DRAFT FINAL
March 1981
STORAGE/SEDIMENTATION FACILITIES
FOR CONTROL OF
STORM AND COMBINED SEWER OVERFLOWS
DESIGN MANUAL
by
W. Michael Stallard, William G. Smith, Ronald W. Crites, and John A. Lager
Metcalf & Eddy, Inc.
Palo Alto, California 94303
Contract No. 68-03-2877
Project Officer
Richard Field, Chief
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
Metcalf & Eddy Engineers
Palo Alto. California
-------
Metca!f& Eddy, Inc.
Engineers & Planners
Fox Plaza, Suite 418 1390 Market Street San Frar...ico, CA 94102
(415) 861-0240 TELEX 33-4427
J-6642.3
March 16, 1981
Mr. Richard Field, Chief
Storm and Combined Sewer Section
USEPA - MERL
Edison, NJ 08837
Subject: Contract 68-03-2877 Storage/Sedimentation Manual
Drar Rich:
Under the terms of the subject contract we are pleased to
furnish herewith five copies of the draft final report titled,
"Storage/Sedimentation Facilities for Control of Storm and
Combined Sewer Overflows: Design Manual". Please note that
while all figures, tables and examples are included, we have
not attempted to replay the draft text to provide an exact
placement at this time due to the cost involved and the small
benefit accomplished.
We understand that you plan to visit our San Francisco office
in the week of April 20-24, 1981 to pass on your review
comments and that we will be authorized up to $10,000 for in-
corporation of addition material from the Swedish and German
draft manuals as requested in our letter dated March 9, 1981.
Best regards,
A. Lager
President
JAL:lm
cc: R. Crites
R. Ad van i
PALO ALTO CHICAGO BOSTON
-------
DRAFT FINAL
March 1981
STORAGE/SEDIMENTATION FACILITIES
FOR CONTROL OF
STORM AND COMBINED SEWER OVERFLOWS
DESIGN MANUAL
by
W. Michael Stallard, William G. Smith, Ronald W. Crites, and John A. Lager
Metcalf & Eddy, Inc.
Palo Alto, California 94303
Contract No. 68-03-2877
Project Officer
Richard Field, Chief
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
PRELIMINARY COPY
NOTE: This document is preliminary only and
is not intended for any purpose except review
and comment by the owner and its agents.
-------
CONTENTS
Chapter Page
1 INTRODUCTION AND USERS' GUIDE
Stormwater, Urbanization, and Control 1-1
Purpose of the Manual 1-6
Users's Guide 1-6
Chapter 1 - Introduction and Users' Guide 1-6
Chapter 2 - Urban Stormwater - An Overview 1-7
Chapter 3 - Design of Detention Facilities 1-7
Chapter 4 - Design of Retention Storage Facilities 1-8
Chapter 5 - Stormwater Management System Integration 1-9
Chapter 6 - Regional Implementation of
Storage/Sedimentation 1-9
Appendixes 1-10
References 1-10
2 URBAN STORMWATER - AN OVERVIEW
Urban Hydrology 2-1
The Urban Hydrologic Cycle 2-1
Hydrology 2-4
Physical Characteristics of the Watershed 2-7
Land Use 2-7
Stormwater Drainage System 2-8
Hydrographs 2-8
Urban Stormwater Pollution 2-12
Stormwater Pollutants 2-15
Suspended Solids 2-15
Oxygen-Demanding Material 2-15
Pathogenic Bacteria 2-15
Nutrients 2-18
Toxic Substances 2-18
Sources of Stormwater Pollutants 2-23
Pollutant Concentrations . . 2-25
Stormwater Control 2-33
Storage/Sedimentation Controls 2-36
Categories of Storage/Sedimentation 2-36
References 2-37
3 DESIGN OF DETENTION FACILITIES
Introduction 3-1
Onsite Detention 3-4
Design Considerations 3-4
Tributary Area 3-6
Storage Area and Volume 3-6
Structural Considerations 3-6
Responsibility of the Owner 3-7
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CONTENTS (Continued)
Chapter Page
Design Procedure/Example 3-7
Step 1 - Identify Functional Requirements 3-7
Step 2 - Identify Site Constraints 3-9
Step 3 - Establish Basis of Design 3-9
Step 4 - Select Detention Option(s) 3-14
Step 5 - Estimate Costs and Cost Sensitivities 3-16
Step 6 - Complete Design 3-16
Operation and Maintenance Considerations 3-16
Inline Storage 3-17
Concept 3-17
Design Considerations 3-18
Size of Sewers 3-18
Slope of Sewer 3-19
Peak Flows 3-19
Controls 3-19
Resuspension of Sediment 3-19
Design Procedure/Examples 3-20
Step 1 - Identify Functional Requirements 3-20
Step 2 - Identify Site Constraints 3-22
Step 3 - Establish Basis for Design 3-22
Step 4 - Select Storage Locations 3-23
Step 5 - Estimate Costs and Cost Sensitivities 3-24
Step 6 - Complete Design 3-24
Operation and Maintenance Considerations 3-24
Costs 3-25
Downstream Storage/Sedimentation Basins 3-25
Concept 3-26
Design Considerations 3-26
Storage 3-27
Treatment Efficiency 3-31
Disinfection 3-33
Site Constraints . . 3-38
Design Procedure/Example 3-43
Step 1 - Identify Functional Requirements 3-43
Step 2 - Identify Site Constraints 3-45
Step 3 - Establish Basis of Design 3-45
Step 4 - Select Main Treatment Geometry 3-49
Step 5 - Identify and Select Pretreatment Components 3-60
Step 6 - Detail Auxiliary Systems 3-60
Step 7 - Estimate Costs and Cost Sensitivities 3-61
Step 8 - Complete Design 3-63
Operation and Maintenance Considerations 3-63
Costs 3-64
Capital Cost Breakdown - Illustrative Examples 3-65
Operation and Maintenance Costs 3-68
References 3-69
ii
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CONTENTS (Continued)
Chapter Page
4 DESIGN OF RETENTION STORAGE FACILITIES
Design Considerations 4-3
Size 4-3
Location 4-8
Design Procedure 4-9
Step 1 - Quantify Functional Requirements 4-10
Step 2 - Identify Required Waste Load and Flow Reduction 4-10
Step 3 - Determine Preliminary Basin Sizing 4-11
Step 4 - Identify Feasible Pond Sites 4-15
Step 5 - Investigate Most Promising Sites 4-19
Step 6 - Establish Basin Sizes 4-19
Step 7 - Design Solids Removal Technique and Facilities 4-22
Step 8 - Determine Configuration 4-25
Performance 4-25
Operations 4-32
Costs 4-37
References 4-42
5 STORMWATER MANAGEMENT SYSTEM INTEGRATION
The Integration Process 5-2
Identify Existing System and Needs 5-2
Establish System Needs 5-3
Identify Applicable Control Alternatives 5-3
Determine Control Method Compatibility 5-5
Retrofitting of Existing Flood Control Facilities 5-6
Sedimentation Basin Integration 5-7
Location 5-8
Functional Compatibility 5-10
Process Compatibility 5-11
Flood Control Retrofit 5-13
Facility Modifications 5-13
Functional Compatibility 5-15
Process Compatibility 5-16
Retention and Attenuation Facility Integration 5-17
Site Potential 5-17
Location 5-18
Functional Compatibility 5-19
Process Compatibility 5-20
References 5-21
6 REGIONAL IMPLEMENTATION OF STORAGE/SEDIMENTATION
European Practice 6-1
Scotland 6-2
Bavaria 6-6
Sweden 6-7
iii
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Chapter
Appendix
A
B
C
D
CONTENTS (Concluded)
United States Practice
Montgomery County, Maryland
Orange County, Florida
Elements of an Areawide Stormwater Mangement Program
Element 1 - Central Authority to Regulate Stormwater
Element 2 - Master Plan for the Watershed
Element 3 - Implementation Guidance
Element 4 - Enformcement Procedures
Summary
References
REFERENCES [References are included at the end of
each chapter in the draft report]
SOIL CONSERVATION SERVICE RUNOFF ANALYSIS METHOD
ASSESSMENT METHODS
INFILTRATION MEASUREMENT TECHNIQUES
Page
6-11
6-12
6-13
6-16
6-16
6-17
6-17
6-17
6-17
6-18
iv
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Chapter 1
INTRODUCTION AND USERS' GUIDE
As municipal wastewater treatment is upgraded in accordance with the
Federal Clean Water Act, urban stormwater runoff and combined sewer
overflows (CSO) are emerging as signficant sources of surface water
pollution in the United States. A 1975 survey of 56 public agencies
located throughout the United States revealed that "...control [of]
stormwater pollution from sources other than erosion to make significant
improvements in existing wet weather quality of streams, lakes, etc."
ranked second only to flood control as a stormwater management goal [1].
Temporary storage of runoff, a widely used method of flood control, is
gaining application in the United States as a means of reducing the
pollutant load of stormwater runoff. Existing flood control facilities
may be retrofitted and new flood control facilities designed to enhance
pollution removal. The purpose of this design manual is to summarize
applications of storage facilities to stormwater control and to present
step-by-step procedures for analysis and design of stormwater and CSO
storage/sedimentation treatment facilities.
STORMWATER, URBANIZATION, AND CONTROL
Urban runoff is not a new problem. .Among the earliest examples of
public works are urban drainage systems. The early sewers were designed
to minimize flooding by rapidly transporting converging runoff through
developed low-lying areas. The stormwater was discharged to natural
drainage channels or streams with little regard to downstream effects.
More recently, it has been recognized that urban areas have significant
impacts on stormwater runoff and on downstream areas through the runoff.
The response of a watershed to precipitation under undeveloped
conditions and urbanized conditions without stormwater controls 1s
Illustrated In Figure 1-la and b.
1-1
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LAND USE
HTDROLOQIC RESPONSE WATER QUALITY RESPONSE
UNDEVELOPED
LAND
DEVELOPED LAND
WITHOUT STORAGE
CONTROLS
UNDEVELOPED
HYDROGRAPH
DEVELOPED
.HYDROORAPH
UNDEVELOPED
POLLUTANT LOAD
DEVELOPED
POLLUTANT LOAD
OVERALL
WATERSHED
RESPONSE
ro
DEVELOPED LAND
WITH STORAGE
CONTROLS
ATTENUATED
HYDROGRAPH
DEVELOPED
POLLUTANT LOAD
POLLUTANT LOAD
CAPTURED
LOAD NOT
CAPTURED
Figure 1-1. Response of a watershed to precipitation
under different conditions [1].
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The hydrographs show rate of flow of runoff at a given point versus
time. Under natural conditions, rain falling on the ground surface may
do one of three things: 1t may be Intercepted and held on vegetation,
roots, and ground as the surfaces are wetted; it may Infiltrate Into the
ground surface and percolate downward to become part of the groundwater;
or 1t may collect on the surface, either in puddles or as runoff. The
paved areas and buildings that characterize an urban environment prevent
infiltration. Urban areas usually have much less vegetation so that
interception is reduced. The net results are increases in the
proportion of rainfall that becomes runoff and in the rate of runoff
flow.
Precipitation, falling through the air and flowing over the ground
surface, captures, dissolves, and suspends materials present and carries
them along, usually to a receiving water. Areas of high density human
habitation are characterized by the discharge of waste materials to the
air and ground surface as well as to water bodies. Runoff from urban
areas picks up this waste material. It may contain as much as three to
four times the concentration of suspended material as is typically found
in raw domestic wastewater.
In addition to the pollution content of urban runoff itself, the runoff
in many older cities of the United States 1s combined with municipal
wastewater 1n the sewer system. Sewers were originally constructed for
stormwater conveyance. When human and industrial wastes came to be
recognized as urban problems in the mid and late 19th century, these
wastes were introduced into the storm sewer system. When overflows of
combined sewers occur, a mixture of runoff and municipal wastewater is
spilled. A typical combined sewer system is illustrated in Figure 1-2.
The cost of controlling stormwater pollution can be substantial. The
authors of the 1978 Water Pollution Control Needs Survey concluded that
$87.4 billion is needed by the year 2000 to bring combined sewer
overflows and urban stormwater runoff into compliance with the
1-3
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requirements of the Federal Water Pollution Control Act Amendments of
1972 [3].
The major problem in controlling stormwater runoff is its variability.
Rainfall and runoff occur as intermittent and random events, varying
widely in volume and rate of flow during single events and from event to
event. Transport and treatment facilities for runoff control, sized to
handle some medium storm size, are idle during dry periods and overflow
during large storms.
Temporary storage of excess runoff can be an effective and economical
method of controlling stormwater flooding and pollution. Storage
capacity can reduce flowrates and enhance pollutant removals. Excess
runoff stored during large storms or during more intense rainfall
periods can be released slowly when capacity in the drainage and
treatment system or stream channel is available. The effect of storage
and controlled release on the stormwater hydrograph of Figure 1-lb is
illustrated in Figure l-l(c). Because the peak rate of flow is much
less, smaller capacities are needed to transport and to treat the same
quantity of flow, and overflows or flooding occur less frequently.
When the storage capacity is exceeded, storage basins may provide
sedimentation treatment to the overflow. The ability of flowing water
to carry heavier solid materials is.directly related to the velocity of
flow. In traveling through a storage facility, the stormwater slows, so
that the heavier solids are deposited. This is considered a problem in
the operation of storage facilities for flood control. By carefully
designing inlet and outlet structures to maximize the sedimentation
effect, and providing some method of removing and disposing of captured
solids, the perceived operations problem can contribute to water quality
improvement.
1-4
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DRAIN ' \ , \ \ i
\W->
NTERCEPTOR
TO WATER POLLUTION
CONTROL PLANTS FOR
TREATMENT
COMBINED SEVER OVERFLOW
MIXTURE OF MUNICIPAL
IASTEWATER AND STORMWATER
DISCHARGING INTO THE
RECEIVING WATERS
Figure 1-2. Common elements of an Interceptor and transport system [4],
1-5
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PURPOSE OF THE MANUAL
In recent years, there has been a large commitment by the U.S.
Environmental Protection Agency (EPA) to identify pollution sources
other than municipal wastewater discharges and to develop viable methods
for their control. A large amount of information on stormwater runoff
and, particularly, CSO characteristics, receiving water impacts, and
treatabilities, has been developed over the past decade. The purpose of
the design manual is to summarize the existing information and to detail
step-by-step analysis and design procedures for stormwater and CSO
storage/sedimentation treatment facilities.
USERS' GUIDE
This manual is organized to present, in a logical sequence, specific
application methods and design procedures for storage/sedimentation
control of stormwater pollution. It is, however, a design manual, and
sequential reading of the material contained herein is not necessary.
As an aid to ready use of the manual, the following description of
chapter contents is provided.
Chapter 1 - Introduction and Users' Guide
Introduction. The problems of flooding, groundwater loss, and water
pollution that may result from urban stormwater are briefly introduced.
The importance of stormwater as an urban pollution source is emphasized.
The potential of storage/sedimentation as a control method to deal with
each problem is discussed.
Purpose of the Manual. The specific aims of the design manual are
discussed.
1-6
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Users' Guide. A brief summary of the purpose, content, and organization
of each of the chapters 1s presented as a quick reference for the
potential user of the manual.
Chapter 2 - Urban Stormwater - An Overview
Urban Hydrology. A brief introduction to the hydrologic cycle and the
effects of urban development on stormwater runoff is presented. The
concept of hydrographs is introduced.
Urban Stormwater Pollution. The importance of urban stormwater as a
problem source, particularly as a pollution source with relation to
other urban pollution sources, is discussed. The types of pollutants
encountered, their possible sources, potential water quality impacts,
and the range of concentrations are presented. Key definitions include
stormwater, runoff, hydrology, hydrograph, combined and separate sewers,
and first flush.
Stormwater Control. The use of stormwater control devices, particularly
of storage as a method of controlling flooding and pollution, is
described. Various storage concepts, such as detention and retention
storage and the ability of storage facilities to provide sedimentation
treatment of overflows, are discussed. In addition, multiple benefits
and cost ranges of such facilities are included.
Chapter 3 - Design of Detention Facilities
Introduction. The concept of detention, storage of stormwater, and CSOs
is presented in detail. Factors such as area hydrology, site
availability, and outflow rates that Influence detention basin sizing
are discussed.
1-7
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Onslte Detention. The basic concept and specific design consideration
of runoff storage ahead of the drainage system are presented. A
recommended design procedure with an illustrative example is included.
Operation and maintenance considerations are discussed, as well as
capital and operational costs.
Inline Storage. The use of excess sewer capacity for storage of runoff
is discussed. Design considerations, including sewer line and overall
system size, type of regulator and control, and solids' capture are
presented. The design procedure and example are included. Operation
and maintenance considerations discussed include staff capabilities,
impact of solids on downstream treatment facilities, and consequences of
system overflows. Costs of facilities and operation are discussed.
Downstream Storage/Sedimentation Basins. Storage/sedimentation basins
located at the end of all or part of a sewer system are examined.
Various basin configurations are presented. Considerations of design
for flow capture versus sedimentation and removal of captured material
are described. A design procedure and illustrative example are
included. The operational flexibility of storage/sedimentation basins
is emphasized in the discussion of operation and maintenance
requirements. Design nomographs and cost curves are included.
Chapter 4 - Design of Retention Storage Facilities
Introduction. The concepts of stormwater and CSO retention and disposal
are presented in detail. Retention basin sizing considerations
including area hydrology, site availability, evaporation and percolation
rates, and disposal options are discussed.
Design Considerations. Design considerations for ponds including size
considerations, soil permeability, location, and multiple uses of the
pond area are covered.
1-8
-------
Design Procedures. A step-by-step design procedure for wet and dry
ponds Is Included. Analysis of flood and pollution impacts, control
goal, preliminary sizing and location, and final design are discussed.
An example is included.
Performance. The efficiency of retention facilities in reducing
pollutants is discussed. Pollutant removal mechanisms through soils are
covered, along with possible ground water impacts.
Operations. Operations and maintenance considerations for retention
facilities are included. The considerations include maintenance of soil
permeability, insect and aquatic weed control, and erosion control.
Costs. Curves for estimating construction cost of retention facilities
are presented.
Chapter 5 - Stormwater Management System Integration
The role of storage/sedimentation facilities in an integrated stormwater
management plan is discussed. The concurrent growth of stormwater
control systems and urban areas is examined. Retrofit of existing flood
control and drainage facilities to maximize pollution control is
discussed. Examples of urban stormwater control are described to
illustrate the several points.
Chapter 6 - Regional Implementation of Storage/Sedimentation
For many generations, control of pollution from combined sewer overflows
by storage/sedimentation has been practiced in Europe. In many cases,
the Europeans have developed simple and easy to follow guidelines for
design of these facilities. This approach is made possible because the
guidelines are applied to very limited areas in which storm patterns,
Vand use, pollutant washoff functions, and water quality impacts are
sufficiently similar to allow generalization. This same approach is
1-9
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being developed 1n some areas of the United States. The final chapter
of the manual looks at this regional guideline approach to stormwater
management. It covers European practice 1n Scotland, Germany (Bavaria),
and Sweden, and American practice 1n Montgomery County, Maryland, and
Orange County, Florida. Each 1s presented on a case study basis,
Including an evaluation of Its effectiveness.
Appendixes
Appendix A - References
Appendix B - Soil Conservation Service Runoff Analysis Method. The SCS
hydrologic analysis method Is summarized from Urban Hydrology for Small
Watersheds [5].
Appendix C - Assessment Methods. Several desk top and computer
hydrologic and stormwater pollution analysis methods arelisted.
Appendix D - Infiltration Measurement Techniques. Soil infiltration
rate testing procedures for use in investigating stormwater retention
sites are presented.
REFERENCES
1. Poertner, H. APWA Manual on Stormwater Management (Draft). 1980.
2. Armandes, C., and P. Bedient. Stormwater Detention in Developing
Watersheds. Journal of Environmental Engineering Division, ASCE.
April 1980.
3. U.S. Environmental Protection Agency. 1978 Needs Survey Methodology
for Control of Combined Sewer Overflow and Stormwater Discharge. EPA-
430/9-79-003. 1979.
4. Lager, J.A. et al. Urban Stormwater Management and Technology:
. Update and Users1" Guide. EPA-600/8-77-014. September 1977.
5. Soil Conservation Service, U.S. Department of Agriculture. Urban
Hydrology for Small Watersheds. Engineering Division. Technical
Release No. 55. January 1975.
1-10
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Chapter 2
URBAN STORKWATER: AN OVERVIEW
Stormwater control and disposal have been recognized as serious urban
problems for many centuries. Traditionally, the major goal of
-Stormwater control has been to reduce the incidence and severity of
flooding. Other goals of Stormwater control include soil erosion and
sedimentation control, and protection and enhancement of Stormwater as a
groundwater recharge source. Increasingly, Stormwater also is being
recognized as a significant source of urban water pollution.
This chapter describes the way in which Stormwater is generated and the
effects of urbanization on that process. Urban hydrology is briefly
covered. The pollutants often found in urban runoff and CSOs are
described. Commonly applied Stormwater control methods are also
included.
URBAN HYDROLOGY
Hydrology is the study of the occurrence, distribution, movement, and
properties of the water of the earth. Water is constantly moved from
the oceans to the air, onto and into the land, and back to the oceans.
This process is called the hydrologic cycle. A rigorous and thorough
examination of hydrology and the hydrologic cycle is not within the
scope of this manual. Some understanding of hydrology is necessary,
however, when considering the hydrologic subcycle of urban Stormwater
generation.
The Urban Hydrologic Cycle
The global hydrologic cycle, that movement of water from ocean to air to
land to ocean, is a closed system, and the total quantity of water
present is constant. For a small area, such as an urban watershed, the
2-1
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system is open with inputs (such as rainfall and entering surface water
and groundwater flows) and outputs (such as evapotranspiration and
existing flows). A hydrologic cycle is illustrated in Figure 2-1.
CLOUDS AND IATEI VAPOR
CLOUDS AND WATER VAPOR
sum
(WATER TABLE)'
ZONE IF SATURATION
LEBEND: T - TRANSPORTATIBI
£ - EVAPORATION
P - PRECIPITATION
ft - SURFACE RUNOFF
<5 - (ROUNOWATER FLtW
/ - INFILTRATION
Figure 2-1. The hydro.logic cycle [1].
2-2
-------
Water entering the system may follow one of a number of paths through
the system. For instance, precipitation falling on an urban area may be
intercepted by vegetation and surfaces. It may then be evaporated and
'exit the system. Alternately, some precipitation may infiltrate into
the soil to become part of the groundwater. Groundwater may be pumped
to the surface and used, where a portion may evaporate. Groundwater may
also surface to become part of surface water flow or may flow in the
ground. Various disposition paths for water entering an urban
hydrologic system are shown in Figure 2-2.
EVAPOTRANSMRATION
INFILTRATION
S
k
SOIL MOISTURE
STORAGE
IROUNOIATER
RESERVOIR
EVAPORATION
INFILTRATION
MAINTAINS ORY-IEATHER
STREAN FLOW
UNDERGROUND FLO!
OF 8ROUNOIATER
INFILTRATION
SURFACE
RUNOFF
STREAHFLOI
ENERATION
f
\
EVAPORATION
ETORN TP OCEAN
Figure 2-2. Flow diagrams indicating disposition of
infiltration, depression storage, and surface runoff [1].
2-3
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The control of urban stormwater is concerned primarily with the
precipitation to runoff flow path and with the factors that influence
"the proportion of precipitation that enters that path. The ground
cover, topography, geology, and meterology of an area all help to
determine that proportion. The factors and their responses to
urbanization are discussed in the following sections.
Hydrology. The occurrence, rate, and pollutant load of runoff are
related directly to the quantity and intensity of rainfall. Runoff
results only when the rainfall exceeds the capacity of the earth surface
to retain it. Rainfall is, of course, intermittent, and precipitation
varies geographically, seasonally, and temporally. The large scale
geographic variations illustrated in Figure 2-3 result from differences
in cyclonic storm patterns and geographic features. In addition,
rainfall within a single storm event may vary significantly in space and
time.
The seasonal variability is apparent in Figure 2-4, in which the average
monthly precipitation rates are shown for selected cities in the United
States. The seasonal variation in types of storms is also important.
Many areas of the United States are subject to high intensity, short
duration summer storms (thunderstorms), while other areas receive the
major part of their precipitation from large, slow moving winter storms.
Even areas where the precipitation distribution is fairly uniform
throughout the year may have seasonal runoff because of snowfall storage
and subsequent melting. For a given area, the occurrence interval,
duration, and intensity also vary during a rainfall season and over
numbers of years.
2-4
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Average Annual Precipitation, in
ro
in
Period 1899-1938
Figure 2-3. Mean and annual precipitation [1].
-------
ro
o>
LEGEND
T mean monthly temperature (°F)
P mean monthly precipitation (in)
Figure 2-4. Precipitation and temperature distributions [1].
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Physical Characteristics of the Watershed. Next to rainfall quantity
and intensity, the runoff quantity from a given area 1s most dependent
-on the surface characteristics of the basin. When a limited amount of
'rain falls on a watershed, a portion is intercepted by the vegetation
and wetted surfaces; the remainder infiltrates into the soil. As the
quantity of rainfall increases, the ability of the soil to infiltrate
water is exceeded. The excess rainfall fills surface depressions.
After depression storage is satisfied, runoff begins. In nonurbanized
watersheds, the quantity of water retained can be significant.
The response of the watershed to precipitation is changed by
urbanization. Removal of vegetation increases runoff as open soil areas
are covered with essentially impervious structures. The added runoff
due to urbanization is more noticeable on watersheds with highly
permeable soils, such as sands or gravels, than those with clay soils,
with low infiltration rates. Depression storage is reduced by grading
and paving. In addition, urbanization tends to reduce the surface
roughness and to channelize flows, decreasing the travel time the runoff
takes in reaching the receiving water. Larger overall quantities of
runoff and higher peak rates of flow result.
Land Use. The type and intensity of land use directly affect the
quantity of runoff, the rate of runoff, and the quantity of pollutants
in the runoff. It has been shown that the shape and size of the runoff
hydrograph for urban areas are related to the population density. The
percentage of area covered by impervious structures is much less for
residential than for commercial or industrial areas. In addition., air
quality, type of pavement, quantity and type of vehicle traffic, and
street cleaning and other maintenance practices influence pollutant
loads in the runoff. Other factors that may influence runoff quality
are vegetation and soil type, and construction practices.
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Stormwater Drainage System. The fourth major Influence on the quantity
and rate of discharge of stormwater runoff is the conveyance system.
Paving and channelization of natural water courses reduce the
'opportunity for infiltration, as the runoff is carried away more
rapidly. Once in the storm or combined sewer, stormwater continues to
move rapidly, increasing the peak rate of discharge. However, sewers
can impede the flow, and their impact in damping the peak rate of
discharge may also be significant.
Hydrographs
A hydrograph is a graphical method of describing the time distribution
of surface flows at a given point. A hydrograph of stormwater flow may
represent only that portion of precipitation that becomes runoff, or may
include some groundwater that has infiltrated the sewer, and also
municipal wastewater flows in the case of combined sewers.
A typical hydrograph shape and important features are shown in Figure 2-
5. The lag time (L) is the time interval from the center of the mass of
the rainfall excess to the peak of the resulting hydrograph. Time to
peak (T ) is the time interval from the start of rainfall excess to the
peak of the resulting hydrograph. Time of concentration (T ) is the
time interval from the end of the rainfall excess to the point on the
falling limb of the hydrograph where the recession curve begins.
2-8
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i/h
1.200
1.000
t.O 2.0 3.8 4.0 9.1
200
1.0
Figure 2-5. Hydrograph properties [2].
2-9
-------
The small rectangle at the upper left corner of Figure 2-5 is called a
hyetograph. It represents the average intensity and duration of the
rainfall event that generated the hydrograph. It is divided into two
parts: the first, losses, includes the quantity of precipitation that
was intercepted, infiltrated, or stored; the second is the rainfall
excess, or direct runoff portion. The rainfall excess portion of the
hyetograph is actually the quantity of runoff divided by the basin area:
mm/h x h = mm over the basin.
The quantity of runoff is also equal to the area under the hydrograph
curve.
The effects of urbanization are to increase the rainfall excess portion
of the hyetograph and to increase the peak of the hydrograph.
Hydrographs and hyetographs for the same rain event on a watershed under
developed and undeveloped conditions are shown in Figure 2-6. As can be
seen, the lag time, time to peak, and time of concentration all decrease
with urbanization.
Ideally, hydrographs are recorded by gaging stations at the location of
interest. In actual practice, gaging stations seldom exist at exactly
the right locations. In addition, it is often desirable to have
hydrographs for a single location for several sets of conditions.
Several methods for calculation of synthetic hydrographs have been
developed, ranging from the Rational Runoff Formula to highly
sophisticated computer simulations of watersheds. An exhaustive
discussion of these methods is not within the scope of this manual;
however, Appendix B contains a description of the simplified approach
developed by the Soil Conservation Service of the U.S. Department of
Agriculture. The method does not consider rainfall intensity or
antecedent soil moisture conditions and its use should be tempered
accordingly. Some additional discussion of hydrologic analysis methods
is also included in Appendix C, Assessment Methods.
2-10
-------
/k
'RE-DEVELOPMENT
KYDROGRAPM
POST-OEVELOPMENT
NYDR06RAPN
1.100
1 .000
POST-DEVELOPMENT
HYDROfiRAPH
PRE-DEVELOPNENT
HYDR06RAPM
8.0 8.0
THE II IOURS
Figure 2-6. Hydrographs for watershed under predevelopment
and postdevelopment conditions.
2-11
-------
URBAN STORMWATER POLLUTION
Stormwater is water that results from precipitation. Falling through
the air, precipitation dissolves and collects pollutants such as smog,
dust, and particulate matter, vapors, gases, etc. Runoff, that portion
-of the Stormwater that collects and flows on the earth's surface, picks
up additional pollutants from the surface. In urban areas, the
quantities of pollutants available to be dissolved and suspended are
large and the pollutant load of runoff is significant. In addition,
many cities are served by combined sewer systems, in which the runoff
and raw municipal wastewater flow in the same conduit.
If the combined flow exceeds the sewer capacity, a mixture of Stormwater
and municipal wastewater overflows. Urban runoff and sewer overflows
are usually discharged to receiving water bodies. There, the pollutants
may severely degrade water quality and/or threaten public health.
Urban water quality problems usually result from a combination of
pollutant sources, including Stormwater runoff and combined sewer
overflows. The impact of an urban Stormwater discharge on the receiving
water must be evaluated in light of other point and nonpoint discharges.
The relative severities of the impacts of various discharges are shown
in Table 2-1. Examples of receiving water beneficial uses and
associated instream water quality standards are shown in Table 2-2.
2-12
-------
Table 2-1. WATER POLLUTANTS CONTRIBUTED BY VARIOUS SOURCES [3]
Source categories
Urban sources
Runoff
Storm sewers
Combined sewer overflows
Separate sanitary sewer overflows
Construction
Residual wastes disposal
Rural sanitation
Landfills
Sludge disposal
Dredge spoils disposal
Hydrographic modifications
Dredging
Maintenance facilities
Channel modification
Dams
Groundwater
Brine
Deicing salts
Agriculture
Livestock production
Crop production
Manure disposal
Windborne loadings
Tile drainage
Silviculture
Forestry management
Forest harvesting
Recreation
Mining
Surface
Subsurface
Miscellaneous
Atmospheric
Spills
Benthic loads
Organic
matter
44
44
44
44
0
4
44
44
4
44
44
0 '
4
0
0
44
4
44
0
44
4
44
4
0
0
0
44
44
Sedi-
ment
44
44
44
44
44
4
4
44
44
44
0
44
4
0
p
44
44
44
4
4
0
44
44
44
44
4
4
44
Nutrients
44
44
44
44
4
44
44
4+
4
4
44
0
4
0
0
44
44
44
0
44
44
44
0
0
0
4
4
44
Micro-
organisms
4
4
44
44
4
44
0
0
0
.0
4
0
0
0
0
44
0
44
4
4
0
0
4
0
0
0
0
4
Trace
metals
44
44
44
0
4
4
44
44
44
44
0
0
0
4
4
0
4
0
4
0
0
0
0
44
44
4
44
4
Toxic
organics
4
4
4
0
4
0
44
4
44
44
0
0
0
0
4
4
44
0
4
44
4
4
0
0
0
44
44
44
Salts
44
44
44
0
0
0
4
0
0
0
0
0
0
44
44
4
0
4
0
4
0
0
0
0
44
0
0
0
Acid
wastes
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
44
44
4
44
0
Note: ++ = severe; + » moderate; 0 = slight or none.
2-13
-------
Table 2-2. BENEFICIAL USES AND WATER QUALITY CRITERIA
Beneficial
Miter use
Domestic'
Mater supply
Body contact
recreation
Shell fishing
Fish habitat
Type of impact
Health
Aesthetics
Health
Aesthetics
Health
Survival
Health
Survival
Pollutant of concern
Bacteria
Toxic substances
Total dissolved solids
Nitrates
Color, taste, odor
Bacteria
Solids
Floa tables
Turbidity
Color
Bacteria
Toxic substances
Dissolved oxygen
Total dissolved solids
Toxic substances
Bacteria
Toxic substances
Dissolved oxygen
Turbidity
EPA water
quality standard
200 MPN/100 irt fecal conforms
Varies
250 mg/L for chlorides and sul fates
10 mg/L as N
Free of
200 MPN/100 ml fecal
None that settle to
No nuisance
No nuisance
No nuisance
14 MPN/100 ml fecal
Varies
5.0 mg/L
Varies
Varies
col i forms
form objectional deposits
col i forms
5.0 mg/L
Shall not reduce depth of compensation point by
Aesthetic
enjoyment
Toxic substances
Eutrophication Nutrients
Others Floatables
Solids
Odor and color
more than 102
Varies
Varies (50 mg/L P)
Varies
No objectionable deposits
No nuisance
2-14
-------
Stormwater Pollutants
The most common stormwater pollutants are suspended solids, oxygen
demanding material, pathogenic bacteria, nutrients, and toxic
substances. Each of the pollutants affects the water in a different
way, and its impacts are varied as to duration and locale. The time
period and location of most severe impacts are shown in Figures 2-7 and
2-8, respectively. The pollutant categories are discussed and
concentrations for each are reported in the following section.
