of direction. For example, the samples from the
influent combined flow pass through a least seven ells, two valves, five
unions, and three tees before reaching the sample pump. The discharge from
the pump goes through similar fittings, including a 6 m (20 ft) horizontal
run and a long vertical run before reaching the sampler. Sediment can build
up in horizontal pipe runs and, if velocities are low, the vertical pipe run
could act as a tube settler. The sampling system flow velocities range from
0.82 to 1.04 m/s (2.7 to 3.4 ft/s) in the sampling loop.
In general, samplers should be located as close to the point being sampled as
practical, to reduce long piping runs. Sample intake locations should
consider the occurrence of stratified solids flow in the stream being
sampled. Sample velocities should be uniform throughout the sampling system
and should at least match the velocity of the stream being sampled, or be at
least 1.2 m/s (4 ft/s). These considerations are viewed as critical in
obtaining representative samples of the heavy solids fraction.
Clear Overflow Weir--
The clear overflow weir in the swirl has experienced level changes that can
result in an uneven flow distribution over the weir. The problem is assumed
to be caused by the heating and cooling of the suspension rods holding up the
weir. Protection of the swirl by a sunshade has been suggested.
Performance
Data on the performance of the Lancaster swirl regulator/concentrator are
limited: only 4 storms of the 10 storms monitored have sufficient flow and
quality data to permit mass removal evaluations. The sampling data collected
306
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for settleable solids may not be representative because of the low sampling
velocities and long piping runs to the samplers. Little BOD data were
collected. Therefore, suspended solids was the only parameter evaluated. The
potential swirl effectiveness, however, may be inferred from the results of
the Syracuse, New York, swirl studies [2, 3], until the Lancaster facilities
collect a statistically reliable data base.
Mass removal of suspended solids is achieved by two processes: (1) the flow
split between "the clear overflow and the foul overflow, and (2) the
concentration effect of the swirl motion. The mass suspended solids removals
for the four storms ranged from 17 to 80%, with hydraulic flow splits ranging
from 15 to 48%, respectively. The size of the storms ranged from very small,
about 10% of the design flow capacity, to a storm that equaled or exceeded the
design flow capacity. A summary of the storm data and sw.irl effectiveness is
presented in Table 114.
Table 114. SWIRL REGULATOR/CONCENTRATOR EFFECTIVENESS, LANCASTER
Storm flows
Storm
date
3/5/79
4/4/79
5/23/79
5/26/79
Storm Ratio to
duration, Peak, design flow.
h ft3/s %
1.7
1.1
2.2
0.5
6.8
6.5
>40C
4.0
17
16
>100
10
, Average ,
ft3/s
3.5
3.8
>10.4
2.3
Ratio to
design flow,
%
9
10
>26
6
Mass
removal
78 ;-
76
17
80
Concentration
reduction
64
54
2
60
Hydraulic h
flow split, %°
40
48
15
49
a. Flow-weighted averages.
b. Ratio of foul underflow volume to total influent volume. These values are also representative of the mass removal
of suspended solids resulting from the flow split.
c. Estimated to be at least 40 ft3/s; flowmeter reads zero during surcharged pipe flow condition.
The size of the storm inflow volume and the split between the foul underflow
and the clear overflow significantly affect the overall performance of the
unit. For example, a very small storm could have up to 50% of its total
storm volume diverted through the foul outlet, thereby netting at least a 50%
mass removal independent of any concentration effects. Since the foul
underflow rate3is controlled and roughly the same for all storms 0.3 to 0.6
m/s (1 to 2 ft /s), large storms would have a relatively small fraction of
the total storm volume diverted through the foul outlet. The corresponding
mass removal by the flow split is therefore much smaller.
Other considerations, such as timing of the influent load (first flush), the
flowrate, and the storm duration, affect performance. For example, during the
storm of May 23, 1979, 76% of the total mass suspended solids load reached the
swirl in the first 14 minutes of the 2.2 hour runoff event. This corresponds
with 50% of the total flow volume during the same time and the foul underflow
volume was only 2% of the total inflow volume. Even if the concentration of
the foul underflow were ten times the concentration of the influent flow, only
about 16% of the total influent load during the 14 minute operation period
would have been removed (76% x [2% x 10]).
307
-------
Tests conducted on a 3.7 m (12 ft) diameter swirl concentrator in Syracuse,
New York, indicated that the mass suspended solids reduction averaged 52% for
11 different storms. The removals ranged from about 30 to 80%. The estimated
average mass removal obtained by the flow split was about 33%. Therefore,
the average expected contribution from solids concentration could be about an
additional 20% removal [2, 3].
COSTS
The swirl regulator/concentrator is the least expensive structural alternative
for controlling combined sewer overflows. Until a reliable and much larger
performance data base is available, complete judgment on the cost
effectiveness of the swirl is not possible. Therefore, none are presented in
this text. Since the Lancaster project was a demonstration project, the
actual demonstration capital costs and the operation and maintenance costs of
the project may be higher than would be required for a normal prototype
facility.
Construction Cost
The total cost of the Lancaster swirl regulator/concentrator demonstration
project was about $1,100,000 (ENR 3000). In addition to the construction cost
of the swirl facilities, this cost includes inspection and administration,
engineering, equipment, supplies, laboratory charges, operating salaries,
utilities, repairs, and the project report.
The construction costs of the swirl facilities were estimated at about
$692,000, of which the swirl regulator/concentrator cost was estimated at
about $168,000 or about $148,000/m3-s ($6,500/Mgal-d) of design capacity. The
other construction costs include the grit swirl and grit handling system, the
control building (which also houses the grit swirl), the instrumentation, and
the disinfection system. The costs are summarized in Table 115.
Table 115. SUMMARY OF SWIRL FACILITIES
CONSTRUCTION COSTS, LANCASTER3
Item
Cost, $
Swirl concentrator 168,000
Swirl degritter and
grit handling system 56,000
Control building 255,000
Instrumentation 146,000
Disinfection system 67,000
Total 692,000
a. ENR 3000.
308
-------
Operation and Maintenance Costs
The operation and maintenance cost for the first year of operation of the
demonstration facilities is estimated at about $58,000. The cost includes
operating personnel salaries, supplies, utilities, and a budget for equipment
repairs or replacement.
There is a large potential for startup problems and facilities debugging
efforts with any newly constructed facility. The annual operation and
maintenance budget after the startup period and the demonstration period is
expected to be about one-half the first year cost, about $26,000.
309
-------
REFERENCES
SECTION 1
2.
3.
2.
3.
4.
5.