Suspended Solids. Deposition of particulate matter within a receiving
^>"
water generally results in both short-term and long-term localized
impacts. The solids settle near the discharge point, forming sludge
banks and smothering bottom dwelling organisms. The organic fraction
may undergo decomposition, depleting dissolved oxygen and producing
gases and odors. Floating material (oil, scum, debris) or suspended
solids may block oxygen tranfer and make the receiving water
aesthetically objectionable. Excessive solids can also render the water
unacceptable for agricultural irrigation.
Oxygen-Demanding Material. Degradable organic matter and certain
nitrogen compounds exert a demand on the dissolved oxygen of the
receiving water as they are organically degraded. As the dissolved
oxygen concentration declines, conditions suitable for a balanced
population of fish and lower aquatic species may be violated. If the
dissolved oxygen level reaches zero and anaerobic conditions develop,
discoloration of the water, gas production, and odors may occur.
Pathogenic Bacteria. Stormwater runoff, and particularly combined sewer
overflow, may contain significant numbers of bacteria. Typically
measured by indicator organisms, excessive concentrations of human
bacteria in receiving water may represent a threat to public health and
2-15
-------
-io8
SECONDS
o4 io*
10"
IOT I01
10'
FLOATABLES
BACTERIA
DISSOLVED OXYGEN
SUSPENDED SOLIDS
NUTRIENTS
ACUTE TOXIC EFFECTS
DISSOLVED SOLIDS
LOHGTEftM
TOXIC EFFECT
I
HOUR
DAY MONTH
WEEK SEASON
YEAR
DECADE
Figure 2-7. Time scale, storm runoff water quality problems [4],
2-16
-------
HYDRAULIC DESIGN
FLOATABLES
I I
BACTERIA
SUSPENDED SOLIDS
DISSOLVED OXYGEN
NUTRIENTS
TOXIC EFFECTS
DISSOLVED SOLIDS
.0"
!»rn
csom
10 '
I SOOFT)
10"
10'
10*
10'
EFFECTIVE DISTANCE-MILES
-LOCAL-
REGION-
-BASIN-
Figure 2-8. Space scales, storm runoff water quality problems [4].
2-17
-------
may prevent water supply and recreational use of the water. Pathogenic
bacteria found in significant concentrations in stormwater runoff
include Vibris chloerae, Salmonella typhi, Salmonella paratyphi, and
Shigella dysenteriae. The concentration of organisms rapidly declines
as a result of initial dilution and die-off with time and distance from
the point of discharge. However, the short-term health risk near the
point of a combined sewer overflow can be great.
Nutrients. The discharge of nutrients (usually nitrogen and phosphorus
compounds) into lakes, ponds, and reservoirs and into sluggish and
shallow streams may promote excessive growth of algae and aquatic weeds.
The results may be dissolved oxygen depletion, interference with
recreational use, odors, and other aesthetically objectional conditions.
Toxic Substances. Toxicity problems fall into two categories:
(1) stable chemical pollutants, such as metals, pesticides, and
persistent organics; and (2) reactive toxic pollutants, such as ammonia
and the byproducts of effluent chlorination. Because stable chemical
pollutants do not decay, their impacts are felt over a long period, and,
for stream discharges, over long distances. Although the concentrations
may be diluted quickly in the stream to less than lethal levels, the
long-term cumulative effects on aquatic organisms may also be serious.
The toxic effects of ammonia and effluent chlorination byproducts are
more immediate and local. These compounds are generally reactive and
degrade with the passage of time. Concentrations of some EPA designated
priority pollutants are shown in Tables' 2-3 and 2-3A for liquid and
sediment fractions, respectively. Note the characteristically high
copper, lead, and zinc values for highway runoff.
2-18
-------
Sources of Stormwater Pollutants
An understanding of the potential sources of stormwater pollutants is of
importance when studying the impact of urban runoff and combined sewer
overflows. The accumulation of the various pollutants in runoff and in
CSO can be attributed to several sources and the individual effects are
difficult to separate. However, a qualitative knowledge of the probable
sources enables an investigator to concentrate on expected problem areas
and evaluate source controls that could be used to curtail an adverse
pollutant loading before it reaches the sewer system. The principal
sources of runoff pollutants are as follows:
1. Street pavement. The components of road surface degradation
can become part of the urban runoff loading. The aggregate
material is the largest contributor and additional quantities
will come from the binder, fillers, and any substance applied
to the surface. The amount of pollutants will depend on the
age and type of surface, the climate, and the quantity and
type of traffic.
2. Motor vehicles. Vehicles can contribute a wide variety of
materials to the street surface runoff. Fuels and lubricants
spill or leak, particles are worn from tires or brake linings,
exhaust emissions collect on the road surface, and corrosion
products or broken parts fall from vehicles. While the . .
quantity of material deposited by motor vehicles is expected
to be relatively small, the pollution potential is important.
Vehicles are the principal nonpoint source of asbestos and
some heavy metals, including lead.
3. Atmospheric fallout. Air pollutants include dust,
contaminants, and particles from industrial stacks and vents,
from automobiles and planes, and from exposed land. The
2-23
-------
airborne matter will settle on the land surface and wash off
as contaminated runoff. The potential significant of dustfall
was indicated during a study done in Cincinnati: during the
study period, 506 Ib/acre (567 kg/ha) of dustfall was measured
at a monitoring station and 730 Ib/acre (818 kg/ha) of
suspended solids was measured in storm runoff.
4. Vegetation. Leaves, grass, clippings, and other plant
materials that fall or are deposited on urban land will become
part of the runoff problem. Quantities will depend on the
geographic location, season, landscaping practices, and
disposal methods.
5. Land surface. The type of ground cover found in a drainage
basin and the amount of vehicular and pedestrian traffic is a
function of land use and will affect the quality of storm
runoff.
6. Litter. Litter consists of various kinds of discarded refuse
items, packaging material, and animal droppings. Although the
quantities are small and not significant sources, of pollution,
the debris is highly visible in a receiving stream and can be
a focal point for citizen complaints.
7. Spills. These obvious surface contaminants can include almost
any substance hauled over city streets. Dirt, sand, and
gravel are the most common examples. Industrial and chemical
spills are potentially the most serious.
8. Antiskid compounds and other chemicals. Cold weather cities
employ large amounts of substances designed to melt ice during
the winter. Salts, sand, and ash are the commonly used
agents. A variety of other chemicals may be used as
2-24
-------
fertilizers, pesticides, and herbicides. Most of these
materials will become part of the urban runoff.
9. Construction sites. Soil erosion from land disturbed by
construction is a highly visible source of solids in storm
runoff. Important urban sites will include large-scale
projects such as highway construction and urban renewal. The
construction methods and control measures will influence
pollutant quantities.
10. Collection network. Storm sewer networks using natural or
improved earthen channels will be subject to erosion of the
banks. Collection networks, both separate and combined sewer
systems, also tend to accumulate deposits of material that
will be dislodged and transported by storm flows. In combined
sewer systems, the overflow is a mixture of polluted runoff
and raw domestic and industrial waste.
It is obvious from this list that there are many potential sources of
pollutants within each basin and the sources vary in importance. The
quantities that accumulate are a function of natural conditions and
urban development. Most of the sources exist concurrently in the urban
environment and their effects cannot be isolated.
Pol 1utant Concentrati ons
Average concentrations for the pollutants in stormwater runoff and
combined sewer overflows measured in several cities are listed in Tables
2-4 and 2-5 respectively. The magnitude of the stormwater problem is
indicated by the concentrations; the wide variations in concentrations
that may be anticipated are represented by the ranges.
2-25
-------
Table 2-4. AVERAGE POLLUTANT CONCENTRATIONS IN STORHWATER RUNOFF [5]
mg/L Unless Otherwise Noted
City
Atlanta,
Georgia
Des Moines,
Iowa
Durham,
North Carolina
Knoxville,
Tennessee
Oklahoma City,
Oklahoma
Tulsa.
Oklahoma
Santa Clara,
California
Pullach,
Germany
Average (not
weighted)
Range
TSS
287
419
1.223
440
147
367
284
158
415
147-1,223
VSS
104
122
--
70
53
88
53-122
BOD
9
56
7
22
12
20
11
20
7-56
COD
48
--
170
98
116
86
147
125
113
48-170
Kjeldahl
nitrogen
0.57
2.09
0.96
1.9
2.08
0.85
--
1.41
0.57-2.09
Total
nitrogen
0.82
3.19
--
2.5
3.22
--
5.8
--
3.11
0.82-5.8
Phosphorus
0.33
0.56
0.82
0.63
1.00
--
0.23
--
0.62
0.33-1.00
Ortho-
phosphate
--
0.15
--
0.30
1.00
0.38
--
--
0.46
0.15-1.00
Lead
0.15
--
0.46
0.17
0.24
--
0.75
--
0.35
0.15-0.75
Fecal
collformsa
6,300
-
230
20.300
40.000
420
13,500
230-40,000
a. Organisms/100 ml.
2-26
-------
Table 2-5. AVERAGE POLLUTANT CONCENTRATIONS IN
COMBINED SEWER OVERFLOWS [5]
mg/L Unless Otherwise Noted
.City
Des Moines,
Iowa
Milwaukee,
Wisconsin
New York City,
New York
Newton Creek
Spring Creek
Polssy, France c
Racine,
Wisconsin
Rochester,
New York
Average (not
weighted)
Range
TSS
413
321
306
347
751
551
273
370
273-551
VSS
117
109
182
387
154
--
140
109-182
BOD
64
59
222
111
279
158
65
115
59-222
COD
--
264
481
358
1.005
--
367
264-481
Kjeldahl
nitrogen
4.9
.. .
--
2.6
3.8
2.6-4.9
Total
nitrogen
4.3
6.3
16.6
43
--
--
9.1
4.3-16.6
P04-P
1.86
1.23
4~5'>
17b
2.78
--
1.95
1.23-2.78
OP04-P
1.31
0.86
_»
0.92
0.88
1.00
0.86-1.31
Fecal
Lead conforms'
--
0.60
.-
201 ,000
0.14 1,140,000
0.37 670,000
0.14-0.60 201,000-1.140,000
a. Organisms/100 ml,
b. Total P (not Included in average).
c. Not included in average because of high strength of municipal wastewater when compared to the United States.
2-27
-------
Because the characteristics of runoff and combined wastewaters are so
variable, it is not possible to establish typical pollutant
-concentrations for either. However, as an aid to understanding the
magnitude of separate storm and combined sewer overflow problems, the
constitutent concentrations from such overflows are compared in Table 2-
6 to sanitary wastewater and background levels of the pollutants that
might be present in the receiving stream before a discharge.
Table 2-6. COMPARISON OF STORMWATER DISCHARGES
TO OTHER POLLUTANT SOURCES L5J
mg/L Unless Otherwise Noted
Kjeldahl Total Total Fecal
TSS VSS BODs COD nitrogen nitrogen P04-P OP04-P Lead coliformsa
Background 5-100 -- 0.5-3 20 0.05-0.5^ 0.01-0.2° <:0.01 <0.1 <1
levels
Stormwater
runoff
Combined
sewer
overflow
Sanitary
wastewater
415
370
200
90
140
150
20
115
200
115
367
500
1.4
3.8
40
3.10
9.10
40
0.6
1.9
10
0.4
1.0
0.05-
1.27
0.35 13,500
0.37 670,000
750,000
a. Organisms/100 ml.
b. N03 as N.
c. Total phosphorus as P.
Pollutant concentrations and pollutant masses emitted vary during single
storm events and from event to event. The variations depend on a number
of factors, including rainfall intensity, land use, time from beginning
of runoff, catchment size, dry-weather pollution abatement practices,
and drainage system type. Although much information on stormwater
pollution has been developed over the past 10 years, the large number of
variables and the high degree of variation among urban areas makes
accurate prediction of stormwater pollutants almost impossible without
2-28
-------
sampled data specific to the area being studied. It is important for
the control facility designer to understand the variables and their
interplay in order to develop adequate sampling programs, and to make
-intelligent decisions as to which pollutants are more important to
control and what control methods might be most effective.
Pollutant concentrations as a function of time from start of overflow
are illustrated in Figure 2-9. As can be seen, pollutant
concentrations usually are high at the beginning of an overflow and
tail off as the overflow continues. This high pollutant concentration
in the initial flow is usually referred to as a first flush phenomenon.
The first flush is attributed to two types of flushing actions: (Da
street flush and (2) a sewer flush.
Street flush refers to the higher concentration of pollutants that may
occur in early runoff as a result of dissolution and suspension of the
most easily removed surface pollutants. These easily suspended and
dissolved materials are not available for subsequent runoff to pick up;
therefore, the concentrations of pollutants in later flows are less.
The magnitude of the street flush depends on the time between flushes
or storms and also on the street sweeping interval. Although this
dependence is not defined by a simple mathematical function, it is
illustrated in Figure 2-10, which shows the increase in average . .
pollutant concentrations with decreasing street sweeping frequency for
Des Moines, Iowa.
Although the more significant flushing of pollutants from streets and
other surfaces usually occurs at the beginning of runoff, a secondary
street flush may occur at any time if the intensity of rainfall should
suddenly increase. The ability of runoff to suspend pollutants is
directly related to the velocity of runoff flow. At the same point in
a given storm, a more intense rainfall will usually dislodge and carry
a greater quantity of pollutants than less intense rainfall. -
2-29
-------
ro
i
Bt
If!
400
>00
« 400
at
ui
S 300
2 too
5 too
w
a
JC
w
S 400
M
300
200
100
H 1 1 1 1-
-\IIIIIh
I
RACINE.WISCONSIN
AVI. OF 10 STORIS
MILWAVKEE.WISCONSIN
AVO. OF 07 STORMS
ROCHESTER. NEW TORI
»V8. OF 12 OVERFLOW
CITED OVER 10 IONTH
PERIOD
1 1 1 1 1 1-
H 1
H 1
200
190
too
80
200
ISO
too
00
200
190
too
00
0 0.9 1.0 1.0 2.0 2.9 3.0 3.9
THE FROM START OF OVERFLOW, h
0.9 1.0 1..9 2.0 2.9 3.0 3.9
ME FROM START OF OVERFLOW, h
Figure 2-9. Combined sewer overflow quality
versus time for selected cities [5].
-------
70r
60
50
_J 40
30
20
10
BOD
EDO
500
400
^
300 E.
200
100
12 16 20 24 26 32 36 40
STREET SWEEPING INTERVAL, d
Figure 2-10. Effect of street sweeping frequency on mean BOD
concentration in urban stormwater runoff, Des Moines, Iowa [5].
The second type of flushing action is called a sewer flush. In sewers
with flat slopes, where the solids-carrying velocity is not maintained,
solids may be deposited during low flow periods. Deposition usually
occurs in separate storm sewers at the end of a runoff event, and the
'solids are primarily inorganic. In combined sewers, organic solids
from the sanitary wastewater are deposited during dry weather. When
the next storm of sufficient size takes place, the solids are flushed
from the system by the higher flows. This flushing action soon after
runoff begins results in a higher concentration of pollutants in the
early discharge.
2-31
-------
Although the Initial concentration of pollutants will vary as a
function of sewer hydraulics and time since the last flush, the ratio
of initial pollutant concentration to the concentration near the end of
the overflow depends also on the size of the catchment. On very small
catchments, a high percentage of the total BOD5 discharged during an
overflow may occur in the first minutes of overflow. On larger
catchments, the effect is dampened as the highly concentrated sewer
flush from various subcatchments arrive at the overflow point at
different times.
The separate subcatchments exhibit a well-defined first flush
individually. As the runoff is summed over the entire basin, however,
much of the effect is lost. Nussbaum noted that a storage volume of
o
185,000 gal (700 m ) would be required to contain the first flush
pollution from a large catchment in a German example. The same
quantity of pollutants might be stored in four 11,888 gal (45 m )
subcatchment basins [6].
The EPA has been developing, for some years, a data base of stormwater
and combined sewer overflow pollutant characteristics. Storm, runoff,
and pollutant concentrations are, of course, highly variable. The
process of data collection is very slow. Until the EPA data base is
expanded significantly, the only viable method of predicting the
characteristics and treatabilities of stormwater pollutants for a
specific watershed is to analyze data collected in that watershed or in
a very similar watershed nearby. Analysis techniques and models are
discussed in Appendix C.
2-32
-------
STORMWATER CONTROL
Control of urban stormwater has been practiced for a number of
centuries. The cloaca maxima, a storm drain constructed for the
ancient Roman Forum, is still in use today. The primary goal of urban
stormwater control has been to reduce the incidence and severity of
flooding. In addition to simple drainage facilities, the approach to
flood control has often been to attenuate, or slow, the flow of runoff
so that the peak rate of flow is reduced and the urban runoff
hydrograph is made to more closely approximate the predevelopment
hydrograph. Flow attenuation may be accomplished by increasing
roughness of flow surfaces or by constructing storage facilities in
which runoff is temporarily stored and released slowly. Storage
facilities may or may not allow infiltration, so that runoff volumes
are reduced. The urban hydrograph from Figure 2-6 is reproduced in
Figure 2-11, along with the storage facility attenuated hydrograph for
the same watershed and storm.
Reduction of stormwater pollution is also emerging as a major goal of
urban stormwater control. The aim here is to reduce the masses of
pollutants discharged in the stormwater to a level that can be safely
assimilated by the receiving water. A variety of techniques have been
applied, ranging from physical removal of solid material to biological
or chemical treatment. The technique applied depends on the pollutant
or pollutants of concern, the distribution of pollutants in the runoff,
and the assimilative capacity of the receiving water.
2-33
-------
i. too
1.000
too
00
400
200
1.1 2.1 I.I 4.1 1.0
Figure 2-11. Hydrograph of a watershed showing
effects of storage.
Generally, the techniques for controlling pollution from stormwater
runoff and combined sewer overflows can be grouped into three main
categories: (1) source control, (2) collection system control, and
(3) downstream storage and treatment. In source control options, the
quantity and pollutant concentrations of runoff are attenuated and/or.
reduced before it enters the sewer system. In collection system
control options, the objective is to ensure that the sewer system
operates as efficiently as possible to reduce overflows. Downstream
storage and treatment options are designed to attenuate stormwater
flows, alternatively dispose of excess flows, or remove pollutants from
stormwater after its collection. The range of options for control of
storm and combined sewer overflows is shown in Table 2-7. The least
expensive method of stormwater management often consists of a
combination of control techniques.
2-34
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Table 2-7. STORMWATER CONTROL TECHNIQUES
Source control
Collection system control
Downstream storage and treatment
Rooftop and plaza ponding'
Increased depression storage
Detention/retention ponds8
Blue-green storage1
Roof leaders to pervious areas
or seepage pits
Parking lots drained to
pervious areas
Porous pavement
Rooftop gardens or sod roofs
Reduced air pollution
Improved street cleaning
Inline flow regulators/Inline storage8 End-of-pipe storage8
Hydraulic concentrators
Polymer injection for hydraulic
friction reduction
Regular sewer flushing
Sewer separation
» I/I control
Storage/sedimentation treatment9
DAF treatment
CAP treatment8
Biological treatment
Screens/filtration
Disinfection
a. Storage/sedimentation techniques.
Process performance of stormwater management techniques is a function
of the nature of the pollutant to be removed and the distribution of
the pollutant loading with time. Commonly used stormwater control
techniques, the pollutants they are most effective in removing, and the
ranges of their effectiveness are shown in Table 2-8.
Table 2-8. COMMON STORMWATER AND COMBINED SEWER OVERFLOW
TREATMENT PROCESSES, POLLUTANT REMOVAL EFFICIENCIES, AND COSTS [5]
Pollutant removal efficiency, X
Sedimentation
Sedimentation with
chemical addition
Nicroscreens
Dissolved air
flotation0
High rate filtration^
Biological treatment
Suspended
solids
20-60
68
50-95
45-85
50-80
40-95
BOD5
30
68
10-50
30-80
20-55
40-90
Total Kjeldah
Phosphorus nitrogen
20
20
55
50
--
38
--
30
35
21
--
Costs9
]
Capital
$400-70 ,000/acre
$25, 000-52 ,000/Mgal-d
$40 ,000-140 ,000/Hgal-d
$68 ,000-1 34 ,000/Mgal-d
$160-17,000/acre
Annual O&M
$10-850/acre
$0.1/1.000 gal
$0.1-0.44/1.000 gal
$90. 000-268 ,000/yr
$0.09-0.40/1.000 gal
a. ENR * 4000.
b. Efficiencies Include both prescreening and dissolved air flotation with chemical addition.
c. .Includes chemical addition.
2-35
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Storage/Sedimentation Controls
The temporary storage of urban runoff is often a flexible and effective
method of reducing stormwater pollution as well as flooding. Such
pollution usually occurs during peak rainfall and runoff periods when
the infiltration capacity of the ground surface and the transport
and/or treatment capacities of the drainage system are exceeded.
Storage facilities can reduce the peak rates of flow so that more
rainfall is infiltrated and more runoff is treated before discharge.
When runoff amounts exceed the storage capacity, storage basins can be
designed to provide treatment by sedimentation to the excess.
Categories of Storage/Sedimentation
Generally, the various techniques of storage and subsequent disposal of
stormwater fit into one of two broad categories:
Detention storage. Excess runoff is stored temporarily. It
is returned to the sewerage system at a reduced rate of flow
when the sewer or treatment capacity is available or is
discharged, again at a reduced rate of flow. The rates of
flow are reduced, but the total volume of stored runoff
ultimately is discharge and may be passed through the
drainage system and treatment facilities before discharge.
Retention storage. Stored runoff is not returned to the
sewers, but is allowed to evaporate and percolate into the
ground. The total volume stored, along with its pollutant
load, is removed from the drainage/treatment system. The
effects of the stored runoff on the groundwater and the
possibility of indirect discharge to surface waters by
groundwater transmission are considerations of retention
storage.
2-36
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Sedimentation of solids associated with storage occurs during overflows
of the storage facility as the flow velocities are reduced. Reduction
of the velocity decreases the solids carrying capacity of the runoff.
The amount of solids deposition depends on the detention time of the
flow. Sedimentation can occur in either retention or detention storage
facilities. In the latter, the effluent quality is not as consistent
as that of primary sedimentation facilities, but can be improved with
the addition of flocculating agents.
Information needs and design methodologies for detention and retention
storage facilities are detailed in Chapters 3 and 4 of this manual,
respectively.
REFERENCES
1. Viessman, W., et al. Introduction to Hydrology. Intext
Educational Publishers. 1972.
2. Wanielista, M. Stormwater Management Quantity and Quality. Ann
Arbor Science. 1978.
3. U.S. Environmental Protection Agency. Benefit Analysis for
Combined Sewer Overflow Control. EPA 625/4-79-013. 1979.
4. U.S. Environmental Protection Agency. Areawide Assessment
Procedures Manual. EPA 600/9-76-014. 1976.
5. U.S. Environmental Protection Agency. Urban Stormwater Management
and Technology: Update and Users' Guide. EPA 600/8-77-014. 1977.
6. Nussbaum, G. Remarks on the Treatment of Rainwater 1n the Sewer
System. Source Unknown.
2-37
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Chapter 3
DESIGN OF DETENTION FACILITIES
INTRODUCTION
Hany communities throughout North America have stormwater runoff and
flooding problems according to a recent APWA survey [I], To alleviate
these problems, planning for stormwater detention 1s often part of the
overall stormwater management plan. For example, of the drainage
masterplans reported, more than half Included the development of
detention facilities. Two hundred nineteen public agencies reported
having detention facilities; the number and type are listed in Table 1A.
A total of 12,683 facilities were reported, an average of 58 per
community. Nearly 40% of those communities without detention facilities
said that facilities are being built, are 1n the planning stage, or have
been considered and are a priority item for the near future.
Table 1A. DETENTION FACILITIES IN USE IN
THE UNITED STATES AND CANADA [f]
Total In use
Type of facility
Dry basin
Parking lot
Pond
Rooftop storage
Underground tank
Oversized sewer
Underground tunnel
Other
Total
No.
6.053
3.134
2.382
694
160
135
9
116
12.683
I
47.
24.
18.
5.
1.
1.
0.
0.
8
7
8
5
3
0
1
9
Private
No.
4.913
2.982
1.199
644
142
83
8
64
10.035
X
81
95
50
93
89
61
89
55
79
Public
No.
1.140
152
1.183
50
18
52
1
52
2.648
%
19
5
50
7
11
39
11
45
21
Objectives reported by the public agencies for establishing detention
facilities are given 1n Table IB [I], Reducing the cost of drainage
systems and reducing pollution from stormwater are two of the top seven
objectives on the list.
3-1
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Table IB. OBJECTIVES IN REQUIRING DETENTION [»]
Objective Rank8
Reduce downstream flooding 100
Reduce cost of drainage systems 71
Reduce onslte flooding 70
Reduce soil erosion 66
Capture silt 64
Improve onslte drainage 63
Reduce pollution from stormwater 56
Improve aesthetics 53
Enhance recreational opportunities 51
Replenish groundwater 42
Supplement domestic water supply 36
Capture water for Irrigation 35
Other 22
a. In order of Importance using 100 as
"most Important."
3-2
-------
Stormwater detention 1s the storage of excess runoff for delaying and
controlling the release rate to attenuate peak flows 1n the surface
drainage and discharge system. This storage, with the resulting
sedimentation that occurs due.to Increased detention times, can also be
considered to be a treatment process.
Detention facilities may be analyzed and designed on the basis of one or
more of several possible criteria:
Providing a specified detention time for runoff from a storm
of given duration or return frequency
Containing a given volume of runoff from the tributary area,
such as the first 0.5 In. (1.3 cm) of runoff
Containing the runoff from a given volume of rain, such as the
runoff from 0.75 in. (1.9 cm) of rain
Containing a specified volume
The sizing of detention facilities also requires consideration of
additional parameters such as design storms, site constraints, and
outflow rates. Design storms may be selected either from actual
historical rainfall records or from synthesized or statistical rainfall
Intensity-duration-frequency relationships. Actual historical rainfall
records selected may be based on the hourly Intensity, storm duration,
total rainfall for the storm, or any combination of these. Site
constraints to be considered for detention storage facilities Include
tributary area, topography, local land use, and area available for the
3-3
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structure or basin. The outflow rate from the detention facility may be
based on the capacity of the drainage channel or conduit downstream, the
capacity of a treatment facility downstream, or a regulatory limitation.
An example of a regulatory limitation might be that the rate of runoff
from a developed piece of property be no more than that before the
property was developed.
ONSITE DETENTION
Onslte detention refers to the storage of stormwater runoff at or very
near the site of Its origin, and Its subsequent discharge at a
predetermined release rate. The Intent of onsite detention 1s to
utilize existing or proposed Impermeable areas or structures to store
and control runoff. Typical examples of such onsite storage Include
rooftops, plazas, parking lots and streets, underground structures, and
multipurpose detention reservoirs.
Many municipalities, having faced the results of increased stormwater
runoff volumes and rates from urban development, are now enacting
ordinances requiring developers to limit the rate of stormwater runoff
from developed areas. An example of such an ordinance 1s that enacted
by the Metropolitan Sanitary District of Greater Chicago (MSDGC).
Typical compliance facilities are shown in Figure 3-0.
The MSDGC ordinance limits the peak rate of runoff from newly developed
areas to that which would occur on the land in its undeveloped state as
a result of the 3 year frequency rainstorm. Any amount exceeding that
must be stored for gradual release. Detention facilities must be
designed to handle the 100 year storm without flooding.
Design Considerations
Factors to be considered in the design of onsite storage facilities are
(1) tributary area, (2) storage area and volume, (3) structural
Integrity, and (4) responsibility of the owner.
3-4
-------
Left - Northbrook, Illinois
detention storage pond.
Below - Downers Grove, Illinois
swale with throttling outlet
retards runoff with detention
storage
$$!&%£&&'& - ^SPS^P^^^^
^y^v^'^^w ;i^«/K^^^'^^-
SiUiJsM^^i^a >H'*.-£X ^feiM-''-''-;:'.^ ''.''
Mfcf*^^^S^Qi^^>
^*^a«-m^.% ^^^Beab^
;:Cfe
^v-i*r&,;Ci^>:.,-«. j^r*.-'i5g
-------
Tributary Area. The size of the tributary area determines the volume of
water that will have to pass the discharge point. The runoff volume 1s
a function of the amount of rainfall, the extent and nature of the land
cover, and the size of the tributary area.
In the cases of rooftop and parking lot storage, the tributary area 1s
the actual rooftop or parking lot area. However, the tributary area for
plaza, underground structure, and multipurpose detention facilities
usually Includes additional area surrounding the facility Itself.
Storage Area and Volume. The area and volume available for detention
storage depend on the topography of the particular site. In the case of
rooftops, the area available is fixed by the geometry of the building.
The volume is limited by the rooftop area and the depth of water that
can be supported without endangering the structure.
For parking lots, plazas, and multipurpose detention reservoirs, the
storage area is less well defined since all or only portions of the area
may be used depending upon the nature of the site or the desires of the
builder. There is usually more latitude available for Increasing the
detention volume for these facilities by adjusting the allowable water
depth.
Underground structures may include concrete, fiber glass, or metal tanks
and oversized pipes for storage. The storage volume depends on the
surface area and depth of the tank or on the diameter and length of
oversized pipes. The desired storage volume can be accommodated by
varying the dimensions as necessary. In most cases, the depth of such
structures 1s limited by the topography and the location of the sewer to
which the structure must discharge.
Structural Considerations. Each of these facilities requires somewhat
different structural considerations. Rooftops are limited by the
building code design load and the need to prevent leaks Into the
structure below. The pavement and base for parking lots must be
3-6
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carefully designed and constructed. The structural Integrity of the
pavement can be jeopardized and the service life drastically reduced
because of the presence of water 1n the pavement base. Plazas,
underground structures, and multipurpose detention reservoirs must be
designed to allow access for sediment and debris removal. Outlet
structures must also be designed to fit the specific application.
Responsibility of the Owner. Even when the designer has provided for
maintenance of a detention facility, it is not always easy to determine
who is responsible for performing the maintenance. For rooftop
detention or for a basin or tank serving an apartment complex or a
commercial complex, the ownership and the maintenance responsibility 1s
easily determined. However, for the other types of facilities, this may
not be true. A basin may be owned by a number of individuals as part of
their house lots. The homeowners, individually, are not a legal entity
that can be forced to maintain such a structure. To prevent this, a
developer can donate the detention facility to the municipality, which
must then provide the maintenance.
Design Procedure/Examples
A suggested design methodology is shown in Figure 3-1. Each of the
Indicated steps are discussed below and examples are introduced where
applicable.
Step 1 - Identify Functional Requirements. As noted previously, the
intended operational function of an onsite detention facility determines
its design emphasis. The design emphasis can also be dictated by the
type of development planned for the site.
Information that must be determined at this point includes:
The type of development planned for the site
The reason that stormwater detention 1s needed or desired
3-7
-------
IDENTIFY FUNCTIONAL
REQUIREMENTS
I. CONTIIOl OF IUIOFF
KITE
I. IEDUCTIOI OF
IUIOFF fOLDIE
-N
CO
00
IDENTIFY SITE
CONSTRAINTS
i. mi
1. HfDRIULIC
e. EiiiioNiEini
o. sTiuuvm
/
t
"--
! YES
1
1 f^
N
-J^/
ESTABLISH BASIS
OF DESIGN
A. OESItl STOIH
1. IIFLOI IITE
C. OUTFIOI IITE
0. STOIICE VOLUME
/\
'-. r>
. > ^
-N,
~ i/
SELECT DETEN
1. OPEIITI
1. IILET/0
C. IIEI/OE
0. CLEIIII
<:\
,i i
i
r
TIOI
till
ITLE
PTI
1 AC
1 OPTION(S)
CIICEPT
r ions
CESI
7
SHIFT TO ALTERNATE
SITE OR METHOD
FACILITIES
SATISFY
OBJECTIVES?
ESTIMATE COSTS AND
COST SENSITIVITIES
I. CIPITIl
I. OPEIITIOI 110
MIIITEIINCE
Figure 3-1. Onsite detention design methodology.
AWBERC LIBRARY U.S.
-------
The type of development planned for the site greatly affects the types
of stormwater detention facilities that can be employed. Commercial
complexes can Incorporate combinations of rooftop, parking lot, plaza,
and underground structures for stormwater detention. Residential
developments usually Incorporate rooftop detention (only 1f flat roofs
are used), underground structures, and multipurpose reservoirs.
Step 2 - Identify Site Constraints. Sites for onsite detention
facilities should be Identified with respect to at least the following
criteria:
Accessibility to the conduit or channel to which it
discharges.
Total area usable for storage and its dimensions,
configuration, and topography.
Hydraulic and hydrologic data on rainfall intensity, flow
levels in the conduits or channels to which flow 1s
discharged, and allowable discharge rate.
Environmental setting such as proximity to residences, land
use, and visual exposure.
Geotechnical conditions that could affect load bearing
capacity, side slope stability, adjacent structures, and
ground water supplies.