CH2M Hill, Inc. 1978 Needs Survey, Cost Methodology for Control of
Combined Sewer Overflow and Stormwater Discharge. USEPA Report No. EPA-
430/9-79-003. February 10, 1979.
Areawide Assessment Procedures Manual, Volumes I and II. USEPA Report
No. EPA-600/9-76-014. July 1976.
Heaney, J.F. et al., (Vol. I and II), and M.J. Manning, et_al., (Vol.
III). Nationwide~Evaluation of Combined Sewer Overflows and Urban
Stormwater Discharges, Volumes I, II, and III. USEPA Report No. EPA-
600/2-77-064a, b, and c. NTIS Nos. PB 273 133, PB 266 005, and PB 272
107. 1977.
Lager, J.A., et al_. Urban Stormwater Management and Technology: Update
and Users' GuTJe. USEPA Report No. EPA-600/8-77-014.. NTIS No. PB 275
654. September 1977.
Lager, J.A. and W.G. Smith. Urban Stormwater Management and Technology:
An Assessment. USEPA Report No. 670/2-74-040. NTIS No. PB 240 687.
December 1974.
SECTION 2
Assistance Available from the Soil Conservation Service. United States
Department of Agriculture Soil Conservation Service, Agriculture
Information Bulletin 345. U.S. GPO: 1974 0-552-498.
USDA Soil Conservation Service. Stormwater Management Cost Study.
July 1977.
USDA Soil Conservation Service. Erosion and Sediment Control Handbook
for Connecticut. Revised 1976.
USDA Soil Conservation Service, Engineering Division. Urban Hydrology
for Small Watersheds, Technical Release No. 550. January 1975.
White, C.A., and A.L. Franks. Demonstration of Erosion and Sediment
Control Technology, Lake Tahoe Region of California. USEPA Report No.
EPA-600/2-78-208. NTIS No. PB 292 491. December 1978.
310
-------
6. Wanielista, M.P. and E.E. Shannon. Orlando Metropolitan 208 Study:
Stormwater Management Practices Evaluations. July 1977.
7. Pitt, R. Demonstration of Non-Point Pollution Abatement Through
Improved Street Cleaning Practices. USEPA Report No. EPA-600/2-79-161.
January 1979.
8. McCuen, R.H. On-Site Control of Nonpoint Soruce Pollution.
Proceedings: Stormwater Management Model (SWMM) Users Group Meeting,
November 13-14, 1978. USEPA Report No. EPA 600/9-79-003. November
1978.
9. Tahoe Regional Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan. Volume III: Assessment of Water Quality and
Environmental Impacts. January 1978.
10. Griffin, D.M., Jr., T.J. Grizzard, C.VI.-Randall, and J.P. Hartigan. An
Examination of Nonpoint Pollution Export From Various Land Use Types.
International Symposium on Urban Stormwater Management, Lexington,
Kentucky. July 24-27, 1978.
11. Characklis, W.G., et_aV..', Maximum Utilization of Water Resources in a
Planned Community - Stormwater Runoff Quality: Data Collection,
Reduction and Analysis. USEPA Report No. EPA-600/2-79-050b.
12. Stormwater Management Master Plan for Davis County, Utah - A Case
History in 208 Water Quality Management Planning. USEPA Report No. EPA-
440/3-77-023. May 1978.
13. Montgomery County Stormwater Management Control by Land Use, 1977.
Data provided by Montgomery County.
14. USDA Soil Conservation Service. Stormwater Management Cost Study,
Montgomery County, Maryland. July 1977.
15. Draft Watts Branch Watershed Storm Water Management Concept Plan.
Montgomery County Department of Environmental Protection. July 1977.
16. Davis, W.J. Sediment Basin Trap Efficiency Study. Montgomery County,
Maryland. Presented at the 1978 Winter Meeting of the American Society
of Agricultural Engineers, Chicago, December 20, 1978.
17. Rummel, Klepper & Kahl. Preliminary Engineering Report for Crabbs
Branch Storm Water Management Project, Montgomery County,, Maryland.
August 1978.
18. Matz, Childs & Associates, Inc. Report on Storm Water Management for
Wheaton Branch Watershed of Sligo Creek. June 1973.
v
19. Kramer, Chin & Mayo - Water Resources Engineers, Inc./Yoder, Trotter,
Orlob & Associates. Drainage Master Plan, City of Bellevue. July 30,
1976.
311
-------
20. Stottler, Stagg and Associates/Brevard Engineering Company. Orlando
Metropolitan 208 Study: Stormwater Management Practices Manual.
November 1977.
21. Wanielista, M.P. Stormwater Management, Quantity and Quality. Ann
Arbor Science, Ann Arbor, Michigan. 1978.
22. McCuen, R.H. Stormwater Management Policy and Design. Department of
Civil Engineering. University of Maryland, College Park, Maryland.
1979.
23. City of Bellevue Department of Public Works Storm and Surface Water
Utility. Guidelines for Stormwater Runoff Detention Facilities.
December 1975.
24. Orange County, Florida. Orange County Ordinances, Section 9.6 -
Stormwater Management.
25. Durham Planning and Zoning Commission. Amendments to Durham
Subdivision Regulations, Section IV.A.I. 1976.
26. Haddam Planning Commission. Subdivision Regulations of the Town of
Haddam, Connecticut, Sections 2.3.9 and 4.6. 1973.
27. Boulder, Colorado. Storm Drainage and Flood Control Regulations,
Chapter 8. 1974.
28. Tahoe Regional Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan, Volume II - Handbook of Best Management Practices.
January 1978.
29. Leiser, C.P. Computer Management of a Combined Sewer System. USEPA
Report No. EPA-670/2-74-022. NTIS No. PB 235 717. July 1974.
30. Metropolitan Engineers. Annual Overflow Volume Reduction - Total
System, Seattle/Metro Combined Sewers. February 1978.
31. Mahida, V.U. and F.J. DeDecker. Multi-Purpose Combined Sewer Overflow
Treatment Facility, Mount Clemens, Michigan. USEPA Report No. EPA-
670/2-75-010. NTIS No. PB 242 914. May 1974.
32. Lager, J.A., et al_. Urban Stormwater'Management and Technology:
Update and Users' Guide. USEPA Report No. EPA-600/8-77-014. NTIS No.
PB 275 654. September 1977.
33. Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan, on
Preliminary Design of the Hancock Street Combined Sewage Overflow
Storage and Treatment Facility. March 16, 1973.
34. Sullivan, R.H. et a]_. Relationship Between Diameter and Height for the
Design of a Swirl Concentrator as a Combined Sewer Overflow Regulator.