Structural requirements
Step 3 - Establish Basis of Design. During this step, it is necessary
to determine the inflow rate, allowable discharge rate, and storage
volume needed to meet the requirements established in Step 1.
3-9
-------
More than 45 different methods of predicting runoff rates and developing
Inflow hydrographs were reported in the APWA survey [/]. The Rational
Formula, the most commonly accepted method, was approved for use by 80%
of the respondees. Of interest, however, was the fact that the Rational
Formula was not acceptable to 20% of the communities. Other popular
methodologies accepted include (in order of stated preference): the
unit hydrograph using recorded data, modified Rational Formula, Curve
Number Method of the U.S. Soil Conservation Service, unit hydrograph
method using synthetic data, and rainfall-runoff simulation models. The
latter was accepted by just over 40% of the respondees.
A simple and frequently used method for calculating the peak runoff rate
is an empirical formula, which is commonly called the rational formula:
Q = C1A C3-1)
where Q = runoff, ft3/s (m3/s)
C = Runoff coefficient, which depends on type of ground cover,
usually 0.15 to 0.95
i = rainfall intensity, 1n./h (cm/h)
A = tributary area, acres (ha)
The design storm for small areas is usually expressed in terms of a
specified hourly rainfall intensity based on the time of concentration
for the tributary area and a selected return interval. This information
is available from intensity-duration-frequency curves established for
the general area based upon historical rainfall data. (See Department
of Commerce Technical Paper No. 40 for an example [2]). The time of
concentration is usually taken as the travel time for rain falling at
the point farthest from the outlet to reach the outlet. For more
detailed information on this method, the reader is directed to any basic
hydrology textbook.
The allowable discharge rate may be based on the capacity of the conduit
or channel at the discharge location, or on building regulations. In
3-10
-------
the latter case, for example, a municipal ordinance may limit the
discharge to that occurlng before development occurs.
Specific criteria to be considered during the design and siting of
various kinds of detention facilities are described here.
Parking Lots and Streets. The parking area should be graded to create
multiple storage areas like saucers. At each low point, a catchbasin or
inlet is used to control the outflow. The outflow control can be
accomplished either by restricting the size or" the outlet pipe or by
using a special cover with drilled holes. As a rule, the maximum depth
of the detained water at the low point should not exceed 12 in. (30 cm).
Thus, the depth of the stored water varies between zero at the edges to
12 in. at the low point.
Almost half of the public agencies responding to the APWA survey
indicated that they permit designs to provide for some street flooding.
Flooding depths ranging from 6 to 8 in. (15 to 20 cm) were most commonly
permitted. The full range of responses is shown In Table 1C.
Table 1C. DEPTH OF FLOODING ALLOWED ON STREETS [/]
Depth of flooding
permitted. In.
0
1-2
3-5
6-8
9-17
18 or more
No answer
Total
Responses
No.
138
14
34
74
16
6
43
325
X
42
4
10
23
5
2
13
Roof Storage. By placing a parapet wall around the edge of a flat roof,
stormwater may be stored on the roof without concern for the structural
Integrity of the roof. Most building codes require roofs to withstand
20 to 30 lb/ft2 (958 to 1,436 N/m2) live loads [£]. This is equivalent
to 4 to 6 In. (10 to 15 cm) of standing water. The detention 1s
3-11
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controlled by a simple drain ring set around the roof drains. As the
roof begins to pond, flow Is controlled by orifices or si Its 1n the
ring; extreme flows overflow the ring to prevent structural damage to
the roof.
Multipurpose Detention Basins. Multipurpose basins are usually used to
store stormwater near the site where It 1s generated. The required
basin size Is determined by calculations based on the design storm.
Such basins are generally 3 to 5 ft (0.9 to 1.5 m) deep. To prevent
water standing 1n the basin between storms, the Invert of the outlet
structure 1s at the same elevation as the bottom of the basin. The
outlet structure should Incorporate not only an orifice for controlling
the outflow rate but also an overflow grating to allow discharge 1f the
orifice 1s blocked with debris. Typically, a 1 to 2 ft (0.3 to 0.6 m)
freeboard should be provided. Sides should be sloped (generally 1-1/2:1
or flatter) and planted with grass that can withstand periodic flooding.
These basins can be used as baseball or football fields, tennis courts,
or playgrounds. In an Oak Lawn, Illinois, detention basin, a concrete
paved area (antleroslon section at the Inlet) 1s flooded during winter
months to serve as an Ice-skating rink
Underground Structures. Concrete, fiber glass, or metal storage tanks
can be constructed underground to serve as detention facilities. Such
structures may be located beneath parking lots, buildings, or sidewalks
and planter strips. Oversized storm sewer pipes can be used In place of
storage tanks. Access must be provided to allow removal of any debris
that collects in the tanks.
Plazas. The basic design approach for plaza storage Is the same as for
other forms of detention. The outlet must be constructed to allow
runoff to accumulate during peak storm conditions. The depth that can
accumulate on plazas should be limited to about 0.75 1n. (1.9 cm) so
pedestrians can still pass, but 1t Is possible to design plazas so that
portions can be flooded without Inconvenience
3-12
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The use of Intensity-duration-frequency curves and the rational formula
to determine the stormwater detention storage volume 1s Illustrated 1n
Example 3-1.
EXAMPLE 3-1. DETERMINATION OF STORMWATER DETENTION VOLUME USING RAINFALL INTENSITY-DURATION-
FREQUENCY DATA AND THE RATIONAL FORMULA
Specified Conditions
1. Stornwater ordinance which states:
The rate of runoff from the land In Us natural state must not be Increased
after the land Is developed. The rate of runoff before development, or the
allowable release rate, 1s assumed to be constant and Is defined mathematically
as the runoff resulting from a 3 year storm and a runoff coefficient of 0.15,
using the rational formula. The runoff rate after development 1s calculated
based on the 100 year storm In 24 hours, and a composite runoff coefficient
to be calculated based on the extent and nature of land cover. The difference
between the runoff rate after development and the allowable release rate Is
to be stored or detained on the site.
2. Site to be developed Is zoned for single family residential use.
3. Lot size 1.0 acre; presently undeveloped.
Assumptions
1. Lot Is located near the Chicago metropolitan area.
2. Runoff coefficient after development will be 0.35.
Solution
1. Determine the undeveloped allowable release rate and the developed runoff rate.
a. Select the appropriate design storm rainfall amounts from reference [ ].
1) For the Chicago metropolitan area, the 3 year, 24 hour rainfall
3.25 1n./24 h.
2) The corresponding 100 year, 24 hour rainfall - 6.00 1n./24 h.
b. Using the rational formula (see page ), calculate the allowable release rate.
1) Rainfall Intensity « 3.25 in./24 h « 0.14 1n./h.
2} For an area of 1.0 acre and a runoff coefficient of 0.15, the resulting
allowable release rate is:
Q C1A « 0.15 x 0.14 In./h x 1.0 acre
* 0.0203 «3/s
c. Using the rational formula again, calculate the developed runoff rate.
1) Rainfall Intensity - 6.00 1n./24 h « 0.25 1n./h.
2) For the same lot with a developed runoff coefficient of 0.35, the resulting
developed runoff rate 1s:
Q = CIA - 0.35 x 0.25 In./h x 1.0 acre
0.0875 ft3/s
2. Determine the detention storage volume required to meet the ordinance conditions.
a. Determine the undeveloped runoff volume for a 24 hour period.
Undeveloped runoff volume « 0.0203 ft3/s x 86,400 s/d
1,755 ft3/d
.b. Determine the developed runoff volume for the same period.
Developed runoff volume 0.0875 «3/s x 86,400 s/d
» 7.560 ft3/d
c. The required detention storage volume is the difference between the 'developed and'
undeveloped runoff volume for a 24 hour period.
Required detention storage volume 7,650 ft3 - 1,755 ft3
5,805 ft3 or 0.1333 acre-ft
Comment
1. The units of the parameters used In the rational formula produce runoff In acre-1n./h,
which Is essentially equivalent to ft3/s (less than IX difference).
2. The use of the national rainfall data, such as In reference [ ]. Is useful for
preliminary analyses but local rainfall Intensity-duration-frequency data should be
used whenever It Is available.
3-13
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Step 4 - Select Detention Opt1on(s). This step requires that one or
more detention methods be selected to meet the constraints Identified 1n
Steps 1 and 2. Depending on the type of development contemplated, one
or more different detention options may be required to develop the
required storage volume. For example, a combination of rooftop and
surface detention might be required for a single-family residential site
to meet the discharge requirements.
The detention opt1on(s) selected must satisfy the functional
requirements established In Step 1. The outlet control works for the
optlon(s) selected must limit the discharge to that determined in
Step 3. The option(s) must also meet the site constraints identified in
Step 2. In addition, consideration must be given to access to the
detention unit for cleaning. Leaves, twigs, grass, dust, and eroded
soil are only some of the items that find their way into detention
facilities in various quantities. Depending upon the detention option
selected, provision must be made to remove these sediments and debris.
The selection of detention options that will meet the storage volume
identified in Example 3-1 is illustrated in Example 3-2.
3-14
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EXAMPLE 3-2. SELECTION AND SIZING OF DETENTION STORAGE FACILITIES
Specified Conditions
1. Same as 1n Example 3-1.
2. Site located adjacent to an existing creek. Stormwater runoff will be discharged to the creek.
.Assumptions
1. The necessary detention storage will be accommodated through (1) rooftop storage,
(2) depression storage, and (3) underground storage. The priority for the storage modes
1s 1n the order listed.
2. The house will have a flat roof with a surface area of 2,800 ft2. The roof will be designed
to support a live load of 16 lb/ft2 In addition to other loads required by the local
building code.
3. During the landscaping of the site, a dperessed area 50 ft by 50 ft by 1.5 ft deep will be
prepared for despresslon storage. Side slopes will be 2 ft horizontal to 1 ft vertical.
4. Any additional storage required will be contained In an underground tank made up of oversized
storm sewer pipe (5.0 ft in diameter).
5. Discharge controls will be Included to limit the runoff rate to the allowable release rate.
Solution
1. Determine storage capacity of the rooftop.
a. Determine the maximum water depth allowed.
1) Based on allowable live load capacity of 16 lb/ft2, the depth of water 1s:
Depth > (16 lb/ft2)/(62.4 lb/ft3) x 12 In./ft = 3.08 1n.
Say 3.0 1n.
b. Determine the storage volume available.
1) Storage volume = 2,800 ft2 x (3 1n.)/l2 in./ft) * 700 ft3
2. Determine storage capacity of the depressed area.
a. Dimensions of the bottom of the depressed area:
Length = 50 ft - 2(2 x 1,5 ft) = 50 - 6 ft = 44 ft
b. Average surface area:
Areaavg B (50 x 50) 4 (44 X 44) . ^ ft2
c. Storage volume of depressed area:
Volume - area x depth = 2,218 ft2 x 1.5 ft
= 3,327 ft3
3. Determine length of 5 ft diameter pipe needed to provide the remaining storage.
a. Determine remaining storage required:
Remaining storage - 5,805 ft3 - (700 ft3 + 3.327 ft3)
* 1,778 ft3
b. Determine storage volume per foot of 5.0 ft diameter pipe:
Unit storage volume = ^- = "(54ft) « 19.63 ft3/ft
c. Determine pipe length required:
U"9th " liffiftS/ft -90-58 ft
say 91 ft
Comment
1. A cost-effectiveness analysis should be made to determine the least cost means of
providing the required detention storage. Local cost data should be used whenever
possible.
2. Consideration should be given also to the maintenance requirements of any storage
methods to be Implemented. The maintenance costs should be Included 1n the cost-
effectiveness analysis. The responsibility for the maintenance should also
be determined.
3-15
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Step 5 - Estimate Costs and Cost Sensitivities. Detailed cost
estimates should be prepared to determine the least cost option(s) to be
Incorporated. These costs can then be compared to the cost difference
between providing an outfall sewer able to convey the peak runoff
(usually based on a 5 or 10 year design storm) with that to convey the
reduced flow.
In some cases, no additional outfall sewers may be required. Then, the
objective 1s to minimize the onslte detention cost by selecting the
detention options that result In the lowest construction cost.
Step 6 - Complete Design. The final step 1s to confirm that all
objectives have been met. Several Iterations may be required to
complete the design. This 1s particularly true once site-specific costs
for the various detention options have been developed.
Operation and Maintenance Considerations
The major operation and maintenance goal of onsite detention facilities
1s to provide readily available, nuisance-free storage for stormwater.
Such units should also utilize as little unproductive space as possible
1n any development while minimizing any visual impact.
Onsite detention facilities should not require extensive maintenance
after each use. Debris removal, care of the landscaping (1f any), and
inspection and maintenance of the outlet structure are all part of the
routine operation of a detention facility.
Mosquito and algae problems can be eliminated by ensuring that detention
facilities drain completely and dry out between uses. Detention ponds
look best when a grass cover is kept on the basin slopes and floor.
However, 1f the basin needs to be vegetation-free for any reason, visual
screening can be provided by sight barriers such as trees.
3-16
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Safety features must also be considered. Hazardous areas must be fenced
to restrict access. Debris must be removed whenever 1t collects to
prevent Interference with the operation of the outlet structure and
eliminate hazards to users 1n a multipurpose facility.
INLINE STORAGE
Overflows from combined sewer systems without stormwater detention
controls generally occur whenever the rainfall Intensity exceeds 0.02 to
0.03 1n./h (0.05 to 0.07 cm/h). This occurs because the peak treatment
rate at plants serving combined sewer systems 1s usually about 1.5 times
the dry-weather flow. Because combined sewers are designed to carry
maximum flows occurring, say, once 1n 5 years (50 to 100 times the
average dry-weather flow), during must storms there will be considerable
unused volume within the major conduits.
In this section, design considerations and procedures for implementing
Inline storage are presented. Examples are used to illustrate the
design procedures and applications. Planning level costs and cost
considerations are given also.
Concept
Inline storage is the use of unused volume in interceptors and trunk
sewers to store stormwater runoff. This is a particularly attractive
option in large cities, especially those with large, flat combined
sewers, for controlling urban runoff. Inline storage Is provided by
damming, gating, or otherwise restricting flow passage just downstream
from a regulator diversion to create additional storage by backing up
the water in the upstream Hnes. Inline storage usually requires a
sophisticated monitoring and control system to maximize the storage
effectiveness. The writers know of no location where Inline storage has
been applied to other than combined sewers. The objective of Inline
storage In these applications has been to maximize the volume of flow
directed to a downstream treatment plant while minimizing the volume and
pollutant load overflowing to the receiving water.
3-17
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The basic elements of a monitoring and control system may Include all or
combinations of the following: (1) remote sensors (rain gages, flow
level and selected quality monitorssuch as DO, TOC, SS, and/or pH
probes, gate Hm1t switches and position monitors); (2) signal
transmission (leased telephone wires, pneumatic circuits); (3) display
and logging (central computer, graphic panels, warning lights);
(4) centralized control capability (control of system gates and/or pumps
from a central location); and (5) 1n the case of fully automated
control, a computer program that makes decisions and executes control
options based on current monitoring data and memory Instructions
Inline storage systems have been developed and successfully Implemented
In Seattle, Minneapolis-St. Paul, and Detroit [/. 7/£, ?]. They are
being Implemented also as part of the overall storm and combined
wastewater overflow abatement programs 1n Chicago and San Francisco.
Design Considerations
Functionally, the application of Inline storage differs little from
onsite detention other than the location where storage occurs. However,
while onsite detention 1s used to minimize the cost of constructing new
storm sewers to serve a developing area, inline storage is used to
decrease the frequency and volume of overflows from combined sewer
systems. Inline storage can also be used to selectively capture and
direct to the treatment plant a portion of the stormflow (i.e., a first
flush contaminant load).
Size of Sewers. To make most effective use of the unused volume in
combined sewers, the conduits used for Inline storage should be as large
as possible. The larger the diameter of the conduit, the more storage
volume is available per unit length. For example, the storage volume
per unit length of a 12 ft (3.7 m) diameter pipe 1s 1.44 times as great
as that of a 10 ft (3.0 m) diameter pipe. Thus, the storage usually
takes place as close to an overflow or diversion structure as possible.
3-18
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Slope of Sewer. Essential to effective use of Inline storage are sewers
with flat sewer grades In the vicinity of the Interceptor. This allows
the backwater effect created by the storage to extend as far upstream
from the regulator location as possible.
Peak Flows. To prevent surcharging of the sewer system and ponding of
runoff on streets, 1t 1s necessary to allow overflow structures to pass
the design flow unrestricted when required. Thus, during large storms,
Inline storage may be affected only during periods when inflows are less
than the designed capacity of the sewer. This Is usually during the low
rainfall intensity portions of the storm. For smaller storms, the full
flow capacity of the sewer may never be reached so that inline storage
1s affected throughout the storm.
Controls. The control system for effecting inline storage is usually
quite sophisticated. Not only are controls required to operate the
regulator itself but also for monitoring conditions upstream and
downstream of the regulator. This requires that level sensors and
overflow detectors be used as a minimum. It may also require rain gage
networks, water quality sensors, and flowmeters. If more than one
inline storage location is involved, 1t may also require a computer to
monitor and control the operation of the storage to maximize Its
effectiveness.
Resuspension of Sediment. One side effect of inline storage is the
settling of material in the sewers as the flow velocity is reduced. To
prevent problems later on, this sediment must be resuspended and
transported downstream to the treatment plant or the overflow. The
operation of inline storage lends Itself to remedying this situation.
During periods when there is high flow 1n the sewer during a storm, the
velocity 1s usually sufficiently high to resuspend and transport any
sediment to the overflow. Also, when the flow in the Interceptor
downstream of the regulator 1s reduced,.the regulator can Increase the
diverted flow so that the velocity 1n the sewer 1s sufficient to
transport any sediment.
3-19
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Design Procedure/Examples
A suggested Inline storage design methodology 1s shown 1n Figure 3-2.
Each of the Indicated steps are discussed below and examples are
Introduced where applicable.
Step 1 - Identify Functional Requirements. The frequency with which
overflows to the receiving water occur at various locations 1n the sewer
system determines the design emphasis for Inline storage facilities.
Locations where frequent overflows occur or where only small amounts of
rainfall initiate overflows are prime candidates for inline storage
implementation.
Identification of the need for overflow control is usually based on one
or more of the following:
Overflow frequency reduction
Overflow volume reduction
Overflow quality Improvement
Typically, for combined sewer systems, the major concerns with overflows
are the contamination of the receiving water and the resulting public
health aspects of that contamination. This 1s particularly true where
NPDES permits have established receiving water quality limits. Reducing
the Impacts of the overflows on the receiving water can be effected by
(1) reducing the frequency with which overflows occur allowing the
receiving water to recover from the shock load, (2) reducing the volume
of stormwater that reaches the receiving water so that Individual shock
loads are smaller, or (3) Improving the quality of the stormwater that
overflows by diverting more of the highly contaminated flow to treatment
facilities.
3-20
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IDENTIFY FUNCTIONAL
REQUIREMENTS
I. OURFLOI FREQUENCY
REDUCTION
I. OUIFLOI IOIUIE
REDUCTION
C. QURFLON OUILITt
IIMOVEIENT
ro
IDENTIFY SITE
CONSTRAINTS
I. «R£»
0. STSTEN HTORIULICI
C. STSTEN COMF1 GURU 101
0. ENIIRONNENTIL
E. STRUCTURAL
-N
ESTABLISH BASIS
OF DESIGN
I. FLO! CMIRUTERISTICS
I. STORAGE CIPUITY
C CONTROLS
D. SEOINENT NESNSPEISION
FACILITIES
SATISFY
OBJECTIVES?
ESTIMATE COSTS AND
COST SENSITIVITIES
«. CIPITIL
R. OPERtTION IND
NIINIENINCE
C. fUUE ENBINEERINfi
SELECT STORAGE
LOCATIONS
I. OPERITIONIL CONCEPT
I. CONTROL IETNODS
C. ICCESS
SHIFT TO ALTERNATE
SITE OR METHOD
Figure 3-2. Inline storage design methodology.
-------
Step 2 - Identify Site Constraints* Sites for Inline storage
Implementation should be Identified and cataloged with respect to at
least the following criteria:
t Accessibility to the sewer, the Interceptor, and the overflow
point.
Total usable area for construction and operation of the
necessary controls.
Hydraulic data on receiving water levels at the overflow
point, flow depth ranges and capacities for the trunk sewers
and Interceptor, any stage and pumping requirements for the
proposed facility, stage and corresponding storage volume
within the sewer system, and the frequency and volume of
overflows.
Environmental setting such as proximity to residences or other
structures, local and surrounding land uses, and visual
impacts.
Geotechnical conditions and probable structural requirements
(I.e., special foundation needs, hazards to adjacent
structures and utilities, etc.).
Accessibility to utility services and construction and
operation activities.
Step 3 - Establish Basis for Design. The purpose of this step 1s to
determine flow characteristics and loadings needed to evaluate the
effectiveness of inline storage in meeting the guidelines established 1n
Step 1. Representative inflow characteristics may be developed from dry-
weather flow data and analyses of historical overflow samples. Direct
field measurements may be required.
3-22
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Storage requirements for the drainage areas must be determined. The
relative effectiveness of storage related to overflow volume and/or
frequency must be evaluated for each area. The areas must be ranked by
relative effectiveness to establish the priority for further evaluation.
The alternative methods available for controlling the Inline storage
must be reviewed. For each site, a control method must be selected that
will satisfy the needs for that particular site. If more than one site
will be Involved 1n the Inline storage system, the means for monitoring
conditions at each site and for coordinating the controls to optimize
the effectiveness must be selected. This could Include simple
supervisory control by an operator or full computerized control.
Provisions should be Incorporated to resuspend ancJ transport any
sediment deposited during Inline storage.
For example, auxiliary controls within the sewer to redirect flow to a
single barrel of a multibarrel sewer may be required. The quantity of
sediment 1n a sewer at the end of a storm is a function of the quantity
and characteristics of suspended solids in the flow, the length of time
the inline storage was in operation, and the hydraulic characteristics
of the sewer.
Step 4 - Select Storage Locations. Based on the design criteria
established in Step 3, the site constraints identified in Step 2, and
the functional requirements from Step 1, the alternative sites should be
ranked. Detailed evaluation of the sites should be performed in
accordance with this priority ranking. The operational concept, storage
control method, and projected effectiveness should be determined for
each specific site. Based on the functional requirement for that.site.
Operational concepts to be considered should include:
Storm anticipation so that runoff from short duration storms
or "spot cell" storms can be completely captured and treated.
3-23
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Selective detention of flow from portions of the system to
allow sewer Inspection, maintenance, or modification.
Selective overflowing to cause overflows to occur at points
that will have the least effect on the receiving waters.
First flush Interception of the more heavily contaminated
stormwater from the early part of a storm.
Auxiliary systems for sediment transport and regulator control must be
determined. This should include air handling and odor control, energy
(power, lighting, heating), and instrumentation for controls.
Availability of utilities needed to operate the storage facility must be
assessed.
In addition, an operation plan and a maintenance schedule should be
prepared at this time.
Step 5 - Estimate Costs and Cost Sensitivities. Once the location has
been selected and the inline storage facility preliminary designed, a
detailed cost estimate should be prepared with emphasis on component
systems and following value engineering guidelines. Operation and
maintenance costs should also be estimated based on the operations plan
and maintenance schedule from Step 4. A cost-effectiveness analysis can
then be prepared evaluating each site and establishing the priority for
implementing construction.
Step 6 - Complete Design. The final step 1s to confirm that all
objectives have been met. Several iterations back through previous
steps may be needed to reach the most cost-effective solution.
Operation and Maintenance Considerations
The major operation and maintenance goal of Inline storage facilities 1s
to provide a system that 1s available and will operate as designed
3-24
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whenever needed (even following prolonged periods of non-use). Adequate
Information on the operation and effectiveness of the facility should
also be Included.
Experience has shown that frequent, periodic maintenance and equipment
exercising is required. The nature of the atmosphere and conditions
found where much of the equipment is located dictates this. Corrosion
and clogging by debris are typical of the problems encountered by the 1n-
sewer portion of such facilities. The Instrumentation needed to control
the operation of the regulator can be another source of problems.
However, recent advances in the manufacture and design of
instrumentation and controls have greatly reduced the problems.
Costs
Construction costs for inline storage have been reported for selected
demonstration sites [£"]. However they are highly site specific.
Adjusted to ENR 4000, the range of unit costs 1s from $0.04 to $0.84 per
gallon ($10.60 to $22.90 per m ) of storage volume. They also vary
considerably depending upon the complexity of the flow regulators and
control systems. For example, the cost of the control and monitoring
system was 47% of the $0.84 per gallon for that demonstration project.
Detailed operation and maintenance cost data are limited. No rule-of-
thumb guides exist at present. Operation and maintenance costs must be
estimated for specific facilities from the operation plan and
maintenance schedule. For planning level studies, estimates can be
developed from costs reported for operating facilities C$3.
DOWNSTREAM STORAGE/SEDIMENTATION BASINS
The frequency of overflow operations to the total frequency of facility
activations determines the design emphasis for downstream
storage/sedimentation basins. By definition these facilities are
Intended to be end-of-the-p1pe controls; discharges (overflows) are
3-25
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expected to be directly to receiving waters with or without further
treatment (such as disinfection, ultraflne screening, filtration, etc.).
It is the most common and, perhaps, effectively practiced (in terms of
operating installations and length of service) method of urban
stormwater runoff control. Conversely, since it parallels historic
sanitary engineering practice, it is frequently criticized for lack of
innovation, due to its simplicity, and high cost, due to Its structural
orientation.
In this section, design considerations and procedures for downstream
storage/sedimentation basins are presented and illustrated by example
and through references to designed and operated facilities. Planning
level costs and cost considerations are given.
Concept
Conceptually, a downstream storage/sedimentation basin differs from
other detention facilities only in its locationimmediately upstream of
the receiving water. However, many of its design features are dictated
by its function (interception of tributary flows), performance
expectations, and environmental setting. Facilities may be online (no
bypass option) or offline (use of facility is by operator's choice).
The general application is on combined sewer systems with post-storm
dewatering back to the collection system for treatment at the dry-
weather plant. When discharging to confined waters, the facilities will
normally include disinfection capabilities.
Design Considerations
Functionally, the applications of downstream storage/sedimentation
facilities vary from essentially total containment, experiencing only a
handful or less of overflows per year, to pass-through treatment systems
where total containment is the exception rather than the rule. In the
former, the major concerns are the usable storage volume, the provisions
for dewatering, and post-storm cleanup. In the latter, performance
3-26
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hinges on treatment effectiveness and design considerations Including
loading rates, Inlet and outlet controls, short circuiting, and sludge
and scum removal systems.
In the case of offline facilities, the option exists to selectively
capture a portion of the stormflow (i.e., first flush contaminant load)
and bypass the balance to avoid the loss of captured solids through
turbulance and resuspension. Examples of representative CSO
storage/sedimentation basins and auxiliary support facilities are shown
in Figure 3-3.
Storage. Required storage volumes to accomplish a specified level of
control are best approximated through use of simplified continuous
simulation models. A summary of representative models and their
application is presented in Section 4 of Urban Stormwater Management and
Technology: Update and Users' Guide [$"]. Where the level of control
objective is high (i.e., storage-treatment capacity is large compared to
runoff volume) and urban development is intense (I.e., runoff
coefficient is a function of impervlousness), two particularly useful
models are EPAMAC [/o] and STORM [//.]. The former, developed by writers,
is an extension of Metcalf & Eddy's Simplified Stormwater Model ['/*] and
the areawide planning model ABMAC [/.?]. Because of the writers'
familiarity with EPAMAC and its availability, it has been used as the
base model for examples in this text. In cases where short-term flow
dynamics are of major concern or where pervious areas of the watershed
play a significant role, other, more detailed, models such as SWMM \_jf\
and NFS [./$] may be required. Discussions of their application,
however, 1s beyond the scope of this text.
EPAMAC operates on an hourly timestep and is designed for applications
on combined sewer systems. Inputs include hourly rainfall data,
watershed subareas, runoff coefficients, runoff quantity and quality,
routing time offsets, and network flow routing. At each network control
node, the user specifies the available storage volume and dewatering
(treatment) rate with its associated operating rules (i.e., pumps
3-27
-------
STORAGE/SEDIMENTATION
r
I
SCREENS
CL,
COARSE FINE
1
mmmmmmxL
^mmmmmm
i
w
,'.'.
OVERFLOI ^
w
HX1 ft>
I
DRAIN RETURN
TO INTERCEPTOR
SIX PARALLEL BASINS
STORAGE CAPACITY 1.3 Hgal
BOSTON (COTTAGE FARM)
STORAGE/SEDIMENTATION (OPEN)
^
\i|x;>:;x
\lii
s X
rNL
CD~~^
DRAIN
RETURN TO
INTERCEPTOR
K ^ >
/^ OVERFLOW ^
1
4
L ONE BASIN
STORAGE CAPACITY
2.B Hgal
CHIPPEWA FALLS
FINE SCREEN
STORAGE/SEDIMENTATION (BURIED)
^ -If
u
* (7\
^ VI/
DEWATER TO
b
INTERCEPTOR
L,,
2
_
OVERFLOW ^
i ONE BASIN
STORAGE CAPACITY
3.B Mgal
MILWAUKEE (HUMBOLT AVE.)
Figure 3-3. Representative process schematics.
3-28
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STORAGE/SEDIMENTATION (OPEN)
STORAGE CAPACITY 27 Nga I i- CL,
TUNNEL
STORAGE
-&
L
S SEDIMENTATION/
IESUSPENSION
ASINS IN SERIES
L-O
POST 4TORM OEIATER
TO INTERCEPTOR
iC
L
OVERFLOI
CONTACT
MS IN
AERATED RETENTION BASIN
LONG-TERM OEIATER TO
TREATMENT AND REUSE
MOUNT CLEMENS
DEGRITTING
CYCLONE
DRAIN AND PUMPED
RETURN TO
INTERCEPTOR
COARSE
SCREEN
STORAGE/SEDIMENTATION
(ENCLOSED)
OVERFLOI
6 PARALLEL I* SINS
STORAGE CAPACITY 10 M|a I
NEW YORK CITY (JAMAICA IAY)
STORAGE/SEDIMENTATION (COVERED)
DRAIN RETURN TO
INTERCEPTOR
3 IASINS IN SERIES INTERCONNECTED
IY OVERFLOI IE IRS (i/e., IASINS
FILL SEQUENTIALLY) STORAGE
CAPACITY 23 H|al
SACRAMENTO (PIONEER IESERVOII)
Figure 3-3 (Continued)
3-29
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STORAGE/SEDIMENTATION
DRAIN
RETURN TO
INTERCEPTOR
(ENCLOSED)
EFFLUENT SCREENS
OVERFLOW
2 PAIRS OF BASINS IN SERIES.
DESIGNED TO FILL SEQUENTIALLY
STORAGE CAPACITY 3.5 Up I
SAGINAN (HANCOCK STREET)
FINE SCREENS
GRIT CHAMBERS
COARSE SCREENS
©_
STORAGE/SEDIMENTATION I
(BURIED)
DEWATER TO
DRY -WEATHER
BASINS AS
NECESSARY
OVERFLOW
16 PARALLEL BASINS
(TYPICALLY 3 IN SERVICE
IN DRY WEATHER)
STORAGE CAPACITY IS Mga 1
CONTINUOUS SLUDGE
PUMPING AVAILABLE
NOTE : JOINT DRY-WEATHER/WET-WEATHER
PRIMARY TREATMENT FACILITY
SAN FRANCISCO (SOUTHWEST IPCP)
©
LEGEND
PUMPS
CL, LOCATION OF CHLORINE OR
' HYPOCHLORITE ADDITION
Figure 3-3 (Concluded)
3-30
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started and stopped as a function of filled storage volume that
tlmestep). When the hourly storage-treatment capacity 1s exceeded, an
overflow (discharge) occurs.
The user selects trial storage volumes and associated dewaterlng
(treatment) rates to fit the constraints of his system and through
Iterative analyses selects the combination that best satisfies his needs
-(I.e., overflow frequency, site limitations, and cost). Note that
storage may be distributed over a series of nodes to represent upstream
and inline storage options; however, only one network control node 1s
allowed per run. For example, 1f storage is to be provided within a
watershed through a sequential series of dispersed storage elements such
as an upstream surface detention basin, an intermediate zone of inline
storage, and a downstream storage/sedimentation basin, three sets of
computational runs would be required. The first would consider only the
upper watershed and its storage basin. The computed discharge from this
basin would constitute a lateral inflow for the second run. The second
run would consider the additional tributary area to the inline storage
facility plus the lateral inflow generated by the first run. In turn,
the second run would produce a lateral Inflow for the third run, which
would reflect the storage benefits of both the upper surface basin and
the inline storage in the intermediate zone. Finally, the third run
would add any new tributary area flows to the lateral Inflow generated
by the second run and permit the sizing of the downstream basin.
Treatment Efficiency. Storage units may alter the wastewater
characteristics of the applied stream by gravity separation. The
changes (removals) approach a maximum under quiescent or lightly stirred
conditions and may reduce to negligible (or in the extreme of
resuspenslon, a net increase rather than removal) levels under turbulant
conditions. In its simplest sense, wastewater 1s made up of water plus
partlculates, some of which have a lower specific gravity than water,
some of which are heavier, and some of which are the same. When the
cohvectlve forces transporting these partlculates are relaxed, those
lighter than water start to rise and those heavier start to fall. This
3-31
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new movement may Increase collisions between particles and by adsorption
or flocculation, large particles are formed that 1n turn further the
separation. The movement continues until the particles hit the floor of
the chamber forming a sludge, the surface forming a scum, or until the
convective forces again predominate and they are carried onward.
»
Sedimentation theory and convective forces are lucidly described in
wastewater engineering texts; however, there are two major problems:
(1) wastewater tends to be quite heterogeneous with Its makeup of
heavier than, equal to,, and lighter than particulates changing from one
moment to the next; and (2) the theory reflects Idealized situations to
which a myriad of modifiers must be applied to reflect real world
conditions. In compensation for these shortcomings, however, the theory
gives the practitioner a concept of what should be happening under what
controls to assist his judgment in ferreting out the best design
decisions.
Controllable design variables in general order of Importance are surface-
loading rates, detention time, basin geometry, Inlet and outlet design,
and rapid sludge removal. Potentially controllable parameters adversely
affecting performance are temperature differentials between the incoming
flow and the basin contents, turbulence generated by flow variations,
and wind induced currents.
Generally, noncontrollable but important parameters of the raw
wastewater are its suspended solids concentration (high concentrations
tend to settle more efficiently), the proportion of settleable sol Ids,
and its age or septicity. A long documented but little used test to
reflect these unique characteristics of a particular waste is the
settling column test described by Metcalf 4 Eddy [/6], Camp [A,?], and
others [//, /?]. It 1s the writer's belief that these tests will
provide a valuable aid 1n projecting storage/sedimentation performance
in urban stormwater systems design. Application and use of settling
column tests are illustrated under the design procedure discussion to
follow. Tests are normally run to record total suspended sol Ids removal
3-32
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as a function of time and depth; however, 1t should be noted that
settling column tests can be run on the basis of any quality parameter
for which removals are accomplished by sedimentation.
Typical removal efficiencies for total suspended sol Ids as related to
surface loading rates and detention times are shown 1n Figure 3-4. Each
plot represents a "best fit" curve representation of a broad data
^scatter from a number of Installations over a large number of events.
The degree of scatter and limitations of theoretical approaches are
Illustrated 1n Figure 3-5, which fs based on 24-hour Influent and
effluent samples from a primary treatment plant 1n San
Francisco receiving storm and sanitary flows from a combined sewer
system [5.O], Thus, even a single plant will exhibit wide day-to-day
fluxes 1n efficiency under the same surface loading rate,'"and NPDES
permit requirements must acknowledge this variability. The potential
for removal efficiencies to vary during individual storm events 1s shown
1n Table 3-1 t?o], wherein performance over the first 2 storm hours
(first flush) is compared to the average performance over the entire
storm event.
Representative removal efficiencies associated with plain sedimentation
of wastewater 1n conventional plants are listed 1n Table 3-2. Because
of the limited data base, Independent performance ranges cannot be
presented for urban stormwater, but the presumption is that they will be
similar. In San Francisco, comparison of heavy metals between filtered
and nonfiltered stormwater samples indicated that the majority of heavy
metals were associated with the soils fraction B0]; thus, effluent
quality improvement would be expected to be associated with
sedimentation. In the limited number of storm composites measured
before and after treatment, however, a conclusive trend was not
apparent.
Disinfection. Where disinfection Is required in a storage/sedimentation
basin, a minimum contact period of 15 minutes at peak flow Is commonly
specified. Further, since the consumption of disinfectant (typically
chlorine or a chlorine derivative) and Its effectiveness are adversely
3-33
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0
10
FROM IEFT18A]
CONVENTIONAL PRIMARY TREATMENT
ITH MECHANICAL SLUDGE REMOVAL
20
FROM IEF.[41
TYPICAL CSO STORACE/SEOIKENTATION
ITHOUT MECHANICAL SLU06E REMOVAL
I
J_
I
500
1000 2000 1000
1= SURFACE LOAOIN6 RATE. |il/lt'-f
I
I
4000
3.6 1.1 0.* 0.6
DETENTION THE (ASSUNIN8 10ft AVG DEPTH). Mur«
0.45
Figure 3-4. Typical TSS removal efficiencies
by sedimentation.
3-34
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100
80
60
ta
v>
II 40
20
NORTH POINT IPCP
WET-WEATHER 1977-1878
(DAILY COMPOSITE VALUES
FOR RAINFALLS 0.10 in.)
LEAST SQUARES FIT
SOO
1000
2000
3000
S= SURFACE LOAOIN6 RATE, pl/ft'-d
Figure 3-5. Experienced TSS removal
efficiency variations.
4000
3-35
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Table 3-1. PILOT PLANT PERFORMANCE ON RICHMOND-SUNSET
STORMWATER (CSO) FLOWS
Total storm
Test
duration
Date* h
2/28/79
3/16/79
3/26/79
a.m.b
p.m.
a. Average
b. Morning
7
14
22
.5
.5
.5
of all
shower
Surface-loading
rate,
gal/ftz-d
1
1
2
,500-2,400
.600-2,400
.000-2.400
grab samples over
lasted only 1 hour
Avg TSS
J
Influent Effluent
128
111
98
first 2
; main
87
78
49
Hvg ret
32
30
50
Avg
Influent
176
173
255
152
First flush8
TSS
Effluent
92
105
118
42
vg removal ,
%
48
39
61
72
hours of storm unless otherwise noted.
storm started 4-1/2 hours later.
3-36
-------
Table 3-2. COMMON REMOVAL EFFICIENCIES
ASSOCIATED WITH PRIMARY SEDIMENTATION OF
SANITARY WASTEWATER
Wastewater Removal efficiency, %
BOD 25-40
TSS 40-70
Settleable solids 85->95
Bacteria 25-75
Total nitrogen 5-25
Total phosphorus 5-20
Grease and oil 40-60
3-37
-------
Impacted by solids in the flow, where detention periods dictated by
storage requirements are significantly longer than the contact period
required, multl staged basins should be considered with the disinfectant
added to the final stage(s) only. In this manner, the benefits of the
partially clarified wastewater will be realized. Common dosage
requirements are 15 to 30 mg/L of available chlorine. Flow control must
be established and the flowrates known 1n order to effectively pace the
dosage. Because of the rapid changes 1n flow typically received 1n
storage/sedimentation basins, pacing disinfection additions solely by
effluent residual monitoring has not been effective
Site Constraints. Whereas approximately one out of ten conventional
wastewater treatment plants is uncovered, the reverse is the general
rule for downstream storage/sedimentation basins serving combined
sewered areas. This is because usable land along waterfronts within the
urban core 1s typically very highly developed or recreation oriented;
thus, in the public's mind at least, 1t is Incompatible with open raw
sewage basins. Historically, treatment plants have been built in quasi-
isolated areas and development has encroached on the sites In a
forwarned condition. Conversely, CSO systems and their associated urban
development exist, and it is the treatment facility that must do the
encroaching. In smaller communities (I.e., Chippewa Falls,
Wisconsin [*/], and Mt. Clemens [**], comparatively Isolated centrally
located areas have been found and open basins constructed and operated
without reported nuisance. In the majority of cases, however (i.e.,
Boston, Milwaukee, New York, Sacramento, Saginaw, and San Francisco),
the facilities are covered and in some cases buried.
In San Francisco, problems with limitations In usable waterfront space
were coupled with the need to Intercept a multitude of dispersed
overflow points 1n arriving at an Innovative storage- transport concept.
In this case, large, elongated downstream storage/ sedimentation basins-
super sewers were constructed that combined storage/sedimentation
functions (I.e., pretreatment for overflows) with Interception and
transport functions. The North Shore consolidation project, for
3-38
-------
example, provides a monolithic box conduit and tunnel structure snaking
along 3 miles of waterfront while Intercepting seven overflow points,
providing 23 Mgal (87 m ) of storage, conveying flows to a central
location for pumping to treatment, and pretreatlng necessary overflows
(an average of four per year) by sedimentation and skimming. The fact
\hat "super sewers" can provide significant treatment during periods of
overflow 1s demonstrated 1n operating data taken from the 2 mile long,
=62 Mgal (2
Table 3-3.
-=62 Mgal (235 m ) capacity, Red Run Drain near Detroit [£2] and shown In
Photographs of typical facilities are shown 1n Figure 3-6. Even covered
facilities vary 1n that some permit operator access during operations
(I.e., walkways and working space above the free water surface), where
others can be entered (as in sewers) only in a dewatered condition. The
net Impact 1s frequently a doubling and redoubling of the basic
functional cost; however, the alternatives are typically no more
acceptable than open sewers would be.
Odor generation from stored urban stormwater (CSO), to the writer's
knowledge, has not been well investigated and documented. Contributing
factors would be elevated temperatures (acceleration of decay) and
turbulance (release of gases from solution). Limited studies conducted
at San Francisco [AC] for two storms showed that for 1,500 gallon
(5,678 L) samples of first flush stormwater stored in covered but vented
storage, hydrogen sulfide generation peaked 24 hours after collection
but that concentrations had not reached a distiguishable level (0.3 ppm-
volumetric), even after 48 hours. Highest odor potential should be
expected during dewatering operations as settled sludge 1s exposed.
Options for sludge removal systems, performance assessment under
variable flowrates, and other design details are discussed in the
following section.
3-39
-------
Table 3-3. PERFORMANCE OF THE RED RUN CSO
SEDIMENTATION/TRANSPORT BASIN
Storm
date
3/14-15/78
3/21-22/78
5/8/78
5/13/78
5/30/78
Total
Influent,
mg/L
116
52
238
114
294
suspended
Effluent
mg/L
102
36
168
38
152
solids
, Removal ,
%
12
31
29
67
48
Volatile
Influent,
mg/L
62
32
128
64
170
suspended
Effluent,
mg/L
36
20
54
4
26
solids
Removal ,
%
42
38
59
94
85
3-40
-------
. .--^s
Above and left - Milwaukee
(Humbolt Ave.) storage/
sedimentation facility
Left - Saginaw (Hancock St.)
storage/sedimentation facility
is under this two level
parking garage in a central
business district
Above - Sacramento (Pioneer Reservoir)
has exposed roof with sidewalls
screened by embankment (photo courtesy
of Sacramento Regional County Sanitation District)
Figure
Site Impacts of typical downstream
storage/sedimentation basins.
3-41
-------
^^f- ^- \t^^3*
^^tr^^-.li^Sr^"^"^^
Above - Boston
(Cottage Farm)
storage /sedimentation
facility buried under
embankment
Above and left - San
Francisco North Shore
Consolidation Project "super
sewer" storage/transport
basin under construction
(Photos courtesy of San Francisco
Clean Water Program Government
Affairs')
Left - New York City (Jamaica Bay)
storage/sedimentation facility enclosed
with superstructure
Figure 3-6 (Concluded)
3-42
-------
Design Procedure/Example
A suggested design methodology 1s shown 1n Figure 3-7. Each of the
Indicated steps 1s discussed below and examples are Introduced where
applicable.
Step 1 - Identify Functional Requirements. As noted previously, the
Intended operational function of the downstream storage/sedimentation
basin will determine its design emphasis (I.e., does its treatment
function rank primary or secondary with respect wO storage). Obviously,
a facility that will spill or discharge under all but the smallest of
storms (I.e., a typical detention-chlorination facility) should be
designed as a treatment facility; whereas discharge frequency of a few
times a year would warrant a design predicated mainly on construction
and operational economics. Area hydrology, system hydraulics, and
overflow frequency objectives will determine the volumetric capacity
required. As stated earlier, simplified continuous simulation models
are generally best suited to this task and detailed user manuals [5) 10]
have been prepared.
Answers to be provided by the model or developed from the model output
include:
Design volumetric capacity or matrix of storage-dewatering
(treatment) rate combinations that meet overflow frequency
criteria.
Frequency distribution of unit operations by month, year, and
period of record.
Frequency distribution of operation durations, storage volumes
utilized, treatment rates experienced, overflow events,
overflow rates, overflow durations, and between storm downtime
availability.
First cut assessment of sol Ids applied, solids retained or
diverted, and solids overflowed.
3-43
-------
IDENTIFY FUNCTIONAL
REQUIREIENTS
i. miinnc CAPACITY
I. rifllTIRLE/SETTtEABLE
IEIOIU EFFICIEICT
C. IISIIFECTIOI
t>
IDENTIFY SITE
CONSTRAINTS
i. AREA
1. NTDRIULIC
c. EitiRomiiTAL
0. STRUCTUIU
-N
-I/
ESTABLISH BASIS
OF DESI6N
». IIFIVEIT
CHIRtCTERISTICt
1. OESICI LOIltlt
IITES
C. PERFORMANCE
ESTIMATE!
\r--j
DETAIL AUXILIARY
SYSTEMS
SLIIEE PROCESSINI
IISINFCCTIOI
All IJIIIINS
ENEICT AND CONTROL
NT
IDENTIFY AND SELECT
PRETREATMENT COMPONENTS
A. COURSE SCREENINS
I. FINE SCREENINS
C. SRIT REMOVAL
D. FLOI MEASUREMENT
SELECT IAIN
TREATMENT GEOMETRY
A. OPERATIONAL CONCEPT
I. CONPUTNENTUIZITIOI
C. INLET/OUTLET fORKS
I. SLUDGE/SCUM REMOVAL
SYSTEM
ESTIMATE COSTS AND
COST SENSIBILITIES
I. CIPITU
I. OPERATION tID
MIINTENIICE
C. milE
EICINEERII6
FACILITIES
SATISFY
OBJECTIVES?
SHIFT TO ALTERNATE
SITE OR PROCESS
Figure 3-T. Design methodology,
3-44
-------
Where an NPDES permit has been Issued, It must be consulted to Identify
any restrictions on discharges 1n terms of concentrations, mass loadings
(I.e., basin plan waste load allocations), disinfection requirements,
and reporting criteria. Where NPDES permits have not been Issued,
target criteria must be established through meetings with regulatory
agencies having jurisdiction, and must be supported through cost-
effectiveness analysis.
Step 2 - Identify Site Constraints. Sites for downstream
storage/sedimentation basins should be Identified and cataloged with
respect to at least the following criteria:
t Accessibility to the collection conduit, the Interceptor (for
postevent dewatering), and a suitable overflow point for
discharges.
t Total usable area and Its dimensions and configuration.
Hydraulic data on influent levels, receiving water levels,
(normal and flood), and interceptor levels to Identify stage
and pumping requirements for the proposed facility.
Environmental setting such as proximity to residences, public
facilities, compatible and noncompatible land uses, visual
exposure, and prevailing winds.
Geotechnical conditions and probable structural requirements
(i.e., pile supports, hazards to adjacent structures and
utilities, etc.).
Accessibility to utility services and for construction and
operation activities.
Typically, this information will be used 1n selecting the main treatment
geometry 1n Step 4.
Step 3 - Establish Basis of Design. The purpose of this step is to
determine influent characteristics and loading rates necessary to meet
the requirements set down 1n Step 1. Representative Influent
characteristics may be developed, in the case of CSO systems, from an
3-45
-------
anlysls of dry-weather plant Influent data during wet-weather operations
segregated by (1) storm size (rainfall recorded), (2) seasonal
occurrence, (3) time Into the event, etc., and supplemented by direct
field measurements. :
In addition, 1t 1s recommended that settling column tests be performed
as a basis for predicting basin performance. These tests have found
wide acceptance In the Industrial waste treatment field (where designers
freely admit lack of knowledge of a particular wastes settling
behavior). However, these tests are rarely performed on municipal
wastewaters where (1) the assumption Is made that behavior will be
typical, or (2) that the sedimentation unit process will be followed by
additional unit processes; thus the relative Importance of primary
settling characteristics is small. In urban stormwater management, the
writers believe the additional testing is certainly warranted (I.e., In
addition to knowing the Influent suspended solids concentration, It
would be extremely informative to know if the solids will settle "like
buckshot in alcohol or feathers in molasses" in selecting surface
loading rates and detention times for design).
In translating the idealized (quiescent) settling column results in
design, texts caution that to account for the less than optimum
conditions encountered in the field, the design settling velocity or
surface loading rate obtained from column studies should be multiplied
by a factor of 0.50 to 0.85 and detention times by a factor of 1.25 to
2.0 [l
-------
EXAMPLE 3-3. COMPUTE TSS PERFORMANCE CURVE FROM SETTLING COLUMN TEST RESULTS
Specified Conditions
1. Laboratory test results were obtained from a settling column test of a 2 hour composite
sample of "first flush* CSO. Test column was 6 In. diameter, 10 ft high, with sample
taps at 24 In. centers. The composite sample was premlxed and pumped Into the columri.
Samples were drawn from each tap Initially and repeated at specified time Intervals.
TSS results for the Individual samples were as follows:
TSS results. ng/L
Initial
depth. In.
5
29
53
77
101
Mean values
Elapsed time.
0
202
240
384
384
408
324
30
136
172
148
236
226
184
60
112
122
126
140
154
131
Inutes
90
96
126
132
142
138
127
120
110
118
124
118
118
2. Sedimentation tank depth Is 10 ft.
Assumptions
1. Assume a surface loading rate scale factor of 0.75 to translate the "Idealized" column
results to a field basin.
Solution
1. Calculate TSS removals as a percent of Initial mean concentration.
TSS removal results. I
Initial
depth, 1n.
5
29
53
77
101
Elapsed time,
0 30
58
-- 47
54
-- 27
30
60
65
62
61
57
52
minutes
90
70
61
59
56
57
120
66
64
62
64
2. Plot results to scale and sketch 1n best fit removal curves for 30J, 40%. SOX. 60S, and 70S.
IIIFtCC
1MI I,
m
3-47
-------
3. Compute the percent removal at 30, 60, 90, and 120 minutes by proportionality. For example.
at t « 120 minutes, percent removal
Ah] R! + R2 + Ah? R2 * R3
1
Time Calculation
120 ^fg x ^°- + ^ x 1|°- 6.37 + 60.13
90 reTx-2- + T27*-2-+i2Tx-?-
1 1 70 16 1 30 85 110 20 98
60 T22x~2~*122x1~T22x~rTl2"xF
30 T23 x *i23*-2~ + T23*2-+T?3xr + T!
4. Using 10 ft depth, compute surface loadings corresponding
factors (0.75 surface loading and 1.33 to detention time)
Unsealed (Ideal) performance.
Detention Surface . Removal Detention
time, min loading, gal/ftz-d efficiency, t time, min.
30 3.591 38 40
60 1.796 56 80
90 1.197 61 120
120 898 66 160
Removal
- 66.5
« 61.3
55.5
jyxf - 37.9
to detention times and apply scale
for projected prototype performance
Projected performance
Surface
loading, gal/ftz-d
2.693
1,347
898
674
Removal
efficiency,
38
56
61
66
I
5. Plot projected performance results for use In Example 3-4.
10 t-
40
soo
MOIECUO KIFOMtlCt
1000 1000 1000
I'tUIPtCE LOtOIMt MTEj pl/M'.g
4000
I.I 1.1 « ».«
BUHTION THE OISUIIRB 10 It »»« OEM*). Mull
0.45
Comments
First flush test behavior shows continued good performance at high overflow rates.
Problem 1n completely mixing the sample 1n the settling column at t - 0 Is evident 1n
the test results. Ideally, results should fall within 10% of mean.
3-48
-------
Step 4 - Select Main Treatment Geometry. The geometry of a downstream
storage/sedimentation basin will be governed by the constraints
Identified 1n Step 2, the loading rates selected from Step 3, and the
operational concept developed from the Step 1 frequency analyses. For
example, 1f a large number of the total plant operations will use, say
30% of the storage capacity or less, a compartmented basin with
sequential filling could greatly reduce cleanup operations without any
impact on performance. Also, 1f the characterization data Indicate a
pronounced first flush, segregating this load from the balance of the
storm may be beneficial from both a cleanup and performance aspect.
Basic reasons for dividing storage/sedimentation basins Into
compartments are:
To reduce short circuiting
To facilitate cleanup and sludge removal
t To permit isolation of slug loads
To provide operational redundancy through parallel units
When compartments are linked in series (Saginaw, Sacramento), short
circuiting 1s minimized. When compartments are operated in parallel
(Boston, New York City), longitudinal flow (scour) velocities are
minimized. Camp [|7] notes that it has been common practice to limit
design longitudinal velocities in settling tanks to about 3 ft/min
(0.02 m/s); however, he notes that despite a dearth of experimental data
there is increasing evidence that higher velocities are accompanied by
better removals. Heinke et al. [A$] suggest 8 ft/m1n (0.04 m/s) as a
maximum design value for primary sedimentation tanks based on field
observations. Initial results from Saginaw [£2], as shown in Table 3-4,
suggest potential benefits from the series (two-stage settling)
configuration; however, at present there are no settling column test
results to balance these remarkable removal efficiencies between
Influent characteristics and basin design.
3-49
-------
Table 3-4. PERFORMANCE OF THE HANCOCK STREET
SEDIMENTATION BASIN
Suspended solids
Avg surface Longitudinal
Storm loading rate, velocity Influent, Effluent, Removal,
date gal/«2-d ft/m1n mg/L mg/L %
8/19/78
9/13/78
9/20/78
970
1,235
2,270
3.7
4.7
8.7
896
149
420
62
27
232
93
82
45
3-50
-------
When treatment effectiveness 1s the primary concern, Inlet and outlet
works should be designed as 1n conventional wastewater treatment
practice C/^/aY], (I.e., to minimize density currents, short
circuiting, scour, and turbulance). The Inlet works should spread the
Influent evenly across the vertical cross-section of the tank without
-scouring the sludge blanket. Effluent weir loading rates of 10,000 to
o
40,000 gal/ft-d (125 to 500 m/m-d) are representative of conventional
design [/&]. When the basin's function 1s basically storage, Inlet and
outlet works should be as simple as practical, but effluent and,
probably, Interstage baffles should be provided to minimize scum
carryover.
Two major, and frequently the most controversial, design decisions will
be the degree and means of covering the basins and the means of sol Ids
removal. Both are expected to Impact cost and aesthetics more than they
do performance; however, past practices may have underestimated the
value of continuous sludge removal. Of the 10 downstream
storage/sedimentation basins reviewed 1n a recent state-of-the-art
assessment [/£], all contemplated a batch (fill-operated-drain)
operation; all but two were covered; and none provided for sol Ids
removal until after the event. The potential liabilities of allowing
sol Ids to accumulate in the basins 1s demonstrated from data taken from
the New York City, Spring Creek facility [?£], as shown in Table 3-5.
Based on monthly averages, effluent quality has been poorer than
influent quality 1n 2 or 3 months out of every 10 during periods of
discharge in spite of the fact that average removals range between 35
and 55% on an annual basis. Data from the Cottage Farm facility 1n
Boston [S] and the Whittier Street facility in Columbus [5] exhibit
similar behavior, especially under surface loading rates exceeding
3,000 gal/ft2-d (37.5 m3/m-d).
More Important, perhaps, in the design and performance assessment of
facilities such as those 1n New York and Boston that provide both
storage and treatment (Ignoring for the present their primary function
for overflow disinfection), are the storm events totally or
3-51
-------
Table 3-5. PERFORMANCE OF THE SPRING CREEK
AUXILIARY WATER POLLUTION CONTROL PLANT
Year
1977
1978
^0! ant
Startups
110
52
Events
tntillu
Discharges X Parameter
24
27
78
48
TSS
BOD
TSS
BOD
Monthly averages
iig/L
166
79
162
56
Negative
C**1 a » Damn .1 .--_
aig/L %* V>
103
50
71
31
38
37
56
45
30
30
17
17
a. Removal efficiencies reflect periods of discharge only.
b. Months where average effluent concentrations exceed average Influent concentration,
excluding zero discharge months.
3-52
-------
substantially contained. As a simple Illustration, Table 3-6 reflects
the total facility performance at Spring Creek when totally contained
events are credited as 100% removal. Obviously, this Illustration could
be expanded to account for the efficiency of the downstream plant, flows
retained 1n the basin and subsequently pumped back, etc. However, the
greater the percentage of events totally contained, the less will be the
Impact on net performance of the short-term efficiencies or
inefficiencies during discharge.
Covering downstream storage/sedimentation basins on CSO systems is
frequently a design requirement for environmental compatibility. Design
considerations include cost, equipment access, and the creation of a
potentially hazardous and corrosive environment. Adamski [*?] sums up
an operator's perspective of New York City's experience in covering
wastewater treatment plants with the conclusion that
...covered treatment plants are difficult and costly to build and
operate. That even by improving the design features, certain
difficulties cannot be overcome. The reason for covering is
usually a result of uneducated planners, architects, and citizens
who impose a burden on the operator because the choices open to
them are limited. When evaluating the need for a roof on a sewage
treatment plant, the reason for it must be clearly defined and
remain clear in the course of review and the rhetoric of protest.
If the need is for odor control, then the best solution, whether
operating or structural, should be selected after adequate study of
alternatives. If the need is aesthetic, then the point of viewing
must be kept in mind (whether from the ground or from above). If
the need is land or recreational opportunity then that should be
explored. In all cases, the cost of satisfying these needs should
be spelled out and the ability to choose other needs to satisfy.
Also, the operator should be considered so that his job can be made
easier [57],
3-53
-------
Table 3-6. NET BENEFITS APPROXIMATION OF
SPRING CREEK FACILITIES
Net removal
Year Parameter Calculation efficiency, %
1977 TSS 86 x 100 24 x 38
BOD
86 x 100^ 24 x 37
1978 TSS 25 x 100^ 27 x 56 , ??
BOD 25 x 100^ 27 x 45 ,
3-54
-------
Examples of sludge removal systems are shown 1n Figure 3-8. Typically,
where flushing Is the adopted system (whether through fixed or movable
nozzles), a center dewaterlng troughInvert slope approximately 1%1s
provided running the length of the basin with floor side slopes to the
trough at 5 to 10%. Basins or Individual bays range from 27 ft.'(8 m)
-"(Boston) to 80 ft (24 m) (Sacramento) 1n width, and typically 10 to
20 ft (3 to 6 m) 1n sldewater depth. Boston provides manual cleanup
after dewatering using fire hoses; New York uses traveling bridge
mounted sprays; Saglnaw uses a combination of wall mounted fixed sprays
and strategically positioned high pressure fire nozzle stations (see
Figure 3-8); and Sacramento uses a series of fixed nozzles under
computer control to sequentially flush its 3.6 acre (1.5 ha) floor area.
Under the latter case, approximately 5 Mgal (19,000 m ) of washwater
(strained river water) is used per washdown cycle for the 23 Mgal
(87 m ) capacity reservoir [5?].
Cited advantages of flushing water systems include low cost, thorough
cleaning performance, and minimum of mechanical equipment exposed to the
corrosive environment. Principal disadvantages would appear to be the
inability to remove sludge in other than a dewatered basin condition,
energy requirements for pressurization, and the increased liquid volume
to be treated through the pump-back system.
In lieu of flushing, Milwaukee uses seven mechanical mixers to resuspend
solids from its 0.7 acre (2.8 ha) floor area during dewatering
operations; Columbus uses a traveling bridge mounted scraper blade; and
San Francisco proposes to use conventional chain and flight collectors
1n its Southwest wet-weather primary treatment plant. In the latter
case, virtually all operations (averaging 633 operating hours per year)
will be 1n a flow-through treatment mode as the principal storage is
provided elsewhere in the system. To avoid problems experienced in
earlier stormwater demonstration projects where chains and drives have
significantly corroded (rust bound) under the normal fill and draw
operations, nonmetalUc chains are under consideration. It 1s also
noted that with the capability for continuous sludge removal, tanks may
3-55
-------
Left - Dallas (Bachman) stormwater
treatment facilities showing conventional
sludge scrapper equipment
'
-:*"" "**a
:.'-: <-.--, -~V
Above - New York City (Jamaica Bay)
storage/sedimentation facility showing
flushing equipment suspended from
traveling bridge
Above and right - Saginaw
^Hancock St.) storage/sedimenta-
tion showing typical center
trough and sidewall flushing
system with detail of high
pressure nozzle station
Figure 3-R Typical sludge removal systems,
3-56
-------
m&Mii&£&&£K
IV r--,," -.: V'rr T*r;:-r^?T'
;-.-" 1'C_\--,-'-">'f*$*',* *;*«-f 4'
k-.-ji-.rfcV" T^ffeV-&«»*;*--i
*_"-",'}. .*,-- '.^.i. '3f, _ .,*_'- !-» 'w.- -.^
Above and left - Mount Clemens
storage/sedimentation facilities.
Above picture shows aerated retention
basin with ramp access for heavy
equipment for annual sludge removal.
Left picture shows two of three
sedimentation/resuspension basins with
air header and drop lines
[Note picture of Sacramento (Pioneer
Reservoir) flushing system to be added]
Figure 3-8 (Concluded)
3-57
-------
not have to be dewatered through most of the wet-weather season, easing
maintenance requirements and maintaining a short response time
(read1ness-to-serve) for system activation. In Mt. Clemens, still
another system will use air and water jets from wall mounted headers to
resuspend solids 1n a slurry as a modification of the more typical
"f lushing system
Example 3-4 Illustrates the use of continuous simulation model (EPAMAC)
results and a projected performance curve for assessing the overall
treatment effectiveness of two operational concepts.
1. Multicompartmented basin with all units committed for each
event
2. Same as Concept 1, but with minimum number of basins
committed, based on maximum overflow rate objective
3-58
-------
EXAMPLE 3-4. COMPARE TREATMENT EFFECTIVENESS OF TWO ALTERNATIVE OPERATIONAL CONCEPTS:
MULTICOMPARTMENTED BASIN WITH ALL AVAILABLE BASIN CAPACITY ONLINE, AND
SAME FACILITIES BUT LIMITING NUMBER OF COMPARTMENTS ONLINE TO APPROACH
BUT NOT EXCEED MAXIMUM SURFACE LOADING RATE OBJECTIVE.
VWI IT
(1)
(2)
Specified Conditions
1. Maximum surface loading rate objective Is 3,000 gal/ft2-d.
2. Average annual operating requirements are as follows (from EPAMAC system analysis):
Flowrate, Average annual
Mgal/d hours of operation
>400 0
400 260
320 25
240 38
160 89
80 221
Total 633
Total volume treated 4.940 Hgal
Total TSS applied 6,923.000 Ib
3. The storage/sedimentation facility Is to have five parallel. Identical basins with average
sidewater depth of 10 feet.
Assumptions
1. For purposes of comparing options, assume TSS Influent concentration Is constant.
2. Performance curve developed In Example 3-3 applies..
Solution
1. Compute mean TSS concentration applied.
6,933.000 Ib 1 mg/L . ,,R ..
4,940 Mgal * 8.34 Ib/Mgal 168 m9/L
2. Compute size of facility based on maximum.surface loading rate.
400 Mgal/d . , , ft2
3,000 gal/ftf-d 133««3 ft
3. For Option 1, compute surface loading rates corresponding to design flows (note In Option 2
surface loading rate is always 3,000 gal/ft2-d by definition).
Read removal efficiencies off performance curve (Example 3-3) for each of these rates.
Surface loading rate, Removal
gal/ftZ-d efficiency. X
3.000 35
2.400 41
1.800 48
1.200 56
600 67
2. Compute removal effectiveness of each option.
Option .1
(80 Mga1/d)/24 x 8.34 x 168 x 221 h x 0.67 0.69 x 10s Ib
(160 Mgal/d)/24 x 8.34 x 168 x 89 h x 0.56 0.47 x 106 Ib
(240 Mgal/d)/24 x 8.34 x 168 x 38 h x 0.48 0.26 x 106 Ib
(320 Mgal/d)/24 x 8.34 x 168 x 25 h x 0.41 - 0.19 x 106 Ib
(400 Mgal/d)/24 x 8.34 x 168 x 260 h x 0.35 2.12 x 106 Ib
Total removed « 3.73 x 10* Ib
Net X removed 54X
Option 2
Total TSS applied 6.923 x 106 Ib x 0.35 - total removed* 2.42 x 106 Ib.
5. Compute net effectives* Improvement of Option 1 over Option 2.
(3.73 - 2.42)/2.42 * MI improvement in annual TSS removal by adopting
operating strategy 1 over 2
Comment
For the conditions stated, It 1s apparent that operating strategy 1 (maximizing tankage
online) Is associated with significant benefits. This might not be the case where the
adopted maximum surface loading Is much more conservative, where the performance change
as a function of surface loading rate Is less pronounced, or where the required operating
history Is significantly different.
3-59
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Step 5 - Identify and Select Pretreatment Components. Pretreatment
components are selected on the basis of enhancing performance and/or
operations. Typically, the choices Include coarse screening, fine
screening, grit removal, and flow measurement. The units, 1f provided,
may be located at the facility or upstream of pumps serving the
facility. In practice, the adopted components Include none, some, or
all of the above. The purpose of the coarse bar racks (2 In. to 4 1n.
clear openings) Is to remove heavy objects of all descriptions from the
flow to protect downstream equipment and to prevent travel of objects to
more Inaccessable locations. Finer screens (3/4 1n. to 1-1/2 1n. clear
openings) typically remove rags and finer sol Ids that tend to clog
process piping, valves, and pumps. They also trap many of the
floatables that otherwise might appear 1n the effluent. Where basin
overflows are rare, either or both have been omitted (Figure 3-3).
Separate grit removal normally would be required only where treatment 1s
the primary role of the facility and where grit is to be handled
separately from the sludge. Flow measurement and recording is
recommended for all facilities that discharge frequently; whereas stage
measurement and recording should suffice for basins which seldom
overflow. Flow measurement is essential for pacing chlorine dosages to
reduce the chance for ineffective application and toxic carryovers.
Step 6 - Detail Auxiliary Systems. Auxiliary systems, those which
support and complete the primary function, typically include sludge and
processing, flushing, disinfection, air handling and odor control,
energy (power, lighting, heating), and instrumentation and control.