USEPA Report No. EPA-670/2-74-039. NTIS No. PB 234 646. July 1974.
312
-------
35. O'Brien & Gere, Engineers. Disinfection/Treatment of Combined Sewer
Overflows - Syracuse, N.Y. USEPA Report No. EPA-600/2-79-134. NTIS No.
PB 80-113459. August 1979.
36. Heaney, J.P., et^ al_. Storm Water .Management Model: Level I -
Preliminary Screening Procedures. USEPA Report No. EPA-600/2-76-275.
NTIS No. PB 259 916. October 1976.
37. Metcalf & Eddy, Inc. Development of Planning and Design Principles for
Urban Stormwater and Combined Sewer Overfl ow Treatment Facilities, SFMAC
Documentation. Contract No. 68-03-2877. Work in Progress.
SECTION 3
1. Lager, J.A., et al_. Urban Stormwater Management and Technology: Update
and Users' GuTHe. USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
2. Manning, M.J., _et al_. Nationwide Evaluation of Combined Sewer Overflows
and Urban Stormwater Discharges, Volume III: Characterization of
Discharges. USEPA Report No. EPA-600/2-77-064c. NTIS No. PB 272 107.
August 1977.
3. Pitt, R. and M. Bozeman. Water Quality and Biological Effects on Urban
Runoff on Coyote Creek. USEPA Grant No. R805418. June "1979. Draft.
4. Randall, C.W., T.J. Grizzard, and R.C. Hoehn. Impact of Urban Runoff
on Water Quality in the Occoquan Watershed. Virginia Water Resources
Research Center, Bulletin 80, May 1978.
5. White, C.A. and A.L. Franks. Demonstration of Erosion and Sediment
Control Technology, Lake Tahoe Region of California. USEPA Report No.
EPA-600/2-78-208. NTIS No. PB 292 491. December 1978.
6. Metcalf & Eddy, Inc. City and County of San Francisco Department of
Public Works, DPW Order No. 108,332. Southwest Water Pollution Control
Plant Project, Draft Project Report. May 1979.
7. The News, Runoff Control Study Backed by Builders. The American City &
County, p.30. July 1979.
SECTION 4
1. Kramer, Chin & Mayo - Water Resources Engineers, Inc./Voder, Trotter,
Orlob & Associates. Drainage Master Plan, City of Bellevue. July 30,
1976.
2. Climate of the States, Volume II - Western States. Water Information
Center, Inc. 1974.
313
-------
3.
4.
5.
6.
7.
8.
2.
3.
4.
5.
6.
Stevens, Thompson & Runyan, Inc. Environmental Management for the
Metropolitan Area, Part III Appendix B - Water Quality Analyses.
Municipality of Metropolitan Seattle and the USEPA (Project No. 1GA
00037). December 1974.
Cyre, H. Storm and Surface Water Management Trends in Bellevue,
Washington. Personal letter and communications, May 9, 1979.
City of Bellevue Department of Public Works Storm and Surface Water
Utility. Guidelines for Stormwater Runoff Detention Facilities.
December 1975.
City of Bellevue, Washington. Ordinance No. 2429 (rate classification
for the Storm and Surface Water Utility). May 27, 1977.
Welch, E.G, T. Wiederholm, D.E. Spyridakis, and C.A. Rock. Nutrient
Loading and Trophic State of Lake Sammamish, Washington. Department
of Civil Engineering, University of Washington, Seattle, Washington.
Edmondson, W.T. Lake Washington, Section VI - Washington. Department
of Zoology, University of Washington, Seattle, Washington.
SECTION 5
8.
Climate of the States, Volume I - Eastern States. Water Information
Center, Inc. 1974.
Draft Watts Branch Watershed Storm Water Management Concept Plan.
Montgomery County Department of Environmental Protection. July 1977.
Northern Virginia Planning District Commission and Virginia
Polytechnic Institute and State University. Planning for Nonpoint
Pollution Management. Prepared for EPA Conference on Watershed
Management R&D, Athens, Georgia. October 18-20, 1977.
Smullen, O.T., J.P. Hartigan, and T.J. Grizzard. Assessment of Runoff
Pollution in Coastal Watersheds.
Griffin, D.M., Jr., T.J. Grizzard, C.W. Randall, and J.P. Hartigan.
An Examination of Nonpoint Pollution Export from Various Land Use
Types. International Symposium on Urban Storm Water Management,
Lexington, Kentucky. July 24-27, 1978.
USDA Soil Conservation Service. Stormwater Management Cost Study.
July 1977.
Montgomery County Stormwater Management Control by Land Use, 1977.
Data Provided by Montgomery County, Maryland.
Rummel, Klepper & Kahl. Preliminary Engineering Report for Crabbs
Branch Storm Water Management Project, Montgomery County, Maryland.
August 1978.
314
-------
9.
10.
11.
12.
13.
14.
1.
2.
3.
4.
5.
6.
7.
McCuen, R.H. On-Site Control of Nonpoint Source Pollution.
Proceedings: Stormwater Management Model (SWMM) Users Group Meeting,
November 13-14, 1978. USEPA Report No. EPA 600/9-79-003. November
1978.
Matz, Childs & Associates, Inc. Report on Storm Water Management for
Wheaton Branch Watershed of Sligo Creek. June 1973.
McCuen, R.H. Stormwater Management Policy and Design. Department of
Civil Engineering, University of Maryland, College Park, Maryland.
1979.
USDA Soil Conservation Service, Engineering Division. Urban Hydrology
for Small Watersheds, Technical Release No. 55. January 1975.
Davis, W.J. Sediment Basin Trap Efficiency Study. Montgomery County,
Maryland. Presented at the 1978 Winter Meeting of the American Society
of Agricultural Engineers, Chicago, December 20, 1978.
Cost Information on Stormwater Source Control Facilities. Montgomery
County Expenditure and Appropriation Schedules. January 1, 1979.
SECTION 6
Tahoe Regional Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan, Approved Summary. January 1978.
Tahoe Regitinal Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan, Volume III: Assessment of Water Quality and
Environmental Impacts. January 1978.
California Tahoe Regional Planning Agency (CTRPA). Regional Plan for
Lake Tahoe, California. August 29, 1975.
Holm-Hansen, 0., C.R. Goldman, R. Richards, and P.M. Williams.
Chemical and Biological Characteristics of a Water Column in Lake
Tahoe. Reprint from Limnology and Oceanography, Vol. 21, No. 4, July
1976. pp. 548-562.