Sludge processing considerations include (1) the location and method of
ultimate processing, (2) method of transport, (3) the Impact on existing
facilities, and (4) constraints (I.e., pumping rates, solids
concentration, pretreatment) that must be observed. The simplest and
most practiced solution 1s to return the sludge to the dry-weather flow
Interceptor, sometimes with an Intermediate degrittlng step.
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Flushing water system evaluation Includes source of supply, quantity and
2
rate of application, pressure requirements (typically up to 150 lb/1n.
2
or 11 kg/cm ), distribution system, and method of control. A conceptual
drawing of the Sacramento system as adapted to San Francisco 1s shown 1n
Figure 3-9 \2l]. The design flushing water application rate 1s
"30 gal/min per foot of basin length I
segments to be flushed sequentially.
30 gal/m1n per foot of basin length (21 L/m2-s) with 100 ft (30 m)
When facilities are covered, air handling and gas monitoring (for
explosive, corrosive, and toxic potential) are important considerations.
In selecting air change requirements for enclosed secondary treatment
plants, Adamski [<37] notes that two air changes per hour proved
inadequate to control misting and that later New York City designs
provided for a minimum of six air changes per hour. Common practice in
covered CSO storage/sedimentation basins seems to be 6 to 12 air changes
per hour with the higher figures based on a full liquid depth condition.
Variable rate air handling control through staging or speed control
appears desirable for energy conservation. Standard practices for odor
control should be evaluated
As a rule, instrumentation and control systems should be as simple as
possible and should be designed giving full recognition to the corrosive
environment and the level of operation and maintenance to be provided.
Areas of study recommended are:
Status and performance monitoring
System activation
t System deactivation
Auxiliary system control
Step 7 - Estimate Costs and Cost Sensitivities. Detailed cost estimates
should be prepared with emphasis on component systems and following the
value engineering guidelines. For example, what is the base cost of the
facility to provide the functional requirements identified 1n Step 1 on
the site selected in Step 2? What was the added cost of covering
3-61
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HEADER (INTERNALLY OR
EXTERNALLY MOUNTED)
FLOOR SPRAY
NOZZLES
45-DE6REE
FILLET
ALL SPRAY
NOZZLES
(OPTIONAL
CENTRAL DISCHARGE
CHANNEL
NOTE: CONTROL VALVES
IN EACH SPRAY LINE
NOT SHOWN.
Figure 3-?. Flushing water system concept
3-62
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Including air handling? What was the added cost of pretreatment?
Sludge removal? Instrumentation and control? How much do site
conditions impact the base cost? This analysis should lead to a more
cost effective total design.
"An operations plan and staffing and maintenance schedule should be
finalized at this point and operations and maintenance cost projections
made. As noted 1n earlier state-of-the-art assessments [5", 'V], the
proximity of the storage/sedimentation basin to a fully staffed water
pollution control plant may provide for optimum joint staff utilization.
Step 8 - Complete Design. The final step is to confirm that all
objectives have been satisfied. This may require several iterations
back through earlier steps including a reassessment of the basic
criteria once the site specific costs for compliance are known.
Operation and Maintenance Considerations
The major operation and maintenance goal of downstream
storage/sedimentation basins 1s to provide a facility that is available
to Its full design capacity when needed and for as long as needed.
Secondary goals include clear, prompt, and complete records of
performance (i.e., NPDES compliance reporting), reliability to provide
for reallocation of personnel and facilities 1n nonstorm periods, and
dual use operations (such as backup treatment and/or flow equalization
for dry-weather plants).
Experience has shown that frequent, periodic maintenance and equipment
exercising 1s essential to maintain an effective readiness-to-serve.
For obvious reasons, 1t is recommended that this maintenance be carried
out on a preplanned rather than as available basis. Staffing
requirements will be unique for each facility and operation and
maintenance organization. Questions to be addressed 1n the operations
plan Include:
Will the facility activate unattended?
What operational staffing is necessary to complete the primary
function?
3-63
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What staffing 1s necessary to complete the auxiliary
functions?
Will the mon1toring-report1ng system activate unattended?
What activities require Immediate response (multl shift
availability) and what activities can be deferred and for how
long (to conform to standard shift)?
What emergency conditions could be encountered and how will
they be addressed?
What operational decisions must be made and who has the
responsibility/control?
What operational decisions can be Implemented remote from the
site and which, 1f any, require direct observation?
What are the standard operating procedures to ensure safety of
the public, operators, and equipment?
Recognizing that unit activations (storms) can occur at any time and
generally without warning, adequate provisions must be made for parking,
assembly and briefing, and operating station access. Backup plans for
automated actions should be identified including confirmation feedback
and manual override if necessary.
Again, the operation and maintenance requirements and procedures should
be developed from the operational plan and not from Industry wide
standards since there are none.
Costs
Construction costs of downstream storage/sedimentation basins have been
reported [£, Table 73] for selected demonstration facilities (including
pretreatment and auxiliary systems) and are seen to be highly site
specific. Adjusted to ENR 4000, the range of unit costs 1s from $0.50
to $10.00 per gallon ($0.13 to $2.64 per litre) of storage capacity with
a median value of about $2.50 per gallon ($0.66 per litre). As would be
expected, facilities whose primary function 1s storage fall at a low end
of the cost range and those which are In effect primary treatment plants
rank at the high end of the range.
3-64
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Storage/sedimentation basins estimated by Benjes excluding pretreatment
and auxiliary systems (based on a 20 Mgal or 76,000 m capacity) ranged
from $0.03 per gallon ($0.01 per litre) for open earthen basins, to
$0.42 per gallon ($0.11 per litre) for covered concrete basins.13']*
The discrepancy between Benjes estimates built up using unit costs and
hypothetical basin designs; and actual construction bid costs
demonstrate not only the Impact of pretreatment and auxiliary systems,
but the overriding Importance of specific site conditions.
Another planning level cost source 1s a 1978 USEPA publication-
Construction Costs for Municipal Wastewater Treatment Plants: 1973-
1977 [3A], which presents a regression analysis of construction bid
costs by region, unit process, and construction component. While the
emphasis is on secondary treatment, there 1s a good deal of potentially
applicable information on preliminary treatment, Influent pumping,
primary sedimentation, site work, and special conditions.
Capital Cost Breakdown - Illustrative Examples. Three examples are
presented in Table 3-7: Facilities A and B represent covered basins
where the primary function is storage and Facility C represents a
covered and buried facility where the primary function is treatment.
Facility C 1s also unique in that it provides continuous service as a
dry-weather treatment plant (22 Mgal/d or 1 m /s average dry-weather
capacity) as well as 450 Mgal/d (20 m /s) peak wet-weather flow
capacity. The unit cost summaries at the bottom of the table clearly.
demonstrate the function/cost relationship stressed in basin design:
the storage units are cost-optimized on the basis of volumetric capacity
and the treatment unit 1s cost-optimized on the basis of volume treated
and discharged. The premium cost for burial of Facility C, Included in
the table costs, to facilitate dual use of the site above the tanks 1s
estimated as 18% above the cost of a totally enclosed plant with exposed
superstructures.
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Table 3-7. EXAMPLE CAPITAL COST BREAKDOWNS
Item
General, sltework. and
outside piping
Structural and
architectural
Mechanical equipment,
piping, and plumbing
Heating, ventilating,
and odor control
Instrumentation
Electrical
Total
Cost, per gallon of
storage capacity^
Cost per gallon treated
and discharged0
Facility
Cost,
$ million
3.24
0.11"
0.08
0.12
3.55
0.91
NAd
A [3V]
% of
total
91
^
3*
2
4
100
Facility
Cost,
$ million
2.1
9.6
3.2
0.5
0.4
0.6
17.4
0.76
0.27
B [35]
% of
total
13
59
19
3
2
4
100
Facility
Cost,
$ million
27.1
32.9
14.3
8.4
2.5
7.2
92.4
6.16
0.006
C [36]
% of
total
29
35
16
9
3
8
100
a. Equipment carried under General.
b. Dollars per gallon.
c. Capital cost divided by average annual volume discharged.
d. NA = not available.
3-66
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Example 3-5 Illustrates the use of the USEPA regression cost curves as a
crosscheck for Facility C.
EXAMPLE 3-5. COMPARE COST OF FACILITY C IN TABLE 3-7 WITH EXPECTED COST OF "EQUIVALENT" PRIMARY
TREATMENT PLANT USING REGRESSION CURVES FROM REFERENCE [33]
'Specified Conditions
1. The design flow «= 450 Mgal/d
2. The process train Includes screening, grit removal, and primary sedimentation.
3; No sludge treatment Is Included.
4. An operations and maintenance building (with laboratory) 1s Included.
5. Surface loading rate for Facility C 1s 2,700 gal/ft^-d at 450 Mgal/d.
Assumptions
1. Design surface loading rate for conventional primary sedimentation 1s 900 gal/ft2-d.
2. Primary plant compnent costs without sludge will be 35% of secondary with sludge.
Solution
1. Select appropriate cost curves or regression equations from reference [31].
a. Process - Second order cost curves, page 6-54.
(1) Preliminary treatment C = 5.79 x K^Q1-!?
(2) Primary sedimentation C = 6.94 x 104Ql-04
(3) Laboratory/maintenance building C = 1.65 x I05gl.02
b. Construction component - second order curves, Tables 6-42 through 6-50 Inclusive.
(1) Mobilization C «= 4.77 x 104Ql.l5
(2) Sitework including excavation C = 1.71 x
(3) Pilings, special foundation, dewatering C = 3.68 x
(4) Electrical C = 1.36 x
(5) Heating, ventilating, and air conditioning C » 3.10 x
(6) Controls and instrumentation C = 5.06 x
(7) Yard piping C = 9.96 x
2. Select "equivalent" design flow for conventional plant.
a. Equate on basis of surface loading rates.
Q = (900/2,700) x 450 = 150 Mgal/d
b. This falls outside range of sample data, use Q = 100 Mgal/d, which is upper limit of
sample data.
3. Compute regional and time adjustment factors.
Base costs are 2nd Quarter 1977 = EPA LCAT Index 134
Current cost 3rd Quarter 1980 = EPA LCAT Index 181
Regional multiplier from Table 7-1, page 7-14, for San Francisco 1s 1.3175
Combined multiplier = (181/134) x 1.3175 = 1.78
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4. Compute costs and compare.
Cost. $ million (ENR 4000)
Item Facility C estimate Computed survey cost
General, site work, and outside piping
[Items b.(l),(2).(3), and (7) x 0.35 (for primary)] 27.1 40.2
Structural and architectural
[Items a(1),(2), and 0.4 x (3)] 32.9 50.3
Mechanical equipment, piping, and plumbing
[included under structural and architectural] 14.3
Heating, ventilating, and odor control
[Item b(5)] 8.4 5.9
Instrumentation
[Item b(6)] 2.5 5.4
Electrical
[Item b(4)] 7.2 8.5
Total 92.4 110.3
Comment
The USEPA guide provides an effective tool for quick cost breakdown comparisons, but application
becomes questionable for plant capacities greater than 50 Mgal/d.
Operation and Maintenance Costs. As noted earlier, there are no rule-of-
thumb guides for estimating operation and maintenance costs short of
developing an operation and maintenance cost program for the specific
facility. For planning level estimates, first-cut approximations may be
developed from reported costs of operating facilities [^, (Table 73),
14] or in the case where the primary function is treatment, from
standard sanitary engineering references such as the 1971 USEPA
published regression curves developed by Black and Veatch [33] with
adjustment to reflect intermittant operations.
3-68
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REFERENCES
1. American Public Works Association. Survey of Stormwater Detention
Practices 1n the United States and Canada. Unpublished report.
2. U.S. Department of Commerce. Rainfall Frequency Atlas of the
United States for Durations from 30 Minutes to 24 Hours and Return
Periods from 1 to 100 Years. Technical Paper No. 40. May 1961.
3. Uniform Building Code. 1979 Edition.
4. Poertner, H.G. Better Storm Drainage Facilities at Lower Cost.
Civil Engineering. ASCE, 43, No. 10, pp 67-70. October 1973.
5. 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.
6. Hollinger, R.H. and T.I. Haigh. Field Evaluation of Porous Paving.
USEPA Grant No. 802433. Draft.
7. 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.
8. Fisher, F.M. Contributions of Refractory Compounds by a Developing
Community. USEPA Grant No. 802433. Draft.
9. Finn, R.M., Metcalf & Eddy, Inc. Personal Communication During
Visit to The Woodlands, Texas. February 1979.
10. Smith, W.G. and M.E. Strickfaden. Macroscopic Planning Model:
Application Guide and Users Manual. USEPA Report (in progress).
11. Hydrologic Engineering Center, Corps of Engineers. Urban
Stormwater Runoff: Storm. Generalized Computer Program 723-S8-
L2520, Hydrologic Engineering Center, Army Corps of Engineers.
Davis, California. July 1976.
12. Lager, J.A., et al. Development and Application of a Simplified
Stormwater Management Model. USEPA Report No. EPA-600/2-76-218.
NTIS No. PB 258 074. August 1976.
13. Litwin, Y.J., J.A. Lager, and W.G. Smith. Areawide Pollution
Analysis with the Macroscopic Planning (ABMAC) Model. USEPA Report
by Association of Bay Area Governments. Progress Draft Final
Report. December 1980.
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14. Metcalf & Eddy, Inc., University of Florida, and Water Resources
Engineers, Inc. Stormwater Management Model, Volume I. USEPA
Report No. 11024DOC07/71. NTIS No. PB 203 289. July 1971.
15. Utwin, Y.J. and A.S. Donigian, Jr. Continuous Simulation of Non
Point Pollution. Journal WPCF, page 2348. October 1978.
16. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal,
Reuse. Second Edition. McGraw-Hill. 1979.
17. Camp, T.R. Sedimentation and the Design of Settling Tanks.
Transactions ASCE, Volume III page 895. 1946.
18. Eckenfelder, W.W. and D.J. O'Connor. Biological Waste Treatment.
Pergamon Press. 1961.
19. White, J.B. and M.R. Allos. Experiments on Wastewater
Sedimentation. Journal WPCF Volume 48, No. 7, page 1741. July
1976.
19a Smith, R. Preliminary Design of Wastewater Treatment Systems.
Journal of the Sanitary Engineering Division, ASCE. February 1969.
20. Metcalf & Eddy, Inc. City and County of San Francisco Southwest
Water Pollution Control Plant Project. Final Project Report.
February 1980.
21. Lager, J.A. and W.G. Smith. Urban Stormwater Management and
Technology, an Assessment. USEPA Report No. EPA-670/2-74-040.
NTIS No. PB 240 687. December 1974.
22. Lynard, W.G. et al. Urban Stormwater Management and Technology:
Case Histories. USEPA Report No. EPA-600/8-80-035. August 1980.
23. Southeastern Oakland County Sewerage Disposal System, MI.
Communication from Mr. C. McKinnon, Chief Chemist to Dr. J.
Finnemore, Metcalf & Eddy, on Red Run Drain Retention Basin.
June 1978.
24. ASCE-Manual of Engineering Practice No. 36 Wastewater Treatment
Plant Design. Lancaster Press, Inc. 1977.
25. Heinke, G.W. et al. Effects of Chemical Addition on the
Performance of Settling Tanks. University of Toronto, Canada.
Journal WPCF, Volume 52, No. 12. December 1980.
26. City of New York, Department of Environmental Protection.
Communication from Mr. H. Tschudi, Deputy Director Bureau of Water
Pollution Control to Mr. M.P. Chow, Section Engineer San Francisco
Clean Water Program on Performance Data from Spring Creek Auxiliary
Water Pollution Control Plant. September 1979.
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27. Adamski, R.E. New York City's Experience 1n Covering Sewage
Treatment Plants. Presented at 50th Annual Winter Meeting of the
New York State WPCA. January 1979.
28. Sacramento Area Consultants, Sacramento, California Contract
Documents for Pioneer Reservoir Sump 1 Modifications. Contract
No. 1108, Sacramento Regional County Sanitation District.
September 1977.
29. Caldwell-Gonzalez-Kennedy-Tudor Consulting Engineers. Bayslde
Facilities Planning Project Draft Project Report for San Francisco
Clean Water Program. December 1980.
30. WPCF Manual of Practice No.22. Odor Control for Wastewater
Facilities. 1979.
31. Benjes, H.H., Jr. Cost Estimating Manual - Combined Sewer Overflow
Storage Treatment. USEPA Report No. EPA-600/2-76-286. NTIS
No. 266 359. December 1976.
32. Dames & Moore. Construction Costs for Municipal Wastewater
Treatment Plants: 1973-1977. USEPA Report No. 430/9-77-013, MCD-
37. Janaury 1978.
33. Black and Veatch. Estimating Costs and Manpower Requirements for
Conventional Wastewater Treatment Facilities. USEPA Report
No. 17090 Dan 10/71. October 1971.
34. Consoer, Townsend & Associates. City of Milwaukee, Wisconsin,
Humbolt Avenue Pollution Abatement Demonstration Project. Final
Report Draft. USEPA Project No. 11020-FAU. May 1973.
35. Brown and Caldwell - Sacramento Area Consultants. Pioneer
Reservoir Bid Tablulation and Engineer's Breakdown. February 1978.
36. Metcalf & Eddy, Inc. San Francisco Sowthwest WPCP. Engineer's
Estimate at 75% Design Level. February 1981.
3-71
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Chapter 4
DESIGN OF RETENTION STORAGE FACILITIES
Stormwater retention 1s the storage of excess runoff for complete
removal from the surface drainage and discharge system. Stormwater
retention facilities may take a variety of forms. For Instance, in
Orlando, Florida, perforated aluminum culverts have recently been
installed below street level 1n the downtown area to store and percolate
ground water [1]. The following chapter describes design procedures and
operation considerations for the most common retention storage facility
typethe pond. Stormwater retention ponds may be divided into two
general categories. Dry ponds are earthen basins that are wet only
during and immediately after runoff events. Excess flows are directed
to them and allowed to percolate to the groundwater. An example of a
dry pond is shown in Figure 4-1. Wet ponds are permanent ponds in which
Stormwater is stored by varying the level of the pond. Stormwater also
percolates to the groundwater from wet ponds. Usually, some base flow
is maintained through wet ponds to prevent stagnant conditions.
Early in 1980, the American Public Works Association conducted a survey
of Stormwater storage practices in the United States and Canada [2]. Of
the 12,683 facilities reported, almost 5Q% were dry ponds. An
additional 2,382 facilities were wet ponds. Not all, however, operate
as retention basins. Often the basin contents are released to a surface
water course.
Percolation of Stormwater to the ground water offers a number of
benefits in addition to controlling Stormwater flows. The ground water
is recharged. A total of 1,513 facilities were reported in use for
ground water recharge. This is particularly important in areas where
the ground water basins are being overdrawn and increased urbanization
1s reducing normal Infiltration. In addition, percolation through a
soil column has been shown to be very effective in removing bacteria,
oxygen demanding material, and suspended material from a wastewater.
4-1
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- ""»> mm i--T^ ' f '***>--v~-. .
Figure 4-1. Total capture percolation facility (dry pond) [3],
4-2
-------
DESIGN CONSIDERATIONS
Runoff storage and percolation are the primary ways 1n which dry ponds
reduce pollutant loadings on receiving waters. The pollutant removal in
wet ponds may also result from biological oxidation of suspended and
dissolved organic material in the runoff. As with other stormwater
storage facilities, ponds also allow sedimentation removal of suspended
materials during overflows. Soil characteristics and permeabilities
play an important role 1n design and operation of these facilities. In
addition, ponds frequently are designed to serve multiple purposes,
usually as flood control as well as stormwater pollution control
facilities, and often as recreational facilities. Dry ponds may serve
as playgrounds or athletic fields when not in use for stormwater control
while wet ponds are often also recreational lakes. The other uses may
have as great or greater impact on size, location, and configuration
decisions as pollution control. This section includes a discussion of
design considerations and factors for both types of ponds.
Size
Size requirements include not only volumetric capacity but area needs,
both surface and soil interface, as well. The optimum pond depends on:
The runoff storage volume needed.
The surface area and weir length required to assure adequate
settling during sedimentation operation.
The surface area needed for adequate transfer of oxygen into
the pond water to allow aerobic decomposition of organic
pollutants.
The soil-water interface area needed for adequate percolation
of stored runoff between storm events.
The area needed to serve whateever dual uses the basin may
have.
4-3
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The Ideal pollution control design 1s a balance of storage capacity and
sedimentation removal that will yield the necessary wasteload reduction
for the lowest cost. Sizing determinations, based on these factors, are
the same as for detention storage/sedimentation basins.
The storage volume required is also a function of soil characteristics
of the pond site, particularly soil permeability, subsurface geologic
conditions, stormwater pollutant makeup, and the resting period between
runoff events.
Soil permeability is a term that is used to describe the ease with which
liquids and gases pass through soil. In general, water moves through
soils or porous media in accordance with Darcy's law (Figure 4-2):
q = kdH/dl (4-1)
where q = the flux (flow of water per unit cross-sectioned area,
in./h (cm/h)
k = the permeability (hydraulic conductivity), in./h cm/h)
dH/dl = the toal head (hydraulic) gradient, ft/ft (m/m)
The total head (H) is the sum of the soil-water pressure head (h), and
the head due to gravity (z), or H = h + z. The hydraulic gradient is
the change in total head (dH) over the path length (dl). The
permeability is defined as the proportionality constant, k.
Soil permeability is determined to a large extent by soil texture with
coarse materials generally having higher conductivities. In some cases,
the soil structure may be of equal importance. A well-structured clay
with good stability, for example, can have a greater permeability than a
much coarser soil. The ionic nature fo the soil water and type of
vegetation may also play a part.
4-4
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h
«tFE*tHtE DATUM LtVEL
I I
i'__L"'!
SATURATED FLO!
H -H
Figure 4-2. Schematic showing relationship of total
head (H), pressure head (h), and gravitation head (Z) for
saturation flow [4].
4-5
-------
Percolation, the movement of water through the soil, 1s a distinctly
different property from Infiltration. The Infiltration rate of a soil
is defined as the rate at which water enters the soil from the surface.
When the soil profile is saturated, the infiltration rate is equal to
the effective saturated permeability of the soil profile. When the soil
profile is relatively dry, the infiltration rate 1s higher because water
is entering large pores and cracks. When water is applied, large pores
fill and clay particles swell reducing the infiltration rate to a near
steady state value.
As with permeability, infiltration rates are affected by the ionic
composition of the soil water and by the type of vegetation. Of course,
any tillage of the soil surface will affect infiltration. Factors that
tend to reduce infiltration rates include clogging by solids in the
applied water, classification of fine soil particles, clogging due to
biological growth, and gases produced by soil microbes.
Field measurements of soil infiltration rates and permeabilities are an
essential part of the design of retention/percolation basins.
Generally, the steady state infiltration rate serves as the basis of
selection of the design hydraulic loading rate. The rapidity with which
water moves through a soil profile will be determined by the least
permeable layer in the profile. However, subsurface permeabilities must
also be considered.
There are many available techniques for measuring infiltration including
flooding basins, sprinkler infiltrometers, cylinder infiltrometers, and
lysimeters. The preferred technique is one that approximates the actual
method of water application. In the case of dry ponds, this would be
flooding basins or cylinder infiltrometers. It 1s strongly recommended
that hydraulic tests of any type be conducted with actual stormwater
when possible. The Infiltration rate of a particular site will tend to
decrease with time as stormwater 1s applied due to clogging of the soil
pores by solid material in the stormwater. This clogging generally
4-6
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occurs only at the surface, and the infiltration rate may be returned to
nearly Its original value by scarifying the surface. Selection of a
design infiltration rate must take into account this clogging [4].
Particularly for wet ponds, the biodegradable organic content of the
runoff may also influence pond sizing. Biological stabilization of
organic materials is accomplished with the use of dissolved oxygen, if
available. If the dissolved oxygen is depleted, anaerobic decomposition
occurs, producing odors and discoloring the water. Oxygen is dissolved
into the water at the air-water surface or from oxygen released by algae
in the water. The rate at which oxygen is used is a function of the
organic loading and of the water temperature. The rate at which oxygen
is dissolved depends on the size of the deficit and the surface area
available. Therefore, organic loading per unit surface area may be an
important consideration. The acceptable loading rate depends on
alternative pond uses. Domestic wastewater treatment ponds are usually
loaded at 15 to 35 pounds BOD,- per acre per day (17 to 40 kg/ha-d).
Stormwater retention storage ponds should not be loaded at higher than 5
to 10 pounds BOD per acre per day (6 to 11 kg/ha'd). Organic loading is
usually not a problem for ponds that control Stormwater runoff.
Combined sewer overflows may result in anaerobic conditions in ponds
with small surface areas.
Because of organic loading and insect control considerations, dry ponds
should be designed to allow complete percolation in not more than 7 days
for Stormwater runoff, or 3 days for combined sewer overflows.
In most cases, flood control is a dual purpose of Stormwater
retention/percolation ponds, and the flood control and hydrograph
attenuation needs may actually determine the storage volume required.
The 1980 APWA survey revealed that for detention facilities, the most
popular basis for flood control storage sizing is a 100 year rainstorm,
followed by a 10 year and 25 year storm, 1n that order [2]. Design of
ponds for control of flooding is not within the scope of this manual,
but is adequately discussed in the literature.
4-7
-------
Location
The suitability of various sites within a drainage area for pond
facilities depends on (1) the availability of the site,
(2) compatability of surrounding land uses with a stormwater retention
facility use and other dual use functions, (3) the area required,
(4) the soil characteristics, and (5) the location of the site with
respect to tributary catchment size and to other sewer or drainage
facilities.
Evaporation may also be a factor in the disposition of water from
retention/percolation ponds. For dry ponds, its affect is slight, since
such ponds are usually designed to empty within 7 days, and evaporative
losses over such a short period are small. Evaporation may be a
consideration in maintaining the permenant pool in a wet pond,
particularly in areas subject to seasonal rainfall. In the western
United States, for instance, a retention/percolation pond may operate as
a wet pond during the winter rainy season and be dry during the summer.
The first consideration in identifying potential locations for a
stormwater control facility is the availability of a given site. For
new development areas where urbanization is yet to occur, this is not
usually a problem. For installations in established areas, site
availability can be an important factor. Ideal locations may already be
occupied by buildings or highways. Even if the agency developing the
control facility has powers of eminent domain, exercise of such powers
should be as a last resort. In any case, fair market value of an
already developed site may make it prohibitively expensive. Any
available sites should be identified and evaluated first before
apparently unavailable sites are considered.
4-8
-------
The compatabillty of land uses 1n the area surrounding a site with
stormwater storage/sedimentation and Intended dual uses of a pond 1s
another Important consideration. In order to make such an Installation
acceptable to nearby residents or business owners, commitment to a more
Intense level of operation and facility maintenance than would be
required If the facilities were located in a more remote or less visible
area may be necessary.
Obviously, the size of the facility needed and the site soil
characteristics play very important roles. Preliminary screening of
sites may be accomplished based on information from soil maps. Final
designs must be based on field testing of soil permeabilities.
The location of a site with respect to tributary catchment size is also
an important consideration. First flushes are usually most pronounced
on small catchments. If first flush control 1s the indicated control
methodology, retention/percolation basins should be located accordingly.
In addition, rapid percolation of a stored volume of stormwater runoff,
so that storage volume is available in time for the next storm, usually
requires a large soil-water interface area. If the catchment 1f large,
the necessary pond area will also be large.
Location of the site with respect to the drainage and/or sewer system 1s
another factor. Ideally, locations should be selected to minimize
transport piping from the sewer/drainage facilities, and also to allow
discharge of basin overflows with minimum of outfall construction.
DESIGN PROCEDURE
The following section consists of a step-by-step procedure for design of
retention ponds, Including an Illustrative example. The first two steps
are typically carried out at a planning stage, and are discussed only
briefly.
4-9
-------
Step 1 - Quantify Functional Requirements
Using an accepted hydrologlc analysis method, determine storm
distribution patterns for the urban area being analyzed. This should
Include a statistical distribution of storm volumes, storm intensities,
and storm durations and frequencies. Hourly rainfall data for many
locations in the United States is available from the National Weather
Bureau in Asheville, North Carolina.
Rainfall occurrence must be related to runoff. Many methods are
available, as discussed in Chapter 2 and Appendix C of this manual.
Often, a regional flood control agency will specify the runoff
calculation msthod to be used. A particularly useful method for
application to small urban watersheds is that developed by the Soil
Conservation Service and outlined in Appendix B.
Next, pollutant characteristics should be determined. Actual field data
specific to the catchment being considered or to a nearby and similar
catchment should be collected. Included should be pollutant types and
concentrations. Particle size distributions, specific gravities, and
settleabilities should be determined. If a significant first flush 1s
suspected, particularly on combined sewer catchments, the distribution
of pollutants within runoff events may be important.
Step 2 - Identify Required Waste Load and Flow Reduction
In many cases, regulatory agencies may specify the level of flood or
pollution control required. If not specified, receiving water
beneficial uses and expected water quality Impacts of the stormwater
discharge may be used to calculate the flood control and pollutant waste
load reductions required.
4-10
-------
Step 3 - Determine Preliminary Basin Sizing
Since a dual purpose of most stormwater retention ponds 1s flood
control, preliminary determination of storage volume needs 1s often
based on flood control requirements. Flood control requirements are
usually expressed as control of runoff peak flow from a design size
storm to some specified rate, often the predevelopment rate. The effect
of a storage pond on runoff peak flows 1s estimated by a flow routing
procedure. Graphs that may be used to estimate needed storage volumes
(given the expected peak rate of inflow, the acceptable rate of outflow,
and the expected runoff volume) are shown in Figures 4-3 and 4-4.
Alternative flood routing procedures, either land computation, computer
based simulations, or graphical methods, may also be used.
The effectiveness of the pond in removing heavier sediments (soil
particles) can be estimated using the curve shown in Figure 4-5, based
on Brune's work. Estimating removal efficiencies for lighter materials,
such as organic solids, 1s not so easy, since the effects of eddies and
currents within the basin are more pronounced for such particles.
The required surface area for oxygen transfer should be based on a
surface loading of 5 to 10 pounds BODg per acre per day (6 to
11 kg/ha:d).
Step 2 and Step 3 are iterative steps. The costs of mass load reduction
must be compared with the beneficial uses being protected.
[Example 4-1. To be added]
4-11
-------
I.I
I.I
J.5
t.l
1.9
I.I
0.5
Ell FLOW STRUCTURES IP Tl 180 ft1/!* ft LEASE
UK FLO! STRUCTURES IP II 100ft1/! I'! IELEASE
APPROXIMATE STRUCTURE IOUTIMQ
FOR SINGLE STA6E STRUCTURES
TYPE II IISTRIIUTIIN
24 HOUR RAINFALL
1.1 I.I 2.1 2.1 I.I I.I 4.1
IUNOFF IN IATERSNEO. IN. (Vf)
4.9
9.0
Figure 4-3. Approximate single-stage structure routing for
weir flow structures up to 150 csm release rate and pipe
flow structures up to 300 csm release rate [5].
4-12
-------
1.10
1.10
EIR PLOW STRUCTURES SVER IBf It'/Hl'i
PI PI PLOW STRICTURES IVER 101 It'/tal1*
M B.40
CO
t.lf
t.tt
I.IB
RPPROXIMTE STRUCTURE ROdTINS
FOR SINBLE STAQE STRUCTURES
TYPE II DISTRIBUTION
14 HOUR RAINFALL
B.fB
I i
i ii
B.1B
B.1B
S.20 B.2B 0.90
0.40 B.BO O.BB B.TB B.BB
PEAR RATE BF OUTFLOW B*
RATIB ()
PEAR RATE BF INFLOW Rl
Figure 4-4. Approximate single-stage structure routing for weir
flow structures over 150 csm release rate and pipe flow structures
over 300 csm release rate [5].
-------
100
80
60
40
20
Cotrti Mdiitnt-
z
MEDIAN CURVE FOR NORMAL^
PONDED RESERVOIRS a
ENVELOPE CURVES FOR NORMAL
PONDED RESERVOIRS
10
10
-2
10
1
CAPACITY-INFLOW RATIO
Figure 4-5. Brune's trap efficiency curves [6],
.10'
4-14
-------
Step 4 - Identify Feasible Pond Sites
Topographic and land use maps of the area may be used to locate
apparently feasible sites. Land use plans should be consulted to make
sure that conflictes of land use will not occur in the future.
Based upon the required waste load reduction calculated in Step 2 and
the removal accomplished by sedimentation from Step 3, the volume of
stormwater that must be percolated can be calculated.
EV = ESV0 + EpVp (4-2)
Where E = overall removal efficiency
V = total volume of runoff
E = sedimentation removal efficiency
V = volume of overflow
o
E = percolation removal efficiency
V = volume percolated
Removal of pollutants from water percolating through a layer of soil is
a complex process, and the efficiencies of removal may vary from
pollutant to pollutant. Percolation through soil is very effective in
removing BOD,-, bacteria, and suspended material. The removal of these
pollutants from percolated stormwater may be considered complete. Other
pollutants, such as some heavy metals or salts, may be carried into the
ground water or transported via the ground water to resurface
downgradient. The possibilities of ground water contamination or ground
water transport should be considered when selecting a
retention/percolation pond. See the performance section of this chapter
for a discussion of treatment mechanisms and efficiencies.
4-15
-------
The volume of stormwater that must be percolated to accomplish the
necessary overall pollutants removal may be calculated by substituting
In Equation 4-2 for V . If net evaporation is assumed to be negligable
compared to percolation,
V - Vp + VQ (4-3)
Substitutes and solving Equation 4-3 for V ,
V -
For dry ponds, the maximum runoff volume from a single storm that must
be contained and percolated can be determined by analyzing the rainfall
and runoff probabilities obtained in Step 1. The runoff volume to be
contained is that for the storm in which the sum of all runoffs less
than or equal to that volume is V .
For wet ponds, the single storm volume is the volume, V , divided by the
average annual number of runoff events expected.
Preliminary percolation area sizing of the pond may be performed using
the nomograph shown in Figure 4-6 and soil permeability ranges obtained
from Soil Conservation Service soil maps.
The ultimate infiltration rate, read from Figure 4-6, is 10% of the
initial soil permeability and takes Into account the decrease 1n
Infiltration that will result from surface clogging by stormwater
solids. The required soilwater interface area may be calculated by
Equation 4-5.
A = UTTTT (4-5)
4-16
-------
1 , UUU
m 400
- 200
K
5 100
c
t
X
- 40
£
»
*" 20
ca
3 to
u
111
S 4.0
WASTEWATER APPLICATK
o fa
* o e
0.2
n i
-
-
-
-
-
^ __ " " "
^
f' '
-.--'
/ '
PROBABLE
LONS-TERM
INFILTRAT
,
jj.
>
RANGE OF
ION v
^
/' ;
-:" "
>
V '
-
\
PERMEABILITY RATES OF MOST RESTRICTIVE LAYER IN SOIL PROFILE. In./h
PERMEABILITY! SOIL CONSERVATION SERVICE DESCRIPTIVE TERMS
VERY SLOl
< 0.06
SLOW
0.06-0.20
MODERATE-
LV SLOl
0.20-0.60
OOERATE
0.60-2.0
MODERATE-
LY RAPID
2.0-6.0
RAPID
6.0-20.0
VERY RAPID
> 20.0
MEASURED WITH CLEAR WATER
1 In. -2. 54 en
Figure 4-6. Soil permeability versus ranges of
application rates [4],
4-17
-------
where A = the soil-water interface area required
SV = the calculated storage volume
DI = the design infiltration rate, assumed to be 10% of the
initial rate
ET = the emptying time
The storage volume required is that volume calculated in Step 3. The
permeability may be obtained from soil maps of the site and by reading
the chart at the bottom of Figure 4-6.
The emptying time, ET, is the time required to completely percolate the
storage volume. Ideally, the entire storage volume should be available
at the beginning of each runoff event. However, storm events are
random, and the interstorm time will vary. In addition, alternative
uses will require that the pond be dry some percentage of time. For
purposes of performing a preliminary estimate of bottom area, ET may be
estimated for dry ponds by Equation 4-6.
ET = (1 - N/100) x avg interstorm time (4-6)
where Avg interstorm time = the average time between the end of one
storm and the beginning of the next storm of runoff volume
equal to or greater than storage volume.
N = the percentage of time during the year that alternative uses
require that the pond be dry, %.
For wet ponds, ET is equivalent to average interstorm time. Equation 4-6
may be adjusted to account for seasonal rainfall patterns by calculating
average interstorm time and N for the most critical season. For dry
ponds, ET should not exceed 7 days for stormwater runoff; for combined
sewer overflows, ET should not exceed 3 days for dry ponds.
4-18
-------
As many feasible sites as possible should be Identified. Sites should
then be ranked, based on apparent acceptability. Some subjective
judgment must be used since many difficult to quantify factors must be
considered, such as land use compatibilities and possible alternative
facility uses.
[Example 4-2. To be added]
Step 5 - Investigate Most Promising Sites
Beginning with the highest ranked site, soil borings and infiltration
tests of the site should be accomplished. The two preferred methods of
infiltration testing are flood basins and ring infiltrometers. Each
method is discussed in Appendix D of this manual. Infiltration testing,
particularly using ring infiltrometers, should be conducted on the most
restrictive soil layer underlying the site.
Again, the measured infiltration rate can be expected to decrease with
time as stormwater solids clog the soil surface. The infiltration rate
,used in design should be 10% of the measured infiltration rate at the
soil surface but should not exceed the permeability of the most
restrictive soil layer.
Step 6 - Establish Basin Sizes
The next step is to determine a final basin size based on the measured
infiltration rate and on a readjusted basin emptying time and
percolation area.
The storage volume calculated in Step 3 and the percolation area
calculated 1n Step 4 assumed that, after each runoff event, adequate
time for the basin to empty would be available before the next runoff
event. In practice, however, events are random and all storage volume
may not be available at the beginning of the next storm. Therefore, the
basin volume and percolation area must be adjusted to account for this.
4-19
-------
The maximum emptying time and the measured infiltration rate determine
the maximum allowable depth of ponding, d:
d = ET x DI
(4-7)
For any given storage volume, a direct relationship between available
storage volume, ASV, and interstorm period, t, exists:
ASV = (A x DI x t)
(4-8)
ASV may not exceed the storage volume for dry ponds.
From Step 1, the probability distribution of interstorm periods is
known.
INTERSTORM PERIOD
Interstorm period is related directly to available storage volume by the
formula:
ASV = (A x DI x t)
(4-9)
Therefore, the probability of occurrence for various available storage
volumes is known.
4-20
-------
MAXIMUM FOR DRY POND
AVAILABLE STORAGE VOLUME
Associated with each available storage volume is also a storm whose
runoff would just fill the ASV. Multiplying the probability that the
storm will be exceeded by the probability of having the corresponding
storage volume available will yield a probability of overflow for each
ASV.
AVAILABLE STORAGE VOLUME
RUNOFF VOLUME
RUNOFF VOLUME
4-21
-------
By assuming an acceptable level of containment, say 80%, one may obtain
the runoff volume controlled by the basin size considered. By
performing this calculation for several basin sizes, one may obtain a
basin size runoff volume curve for the assumed level of containment.
The design basin size can be selected, based on the percentage of runoff
to be contained and the associated waste load reduction (Step 2).
The basin sizing process involves a number of repetitive calculations
and can be carried out on a computer.
Note that this process assumes that the storage basin is always full at
the end of the preceding storm event. This is a conservative
assumption.
Excessive percolation area, due to the existence of a restrictive
subsurface soil layer, can sometimes be reduced by installation of
underdrains to collect and discharge percolated stormwater.
[Example 4-3. To be added]
Step 7 - Design Solids Removal Technique and Facilities
The handling and removal of captured solids within the
retention/percolation ponds present the biggest problems in facility
operation. In addition, during overflow conditions and sedimentation,
short-circuiting and scouring of previously settled solids may occur,
reducing overall basin removal efficiency. Very important aspects of
the dry pond design are inlet and outlet structures and solids removal
devices.
4-22
-------
Inlet structures should be designed to provide even distribution of flow
across the head of the basin. Devices Include weir overflows, multiple
pipe Inlets, and hydraulic energy dissipation devices such as stilling
walls. Prescreenlng of stormwater flows 1s often necessary to reduce
the cleanup required 1f high quantities of paper and other gross sol Ids
or floatable materials are present.
As the stormwater flow enters the pond from a confined conduit, the
velocity quickly drops. Since the ability of flowing water to transport
heavy solids is directly related to the velocity, a large quantity of
the suspended material settles out of the stormwater in the first few
feet of the basin. If not removed, a bank of solids may develop,
emitting odors as biodegradable materials are anaerobically decomposed.
Many flood control basins are constructed with forebays to confine the
solids for easier removal. This capture and removal can help extend the
period between diskings of the dry pond bottom or dredging of the wet
pond and reduce operational costs.
The forebay may be designed to act as a detention storage/sedimentation
basin during small runoff events, with return of the captured flow and
solids to the sewers for treatment. A possible inlet/forebay structure
is illustrated in Figure 4-7. Alternatively, a detention
storage/sedimentation basin may be used as a pretreatment facility 1n
front of a dry pond.
The width of the forebay should be based on expected changes in flow
velocity and settleabillty of the stormwater, similar to the design
approach for detention storage/sedimentation basins (Chapter 3).
An alternative to the forebay or pretreatment detention basin 1s to
constuct the pond in a triangular shape, with Inflow at one vertex and
the overflow along the opposite side. In this way, the drop In
velocity 1s gradual along the length of the basin and the deposited
solids are more evenly spread over the basin bottom.
4-23
-------
STILLING WALL
INLET PIPE
DRY POND
FORE BAT
~,
ro
SECTION
Figure 4-7. Inlet structure/forebay.
-------
The velocity of the basin flow together with the settling velocity of
the sediment particles play predominant roles 1n the sediment trapping
performance of a pond during overflow conditions. The velocity of
basin flow depends upon the outflow rate from the basin. Outflow
rates are usually determined by the hydraulic characteristics of the
outflow structure. Commonly used pond outlet forms Include overflow
weirs, sluice gates, orifices, and spillways. Hydraulic texts should
be consulted for the descriptions of the hydraulic characteristic of
the form selected. The selection of the overflow and its design are
usually based on flood discharge requirements.
Step 8 - Determine Configuration
Often, the alternative uses of a dry pond or constraints of the
selected site will determine the pond configurations. If not so
constrained, economical earth construction methods dictate square or
rectangular configurations with length not greater than three times
the width [7]. Side slopes should be shallow enough to allow mowing
or other maintenance of the vegetative cover.
PERFORMANCE
The efficiency of ponds in reducing stormwater pollutant loadings
depends heavily on the soil as a treatment medium. Soils have been
found to be very effective in removing a broad range of pollutants
from wastewater (including suspended material, phosphorus, some
metals, bacteria and viruses) and providing a medium for stabilization
of oxygen-demanding materials. The effect of wet pond retention on
suspended solids for an impoundment at the Woodlands, Texas, is shown
in Figure 4-8. The mechanisms of removal include straining,
biological activity, adhesion, and chemical reaction. Of course,
percolation of wastewater may result 1n degradation of the ground
water. It 1s, therefore, Important to have an understanding of the
removal processes at work in soils and removal efficiencies that might
be anticipated.
4-25
-------
1100
2400
JOOO
1600
1200
100
400
0
2100
2400
2000
1600
1200
100
400
0
140
120
100
60
60
40
20
LAKE INFLOI
140
120
100
10
40
20
0
SOLIDS
CONCENTRATION
6 6 " 10 12 14 16
HIE FlOi START Of STOIB. k
II 20
LAKE OUTFLOI
IISMAR6E
I I 10 12 14
1IIE r«OI STAIT IF ST6II. k
IB II fO
Figure 4-8. Lake Impoundment on storm runoff 1n
the Woodlands, Texas [3].
4-26
-------
Very few Investigations of removal efficiencies and treatment
performance of soil in percolating stormwater have been conducted.
The following discussion is based on observations of land treatment
.systems for rapid infiltration of municipal wastewater treatment
effluent.
Straining in the soil profile effectively eliminates suspended solids
from percolating wastewater. This straining occurs almost exclusively
on the surface. Removed particles tend to fill soil interstices,
further improving the removal by straining. Once retained in the soil
profile, biodegradable solids undergo decomposition. Nondegradable
and slowly degradable solids, however, tend to gradually build up
within the soil, decreasing the infiltration rate. Aerobic
decomposition of retained degradable sol Ids and clearing of surface
soil pores 1s enhanced by disking or plowing the soil surface.
The ultimate fate of an organic compound in the soil environment depends
largely on its ability to be metabolized by soil microorganisms.
Microorganisms growing on the soil particles quickly contact and
stabilize degradable organic compounds as the wastewater trickles
through the soil. If the flow is unsaturated, oxygen will circulate
through the soil pores and the stabilization will be aerobic. If the
flow is saturated for some period of time, available oxygen may be used
up and the process may become anaerobic.
It is important to note that many organic compounds are not susceptible
to microbial degradation. Additionally, a variety of organic compounds
may be unavailable for microbial or enzymatic decomposition because of
environmental factors such as pH, organic matter, moisture content,
temperature, aeration, and cation exchange capacity. Other removal
mechanisms, in addition to biological stabilization at work on organic
molecules in soils, Include volatilization, sorption, and chemical
degradation. Unless removed by these mechanisms, wastewater organics
move through the soil by mass-transport and dispersion and Into the
4-27
-------
ground water. Concentrations of trace organlcs in ground water
downstream from spreading basins at Whlttier Narrows, California, that
receive stormwater, reclaimed wastewater and surface water are presented
1n Table 4-1.
Bacteria, viruses, and parasites present 1n stormwater may pose a threat
to human health due to waterborne disease transmission. Percolation of
wastewater through soil can effectively eliminate pathogenic
microorganisms. Straining at the soil surface and at intergrain
contacts, and sedimentation and sorption by soil particles are the major
removal mechanisms for bacteria. Bouwer and Chaney stated that fecal
coliform bacteria are generally removed after 2 to 3 in. (5 to 7.5 cm)
of travel in soils. Coarse soils and high rates of application may make
100 or more feet (30 or more metres) of travel necessary for complete
removal.
Unlike bacteria, adsorption is probably the predominant factor in virus
removal by soil. Factors such as pH of the media, presence of cations,
and presence of ionizable groups on the virus affect this mechanism.
Once retained in the soil, viruses survive for up to 6 months. Land
treatment sites at which enteric viruses have been detected in the
ground water are listed in Table 4-2. It should be noted that the
systems at Vineland, New Jersey, and Ft. Devens, Maine, operate with
undisinfected primary municipal effluent. Factors that may Influence
bacterial and viral survival in soils are shown in Table 4-3.
Nitrogen compounds in wastewaters applied to soil are quickly oxidized
to the nitrate form under aerobic conditions. Under anaerobic
conditions, nitrates may be denitrified and nitrogen removed as a gas.
Denitrification does require anaerobic conditions and the presence of an
available carbon source. Land application systems may be operated to
maximize denitrlfication; however, the unpredictability of stormwater
makes such operation difficult 1f not Impossible. Therefore, nitrogen
removal by percolating stormwater 1s generally poor.
4-28
-------
TABLE 4-1
TRACE ORGANICS IN GROUND WATER DOWNSTREAM OF
SPREADING BASINS OF WHITTIER NARROWS, CALIFORNIA [8]
Wells
Un chlorinated
Target compound
Vinyl Chloride
Hethylene Chloride
1,1-dichloroe thane
Chloroform
1,2-dichloroe thane
Carbon tetrachlorlde
Bromodi chl oro me thane
Tri chl oroethylene
D1 b romochl orome thane
1,1,2-trlchloroe thane
Benzene
Bromoform
Tol uene
Chl orobenzene
1 ,4-dichlorobenzene
1 ,2- d1 chl orobenzene
Tetrachloroethylene
6-V-U
<1
2.2
<0.1
2.6
<0.2
--C
0.2
2.3
<0.1
<0'.2
0.9
0.4
0.2
<0.1
0.7
<0.1
0.8
8-V-W
<1
39.2
<0.1
7.2
<0.2
0.2
<0.2
1.0
<0.1
<0.2
<0. 1
<0.1
<0.1
<0.1
<0.2
<0.1
1.2
11-V-W
<1
1.6
<0.1
1.8
<0.2
0.6
0.2
1.6
0.4
<0.2
<0.2
0.4
<0.1
<0.1
<0.2
<0.1
1.1
15-V-W
<1
3.8
<0.1
1.0
<0.2
<0.1
0.2
1.2
0.6
<0.2
<0.2
<0.1
<0.1
<0.1
<0.2
<0.1
0.4
16-V-W
50
<0.2
<0.2
>40
<0.1
<0.1
<0.2
<0.1
0.4
12-V-W
<1
1.8
0.8
<0.4
<0.2
<0.1
0.2
<0.2
2.7
<0.2
<0.2
<0.1
<0.1
<0.1
0.7
0.9
0.4
4-29
-------
TABLE 4-2
REPORTED ISOLATIONS OF VIRUS BENEATH
LAND TREATMENT SITES [9]
Distance of virus
Migration, ft
Site location Vertical Horizontal
St. Petersburg, 20
Florida
Cypress Dome, 10 23
Florida
Fort Devens, 60 600
Massachusetts
Vine!and. 55 820
New Jersey
East Meadows, 37 10
New York
Holbrook. 20 150
New York
4-30
-------
TABLE 4-3
FACTORS THAT AFFECT THE SURVIVAL OF ENTERIC BACTERIA
AND VIRUSES IN SOIL [9]
Factor
Remarks
Bacteria Shorter survival 1n add soils (pH 3 to 5)
than In neutral and alkaline soils
Antagonism
from soil
mlcroflora
Moisture
content
Temperature
Sunlight
Organic
Viruses Insufficient data
Bacteria Increased survival time 1n sterile soil
Viruses Insufficient data
Bacteria
and
viruses
Bacteria
and
viruses
Bacteria
and
viruses
Bacteria
and
viruses
Longer survival in moist soils and during
periods of high rainfall
Longer survival at low (winter) temperatures
Shorter survival at the soil surface
Longer survival (regrowth of some bacteria
when sufficient amounts of organic matter
are present)
4-31
-------
In contrast to nitrogen, the behavior of stormwater applied phosphorus
Is controlled primarily by chemical, rather than biological reactions.
Soluble orthophosphate can be chemically adsorbed onto soil surfaces or
directly precipitated. In the adsorption process, orthophosphates react
with Iron, aluminum, or calcium Ions exposed on soil surfaces. Over
time, reactions occur that use adsorbed orthophosphate to form phosphate
minerals with low solubilities. This, coupled with the creating of new
sorption sites, by alternating drying and wetting, regenerates new sites
for adsorption. Phosphorus removals can range from 70 to 99%.
In addition to toxic organic pollutant removals discussed above, soils
have been effective in reducing the concentrations of trace elements in
percolating water over limited period of time. However, the long-term
ability to remove metals is questioned, as soil sorption sites are
thought to become saturated. Removal mechanisms for several trace
elements are shown in Table 4-4.
OPERATIONS
As with most stormwater pollutant control facilities, the major
operational problems with ponds center around handling of captured
solids. Other operational concerns for dry ponds include maintenance of
vegetative cover through alternating wetting and drying periods, and
maximizing availability of the pond for alternative uses.
The problem of solids deposition in intermittently used flood control
basins has been recognized for some time. Often, such facilities are
constructed with forebays in an attempt to concentrate the solids for
easier removal. Dry ponds may be constructed in series with a detention
storage/sedimentation facility ahead of the pond. During small volume
runoff events, the heaviest sol Ids are captured in the detention
facility and returned to the sewers for treatment. The settled overflow
Is allowed to percolate in the ponds. For ponds in which stormwater
control 1s the exclusive use, frequent disking of the pond bottom will
4-32
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TABLE 4-4
REMOVAL MECHANISMS OF TRACE
ELEMENTS IN SOIL [9]
Principal forms In soil
Trace elements
Solution
Principal removal mechanism
Ag (silver)
As (arsenic)
Ba (barium)
Cd (cadmium)
Co (cobalt)
Cr (chromium)
Cu (copper)
F (fluorine)
Fe (Iron)
Hg (mercury)
Nn (manganese)
N1 (nickel)
Pb (lead)
Se (selenium)
Zn (zinc)
Ba+2
Cd*2, complexes, chelates
Co*2. Co43
Cr*3, Cr*6. Cr2o9'2, Cro4'2
Cu42. Cu(OH)*, anlonlc chelates
Fe*2. Fe*3, polymeric forms
Hg°, HgS, HgCl3-, HgCl4'2,
CH^g*, Hg+2
Mn+2
HI*2
Pb*2
Se03-2, Se04'2
Zn*2, complexes, chelates
Precipitation
Strong association with clay fraction of
soil
Precipitation, sorptlon Into metal
oxides and hydroxides
Ion exchange, sorptlon. precipitation
Surface sorptlon, surface complex Ion
formation, lattice penetration, 1on
exchange, chelation, precipitation
Sorptlon, precipitation, 1on exchange
Surface sorptlon, surface complex Ion
formation, ion exchange, chelation
Sorption, precipitation
surface sorptlon, surface complex Ion
Volatilization, sorptlon, chemical and
microbial degradation
Surface sorptlon, surface complex Ion
formation, ion exchange, chelation,
precipitation
Surface sorptlon, ion exchange, chelation
Surface sorptlon, ion exchange, chelation,
precipitation
Ferric oxide-ferric selenlte complexatlon
Surface sorptlon, surface complex ion
formation, lattice penetration. Ion
exchange, chelation, precipitation
4-33
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aid aerobic decomposition of biodegradable solids and disperse the
captured solids through a thicker layer of the soil. This will reduce
surface clogging. No matter what the alternative use of the dry ponds,
disking or scarifying of the surface must be practiced periodically 1n
order to maintain Infiltration.
Vegetative cover on dry ponds serves a variety of purposes. The most
obvious is to make the pond more aesthetically acceptable and to allow
alternative uses such as athletic fields. In addition, vegetation can
supply some removal of pollutants, particularly nitrogen, by plant
uptake and vegetation helps to maintain high infiltration rates. The
effects of vegetation on infiltration rates are illustrated 1n Figure 4-9,
Successfully used vegetation includes fescue, perennial rye, and
bermudagrass.
The alternative uses and the surrounding land uses will, to a large
extent, determine the operational schedule and requirements. Heavy
deposits of organic solids may produce odors as they decompose and
should be prevented. The obvious presence of deposited solids may also
be visually objectionable.
The major operations considerations for wet ponds are sediment handling
and removal, control of floating materials, erosion of the pond banks,
algae and aquatic weed control, prevention of nuisance odors, and
control of insects.
As was the case with dry ponds, the major operational problem with wet
ponds 1s sediment handling and removal. Deposition of materials near
the pond inlets can result 1n buildup of sludge banks, generation of
odors or the deposited solids decompose, and loss of pond volume. In
addition, the infiltration rate of water through the pond bottom may
decrease.
4-34
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2.8
2.4
2.0
£ 1.6
- 1.2
0.4
BARE SOIL PERMANENT
PASTURE
Figure 4-9. Effect of vegetation on soil
Infiltration rates [4],
4-35
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Because wet ponds do contain water all the time, removal of sludge banks
and scarifying of the pond bottom is much more difficult than for dry
ponds. Construction of a forebay or sedimentation basin in front of the
wet pond is the most effective method of control. The pond may also be
periodically dredged or even drained and the solids removed.
Floating materials in the stormwater may also present an operational
problem. Floating materials may clog pond outlets or overflows, may
interfere with oxygen transfer at the pond surface (resulting in -
anaerobic conditions and odors), and may provide conditions suitable to
insert breeding. Floating materials are also unsightly. Floating
materials may be controlled by prescreening and/or installation of a
floating boom (photo) near the pond inlet. The boom must be cleaned
after each runoff event.
Erosion of the pond banks may result from wave action particularly if
the pond is large and exposed to winds. Erosion problems are primarily
the result of neglect of monitoring and maintenance. The maintenance of
proper grass cover and riprap minimizes the problem.
Aquatic plants have both desirable and undesirable effects in ponds.
The weeds may exert a significant oxygen demand as they decompose. In
addition, they are often unsightly. They provide a suitable habitat for
insect breeding. Woody plants and trees tend to weaken dikes by their
root growth. On the other land, marsh treatment systems rely on aquatic
plants to uptake nutrients from wastewater and to promote settling by
enhancing quiescent conditions. Maintenance of 2 ft (0.6 m) of water
depth will discourage growth of weeds. Weeds growing from the pond
bottom should be pulled and removed, rather than sprayed or cut, since
the decaying material may exert a significant oxygen demand.
4-36
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Control of algae growth 1s another Important operational consideration.
Algae blooms, sudden and extreme growth usually of blue-green algae,
tend to die-off with a rapidity equal to that of the growth. The dead
algae then furnish an extremely large and sudden oxygen demand,
frequently producing anaerobic conditions and odor problems. Algae
blooms may be controlled by the use of copper sulfate or certain
herbicides.
Insects commonly found near retention ponds include mosquitoes, midges,
beetles, and dragonflies. Insect generation occurs in sheltered areas
or quiescent portions of the pond where there may be substantial growth
of rooted plants or layers of scum. The basic measures for insect
control are control of weeds and scum and pretention of stagnant
conditions. Insecticides also may be used, as shown 1n Table 4-5. If
insecticides are used, 1 or 2 days of contact time is usually required.
Prolonged contact periods will lessen the dose required for equal
effect.
Examples of wet and dry detention pond operation and maintenance are
shown in Figure 4-10.
COSTS
The costs of constructing dry or wet retention/percolation ponds may be
estimated using the curves shown in Figures 4-11 and 4-12. The cost
curves are based on construction costs for rapid infiltration basins and
storage ponds for disposal of domestic wastewater. These costs should
be used for preliminary estimates only. Actual costs depend on the
initial condition of the site and the dual uses of the pond,
particularly recreation uses. Operation and maintenance costs for
either type of basin are highly site specific and depend on stormwater
pollutant concentrations, frequency of runoff events, and dual facility
uses.
4-37
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TABLE 4-5
SOME INSECTICIDES USED FOR
LAGOON INSECT CONTROL [10]
Insect
Insecticide Application rate
Culex
Mosquitoes
Midges
"Shrlmp-Uke"
Insects;
algal
predators
Dursban 1 ng/L
Naled 1 mg/L
Fentnlon 1 ng/L
Abate 1 mg/L
Diesel oil 6 to 8 gal/acre
MalatMon 2% sprayed around edge
Abate 21 sprayed around edge
BHC Dust, 31 garnna Isomer
Fenthlon As directed on package
(Baytex)
Abate As directed on package
Sursban As directed on package
D1brom-8 As directed on package
4-38
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5kfldg5iS35are
-------
7.000
1.000
v>
e
u
100
10
10 100
FIELD AREA. ACRES
1.000
Figure 4-11. Cost of dry pond construction, ENR 4000 [2],
4-40
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o.eo
0.70
". 0.60
e
u
O.SO
H- 0.40
0.30
0.20
0.10
STORAGE PONDS REQUIRING SUBSTANTIAL EXCAVATION,
EMABANKMENT, AND SPILL«AY IORK
STORAGE PONDS CREATED FROM EXISTING
WETLANDS AND NATURAL LOI AREAS
500 1.000 1.500 2.000 2.500 3.000
TOTAL STORAGE CAPACITY. 1.000 ft3
3.500 4.000
Figure 4-12. Storage pond construction costs, ENR 4000 [3],
4-41
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REFERENCES
1. Wan1el1sta, M.P., et al. An Example of Urban Watershed Management
for Improving Lake Water Quality. Presented at International
Symposium on Inland Waters and Lake Restoration. Portland, Maine.
September 8-12, 1980.
2. American Public Works Association. Survey of Stormwater Detention
Practice in the United States and Canada. Unpublished report.
3. Lynard, W., et al. Urban Stormwater Management and Technology:.
Case Histories, EPA 600/8-80-035. August 1980.
4. USEPA, U.S. Army COE, and U.S. Department of Agriculture. Process
Design Manual for Land Treatment of Municipal Wastewater.
EPA 625/1-77-008. October 1977.
5. Soil Conservation Service. Urban Hydrology for Small Watersheds.
Technical Release No. 55, U.S. Department of Agriculture. January
1975.
6. Chen, C. Design of Sediment Retention Basins. Proceeding National
Symposium on Urban Hydrology and Sediment Control. Lexington, Ky.
July 28-31, 1975.
7. WPCF. Wastewater Treatment Plant Design 1977: A Manual of Practice
(MOP 8). 1977.
8. Garrison, W.E., et al. A Study on the Health Aspects of
Groundwater Recharge in Southern California. CSD of Los Angeles
County, Whittier, CA. September 1979.
9. Metcalf & Eddy-L.D. King. Chino Basin Water Reclamation Study-
Trace Organics Demonstration Project Work Plan. December 1979.
10. WPCF. Operation of Wastewater Treatment Plants: A Manual of
Practice. (MOP 11).
11. Poertner, H.G. Practices in Detention of Urban Stormwater Runoff.
APWA Special Report No. 43. 1974.
12. Reed, S. et al. Cost of Land Treatment Systems. EPA 430/9-75-003.
Revised September 1979.
4-42
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Chapter 5
STORMWATER MANAGEMENT SYSTEM INTEGRATION
Usually, the most economical and effective stormwater management system
consists of a combination of facilities and techniques integrated into
.an overall pollution control plan. Storage/sedimentation facilities are
often the backbone of such an integrated plan. They provide
inexpensive, effective, and flexible stormwater control that can be
constructed singly, in series, or in combination with more advanced
techniques as needed.
The advantages of an integrated control plan include lower overall price
and system flexibility. Smaller control facilities are easier and often
less costly to construct. Smaller sites usually are more easily found.
In addition, facilities needed to control runoff from developed areas
can be constructed as development occurs.
A multiple unit control system allows the level and type of control to
be matched to the catchment. For instance, storage/sedimentation basins
may be used to capture first flushes in areas with combined sewers, or
chemically assisted sedimentation may be needed for control of toxic
runoff from an industrial area. A multiunit control system retains its
flexibility. Additional units or control techniques may be added if the
runoff characteristics or available assimilative capacities change.
Existing flood control facilities also may be retrofitted to enhance
pollution removal in integrated systems.
Of course, stormwater is usually only one of several pollution sources
in an urban area. The compatability of various stormwater control
techniques with other pollution control facilities and of pollution
control with flood control facilities must be considered when developing
a unified pollution control plan. The integration of various control
methods into an effective system is the subject of this chapter. The
5-1
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integration process, retrofit techniques, and three
storage/sedimentation application examples are presented.
THE INTEGRATION PROCESS
The process of integrating stormwater control into a pollution control
system involves initial planning where existing facilities are
identified and goals are determined. Additional steps then involve
selecting control methods that are both applicable and compatible to the
existing facilities and established goals. Each of the steps is briefly
described.
Identify Existing System and Needs
The first step in the integration process is the identification of
existing system components and function. This information is the basis
of plan development and determination of control methods that are most
appropriate for the system.
The watershed characteristics and existing facility components are
examined. Information on the land uses of the watershed(s) and acreage
of each is used to evaluate the potential for development and to
estimate the runoff quality and quantity.
Identification of existing facility location, function, and capacity is
required. The compatibility of existing facilities with proposed
control methods must be determined. The potential for retrofits with
the existing facilities, and whether the existing facilities are in
suitable condition must be established. Storm and sanitary sewer line
location, treatment facility types, and capacities will strongly
Influence the location and points of interconnection for new facilities.
5-2
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Establish System Needs
After identifying the existing system and conditions, the next step is
to establish additional flood and pollution control needs. Plans should
encompass the entire urban wastewater control system including both dry-
and wet-weather facilities, and both developed and undeveloped areas.
Planning over a whole system enables the use of all resources available.
By examining the potential from undeveloped areas, the system can be
planned to expand and accommodate future needs much more easily.
Identify Applicable Control Alternatives
The engineer or planner, at this point in the integration process,
should be familiar with the existing system and the goals of stormwater
management. Alternative control method should be examined, on the basis
of the three factors: (1) physical limitations, (2) effectiveness, and
(3) institutional limitations.
Physical limitations may exclude some storage/sedimentation methods.
Steep slopes or extensive development may preclude the use of retention
ponds. In general, older developed areas impose more constraints on the
number of options available. Land availability will also limit the
control method options. Limited land tends to preclude open ponds, but
storage/sedimentation basins would be applicable because the structure
could be built to physically support a second use above it. A list of
the more common control methods and the physical situations favoring
their selection are shown in Table 5-1.
The control methods also vary as to their effectiveness in reducing
pollutant concentrations and mitigating storm volumes. Retention ponds
are high in effectiveness for both pollutant and flow control. Upstream
or inline storage may mitigate storm flow but be somewhat less effective
in pollution control. The effectiveness of the various
storage/sedimentation control methods are listed in Table 5-2.
5-3
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Table 5-1. STORAGE SEDIMENTATION CONTROL METHOD
VERSUS FLOW OR QUALITY APPROACH
Inline storage
Rooftop ponds
Plaza ponds
Parking lot ponds
Sedimentation basins
Dry retention ponds
Wet retention ponds
Flow attenuation
X
X
X
X
-
X
X
Quality Improvement
.
-
-
-
X
X
X
Table 5-2. STORAGE SEDIMENTATION CONTROL METHOD
VERSUS PHYSICAL AND EFFECTIVENSS LIMITATIONS
Effectiveness
Physical Flow Quality
Inline storage Extra capacity must be Proportional to
present Inline. capacity available
Rooftop ponds Flat roof structures Yes for peaking flows
Plaza ponds Land area for . Yes for peaking flows
development
Parking lot ponds -- Yes for peaking flows Some 1f street sweeping
program In effect
Sedimentation basins Land use conflicts Potentially high Yesup to 60% SS removal;
depending on mode of other parameters vary
operation
Dry retention ponds Large space require- Yes, 100% Yes, 100%
ment, flat terrain
Wet retention ponds Large space requirement Yes, 100% Yes, 100%
5-4
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The third factor determining applicability involves institutional
limitations. The means of implementation, operation, maintenance, and
financial support are crucial to the applicability of a management
system. The institutional authority must exist to ensure the
implementation of the system and continued system operation.
Funding for implementation, operation, and maintenance must be secured.
An institutional organization must exist to collect funds, impose fees,
or make other necessary financial arrangements. If the cost is to be
borne by developers, then the legal and enforcement authority must exist
to confirm that the control methods are applied. Institutional
considerations are discussed in greater detail in Chapter 6.
Determine Control Method Compatibility
Once the applicable control methods are identified, the final step in
the integration process is to assess the process compatibility. Process
questions include (1) which control methods have compatible treatment
methods, (2) will capacity of existing treatment be exceeded due to
installation of control method, and (3) is the method flexible for use
in other treatment process trains.