Goldman, C.R. and E. DeAmezaga. Spatial and Temporal Changes in the
Primary Productivity of Lake Tahoe, California-Nevada between 1959 and
1971. Verh Internet. Verein. Limnol, 19, Stuttgart. October 1975. pp.
812-825.
Goldman, C.R. and T.A. Cahill. Danger Signs for Tahoe's Future.
Reprint from Cry California, Spring, 1975.
Paerl, H.W. and C.R. Goldman. Stimulation of Heterotrophic and
Autotrophic Activities of a Planktonic Microbial Community by Siltation
at Lake Tahoe, California. (Proceedings of the IBP-UNESCO Symposium on
Detritus and its Role in Aquatic Ecosystems, Pallanza, Italy, May 23-
27, 1972). Mem. 1st. Ital. Idrobiol., 29 Suppl.: 129-147. 1972.
315
-------
8. Tahoe Regional Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan, Volume II - Handbook of Best Management Practices.
January 1978.
9. Tahoe Regional Planning Agency (TRPA). Tahoe Regional Planning Agency
An Overview. September 1975.
10. White, C.A. and A.L. Franks. Demonstration of Erosion and Sediment
Control Technology, Lake Tahoe Region of California. USEPA Report No.
EPA-600/2-78-208. NTIS No. PB 292 491. December 1978.
SECTION 7
1. Characklis, W.G., jet jiK Maximum Utilization of Water Resources in a
Planned Community - Stormwater Runoff Quality: Data Collection,
Reduction and Analysis. USEPA Report No. EPA-600/2-79-050b.
2. Hollinger, R.H. and T.I. Haigh. Field Evaluation of Porous Paving.
USEPA Grant No. 802433. Draft.
3. Ward, C.H. and J.M. King. Eutrophication Potential of Surface Waters
in a Developing Watershed. USEPA Grant No. 802433. July 1976. Draft.
4. Hammond, B. and J. Bishop, Jr. Maximum Utilization of Water Resources
in a Planned Community - Chlorine and Ozone Toxicity Evaluations.
USEPA Report No. EPA-600/2-79-050e.
5. Davis, E.M. Maximum Utilization of Water Resources in a Planned
Community - Bacterial Characteristics of Stormwaters in Developing
Rural Areas. USEPA REport No. EPA-600/2-79-050f.
6. Fisher, F.M. Contributions of Refractory Compounds by a Developing
Community. USEPA Grant No. 802433. Draft.
7. Finn, R.M., Metcalf & Eddy, Inc. Personal Communication During Visit
to The Woodlands, Texas. February 1979.
SECTION 8
1. Shannon, E.E. Orlando Metropolitan 208 Study: A Preliminary
Assessment of the Drainage Well Situation in the Orlando Area.
September 1977.
2. East Central Florida Regional Planning Council. Orlando Metropolitan
Areawide Water Quality Management Plan 208, Vol. 1 and 2. June 1978.
y
3. East Central Florida Regional Planning Council. Orlando Metropolitan
Areaswide Water Quality Management Plan 208, Vol. 3. June 1978.
4. Loop, J.A. Memorandum for the Record, Metcalf & Eddy, Inc., Palo Alto,
California. Trip Report for Orlando, Orange County, Florida. June
1978.
316
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5. Orange County, Florida. Orange County Ordinances, Section 9.6 -
Stormwater Management.
6. East Central Florida Regional Planning Council. Orlando fletropolitan
Areawide Water Quality Management Plan 208, Vol. 4. June 1978.
7. Wanielista, M.P. and E.E. Shannon. Orlando Metropolitan 208 Study:
Stormwater Management Practices Evaluations. July 1977.
8. Stottler, Stagg and Associates/Brevard Egnineering Company. Orlando
Metropolitan 208 Study: Stormwater Management Practices Manual.
November 1977.
SECTION 9
1. Pitt, R. Demonstration of Non-Point Pollution Abatement Through
Improved Street Cleaning Practices. USEPA Report No. EPA-600/2-79-161.
January 1979.
2. Pitt, R. and M. Bozeman. Water Quality and Biological Effects on Urban
Runoff on Coyote Creek. USEPA Grant No. R805418. June 1979.
3. Fleming, R.R., editor. Street Cleaning Practice, Third Edition.
American Public Works Association, Chicago, Illinois. 1978.
4. Amy, G., et. al_. Water Quality Management Planning for Urban Runoff.
USEPA Report No. EPA-440/9-75-004. NTIS No. PB 241 689. December
1974.
5. American Public Works Association. Street Cleaning Questionnaire.
Chicago, Illinois. 1975 (unpublished).
6. Scott, J.B. The American City 1970 Survey of Street Cleaning
Equipment. Market Research Report No. 81-1270. American City.
December 1970.
7. Mainstem, Inc. Special Street Cleaning Study. Princeton, New Jersey.
1973. (unpublished).
8. Sehmel, G.A. Particle Resuspension from Asphalt Roads Caused by Car
and Truck Traffic. Atmospheric Environment, Vol. 7, No. 3, p. 291-309.
1973.
9. Cowherd, C., Jr., et al_. Quantification of Dust Entrainment from
Paved Roadways. USEPA Report No. EPA-450/3-77-027. July 1977.
10. PEDCo - Environmental, Inc. Control of Reentrained Dust from Paved
Streets. USEPA Report No. EPA-907/9-77-007. 1977.
317
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SECTION 10
1.
2.
3.
4.
5.
6.
7.
8.
1.
2.
3.
4.
5.
USDA Soil Conservation Service. Erosion and Sediment Control Handbook
for Connecticut. SCS, Storrs, Connecticut. 1976.
Durham Planning and Zoning Commission. Amendments to Durham
Subdivision Regulations, Section IV.A.I. 1976.
Haddam Planning Commission Subdivision Regulation of the Town of
Haddam, Connecticut, Sections 2.3.9 and 4.6. 1973.
Eastern Connecticut Resource Conservation and Development Project, and
Soil Conservation Service. Environmental Review Team Report on
Industrial Sites, Essex, Connecticut. August 1976.
USDA Soil Conservation Service, Engineering Division. Urban Hydrology
for Small Watersheds, Technical Release No. 550. January 1975.
Lager, J.A. et al. Urban Stormwater Management and Technology: Update
and Users' GuTdeT USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
Manning, M.J., et al_. Nationwide Evaluation of Combined Sewer Overflows
and Urban Stormwater Discharges, Volume III: Characterization of
Discharges. USEPA Report No. EPA-600/2-77-064C. NTIS No. PB 272 107.
August 1977.
Tahoe Regional Planning Agency (TRPA). Lake Tahoe Basin Water Quality
Management Plan, Volume III: Assessment of Water Quality and
Environmental Impacts. January 1978.
SECTION 11
Pontius, F.W. Characterization and Treatment of Stormwater Runoff.
Graduate Thesis, Department of Civil, Environmental, and Architectural
Engineering, University of Colorado, Boulder, Colorado. 1977.
Battaglia, G.M. Pollutional Characteristics of Urban Snowmelt Runoff.
Graduate Thesis, Department of Civil, Environmental, and Architectural
Engineering, University of Colorado, Boulder, Colorado. 1977.
City of Boulder Planning Department and Boulder County Land Use
Department. The Boulder Valley Comprehensive Plan, June 1978.
Denver Regional Council of Governments, Clean Water Program.
Report (208 Program). October 1977.
Technical
City of Boulder, Colorado. The Revised Code of the City of Boulder,
Storm Drainage and Flood Control Regulations - Chapter VIII. (Ord. No.
3927 §1 (part)). 1974.
318
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6. Wright-Mclaughlin Engineers. Urban Storm Drainage Criteria Manual,
Volumes I and II. NTIS Nos. PB 185 262, and PB 185 263. March 1969.
7. Poertner, H.G. Practices in Detention of Urban Stormwater Runoff, An
Investigation of Concepts, Techniques, Applications, Cost, Problems,
Legislation, Legal Aspects, and Opinions. American Public Works
Association Special Report No. 43. 1974.
SECTION 12
1. Warburton, J. Seattle's Approach to Evaluating Costs and Benefits of
Combined Sewer Overflow Control Per PGM-61. Prepared for USEPA
Technology Transfer Program Seminar on Combined Sewer Overflow
Assessment and Control Procedures. May 18, 1978.
2. Warburton, J. Combined Sewer Overflow Control Graph - Metropolitan
Seattle Annual Report. April 1978.
3. Metropolitan Engineers. Draft Facility Plan for Upgrading Metro Puget
Sound Plants—System-Wide Volume - Part 1, Basis of Planning. 1977.
4. Metropolitan Engineers. West Point Facilities Plan Appendix B2,
Combined Sewer Overflow Analysis. 1978.
5. Tomlinson, R.D., et_ al_. Fate and Effects of Sediments from Combined
Sewer and Storm Drain Overflows in Seattle Nearshore Waters, First
Quarter Report: November 1977 - January 1978. USEPA Grant No. R805602.
February 1978. (EPA-600/2-80-111)
6. Gibbs, C.V. and G.W. Isaac. Seattle Metro's Duwamish Estuary Water
Quality Program. Journal of Water Pollution Control Federation. Vol.
40, No. 3, Part 1. March 1968.
7. Metropolitan Engineers. Summary of Draft 201 Facility Plan for Upgrading
Metro Puget Sound Plants. September 1977.
8. City of Seattle Department of Engineering. Proposed City of Seattle
Drainage Management Program. January 1978.
9. Leiser, C.P. Computer Management of a Combined Sewer System. USEPA
Report No. EPA-670/2-74-022. NTIS No. PB 235 717. July 1974.
10. Lager, J.A., ejt _al_. Urban Stormwater Management and Technology: Update
and Users' Guide. USEPA Report No. EPA-600/8-77-014. NTIS No. PB 275
654. September 1977.
11. Metropolitan Engineers. West Point Facilities Plan, Appendix A. Task
B3D, Hydraulic Analysis of Combined Sewer Network. 1977.
12. Municipality of Metropolitan Seattle. Metro Seattle CATAD Computer Tape:
Rainfall and Combined Sewer Overflow Data. 1970-1977.
319
-------
13.
14.
15.
16.
1.
2-
3.
4.
5.
1.
2.
3.
Leiser, C.P. Municipality of Metropolitan Seattle. Maximizing Storage
in Combined Sewer Systems. USEPA Report No. 11022ELK. NTIS No. PB 209
861. December 1971.
Metropolitan Engineers. Final Environmental Impact Statement, City of
Seattle Sewer Separation Program. September 1973.
Metropolitan Engineers. Draft Facility Plan for Upgrading Metro Puget
Sound Plants—System-Wide Volume - Part 2, Facility Planning
Alternatives. 1977.
Metropolitan Engineers. Annual Overflow Volume Reduction - Total
System, Seattle/Metro Combined Sewers. February 1978.
SECTION 13
Feuerstein, D.L. and W.O. Maddaus. Wastewater Management Program,
Jamaica Bay, New York, Volume II: Supplemental Data. New York City
Spring Creek. USEPA Report No. EPA-600/2-76-222b. NTIS No. PB 258
308. September 1976.
Commonwealth of Massachusetts, Metropolitan District Commission.
Cottage Farm Combined Sewer Detention and Chlorination Stations,
Cambridge, Massachusetts. USEPA Report No. EPA-600/2-77-046. NTIS No.
PB 263 292. November 1976.
Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan on Waste
Water Treatment Facilities and Intercepting System. March 8, 1967.
Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan upon the
Recommended Plan for Abating Pollution from Combined Sewage Overflows.
March 21, 1972.
Metcalf & Eddy, Inc. Report to the City of Saginaw, Michigan on
Preliminary Design of the Hancock Street Combined Sewage Overflow
Storage and Treatment Facility. March 16, 1973.
SECTION 14
Spalding, DeDecker & Associates, Inc. City of Mount Clemens, Michigan
Facilities Plan for Combined Sewer Overflows. USEPA Program C-262491
FY1974. December 1974.
Driker Associates, Inc.
Plan. April 1970.
Mount Clemens, Michigan General Development
Mahida, V.U- and F.J. DeDecker. Multi-Purpose Combined Sewer Overflow
Treatment Facility, Mount Clemens, Michigan. USEPA Report No. EPA-
670/2-75-010. NTIS NO. PB 242 914. May 1974.
320
-------
4.
5.
6.
7.
8.
9.
10.
1.
2.
3.
4.
5.
6.
Spalding, DeDecker & Associates, Inc. City of Mount Clemens Amendment
to December 1974 Facilities Plan for Dry Weather Flow Wastewater
Pollution Control. USEPA Project C-262491 FY1974. December 1974.
National Oceanic and Atmospheric Administration, U.S. Department of
Commerce. Climates of the States. Vol. I. 1974.
Spalding, DeDecker & Associates, Inc. Mount Clemens, Michigan Report:
A Complete Pollution Control Program for the City. December 11, 1967.