Control method compatibility with treatment processes is an important
consideration of the integration process. A chemical treatment process
that could upset a biological process should not be used ahead of the
latter. Also, pretreatment requirements for the functioning of some
processes must be incorporated into a system and could be part of
stormwater treatment facilities.
Flexibility of dry- and wet-weather facilities in separated sewer
systems can take advantage of the wet-weather facilities as a
pretreatment process, or standby 1n case of dry-weather facility
failure. Wet-weather facilities might also be used as an effluent
polishing step during dry weather.
5-5
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Another Important compatibility consideration is the effect of sludge
generation by the stormwater management control methods. An integrated
facility must include transportation and processing of the solids in the
overall plan.
RETROFITTING OF EXISTING FLOOD CONTROL FACILITIES
Control of runoff for flood protection is one of the basic services of
urban government. Temporary storage is one of the most commonly applied
control techniques. Many existing flood control detention facilities
may be modified to enhance pollution control of the stormwater as well.
The most common flood control facilities in the United States are wet
and dry ponds [ ]. Basin designs, however, usually fail to maximize the
potential for pollutant removals. Often the inlet and outlet of the
basin are located near each other, so that for small runoff events
essentially direct discharge occurs. Even for large events, short
circuiting through the basin is possible. The outlet is typically at
the same elevation as the inlet, again resulting in short circuiting.
Wind action and flow related turbulence may suspend settled material or
soil from the pond bottom.
Several modifications are possible to increase the pollutant removal
efficiency of existing or planned flood control facilities. Gravel or
cement may be added to the pond wells and bottom to stabilize soils.
Baffles or other energy dissipation devices installed near the pond
inlet may reduce turbulence and short circuiting. Dividing the basin
into several cells may allow settled solids to be more easily contained
and removed. In addition, sequential flow through the cells may reduce
short circuiting. Installation of a storage/sedimentation pretreatment
basin ahead of the pond, as described in Chapter 4, and installation of
a concrete lined forebay near the inlet are methods of accomplishing all
the above.
5-6
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Increasing the Infiltration capacity of the flood control basin will
help reduce pollutants by filtering the stormwater through soil before
it is discharged. Biological activity, straining, and adsorption
combine to effectively remove pollutants. Installation of underdrains
in dry ponds is one method of increasing infiltration capacity. The
pollutant removal performance of wet ponds may be increased by raising
the level of the outlet pipe. The permanent pond size will be increased
and sedimentation performance should improve. Of course, some flood
control capacity will be lost. Altering the outlet elevation relative
to the inlet elevation also can help reduce short circuiting.
Increasing the flow distance between the inlet and outlet pipe is
another method of reducing short circuiting.
Some pollutants, particularly eutrophic nutrients, are uptaken by plants
growing in the pond. If the plants are allowed to die and decompose,
the nutrients are rereleased. Harvesting and proper disposal of such
vegetation will remove those pollutants from the stormwater control
system [ ].
SEDIMENTATION BASIN INTEGRATION
The general principles presented in the integration section can be
practically explained by use of illustrations. The first example places
a sedimentation basin in a developed urban area to mitigate flooding and
stormwater pollution impacts. The process of choosing a location for
the basin and examination of the applicability and compatibility of this
control method are examined.
The setting is Upsilon City (mythical, of course), which is a medium
sized city of 100,000 to 200,000 inhabitants. Upsilon is located on the
Tau River, which receives stormwater overflows and treated wastewater
from the city. Upsilon is situated in the rolling foothills of the
Rocky Mountains; the elevation is approximately 1,800 m (5,900 ft).
5-7
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Upsilon's annual temperature ranges between -9 and 30.5 *C (16 and
87 °F) with an annual precipitation of about 51 cm (20 in.). Fifty-two
percent of the runoff occurs in the spring, between March and June,
creating large runoff volumes. A diagram of Upsilon highlighting
pertinent features is shown in Figure 5-1.
Location
The siting of a sedimentation/storage basin in Upsilon involves three
factors: existing facility interface, land use compatibility, and space
availability. The preliminary phases of integration planning and data
gathering had identified a storage/sedimentation facility as the most
appropriate control method for accommodating the stormwater from the
northwest portion of the city.
The goals of stormwater management for Upsilon are to eliminate flooding
and reduce the pollutants in overflows to the Tau River. The
sedimentation facility was selected as a control method because storage
volume is required to prevent flooding in the urbanized portion of the
city and sedimentation also afforded a pollutant reduction mechanism to
improve overflow quality.
For the basin to interface with existing facilities, it must be located
near existing storm sewers from the northwest portion of the city. It
should also be sufficiently close to the treatment plant for minimum
sludge transmission distance and ahead of the interceptor that
aggrevates the flooding tendency of the urban area between Apple Street
and Interstate 73 during high runoff periods. The areas meeting these
criteria happen to be in a business and industrial center of Upsilon.
The land uses are small commercial business and light-to-medium
industry. Within the area are a few unimproved lots used for parking,
and renovation of some of the older warehouses is planned. The
installation of a sedimentation basin in the area would be compatible
with existing land uses.
5-8
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NEW DEVELOPMENT -^ x ' V
X
SEDIMENTATION BASIN
: FLOOD CONTROL FACILITY
Figure 5-1. Schematic of Upsilon Area.
5-9
-------
Due to limited space availability, the location of the basin was
narrowed to two potential sites: an existing parking lot, or a
warehouse scheduled for demolition and renovation. The basin, 1f
located at the parking site, could be constructed to allow parking above
it thus permitting dual use of the site. Alternatively, the warehouse
site was proposed to have a dual use of a recreational facility for
workers in the area.
Functional Compatibility
In initial planning stages, the functional compatibility between the
basin and existing facilities is briefly considered in selecting a
general area in which to search for compatible locations. Functional
compatibility now must investigate the flow scheme and impacts and
operation and maintenance aspects in more detail.
The flow scheme during the spring runoff period from March through June
is to have the basin offline and connected to the stormwater sewer by
regulators. The primary function of the basin is to prevent flooding.
When stormwater flow in the sewer reaches maximum stage without
flooding, the basin is to be brought online to intercept and hold the
stormwater excess. When the peak flow subsides, detained water will
then be drained back into the sewer, which now can handle additional
flow volume.
The impacts of the sedimentation basin in the stormwater system are to
reduce peak flows and remove solids. Existing drainageways need not be
enlarged due to upstream development creating greater runoff volumes
because the basin accommodates the excess flow. Should wet-weather
treatment become necessary, the basin will help to distribute the flow
volume over a longer period enabling a smaller maximum design capacity
for the treatment plant.
5-10
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The operation of the basin In stormwater mode will be triggered
automatically by sensors of flow volume 1n the main sewer. Sludge
transport lines will convey sol Ids to the dry-weather sludge processing
facilities. Sludge will be pumped. Nozzles and flushing water will
dilute the sludge sufficiently to allow pumping. Drainage of the
facility under normal operations is to the storm sewer. Maintenance and
cleanup operations will occur after each storm. The most frequent
cleanup efforts will be during the spring runoff season.
Alternate drainage path for the facility is through the dry-weather
treatment facility. Current design of the basin has the dry-weather
drainage path option, as well as permitting a potential reverse flow
path from the dry-weather facility to the basin. The path is to be
installed at the same time the sludge line is being constructed. The
pipeline will permit the second mode of operation for the basin.
During the remainder of the year, (July to February) the regulators will
relay storm flows into the basin for detention until the volume can be
pumped to the dry-weather treatment facility. Thus, the basin will also
serve to minimize the number of overflows into the Tau River.
Maintenance access 1s provided. In the event a sludge transport line is
unavailable, trucks would be used to remove any sludge from the
facility. Suitable access arrangements are included in the design.
Process Compatibility
The two flow schemes for the basin have different impacts on the
treatment facilities. Flexibility, capacity, and quality impacts are
discussed. The flexibility of the system is high. The basin is
designed to operate as a peak flow detention facility with the added
ability of being able to route flow to the dry-weather treatment
facility. For dry-weather failure emergency conditions the system could
be additionally modified to transfer some flows to the basin to provide
a minimum of primary treatment to dry-weather flows. Disinfection
5-11
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capacity would need to be retrofitted Into the basin. The emergency
processing of dry-weather flows 1s more economically arranged at the
treatment plant; therefore, His doubtful this last scheme would be
imp!erneted.
Additional flexibility is obtained by using the connection from dry
weather to the offline storage for temporary storage of dry-weather
flows during peak periods. In 10 years the treatment plant is expected
to reach its design capacity. The useful function of the plant is
extended by peak storage of dry-weather volume and then processing it
during low flow periods. This flexibility can only be achieved during
non-storm periods. In all of the operation options, odor control
measures must be implemented; therefore, the dry-weather processing
option in this case does not impose added design cost due to odor
control.
The impacts of the basin on treatment facilities are both positive and
negative. The positive impact includes extending the useful life of the
plant due to mitigating peak flow demands. The negative impact includes
more rapidly using sludge processing capacity. If the sludge process
requires degritted sludge, then arrangements for degritting must be
provided. Options include installation of grit removal equipment at the
basin or introduction of the sludge ahead of grit removal equipment in
the dry-weather treatment facility. Stormwater flows are seasonal and
sporadic. The demands which stormwater imposes on treatment facilities
are highly variable.
In Upsilon, the development of treatment facilities is still undergoing
analysis. To meet flow and quality requirements, Upsilon will have to
construct treatment facilities. Available options are: the dry-weather
plant will be expanded to accommodate some stormwater treatment, or new
separate facilities for wet-weather processing will be constructed. The
storage/sedimentation basin is an Integral part of either option. The
functioning of the basin 1n regard to dry-weather facilities was
previously discussed. The basin will operate in a similar fashion with
5-12
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wet-weather facilities. The basin's primary function 1s to prevent
flooding and by storing flow, permits a lessened peak design volume for
the downstream treatment plant. Sludge would be transferred to expanded
sludge facilities at the dry-weather site.
The Improvement In quality of the discharge to the Tau River will
primarily consist of lowered sediment loadings. The fraction of heavy
metals and other pollutants associated with sediment will also be
reduced In the effluent. During the nonspring period of the year, the
of overflows from the area serviced by the basin will be essentially
reduced to zero, with a corresponding increased quality effluent due to
processing through the treatment plant.
FLOOD CONTROL RETROFIT
The second example of integration of storage/sedimentation facilities
Involves the retrofit of a flood control facility. In the east part of
Upsilon, flood control efforts are totally independent of the stormwater
system in the west side of the city. The Tau River separates the two
stormwater systems. To alleviate the flooding of a low elevation
residential section, a flood control basin was installed. Now, with the
increased concern and regulations regarding stormwater discharges,
Upsilon determined it would be cost effective to retrofit the existing
flood control basin to improve the quality of the stormwater discharge
into the Tau River.
Facility Modifications
The flood control facility is located on city property. Before the
installation of the flood control measures, periodically during spring,
the Tau River swells and water Invades the eastern portion of Upsilon.
To prevent flooding, the embankment along the river was raised and the
flood control basin was installed to collect runoff. Pumps and flap
gates were also Installed to permit pumping stormwater from the basin
5-13
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Into the river. When water levels 1n the Tau River subside, runoff from
the east portion of the city flows by gravity to the river.
The flood control facility, in order to treat stormwater, must undergo
several modifications. The modifications include physical and
operational changes. The current mode of operation is to drain runoff
into the Tau River as quickly as possible. When gravity flow into the
Tau is possible, no buildup of runoff volume occurs in the basin. When
pumping is required, the volume in the basin is allowed to accumulate
sufficiently to prevent high cycling of the pumps. Most of the volume,
however, is kept in reserve for sudden peak runoff occurrences.
The operational changes required involve detaining a volume of
stormwater sufficiently for sedimentation to occur. During low river
level, gravity flow would fill the basin, overtop a weir (or other
restraining device), and continue by gravity flow to the river. During
the pumping phase, effluent from the sedimentation basin would flow to
the pump sump. For peak flush periods, a flow bypass from the entry of
the facility to the sump would prevent velocities in the sedimentation
volume from getting high enough to resuspend the sediment. To maintain
the capacity of the flood control facility, additional pumping capacity
would be required since the volume is being used to achieve
sedimentation rather than being kept empty as reserve storage volume.
The physical modifications to permit the above operation mode include
installation of stilling basin, weir (or other restraining device),
sediment removal equipment, and peak flush conveyance channel. The
stilling basin is required ahead of the flood control basin to dissipate
velocity head in the runoff. The flow then is more uniform as it enters
the sedimentation basin. To create the sedimentation basin, a weir or a
stop log type of flow restraint is installed. The flow then is subject
to detention time in the basin. The proper design of such a system was
outlined in Chapter 4. The flow then proceeds from the basin to the
pump sump.
5-14
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A channel from the stilling well to the sump 1s also a part of the
Upsllon facility. By keeping a sedimentation volume full, the flood
control facility loses the reserve volume for peak runoff events. Also,
sudden large volumes through the sedimentation basin may resuspend
particles. Flows above the maximum design of the sedimentation basin
must be diverted. The added volume, represented by the stilling well
and bypass channel, helps offset the loss of standby volume 1n the
sedimentation basin area. The addition of larger pump capacity makes up
the remaining difference so the overall flood control capacity remains
the same.
Sediment removal pumps and equipment installation 1s another part of
retrofitting this facility. Chain and flight collectors were installed
with screw conveyors to a new classifier and hopper unit for ultimate
truck transport to the dry-weather facilities. Other modifications
include provision for access for cleaning and maintenance work. At the
Upsilon facility, access to the existing structure was adequate and
additional access provisions were included in the stilling well and
sludge facility designs.
Functional Compatibility
The Upsilon flood control facility, as retrofitted, is compatible with
stormwater management goals. Due to the location of the facility,
integration with dry-weather facilities is not currently cost effective.
As a standby facility, it is possible for Upsilon to construct a
connection from the sanitary sewer lines to the facility. In the event
the sewer main under the river were to be damaged, such a connection
would permit primary treatment of the wastewater during the emergency.
As stated, however, a hookup of that type at this time is not deemed
prudent.
Impacts of the retrofit of the facility on flow handling are minor. The
decrease of the storage volume 1s offset by Increased pumping capacity
so a higher volume of stormwater will be pumped at the design storm peak
5-15
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flow, but there Is no practical change 1n the ability of the system to
protect the residential area from flooding. Operation and maintenance
of the facility 1s altered due to Increased cleanup after storms and
higher maintenance requirements. The facility 1s self-actuating so the
beginning of a storm event does not necessitate quick mobilization of
staff. The city has made a commitment to cleanup of stormwater and is
willing to accept the increased operation and maintenance requirements.
Process Compatibility
Since the flood control facility is physically isolated from the rest of
the treatment processes, it is not directly integrated with other
treatment facilities. The flexibility of designing the facility
retrofit to be compatible with handling dry-weather flows is not cost
effective at this time. Upsilon growth is planned for the northwest
sector. The capacity of the sewer mains from the southeast to the
treatment plant is expected to be adequate.
The retrofit of the flood control facility will impact the capacity of
the sludge processing facilities. No other treatment capacity is
influenced by this facility. As discussed in the first example, Upsilon
is planning to expand the sludge processing capacity. Another concern
is where the sludge is entered into the sludge processing train. If the
grit has been removed, then the sludge can enter the process stream
directly. If the sludge is undegritted and the sludge handling
facilities at the dry-weather plant require degritted sludge, then
either degritting is required at the flood control facility or the
sludge must be introduced into the dry-weather treatment plant ahead of
the degritting equipment. In this case, Upsilon decided to introduce
the sludge in front of the degritting equipment.
The quality impacts of retrofitting the flood control facility are to
reduce the amount of sediment and associated pollutants released during
storm events. Except for the largest storms, the flow undergoes primary
treatment. During bypass conditions, over two-thirds of the flow 1s
5-16
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subject to primary treatment. Sampling of the stormwater in Upsilon
demonstrated the characteristic of higher pollutants occuring in early
storm flows. The first flush phenomenon applies to the city's
stormwater system. The retrofit flood control faciity will detain the
first flush volumes for treatment, enabling a greater percent removal on
an average basis.
RETENTION AND ATTENUATION FACILITY INTEGRATION
The third example of integration of facilities is located in the most
western portion of Upsilon. Rolling hills on either side create a
valley which is planned for residential and some commercial business
use. This discussion will first investigate the development plans of
the site. The discussion will then address location constraints on
control methodologies and then functional and process compatibility
issues.
Site Potential
The existing site is a cattle farm consisting of grazing fields and
trees. There are very few improvements on the land from its original
state. A pond is maintained on the property for drinking water for the
cattle. A small creek channel crosses the property and eventually
enters the Upsilon storm sewer. The creek is dry except during the
spring runoff periods.
The zoning ordinances permit the development of this area into single
family residential area with a local commercial business district. The
sanitary sewer lines are to be extended to service the area, as will the
storm sewer; however, the peak runoff volumes from the future
development cannot exceed the volumes at the site under existing
conditions. An ordinance was passed in Upsilon to require all new
development not to increase peak runoff volumes to prevent exceeding the
capacity of the existing storm sewers.
5-17
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The potential retention and attenuation control methods for this site
are numerous. Rooftop storage, parking lot ponding, and low structural
retention/detention basins offer the highest potential for being optimum
stormwater management control techniques. As the site plans are in the
process of being developed, the stormwater control measures can be
easily integrated.
Location
The stormwater control methods selected for the new development will
need to interface to the stormwater sewer system. Due to regulation,
the peak runoff volumes cannot exceed the volumes of the existing site.
Other constraints of pollutant control are suggested, but not legally
enforced in Upsilon. One advantage of the storage sedimentation and
retention control methods is their adaptability. Given new development,
each subsequent site is free to choose the best method for the location
without conflicting with other areas. For an expanding system, the net
effect is that each expanding area adds facilities as required, which
prevents negative impacts on downstream facilities.
With the land use at the site being primarily residential, control
methods need to be selected with compatibility in mind. Rooftop
storage, plaza ponding, and parking lot ponding are all compatible with
commercial business land use. Retention or detention ponds can be
compatible control methods with residential land uses. Other storage
sedimentation facilities requiring more structural components can also
be compatible. In this development, the low structural control methods
were desired as blending more with the natural surroundings.
The low structural controls often require larger land areas. In the new
Upsilon development, there is sufficient land for retention ponds. The
options are open to have dry ponds or wet ponds. The cattle watering
pond, having been in existence for many years, has developed natural
vegetation for pond areas. This portion of the site could be converted
5-18
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to a wet pond with additional storage volume requirements being
accounted for by dry pond storage.
Functional Compatibility
After analysis of the site to optimize the integration of stormwater
control methods, the following combination of controls was selected.
For the commercial business area controls Include: (1) rooftop storage
and controlled rate of release, and (2) parking lot storage with drains
to a small structural basin to aid in grease and sediment removal and to
add additional required volume. The additional volume was required
because parking storage, without becoming a nuisance or hazard, was not
sufficient to handle the business district runoff volume. The designed
rooftop and parking storage are expected to be sufficient to detain the
peak flows and prevent exceeding the storm sewer capacity.
The residential section has both a wet and dry pond to regulate storm
flows. The wet pond was established at the cattle watering pond.
Surrounding land was built up sufficiently to provide for storage above
the normal level of the pond. A dry pond was also established as a
recreational field during periods without rain. Drainage swales
throughout the development have stepped barriers to add to the detention
storage volume available.
The flow impact of these measures 1s to reduce the peak runoff volumes
generated by the more highly impervious surfaces of the development.
The regulated releases from the rooftop storage and parking basin
attenuate the peak volume. The wet pond services a retention basin for
most storms, 1n that water is retained. The large storms exceed the
capacity and continue to the dry pond area.
Operation of these control methods Is automatic. Maintenance 1s
required, however. Channels must be kept free of debris. The rooftop
units must be periodically checked. The parking lot area should be kept
5-19
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swept with normal street cleaning procedures. The dry pond must be
maintained 1n grass free from excess vegetative growth. Growth around
the wet pond must also be checked. The maintenance procedures can be
distributed over a period of time and do not, therefore, represent a
labor Intensive period at any point during the year.
Additional maintenance around the wet pond 1s required after storms with
heavy sediment loading. Dredging or scraping the bottom of the pond
periodically will prevent loss of recreational used to sedimentation.
None of the control methods conflict with the operation of the storm
sewer.
Process Compatibility
Retention and attenuation facilities are compatible with any of the
other treatment processes. Since the control measures in the new
development are primarily flow control measures, little impact of
treatment processes will be observed. The only quality effects will be
street sweeping of the parking lot that will reduce the quantity of
pollutants and the wet retention pond will contain pollutants from part
of the residential section. The chosen control methods, being
independent of the other stormwater management controls, do not impose
constraints on downstream facilities and therefore allow for maximum
flexibility.
Treatment capacity will not be negatively impacted, since the new
development is designed to have the same net runoff effect as the
unimproved property. A slight decrease 1n pollutants 1s expected,
causing a reduction in pollutant loading. Sediment accumulation in the
wet pond will need to be disposed of. The quantity 1s not expected to
be prohibitive, however. Landfill, construction, or other disposal
locations are available.
5-20
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The quality Impacts of this stormwater management system are to slightly
reduce pollutant loadings 1n the stormwater runoff. Groundwater impacts
of retention facilities must also be examined. The stormwater runoff in
this area is low in all pollutants except sediment. Since the sediment
will settle 1n the pond and other pollutant quantities are low, water
seepage 1s not expected to present a ground water contamination problem.
REFERENCES
1. American Public Works Association. Survey of Stormwater Detention
Practices in the United States and Canada. Unpublished Report.
2. Akeley, R. Retention Basins for Control of Urban Stormwater
Quality. Proceedings: National Conference on Urban Erosion and
Sediment Control. EPA 905/9-80-002. 1980.
5-21
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Chapter 6
REGIONAL IMPLEMENTATION OF
STORAGE/SEDIMENTATION
A unified, areawide approach to stormwater pollution control that takes
into account expected land uses, area hydrology, and assimilative
capacities of receiving waters is a necessary part of rational planning
for urban development. Storage/sedimentation is a control technique
that fits very well into such an approach. This chapter presents four
examples of regional stormwater control strategies in which
storage/sedimentation plays an important part. In each case, a
regional governmental agency has established regulations and/or
guidelines for the application and design of storage/sedimentation
control facilities. The existence of clear guidance and requirements
greatly simplifies the process of regulating development and of
complying with the regulation.
EUROPEAN PRACTICE
Many of Europe's cities are served by combined sewer systems. Combined
sewer overflows are recognized as significant sources of urban
pollution. Storage/sedimentation has been applied throughout Europe as
an effective control technique. In some areas, simple application and
design guidelines for storage/sedimentation facilities have been
developed; two such guidelines are examined. Scotland has developed a
very simple combined sewer overflow control strategy that allows
designers to quickly size sewers, treatment plants, and
storage/sedimentation basins. An example of a more sophisticated
guideline is that applied by the German State of Bavaria.
In addition, innovative approaches to storage/sedimentation have been
applied such as the in-lake storage facilities at Huddings, Stockholm,
Sweden.
6-1
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Scotland
Scotland 1s a member of the United Kingdom. Stormwater control there
1s based on British practice that has evolved over the last 100 years.
Combined sewer overflows are the most significant stormwater pollution
source throughout the United Kingdom. Traditional design of combined
sewers has been to provide sewer capacity equivalent to six times the
average dry-weather flow. Excess flows are allowed to overflow
directly to the receiving water. Wastewater treatment plants are
designed to provide biological treatment to three times the dry-weather
flow. Flows greater than three times dry-weather flow but less than
six times dry-weather flow are routed to special stormwater storage
sedimentation tanks. The tanks are sized to provide a minimum of 2
hours detention to the 3 to 6 times dry-weather flow before overflowing
to the receiving water.
Recently, English and Welsh policy has been to gradually separate the
sanitary and storm sewers as areas are redeveloped, and for new
development to be on separate sewers. In 1970, the British Ministry of
Housing and Local Government (BMHLG) published a report on stormwater
disposal practices [2]. In the report, more rational formulas were
suggested for combined sewer and overflow treatment design. In 1977,
the Scottish Development Department published its Report of the Working
Party on Storm Sewage [2], endorsing the BMHLG formulas, but
disagreeing with the idea that sewer separation is always desirable.
The BMHLG formula for sizing of storm sewers is:
Q = DWF + 360P + ZE (6-1)
where DWF = dry-weather flow, gal/d (L/d)
P = the population of the drainage area
E = the dry-weather flow from industries, gal/d (L/d)
6-2
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The Scottish Working Party recommended adoption of the formula, which
reduces to 6 times dry-weather flow for the typical Scottish condition
of gal/capita*d ( L/capita*d) dry-weather flow and no industrial
flow component. Overflow control structures accepted for use Include
the Sharpe & Klrkbride type illustrated in Figure 6-1, the expanding
stilling basin type, and the high side weir type. Swirl concentrators
are being evaluated for this use. Typically, overflows are discharged
directly without additional treatment.
Storage/sedimentation basins for control of combined wastewaters at the
treatment plant are recommended. The 1970 BMHLG design basis for these
basins, endorsed by the Scottish Working Party, is to provide 18 U.S.
gallons (68 L) per capita of storage capacity. On most catchments,
first flush is not seen as a significant problem, and basins are
usually designed as flow-through basins. One or two of the basins at
the treatment works may operate as capture basins if a flushing
phenomenon is noted.
British and Scottish practice does not typically call for pollution
control devices at combined sewer overflow points. The Scottish
Working Party did recognize the need for such basins where the overflow
might result in aesthetically objectionable conditions, where the
overflow is to a very small stream, where the discharge might impact
downstream uses, or if a particularly strong or toxic industrial
wastewater is present in the overflow. Accordingly, the Working
Party's report included design guidance for overflow basins. The
curves shown in Figure 6-2 are used to determine tank size.
The simple design guidelines applied in the United Kingdom have been
very effective in controlling stormwater pollution 1n Scotland. Of the
over 2,000 combined sewer overflows identified in the country, only 6%
were considered unacceptable by the regulatory agencies. The use and
effectiveness of such guidelines are made possible by the facts that
Scotland does not have a lot of heavy Industrial development, that most
6-3
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/"<,»....' .''.. /».».. :* -vy; -v?- ^ '*.-../. *.-. -.*_ .< /> - . .-, ».,.-- /-,. >-. «.-;
SCICEHINCI
XETUtHEO
TO FIOI
I Hit I MIL MFFLE
BIO
(00
0 I A
.0.4B
(1f t-T in. >
SECTION ON BB
OUTFALL TO
IAT.ER OF LEITH
22S (Bin.)
OIA THROTTLE
DRIVE
MOTOR
7.
-1050
(42 in. )
OIA
PLAN ON U
Figure 6-1. Plan and longitudinal section of Sharpe & K1rkbr1de
storm overflow at Keddie Gardens showing evapotransplration
of mechanically raked screen [2].
6-4
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en
-I- DATA FROM 8UCKSBURN
A DATA FROM STONEVIOOO
6*0 8t) 100
TANK CAPACITY. L/eaplta
Figure 6-2. Effectiveness of storm tank capacity on sewerage
system in reducing discharges to watercourses [2].
-------
of the population Is concentrated 1n coastal areas, and by the nature
of Its hydrology. As these conditions change, more refined design
procedures must evolve.
Bavaria
Bavaria is a state in the southern part of West Germany. Munich is its
largest city. Many of the Alpine lakes in the Bavarian region are
experiencing eutrophication due to pollution by untreated discharges
from populated areas. As with much of Europe, the most critical aspect
of Bavaria's stormwater control is control of combined sewer overflows.
Until 1973, the Bavarian policy was to minimize the number of
overflows. Sewers were large and designed to discharge excess flows
just upstream of the dry-weather treatment plant.
Evaluation of the pollutant distributions within overflows revealed a
pronounced first flush phenomenon. Accordingly, the Bavarians have
changed their policy to encourage capture of first flush with a large
number of overflows and capture basins on relatively small catchments.
An attempt is made to keep catchments in the range of 20 to 40 acres (50
to 100 ha), with times of concentration of less than 15 minutes. All
o
flows greater than 0.2 ft /acre-s (15 L/ha-s) are overflowed to a
storage tank of from 10,600 to 17,600 ft3 (300 to 500 m3). When the
tank is full, a signal to the overflow structure directs the additional
excess flow to the receiving stream. If a catchment area of less than
40 acres (100 ha) cannot be defined, Bavarian practice calls for a
sedimentatioi
capita) [3].
3 3
sedimentation pond sized at 160 to 320 ft /acre:capita (10 to 20 m /ha
Guidance for design of stormwater control facilities is Included in a
series of bulletins published by the Technical Wastewater Union e.V.
Guideline A-128 covers sizing and design, of combined sewer overflows.
The basis of the guidelines is to allow only 10% of the total BODg and
suspended solids generated in a combined sewered area to be discharged
without having received biological treatment.
6-6
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The necessary sewer capacity 1s calculated using a curve based on the
hydrologlc character of the area. The rainfall rate whose runoff should
just begin overflow of the combined sewer 1s selected from the curve
shown 1n Figure 6-3. The resulting runoff 1s calculated for that
critical rainfall rate, considering only Impermeable areas.
The guideline includes a step-by-step procedure for designing the
combined sewer overflow structure, which also makes extensive use of
nomographs.
Design guidelines for first flush capture and flow-through sedimentation
basins are discussed, both for inline and offline installations. Again,
curves are provided for sizing of the basins. Operation and maintenance
aspects are also included.
As stated previously, Guideline A-128 is only one of a series of design
bulletins for stormwater control facilities. The availability of these
guidelines, which include charts and graphs based on German and Bavarian
conditions and the specific regulations of the Bavarian state for
control of combined sewer overflows, greatly simplifies the task of the
design engineer.
Sweden
An interesting approach to urban stormwater treatment is that developed
by Dr. Karl Dunkers for protection of Lake Trehorningen at
Huddings/Stockholm, Sweden. The patented system uses a portion of the
lake volume to store and settle urban runoff or CSO before discharge.
The system is shown in Figure 6-3b and diagramed in Figure 6-3c. A grid
of wooden platforms floating on the lake's surface isolate the point of
runoff discharge from the body of the lake. Plastic sheets extend from
the platforms to the lake bottom, forming .a series of rectangular cells.
The cells are baffled to provide a single flow path through the grid.
Stormwater runoff entering the grid displaces lake water and must flow
6-7
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RANGE WITH PARTICULARLY
HIGH DEMANDS ON WATER
MLF-MEAN LOW FLOW IN RECEIVING
WATER
-|Qs eWASTEWATER (ONLY WASTEWATER
AREA) FROM ENTIRE CATCHMENT
0.5 1 2 345 7 10 2030 5070100 200 40070010002000 5000
MLF/Qs TOTAL
Figure 6-3. Determination of critical rainfall
per unit area [4].
6-8
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a*
i
pjm &$£itL*-^ ^*ji!^:^
Figure 6-3b. Pontoon tank storage system at Lake Trehornlngen.
-------
'feed punip(
- - pressure linej
Tank section
pontoons
pontoons
Figure 6-3c. Schematic of Lake Trehornlngen pontoon tank system.
6-10
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sequentially from cell to cell. For most storms, the runoff 1s
contained within the grid system.
Stored runoff is withdrawn from the inlet cell and treated prior to Its
discharge to the lake. Lake water replaces the withdrawn stored runoff
or CSO at the grid's outlet, forcing the stormwater 1n the grid to flow
back toward the inlet cell.
The Dunkers system 1s an excellent example of low cost, but effective
stormwater storage for water quality protection. Eutrophication of Lake
Trehorningen results from phosphorus loading. The treatment plant
removes phosphorus by chemical precipitation. Even after all stormwater
is withdrawn from the grid and treated, the phosphorus removal plant
continues to operate, removing phosphorus from the lake water.
UNITED STATES PRACTICE
The countrywide and statewide approaches to stormwater control are
possible in Scotland and Bavaria because the regions are small and
hydrologic conditions are relatively uniform. The United States and
many of the individual states are much larger and hydrology varies
significantly. Nevertheless, regional regulations and guidelines for
stormwater control are in effect is many areas of the United States.
Traditionally, the primary purpose of the regulations is for control of
flooding from urban land. The guidelines have been expanded to include
control of erosion and sedimentation from new construction and, in some
cases, of urban stormwater pollution. The following sections include
examples of the regional approach to stormwater control in the United
States, the first in the metropolitan Washington, D.C., area and the
second in central Florida.
6-11
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Montgomery County, Maryland
Montgomery County, Maryland, lies along the Potomac River, northwest of
Washington, D.C. Increasing urbanization has created both runoff
quantity and water quality problems. Since 1965, the county has
evolved a policy of control of flooding, erosion, and sediment
deposition at the source by restricting runoff from a 2 year design
storm on new developments to predevelopment rates. Reduction of the
rate and volume of runoff influences the amount of soil erosion and
pollutants wasted from urban surfaces.
All new development and construction in Montgomery County that increase
the impervious area must have stormwater management facilities.
Exemptions are allowed for development sites having minimal land
disturbance or small percentages of impervious area. The regulations
require the control or storage of stormwater runoff in excess of the
predevelopment flow from the 2 year storm. The role of the county is
limited to design review, permit issuance, and inspection during
construction.
Over 800 stormwater storage/sedimentation control devices are in use in
Montgomery County. Most have been constructed on development sites of
less than 10 acres (4 ha). The largest percentage of this number are
detention ponds, although rooftop storage, parking lot storage,
underground storage, and percolation systems are also in use. Control
of runoff from larger drainage areas, of 50 to over 500 acres (20 to
203 ha), is usually accomplished by use of wet or dry detention ponds.