Spalding, DeDecker & Associates, Inc. City of Mount Clemens Combined
Wastewater Retention and Treatment Facilities, Retention Basin Site
Operation and Maintenance Manual. USEPA Project No. C-262088. June
1976.
City of Mount Clemens Combined Sewer Overf 1 ow.Treatment Facility
Operation and Maintenance Manual. USEPA Project C-262491. March 1978.
Lager, J.A., jet al. Urban Stormwater Management and Technology:
Update and Users^Guide. USEPA Report No. EPA-600/8-77-014. NTIS No.
PB 275 654. September 1977.
Lager, J.A. and W.6. Smith.
Technology: An Assessment.
PB 240 687. December 1974.
Urban Stormwater Management and
USEPA Report No. 670/2-74-040. NTIS No.
SECTION 15
Demonstration of a Dual Functioning Swirl Combined Sewer Overflow
Regulator/Solids Liquid Separator. USEPA Grant No. 11023 GSC/S-802219.
In progress (1979).
O'Brien & Gere, Engineers. Disinfection/Treatment of Combined Sewer
Overflows - Syracuse, N.Y. USEPA Report No. EPA-600/2-79-134. NTIS No.
PB 80-113459. August 1979.
Field, R. and R.P. Traver. Development of and Application of the Swirl
and Helical Bend Devices for Combined Sewer Overflow Abatement and
Runoff Control. Presented at USEPA Technology Transfer Seminar Series
on Combined Sewer Overflow Assessment and Control Procedures. 1978.
Field, R., and R.P. Traver. Urban Runoff Flow Regulator/Concentrators.
Proceedings of the Environmental Engineering Conference, ASCE. San
Francisco. July 9-11, 1979.
Sullivan, R.H., et a\_. Relationship Between Diameter and Height for the
Design of a SwirT~Concentrator as a Combined Sewer Overflow Regulator.
USEPA Report No. EPA-670/2-74-039. NTIS No. PB 234 646. July 1974.
Ichthyological Associates, Inc.
Pennsylvania. July 29, 1977.
Conestoga River Studies, Lancaster,
321
-------
7. Meridian Engineering, Inc. City of Lancaster, Pennsylvania, Silo
Demonstration Project. USEPA Grant No. 11023 GSC. 1973.
8. Sullivan, R.H., et al. The Swirl Concentrator as a Combined Sewer
Overflow Regulator Facility. USEPA Report No. EPA-R2-72-008. NTISJio.
PB 214 687. September 1972.
9. Field, R. The Dual Functioning Swirl Combined Sewer Overflow
Regulator/Concentrator. USEPA Report No. EPA-670/2-73-059. NTIS No. PB
227 182. September 1973.
322
-------
GLOSSARY
Aerated lagoon—A natural or artificial wastewater treatment lagoon (generally
from 4 to 12 feet deep) in which mechanical or diffused-air aeration is used
to supplement the oxygen supply.
Benthic—Of or pertaining to the bottom of streams, rivers, lakes, or oceans.
Biological treatment processes—Means of treatment in which bacterial or bio-
chemical action is intensified to stabilize, oxidize, and nitrify the unstable
organic matter present. Trickling filters, activated sludge processes, and
lagoons are examples.
BMP—Best Management Practices. Nonstructural and low structurally intensive
measures for controlling stormwater pollution by correcting the problem at its
source.
BOD—Biochemical Oxygen Demand. The quantity of dissolved oxygen used by
microorganisms in the biochemical oxidation of organic matter and oxidizable
inorganic matter by aerobic biological action. Generally refers to the stan-
dard 5-day BOD test.
Combined sewage—Sewage containing both domestic sewage and surface water or
stormwater, with or without industrial wastes. Includes flow in heavily in-
filtrated sanitary sewer systems as well as combined sewer systems.
Combined sewer—A sewer receiving both intercepted surface runoff and municipal
sewage.
Combined sewer overflow--Flow from a combined sewer in excess of the inter-
ceptor capacity that is discharged into a receiving water.
COD—Chemical Oxygen Demand. The quantity of oxygen required to oxidize
organic matter in the presence of a strong oxidizing agent in an acidic medium.
CSO—Combined Sewer Overflow.
Curve numbers (CN)—A table of numbers, developed by the Soil Conservation
Service to calculate stormwater runoff for various land surface conditions.
The numbers are a function of land use, hydro!ogic soil group, and antecedent
moisture conditions.
Detention—The slowing, dampening, or attenuating of flows either entering the
sewer system or within the sewer system by temporarily holding the water on a
surface area, in a storage basin, or within the sewer itself.
323
-------
Pisinfection—The destruction of the larger portion of microorganisms in or on
a substance with the probability that all pathogenic bacteria are killed by
the agent used.
Diversity—A measure of the relationship between the number of species and the
total population of a biological community. Values determined by the Shannon-
Weaver formula.
Domestic sewage—Sewage derived principally from dwellings, business buildings,
institutions, and the like. It may or may not contain groundwater.
Dry weather flow—Base flow in sanitary or combined sewers including sanitary,
industrial, and inflow/infiltration flows during nonstorm periods. .
Dual treatment—Those processes or facilities designed for operating on both
dry- and wet-weather flows.
Dynamic regulator—A semiautomatic or automatic regulator device which may or
may not have movable parts that are sensitive to hydraulic conditions at their
points of installation and are capable of adjusting themselves to variations
in such conditions or of being adjusted by remote control to meet hydraulic
conditions at points of installation or at other points in the total combined
sewer system.
Equalization—The averaging (or method for averaging) of variations in flow
and composition of a liquid.
Eutrophication—The maturing natural or artificial process of a water body,
characterized by high concentrations of nutrients and periods of oxygen
deficiency.
First flush—The condition, often occurring in storm sewer discharges and com-
bined sewer overflows, in which a disproportionately high pollutional load is
carried in the first portion of the discharge or overflow.
Infiltration—The water entering a sewer system and service connections from
the ground, through such means as, but not limited to, defective pipes, pipe
joints, connections, or manhole walls. Infiltration does not include, and is
distinguished from, inflow.
Inflow—The water discharged into a sewer system and service connections from
such sources as, but not limited to, roof leaders, cellar, yard, and area
drains, foundation drains, cooling water discharges, drains from springs and
swampy areas, manhole covers, cross connections from storm sewers and combined
sewers, catchbasins, stormwaters, surface runoff, street wash waters, or
drainage. Inflow does not include, and is distinguished from, infiltration.
In-system—Within the physical confines of the sewer pipe network or treat-
ment system.
Intercepted surface runoff—That portion of surface runoff that enters a
sewer, either storm or combined, directly through catchbasins, inlets, etc.
324
-------
Interceptor (intercepting sewer)--A sewer that receives dry-weather or wet-
weather flow from a number of transverse combined sewers and additional pre-
determined quantities of intercepted surface runoff and conveys such waters to
a point for treatment.
Intermittent point sources—Any discernible, confined, and discrete conveyance
from which pollutants are or may be discharged on a noncontiguous basis.
Municipal sewage—Sewage from a community.
sewage, industrial wastes, or both.
May be composed of domestic
Natural treatment—The use of wetlands, marshes, swamps, etc., for the
reduction of pollutant concentrations of stormwater or secondary effluent
from sanitary sewage treatment plants.
Nonpotnt source—Any unconfined and nondiscrete conveyance from which
pollutants are or may be discharged.
Nonsewered urban runoff—That part of the precipitation that runs off the
surface of an urban drainage area and reaches a stream or other body of water
without passing through a sewer system.
Offsite storage-^Source detention methods used at a location that is down-
stream or separated from the site or sites being controlled. Offsite storage
is used to control stormwater from developed areas.
Onsite storage—Source detention methods used directly on the site being
controlled.
Overflow—(1) The flow discharging from a sewer resulting from combined
sewage, stormwater, or extraneous flows and normal flows that exceed the sewer
capacity. (2) The location at which such flows leave the sewer.
Physical-chemical treatment processes—Means of treatment in which the removal
of pollutants is brought about primarily by chemical clarification in conjunc-
tion with physical processes. The full process string generally includes
preliminary treatment, chemical clarification, filtering, carbon absorption,
and disinfection.
Physical treatment operations—Means of treatment in which the application of
physical forces predominates. Screening, sedimentation, flotation, and
filtration are examples. Physical treatment operations may or may not include
chemical additions.
Point source—Any discernible, confined, and discrete conveyance from which
pollutants are or may be discharged.
Pollutant—Any harmful or objectionable material in or change in the
physical characteristic of water or sewage.
325
-------
Primary productivity—The rate at which energy-containing materials are formed
by plants usually,-'measured as mg carbon/m2-d. The rate is a function of
water quality, substrate, temperature (seasonal), and solar illumination.
Pretreatment— The removal of material such as gross solids, grit, grease, and
scum from sewage flows prior to physical, biological, or physical-chemical
treatment processes to improve treatability. Pretreatment may include
screening, grit removal, skimming, preaeration, and flocculation.
Regulator—A structure that controls the amount of sewage entering an inter-
ceptor by storing in a trunk line or diverting some portion of the flow to
an outfall.
Retention—The prevention of runoff from entering the drainage system or
downstream water body by storing on a surface area or in a storage basin.
Sanitary sewer—A sewer that carries liquid and water-carried wastes from
residences, commercial buildings, industrial plants, and institutions,
together with relatively low quantities.of ground, storm, and surface waters.
SCS--Soil Conservation Service.
Sewer—A pipe or conduit generally closed, but normally not flowing full, for
carrying sewage, stormwater, or other waste liquids.
Sewerage—System of piping, with appurtenances, for collecting and conveying
wastewaters from source to discharge.
S(3—Specific gravity.
Static regulator—A regulator device that has no moving parts or has movable
parts which are insensitive to hydraulic conditions at the point of instal-
lation and which'are not capable of adjusting themselves to meet varying
flow or level conditions in the regulator-overflow structure.
Storm flow—Overland flow, sewer flow, or receiving stream flow caused
totally or partially by storm runoff or snowmelt.
Storm sewer—A sewer that carries intercepted surface runoff, street wash and
other wash waters, or drainage; designed to exclude domestic sewage and
industrial wastes.
Storm sewer discharge—Flow from a storm sewer that is discharged into a
receiving water.
Stormwater—l\later resulting from precipitation that either percolates into
the soil, runs off freely from the surface, or is captured by storm sewer,
combined sewer, and to a limited degree sanitary sewer facilities.
Surcharge—The flow condition occurring in closed conduits when the hydraulic
grade line is above the crown of the sewer.
326
-------
Surface runoff—Precipitation that falls onto the surface of roofs, streets,
ground, etc., and is not absorbed or retained by that surface, thereby
collecting and running off.
Trunk sewer—That portion of a sanitary, storm, or combined sewer that
accepts flow from collectors and laterals for transport to an interceptor.
Urban runoff—Surface runoff from an urban drainage area that reaches a
stream or other body of water or. a sewer.
Wastewater--The spent water of a community. See Municipal Sewage and Combined
Sewage.
Wet-weather flow--Dry-weather flow plus surface water, stormwater, and/or
excess inflow/infiltration during or after storms.
327
-------
CONVERSION FACTORS
U.S. Customary to SI (Metric)
U.S. customary unit
Hamc
acre
acre-foot
acre-inch
cubic foot
cubic feet per minute
cubic feet per minute per 100 gallons
cubic feet per pound
cubic feet per second
cubic feet per square foot per minute
cubic Inch
cubic yard
degrees Fahrenheit
feet per minute
feet per second
foot (feet)
gallon(s)
gallons per acre per day
gallons per capita per day
gallons per day
gallons per foot per minute
gallons per minute
gallons per square foot
gallons per square foot per day
gallons per square foot per minute
horsepower
Inch(es)
Inches per hour
nile
nil lion gallons
million gallons per acre
million gallons per acre per day
million gallons per day
Billion gallons per square mile
parts per billion
parts per million
pound(s)
pounds per acre per day
pounds per cubic foot
pounds per 1000 cubic feet
pounds per nile
pounds per Billion gallons
pounds per square foot
pounds per 1000 square feet per day
pounds per square inch
square foot
square inch
square Bile
square yard
standard cubic feet per minute
ton (short)
tons per acre
tons per square nile
yard
Abbreviation
acre
acre- ft
acre-in.
ft3
ft3/min
ft3/min-100 gal
ft3/lb
ft3/s
ft3/ft2-min
in.3
yd3
°F
ft/min
ft/s
ft
gal
gal/acre-d
gal/caplta-d
gal/d
gal/ft-min
gal/min
gal/ft2
gal/ft2-d
gal/ft2-min
hp
in.
1n./h
mi
Hgal
Hgal/acre
Mgal/acre-d
Mgal/d
Hgal/mi2
ppb
ppm
Ib
lb/acre-d
Ib/ft3
lb/1000 ft3
Ib/mi
Ib/Hgal
Ib/ft2
lb/1000 ft2-d
lb/1n.2
ft2
in. 2
mi 2'
yd2
std ft3/min
ton (short)
tons/acre
tons/mi 2
yd
Multiplier
0.405
1,233.5
102.79
28.32
0.0283
0.0283
0.00747
62.4
28.32
0.305
16.39
0.0164
0.765
764.6
0.555 (°F-32)
0.00508
0.305
0.305
3.785 ..
3.785 x 10"J
9.353
3.785
4.381 x 10-5
0.207
0.0631
40.743
1.698 x 10-3
0.283
2.445
0.679
0.746
2.54 '
2.54
1.609
3.785
3785.0
8353
0.039
43.808
0.0438
1.461
1461
1.0
1.0
0.454
453.6
1.121
16.018
16.018
0.016
0.282
0.120
4.882 x 10-1
4.882
4.882 x ID"3
0.0703
0.0929
6.452
2.590
259.0
0.836
1.699
0.907
2240
3.503
0.914
Symbol
ha
m3
m3
L
nH
m3/mi n
m3/min-100 L
L/kg
L/s
m3/m2 • mi n
cm3
L
m3
L
°C
m/s
m/s
m
L3
m
L/ha-d
L/capi'ta-d
L/s
L/m-s
L/s
L/m2
Iti3/in2.h
m3/ha-min
m3/m2.h
L/m2.s
kW
cm
cm/h
km
ML
(1)3
m3/ha
m3/mz-h
L/s
m3/s
ML/km2
m3/km2
mg/L
mg/L
kg
g
kg/m3
g/m3
kg/m3
kg/km
mg/L
kg/qp2
kg/m2
kg/m2-d
kg/cm2
m2
cm2
km2
ha
m2
m3/h
Hg (or t)
kg/ha
kg/ha
m
SI
Name
hectare
cubic metre
cubic metre
litre
cubic metre
cubic metres per minute
cubic metres per minute per 100 litres
litres per kilogram
litres per second
cubic metres per square metre per minute
cubic centimetre
litre
cubic metre
litre
degrees Celsius
metres per second
metres per second
metre(s)
litre(s)
cubic metre
litres per hectare per day
litres per capita per day
litres per second
litres per metre per second
litres per second
litres per square metre
cubic metres per square metre per hour
cubic metres per hectare per minute
cubic metres per square metre per hour
litres per square metre per second
kilowatts
centimetre
centimetres per hour
kilometre
megalitres (litres x 10°)
cubic metres
cubic metres per hectare
cubic metres per square metre per hour
litres per second"
cubic metres per second
megalitres per square kilometre
cubic metres per square kilometre
micrograms per litre
milligrams per litre
kilogram(s)
gram(s)
kilograms per hectare per day
kilograms per cubic metre
grams per cubic metre
kilograms per cubic metre
kilograms per kilometre
milligrams per litre
kilograms per square centimetre
kilograms per- square metre
kilograms per square metre per day
kilograms per square centimetre
square metre
square centimetre
square kilometre
hectare
square metre
cubic metres per hour
megagram (metric tonne)
kilograms per hectare
kilograms per hectare
metre
328
-------
TECHNICAL REPORT DATA
(Please read Inductions on the reverse before completing)
1. REPORT NO.
EPA-600/8-80-035
3. RECIPIENT'S ACCESS1ON»NO.
4. TITLE AND SUBTITLE
URBAN STORMWATER MANAGEMENT AND TECHNOLOGY:
Case Histories
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
August 1980 (Issuing Date)
'. AUT
3)
William G. Lynard, E. John Finnemore, Joseph A. Loop,
and Robert M. Finn
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADORES
Metcalf & Eddy, Inc.
1029 Corporation Way
P 0 Box 10-046
Palo Alto. California 94303
10. PROGRAM ELEMENT NO.
55B1C,DU No. B124,Task 10546
11. CONTRACT/GRANT NO.
3-03-2617
JG AGENCY NAME AND ADORES
Municipal Environmental Research Laboratory - Cin., OH
Dffice of_Research and Development
J. S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
final Dec. 1977 - Nov. 1979
14. SPONSORING AGENCY CODE
EPA/600/14
5. SUPPLEMENTARY NOTES
Continuation of EPA-670/2-74-040 and EPA-600/8-77-014.
Field, Chief, Storm and Combined Sewer Section, Edison,
Project Officer: Richard
NJ (201) 321-6674, 8-340-6674
This report is the third in a series on urban stormwater and combined sewer overflow
management. It presents 12 case histories representing most promising approaches to
stormwater control. The case histories were developed by evaluating completed and
operational facilities or ong9ing demonstration projects that have significant in-
formation value for future guidance. Essential elements of the case history evalu-
ations.cover (1) approach methodology, (2) design considerations, (3) costs, (4)
effectiveness, and (5) environmental and socioeconomic impacts. Eight of the case
histories assess Best Management Practices (BMPs) and expand the data base on source
control methodology,.focusing principally on planning and storage alternatives. Special
considerations are given to flood and erosion control measures also having a dual
Benefit of stormwater control. The project sites evaluated are Bellevue, Washington-
Montgomery C9unty, Maryland; Lake Tahoe, California; The Woodlands, Texas; Orange
County Florida; San Jose, California; Middlesex County, Connecticut; and Boulder
Colorado. The remaining four case histories evaluate the control of combined sewage
overflows and document a systems approach in applying unit process -alternatives. The
effectiveness and unit costs of storage and treatment processes are presented, together
.71 I-H evaluations of areawide and systemwide integration of these technologies. Storage
f^ TT d I a-m^T-* f- .-V -P rt-rt -l*>*4-ASW»>-t4-Aj «__ — _ — _T_ *._ _T . / . . i °_ . O
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Drainage, Water pollution, Waste treatment,
lurface water runoff, Water quality, Cost-
Effectiveness, Storage tanks, Storm sewers,
Overflows—^sewers, Combined sewers,
Hydrology, Remote control
b.lDENTIFIERS/OPEN ENDED TERMS [c!COSATI Field/Group
Best management practices
Street sweeping, Source
storage, Planning
Combined sewage overflow
controls, Storage/
treatment, Systems
approach.
13B
PEMENT
19. SECURITY CLASS (ThisReport)'
Unclassified
Release to public
20. SECURITY CLASS (Thispage)
Unclassified
21. NO. OF PAGES
355
22. PRICE
EPA Form 2220-1 (9-73)
329
U.S. GOVERNMENT PRINTING OFFICE: 1980--657-165/0150
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