Most large site control facilities are designed to control several.
different runoff events, up to the 100 year storm flow, while most
small site controls are designed for a 2 year return period.
Facilities designed for control of a simple return period storm usually
do not provide the intended control efficiency for flows from other
return period storms. The Soil Conservation Service (SCS) hydrologic
6-12
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analysis method, outlined 1n Appendix B, 1s used In Montgomery County.
A solid, rather than perforated, riser design is used to maintain pool
levels in wet ponds.
The primary goal of Montgomery County's stormwater control regulations
is to reduce runoff flow from developed sites. Flow reduction
efficiencies for the installed facilities can approach 90% for flows at
or less than the design storm. Reduction of stormwater pollution 1s a
secondary benefit. Pollutant trap efficiencies, as monitored at one
wet detention pond site in Montgomery County, varied from 93% for
orthophosphate and BOD2Q to 99% for ammonia nitrogen and zinc.
Most stormwater control facilities in Montgomery County are constructed
by private developers. When the development is sold, responsibility
for facility maintenance passes to the new owner. Facility maintenance
does represent a cost and is often neglected. Currently, there 1s no
mechanism for enforcement of maintenance after the stormwater facility
is built.
Orange County, Florida
Orange County is in east central Florida with Orlando its largest city.
? 2
Approximately one-tenth of the county's 1,000 mi (2,500 km ) is
surface water. Nearly 1,100 waterways and lakes offer scenic beauty
and recreation, and attract tourism. Stormwater runoff 1s depositing
pollutants, particularly phosphorus and sediments, that are degrading
water quality and increasing the rate of eutrophication. Orange County
has developed a strategy of implementing nonstructural practices and
low structural facilities to control and/or treat stormwater runoff
near Its source.
6-13
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The control strategy 1n Orange County relies heavily on stormwater
retention and percolation. Southern Florida Water Management District
requires the detention and controlled release of the first 1 1n.
(2.54 cm) of rainfall runoff as a flood control measure. Orange
County's stormwater and subdivision regulations require that the first
1 In. (2.54 cm) of rainfall be retained and percolated onsite. The
recently accepted Orlando area 208 study will result in new
regulations, based on soil recharge capacities. The first 0.5 1n.
(1.27 cm) of runoff will be retained on soils with high percolation
rates. The new regulations will provide for retention of the first
0.35 1n. (0.9 cm) of runoff on soils with low percolation rates [6],
Guidance in stormwater facility design in Florida is provided by
"Stormwater Management Practices Manual" [7]. Evaluation of runoff
from a drainage area is based on the SCS soil groupings and hydrologic
analysis method outlined in Appendix B. A nomograph for estimating
necessary size, removal efficiency, and costs for diversion/percolation
basins in Florida is shown in Figure 6-4.
The stormwater facilities in Orange County are designed to have low
manpower requirements. Most facilities have been designed for specific
loadings or as self-activating units. Orange County requires
developers not only to implement stormwater runoff controls, but to
maintain them. In most cases, developers have assumed maintenance
responsibilities. Where they do not, Orange County maintains the
facility and levies a fee against the development for the required
work. For regional control facilities, maintenance is the
responsibility of Orange County, the City of Orlando, or the State
Department of Transportation.
Orange County has one of the most complete regional programs for
control of storm-induced water pollution in the United States.
Although the cumulative effects of Its program have not been
determined, the performance of many of the facilities Implemented has
6-14
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NOTE: IMPEIVIBUS AIEA
CM- 100
o
IBAOIN6S (LB/ACRE-TR)
SS 254
, B005 36.5
BT ACRE-FT M 7.B
1.1
6000 SS
BBO BOD
185 N
26 P
1.00
10
FIRST FLUSH
DIVERSION
VOLUME
(INCHES)
SS
730 BOD
'55 N
21 r
590 BOO
125 N
16 t
2.43
2000 SS
2BO BO
65
2000 4-1.62 7
6000 5000 4000 3000 2000
10 15 20 25 30 ACRES
CONTRIBUTING WATERSHED AREA
BASIN VOLUHE-FOR COMPOSITE
' VALUES
PRESENT VALUE
(20 YR. 7*)
ACRE-FT 4.B6 4.05 3.24 2.43 1.62
100
'CM" VALUES
20 40 60 BO 100 120
3 ACRE-FT CAPITAL AND PRESENT
VALUE ($1000)
Figure 6-4. Size, efficiencies, and cost of diversion/percolation
basins (ENR 4000) [7],
6-15
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T.
111
been well over 90% in reducing annual BODK, phosphorus, and suspended
o CO
solids loads [ ]. Important aspects of Orange County's program are d
that (1) developers are required by ordinance to provide stormwater >j
control, (2) the county provides simple requirements that apply to the
area, (3) design guidance
the facilities is assured.
area, (3) design guidance is provided, and (4) long-term maintenance of S
LU
ELEMENTS OF AN AREAWIDE STORMWATER MANAGEMENT PROGRAM >
<
A unified, areawide program of stormwater management that includes
storage/sedimentation techniques can greatly reduce the environmental
and economic impacts of urban development. Usually, responsibility for
stormwater management rests with local municipalities. An entire urban
area or watershed may encompass several municipalities and unicorporated
urban areas. Fragmented and overlapping authority limits the
effectiveness of stormwater management by encouraging jurisdictional
rivalry, making planning difficult, and diffusing financial resources.
The following section describes elements of a coordinated stormwater
management program designed to overcome these problems.
Element 1 - Central Authority to Regulate Stormwater
Coordination among the agencies within a watershed that have
responsibility to regulate stormwater runoff, from both quantity and
quality standpoints, is of primary importance. An umbrella or joint
powers agency, with a permanent source of revenue, is usually the most
effective way of accomplishing this. Such an agency should have
authority to require construction of stormwater control facilities from
new developments. It should have the authority to require continued
operation and maintenance functions. For already developed areas, the
agency should be empowered to plan, construct, and operate factilities.
6-16
-------
Element 2 - Master Plan for the Watershed
A master plan for stormwater management for the basin should be
developed. Land use plans and other point and nonpoint pollution
sources should be considered along with local hydrology and existing
land uses. The master plan should clearly state the perceived problems
of the area and the goals and policies of the agency In meeting federal,
state, and local flood and pollution control requirements.
Element 3 - Implementation Guidance
The central agency should develop detailed design and implementation
guidelines, such as those used in Orange County, Florida, as an aid to
developers and engineers. Design nomographs and computerized aids may
be developed, based on local watershed data on hydrology, assimilative
capacities of receiving waters, and pollution washoff functions. The
complexity of the guidelines will depend on the complexity of local
conditions and needs. The guidelines should provide for consideration
of new and innovative control techniques.
Element 4 - Enforcement Procedures
The agency's policies should include enforcement and appeal procedures.
Typically, for new developments, enforcement will consist of review of
stormwater control plans before subdivision approval, inspection of
control facilities during construction, and periodic inspection and
monitoring during facility operation. In many cases, the operations
responsibility will be assumed by the control agency.
SUMMARY
Storage/sedimentation facilities, often in conjunction with other
stormwater control techniques, can effectively control flooding and
runoff pollution from urban areas. Coordinated areawide flood control
6-17
-------
programs that made use of detention and retention facilities have been
applied in the United States for many years. The same approach can be
extended to include areawide stormwater pollution control.
The purpose of this manual is to provide design and implementation
guidance to the engineer and public works employee in designing
storage/sedimentation facilities for dual flood and pollution control.
These guidelines may be adapted to specific urban areas to form a basis
for local design guidelines.
REFERENCES
1. Ministry of Housing and Local Government, Technical Committee on
Storm Overflows and the Disposal of Storm Sewage: Final Report.
London. 1970.
2. Scottish Develpment Department. Storm Sewage Separation and
Disposal. Edinburgh. 1977.
3. Field, R. Trip Report: 1978 European Trip, Unpublished Report.
October 15, 1978.
4. Technical Wastewater Union E.V. Guidelines for the Sizing and
Design of Stormwater Discharges in Combined Sewers. Working
Instruction A-128. 1978.
5. Lynard, W., et al. Urban Stormwater Management and Technology:
Case Histories. EPA 600/6-800-035. August 1980.
6. Loop, J. Memorandum for the Record, Metcalf & Eddy, Inc., Palo
Alto, California. Trip Report for Orlando, Orange County, Florida.
June 1978.
7. Stottler, Stegg and Associates/Brevend Engineering Company.
Orlando Metropolitan 208 Study: Stormwater Management Practices
Manual. November 1977.
6-18
-------
APPENDIX A
REFERENCES
[References appear at the end of each chapter
in the draft report]
A-l
MCTCALP t C OOV
-------
Appendix B
SOIL CONSERVATION SERVICE
RUNOFF ANALYSIS METHOD [1]
An urban watershed has substantial areas covered by impervious
structures, such as roads, sidewalks, parking lots, and houses.
Drainage systems, such as paved gutters and storm sewers, may also be
present. These conditions can produce significantly higher volumes and
flowrates when compared with the stormwater runoff characteristics of
the watershed before urbanization.
The Soil Conservation Service (SCS) of the U.S. Department of
Agriculture has developed a simplified method for computing runoff
volume, travel time, and peak discharge rate for agricultural
watersheds. This method is also applicable to the preliminary runoff
analysis of small urban watersheds. Its use is restricted to a first-
cut analysis method due to the fact it does not consider rainfall
intensity or antecedent soil moisture content.
The following is a summary of the SCS runoff analysis method for small
urban watersheds.
RUNOFF VOLUME
Assuming a storm with a constant rate of rainfall, the relationship of
rainfall P, runoff Q, infiltration F, and initial abstraction I is
a
illustrated in Figure B-l. At any time, T, after rainfall begins
Pe-Q (1)
where P£ = potential runoff.
B-l
MCTCALF A COOr
-------
H
O
ACCUMULATED
F + Ia
F + Ia->S
AS T-*oo
TIME, T
P = TOTAL RAINFALL
Pe = POTENTIAL RUNOFF
Q = ACTUAL RUNOFF
F = INFILTRATION AFTER RUNOFF BEGINS
Ia = INITIAL ABSTRACTION
S = POTENTIAL ABSTRACTION
Figure B-l. Schematic curve of
rainfall, runoff, infiltration, and initial abstraction,
B-2
METCALP » B ODV
-------
If it is further assumed that
F/S = Q/Pe (2)
at any time T, from Figure B-l:
F - Pe - Q. (3)
Rewriting (3),
Again from Figure B-l:
Pe = P - Ia. (5)
Empirically, for small watersheds,
Rewriting (4),
I = 0.2 S. (6)
a
Q = (P - 0.2 S)2/(P + 0.8 S) (7)
The storage at saturation, S, is a function of the soil and cover
conditions of the watershed. Using more than 3,000 soil types divided
into four hydrologic groups, the SCS developed runoff curve numbers (CN)
to estimate the the value of saturation storage. The relationship of
storage to curve number is:
S = (1,000/CN) - 10 (8)
The four hydrologic groups are described in Table B-l. Coven conditions
can be divided into agricultural, suburban, and urban land use
classifications. Curve numbers for these classifications and associated
soil groups are shown in Table B-2. A weighted average CN is used for
the watersheds with the combination of cover conditions or soil types.
B-3
MCTCALF 4
-------
Table B-l. DEFINITIONS OF HYDROLOGIC SOIL GROUPS
Group
Definition
A Low runoff potential. Soils having a high infiltration rate and consisting chiefly of
deep, well to excessively drained sands or gravels.
B Soils having a moderate infiltration rate and consisting chiefly of moderately deep to
deep, moderately well to well drained soils with moderately fine to moderately
coarse texture.
C Soils having a slow infiltration rate and consisting chiefly of soils with a layer that
impedes downward movement of water or soils with moderately fine to fine texture.
D High runoff potential. Soils having a very slow infiltration rate and consisting
chiefly of clay soils with a high swelling potential, soils with a permanent high water
table, soils with a claypan or clay layer at or near the surface, and shallow soils
over nearly Impervious material.
Table B-2. RUNOFF CURVE NUMBERS FOR
AGRICULTURAL, SUBURBAN, AND URBAN LAND USE
Land use description
Hydrologlc
soil group
A B C D
Cultivated land*
Without conservation treatment
With conservation treatment
Pasture or rangeland
Poor condition
Good condition
Meadow: good condition
Wood or forestland
Thin stand, poor cover, no mulch
Good cover"
Open spaces, lawns, parks, gold courses, cemeteries, etc.
Good condition: grasscover on 75% or more of the area
Fair condition: grasscover on 50 to 75% of the area
Commercial and business areas (85% Impervious)
Industrial districts (72« impervious)
Residential0
Avg lot size, acres Avg I Impervious^
0.125
0.25
0.33
0.50
1
65
38
30
25
20
Paved parking lots, roofs, driveways, etc.6
Streets and roads
Paved with curbs and storm sewers6
Gravel
Dirt
72 81 88 91
62 71 ~78 81
68 79 86 89
39 61 74 80
30 58 71 78
45 66 77 83
25 55 70 77
39 61 74 80
49 69 79 84
89 92 94 95
81 88 91 93
77 85 90 92
61 75 83 87
57 72 81 86
54 70 80 85
51 68 79 84
98 98 98 98
98 98 98 98
76 85 89 91
72 82 87 89
a. For a more detailed description of agricultural land use curve
numbers, refer to reference [2].
b. Good cover 1s protected from grazing, litter, and brush cover soil.
c. Curve numbers are computed assuming the runoff from the house and
driveway 1s directed toward the street with a minimum of roof water
directed to lawns where additional Infiltration could occur.
d. The remaining pervious areas (lawn) are considered to be 1n good
pasture condition for these curve numbers.
e. In some warmer climates of the country, a curve number of 95 may
be used.
B-4
MBTCAL* t e OOV
-------
If the appropriate curve number 1s known, the runoff, in inches, can be
calculated from Equation (7). The SCS has developed a chart, Table B-3,
and a plot, Figure B-2, of these relationships.
Table B-3. RUNOFF DEPTH FOR SELECTED CNs
AND RAINFALL AMOUNTS
Inches
Rainfall,
Curve number (CN)a
60 65
70
75 80 85
90
95
98
1.0
1.2
1.4
1.6
1.8
2.0
2.5
3.0
4.0
5.0
6.0
7.0
8.0
9:0
10.0
11.0
12.0
0
0
0
0
0
0
0
0
0
1
1
2
3
4
4
5
6
.01
.03
.06
.17
.33
.76
.30
.92
.60
.33
.10
.90
.72
.56
0
0
0.02
0.05
0.09
0.14
0.30
0.51
1.03
1.65
2.35
3.10
3.90
4.72
5.57
6.44
7.32
0
0.03
0.06
0.11
0.17
0.24
0.46
0.72
1.33
2.04
2.80
3.62
4.47
5.34
6.23
7.13
8.05
0.03
0.07
0.13
0.20
0.29
0.38
0.65
0.96
1.67
2.45
3.28
4.15
5.04
5.95
6.88
7.82
8.76
0.08
0.15
0.24
0.34
0.44
0.56
0.89
1.25
2.04
2.89
3.78
4.69
5.62
6.57
7.52
8.48
9.45
0.17
0.28
0.39
0.52
0.65
0.80
1.18
1.59
2.46
3.37
4.31
5.26
6.22
7.19
8.16
9.14
10.12
0.32
0.46
0.61
0.76
0.93
1.09
1.53
1.98
2.92
3.88
4.85
5.82
6.81
7.79
8.78
9.77
10.76
0.56
0.74
0.92
1.11
1.29
1.48
1.96
2.45
3.43
4.42
5.41
6.41
7.40
8.40
9.40
10.39
11.39
0
0
1
1
1
1
2
2
3
4
5
6
7
8
9
10
11
.79
.99
.18
.38
.58
.77
.27
.78
.77
.76
.76
.76
.76
.76
.76
.76
.76
.To obtains runoff depths for CNs and other rainfall amounts not
.shown in this table, use an arithmetic Interpolation.
B-5
MCTCALF « tOOV
-------
100
Cfi
w
CO
w
I
o
w
H
o
o
40 60
PERCENT IMPERVIONS
80
100
Figure B-2. Percentage of impervious areas versus
composite CNs for given pervious area CNs.
B-6
METCALP * CODV
-------
Example. Compute the runoff from 5 Inches of rainfall for a 1,000 acre
watershed to be converted to a suburban development. All the soils are
in hydrologic soil group C. The proposed land use is 50% detached
houses with lot size 0.25 acre; 10% townhouses with lot size 0.125 acre;
25% streets with curbs and gutters, schools, parking lots, plazas; and
15% open space, parks, schoolyards, etc., with good grass cover.
1. Compute the weighted runoff CN:
Land use Percent CN (Table B-2) Product
Detached houses with
lot size 0.25 acre 50 83 4,150
Townhouses with lot
size 0.125 acre 10 90 900
Streets with curbs
plazas, etc. 25 98 2,450
Open space, parks, etc. 1_5_ 74 1,110
100 8,610
Weighted CN = 8,610/100 = 86
2. From Table 3, using CN = 86 and P = 5, interpolate to read
Q = 3.47 inches.
TRAVEL TIME AND LAG
Urbanization of a watershed results in the increase of impervious area,
more points of runoff interception, and hydraulic improvement of
channels, These conditions Increase the runoff velocity and decrease
the runoff travel distance and time.
B-7
MCTCALF * COOV
-------
Travel Time
Travel time is the time of travel of runoff through a portion of or the
entire watershed. Time of concentration is the travel time of runoff
from the hydraulically most distant part of the watershed to the point
of reference. Travel time can be divided into three phases of flow:
overland, channel, and storm sewer or road gutter.
Travel time for overland flow in an urban area may be estimated by
dividing the distance from the uppermost part of a watershed to a
defined channel or inlet of the storm sewer system by the runoff
velocity. The runoff velocity, which may be read from Figure B-3, is a
function of the terrain and slope of the watershed.
Travel times for open channel and storm sewer flow are computed by
dividing the length of the channel or sewer by the velocity calculated
using Manning's equation. If the flow is primarily in shallow road
gutters, the average velocity can be obtained from Figure B-3.
Lag
Lag is the time between the center of mass of an excessive rainfall and
the peak runoff rate. Lag is related to the time of concentration
empirically as:
L = 0.6 Tc (9)
where L = lag
T = time of concentration
B-8
MCTCAL' » BOOV
-------
0.2 0.3 0.5
1 2 3
VELOCITY, ft/s
10
20
Figure B-3. Average velocities for estimating
travel time for overland flow.
B-9
MCTCAUP « CODV
-------
In small urban areas, less than 2,000 acres, the CN method can be used
to determine lag. The equation is:
L = [A0'8 (S+l)°'7]/[l,900 y0*5] (10)
where L = lag, h
£ = hydraulic length of watershed, ft
S = 1,000/CN1 - 10
y = average watershed slope, %
CN1 is the retardance factor and is equivalent to the runoff curve
number. Equation (10) is presented in graphical form in Figure B-4.
Use of Figure B-4 is illustrated in the example which follows.
Two conditions affect the lag time estimated by Equation (10). One is
the hydraulic improvement over the natural condition of the channel or
streambed. The second is the increased amount of impervious area in the
watershed. Both conditions decrease lag by decreasing retardance of
flow and increasing channel capacity. The actual lag is a product of
the lag factors obtained from Figures B-5 and B-6.
Example. A watershed of 1,000 acres has a present-condition CN of 75,
average watershed slope of 4%, and hydraulic length of 13,200 feet.
Urban development is expected to modify about 70% of the hydraulic
length, increase the impervious area to 40%, and increase the runoff CN
to 80. Comute the present- and future-condition time of concentration
using the CN method.
1. Present-condition lag from Equation (10) or Figure B-4, with
CN = 75.
L = (13,200)0-8(3>33+1 )0.7/MOO(4)0.5 = K
B-10
MCTCALF * C OOV
-------
GREATEST FLOW LENGTH, ft
o
o
o o o
o o o
CM m 5T
0 I
000 I
O 00 I
vo o>,_i J
o o o o o
o o o o o
o o o o o
(M m 'ff \D CD
a
o
o
o
LO\O 00
oo o.
IO ^- vO CO
_T
o o o o o ,
00 P
WATERSHED LAG, h
Figure B-4. Curve number method for estimating lag.
B-ll
MBTCALF A BOO
-------
100
0.9
).8 0.7
LAG FACTOR
Figure B-5. Lag factor for hydraulically improved.
100
w
75
50
*£ 2!
rr
££
^
<&
i3*
o>
^j^
^=
cl
0.9
0.8 0.7
LAG FACTOR
0.6
0.5
Figure B-6. Lag factor for
increase in impervious area in a watershed
B-12
MCTCAL* 4 ODV
-------
2. Present-condition time of concentration from L = 0.6 T :
Tr = 1.67(1.45) = 2.42 h
c
3. Future-condition lag
a. Basic future-condition lag with CN = 80:
L = (13,200)0>8(2.5+1)0%7/1,900(4)°-5 = 1.25 h
b. Lag factor for modification of 70% of the hydraulic
length from Figure B-5, hdraulic length lag factor
= 0.59.
c. Lag factor for 40% impervious area from Figure B-6,
impervious area lag factor = 0.76.
d. Future-condition lag = 1.25(0.59)(0.76) = 0.56 h
4. Future condition time of concentration:
TC = 1.67(0.56) = 0.94 h
PEAK DISCHARGE RATE
The SCS method for computing peak discharge or runoff rates, based on
Type II 24 hour rainfall distribution, is applicable to most
agricultural areas of the United States. It does not apply to parts of
the Pacific Coast states, as shown in the map in Figure B-7.
The basic peak discharge is estimated using Figures B-8, B-9, or B-10..
A peak discharge rate in ft /s per inch of runoff is read off the graphs
in these figures. This rate, multiplied by the runoff (in inches) from
3
Table B-3, results in the total peak discharge in ft /s for agricultural
watersheds.
This basic peak discharge rate is adjusted for an urban watershed, where
impervious area, channel hydraulics, slope, swampy or ponding areas, and
watershed shape affect the rate of discharge. Urban adjustment factors
for impervious areas and modification of channel hydraulics are
presented in Figures B-11 and B-12. The urban adjustment factors for
slope are presented in Table B-4.
B-13
MKTCALP * C DDV
-------
LEGEND - STORM DISTRIBUTION
TYPE IA
TYPE I
TYPE II
Figure B-7. Pacific coast rainfall storm distribution.
B-14
MCTCALP C DOV
-------
o
1000
800
600
400
300
200
100
80
g
H 60
£
e
w
M
0
40
30
20
TYPE II STORM
X^
CM
\O 00 O
O
CM
OO
r-l
-------
1000
800
600
400
300
£ 200
o
os
». 100
° 80
1 60
1-1
g 40
* 30
« 20
IM
BE
TYPE II STORM
X
X
8
o
t/3
«
<
td
10
^^
\O OO O
o
CM
o o
o -*
000
VO 00 O
o
o
CM
o o
o o
p-, oo o,
DRAINAGE AREA, ACRES
o
o
o
«N
Figure B-9. Peak rates of discharge for small watersheds
with moderate slope = 4%.
B-16
MITCALF COOV
-------
1000
800
600
400
300
£ 200
100
80
60
a-
5
w 40
PL,
30
ft
£ 20
0
|
o
CO
w
10
8
!TYPE n STORM".
CM
<"">«* \D 00 O
O
CN
OO
f»1%3-
OOO
vooOO
O
O
OO
OO
O
O
OO
OO
DRAINAGE AREA, ACRES
o
o
o
CM
Figure B-10. Peak rates of discharge for small watersheds
with steep slope
B-17
MITCiLF « C DOV
-------
PERCENT OF IMPERVIOUS AREA
_ M U1 -J C
3 ui o ui c
-
/
f
4
i
/
iv
A
'
4
X
&
^1
7
/
\S
!x*
&
f
X
/
/
/
J
*
/
/
X*
\
y
^
X
X
»
/
^
f
s
X1
1
d
/
.
'
/
X
X
0
yl
|x
X*
*^
X
>
/
X
X
x1
x*1
x1
^
X"
^1
x
X
^
^
X
41
!r
X1
X
X
^
^
eg
x*
^
,«
^ ,
/
XJ
x1
M^
c
«^
»
h
jg
ft*
^*
Tfl
>"
^*
cs
^J
&c
X
JL
^*
.rs Ji
^
^1
I.
1.0 1.2 1.4 1.6 1.
PEAK FACTOR
Figure B-ll. Peak discharge adjustment factors
for impervious area.
100
o
ii
o
C£J
M
b
S M
*§
3
Si
B§
a: J
u
a.
S3
vji
1.4
PEAK FACTOR
Figure B-12. Peak discharge adjustment factors for
modification of channel hydraulics.
B-18
METCALF 4 COOV
-------
Table B-4. PEAK DISCHARGE ADJUSTMENT
FACTORS FOR SLOPE
10 20 50 100 200 500 1.000 2,000
Slope, X acres acres acres acres acres acres acres acres
Flat slopes
Moderate slopes
Steep slopes
0.
0.
0.
0.
0.
0.
1.
1.
2.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
20
25
30
40
50
1
2
3
4
5
7
0
5
0
0.49
0.61
0.69
0.76
0.82
0.90
.00
.13
.21
0.93
.00
.04
.07
1.09
0.92
0.94
0.96
0.96
0.97
0.97
0.98
0.99
1.00
1.03
1.06
1.09
1.12
1.17
0.47
0.59
0.67
0.74
0.80
0.89
1.00
1.14
1.24
0.92
1.00
1.05
1.10
1.13
0.88
0.90
0.92
0.94
0.95
0.97
0.98
0.99
.00
.04
.08
.11
.16
1.21
0.44
0.56
0.65
0.72
0.78
0.88
.00
.14
.26
0.91
.00
.07
1.12
1.18
0.84
0.86
0.88
0.91
0.93
0.95
0.97
0.99
1.00
1.05
1.12
1.14
1.20
1.25
0.43
0.55
0.64
0.71
0.77
0.87
1.00
1.15
1.28
0.90
1.00
1.08
1.14
1.21
0.81
0.84
0.87
0.90
0.92
0.94
0.96
0.98
.00
.06
.14
.17
.24
.29
0.42
0.54
0.63
0.70
0.77
0.87
.00
.16
.29
0.90
.00
.08
.15
.22
0.80
0.83
0.86
0.89
0.91
0.94
0.96
0.98
1.00
1.07
1.15
1.20
1.29
1.34
0.41
0.53
0.62
0.69
0.76
0.87
1.00
1.17
1.30
0.90
1.00
1.08
1.16
1.23
0.78
0.82
0.85
0.88
0.90
0.93
0.96
0.98
.00
.08
.16
.22
.31
.37
0
0
0
0
0
0
1
1
1
0
1
1
1
1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
.41
.53
.62
.69
.76
.87
.00
.17
.31
.89
.00
.09
.17
.23
.78
.81
.84
.87
.90
.93
.95
.98
.00
.09
.17
.23
.33
.40
0.40
0.52
0.61
0.69
0.76
0.87
.00
.17
.31
0.89
.00
.09
.17
.24
0.77
0.81
0.84
0.87
0.90
0.92
0.95
0.98
1.00
1.10
1.19
1.24
1.35
1.43
The existence of ponds or swampy areas decreases the peak discharge.
These areas retain large amounts of surface runoff as temporary storage.
The adjustment factors are presented in Table B-5. To account for the
location of the ponds or swampy areas and their sizes, the basic peak .
discharge rate is multiplied by the appropriate factor.
B-19
METCALP * E OOV
-------
Table B-5. PEAK DISCHARGE ADJUSTMENT FACTORS
FOR PONDING OR SWAMPY AREAS
Ponding and swampy areas
occur at the design point
Ponding and swampy areas are
spread throughout the water-
shed or occur 1n central
parts of the watershed
Ponding and swampy areas are
located only In upper reaches
of the watershed
Ratio of drainage
area to ponding
and swampy area
500
200
100
SO
40
30
20
15
10
5
500
200
100
50
40
30
20
15
10
5
4
500
200
100
50
40
30
20
15
10
5
Percentage of
ponding and
swampy area
0
0
1
2
2
3
5
6
10
20
0
0
1
2
2
3
5
6
10
20
25
0
0
1
2
2
3
5
6
10
20
.2
.5
.0
.0
.5
.3
.0
.7
.0
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.0
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.0
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Storm frequency,
2
0
0
0
0
0
0
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0
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0
0
0
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0
0
0
0
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.92
.66
.80
.74
.69
.64
.59
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.48
.94
.88
.83
.78
.73
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.58
.53
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.96
.93
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.87
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.77
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5
0.
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94
87
81
75
70
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95
89
84
79
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70
66
63
59
54
51
97
94
91
88
85
83
81
79
77
75
10
0.95
0.88
0.83
0.76
0.72
0.67
0.63
0.60
0.56
0.51
0.96
0.90
0.86
0.81
0.76
0.71
0.68
0.65
0.61
0.56
0.53
0.98
0.94
0.92
0.88
0.86
0.84
0.82
0.80
0.78
0.76
25
0.96
0.90
0.85
0.79
0.75
0.71
0.67
0.64
0.60
0.55
0.97
0.91
0.87
0.83
0.78
0.74
0.72
0.69
0.65
0.60
0.57
0.98
0.95
0.93
0.90
0.88
0.86
0.84
0.82
0.80
0.78
yr
50
0.97
0.92
0.87
0.82
0.78
0.75
0.71
0.67
0.63
0.59
0.98
0.92
0.88
0.85
0.81
0.77
0.75
0.72
0.68
0.63
0.61
0.99
0.96
0.94
0.91
0.89
0.88
0.86
0.84
0.82
0.80
100
0.98
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0.89
0.86
0.82
0.78
0.75
0.71
0.68
0.64
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0.84
0.81
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0.75
0.71
0.68
0.66
0.99
0.97
0.95
0.93
0.91
0.89
0.88
0.86
0.84
0.82
The peak rates of discharge shown in Figures B-8, B-9, and B-10 are
based on a relationship between watershed hydraulic length and area:
1 = 209 a
0.6
(11)
where 1 = watershed hydraulic length
a = watershed area
B-20
MCTCALF A C DOV
-------
This equation 1s presented 1n graphical form in Figure B-13. For
watersheds that deviate from this relationship, the peak discharge 1s
modified by the following procedure:
Determine the watershed hydraulic length and read off the
"equivalent" watershed area from Figure B-13.
Determine the peak discharge rate from Figure B-8, B-9, or B-
10 for the equivalent area.
Compute the actual peak discharge rate by multiplying the
equivalent peak discharge rate by the ratio of the actual
drainage area to the equivalent drainage area.
Example. A 300 acre watershed is to be developed. The runoff CN for
the proposed development is computed to be 80. Approximately 60% of the
hydraulic length will be modified by the installation of street gutters
and storm drains to the watershed outlet. Approximately 30% of the
watershed will be impervious. The average watershed slope is estimated
to be 4%. Compute the present-condition and anticipated future-
condition peak discharge for a 50 year, 24 hour storm event with 5
inches of rainfall. The present-condition runoff CN is 75.
1. From Table B-3, the runoff for present condition 1s 2.45 in.
and for future condition is 2.89 in.
2. From Figure B-9 for moderate slope3(CN = 75), the present
condition peak discharge is 120 ft /s per inch of runoff. -The
peak discharge is then 120 x 2.45 or 294 ft /s.
3. From Figure B-9 for moderate slope (CN = 80), the future-
condition base discharge for CN = 80 is 133 ft /s per inch of
runoff. The base discharge is then 133 x 2.89 or 384 ft /s.
4. From Figure B-ll, with 30% impervious area and future runoff
CN = 80, read peak factor =1.16.
5. From Figure B-12, with 60% of the hydraulic length modified
and future-condition CN = 80, read peak factor = 1.42.
B-21
METCALF A CODV
-------
20000
10000
50000
o
ro
ro
CO
03
U.
2
a
3000
2000
1000
500
X
X
X
X
10
20 30 50 100 200 300
DRAINAGE AREA. ACRES
500
1000
2000
Figure B-13. Relationship of hydraulic length and drainage
area 1n watershed shape factor.
-------
6. The future-condition peak discharge is:
389U.16H1.42) = 633 ft3/s
7. The effect of this proposed development is to increase the
peak discharge from 294 to 633 ft /s.
REFERENCES
1. Soil Conservation Service, U.S. Department of Agriculture. Urban
Hydrology for Small Watersheds. Engineering Division. Technical
Release No. 55. January 1975.
2. National Engineering Handbook. Section 4, Chapter 9. August 1972.
B-23
MCTCALP » CDOV
-------
APPENDIX C
ASSESSMENT METHODS
The relationship between rainfall and runoff is a complex and variable
phenomenon, sensitive to many factors. For most stormwater analyses,
the planner must model the physical system, usually with some type of
mathematical model, to predict response to varying watershed conditions.
A number of mathematical stormwater simulation models are available.
They range in complexity from the very simple, involving desktop
calculation techniques, to the highly sophisticated, involving computer
simulators.
MODEL CATEGORIES
Assessment models may generally be divided into three application
categories:
1. Preliminary assessment models provide indications of the
existence, source, and nature of a stormwater pollutant
problem. Desktop computational procedures that make use of
simple equations and nomographs are usually adequate. The
data requirements for these models are minimal, consisting of
mean annual precipitation, watershed area, land use,
population density, and sewer types. The results of
preliminary assessment models are used to direct subsequent
study efforts, to screen alternatives, to assess flow and
quality data needs, and to aid in the selection of more
sophisticated models, if necessary.
2. Continuous simulation models may help describe the variation
of pollutant loadings from storm event to storm event.
Variations within a single event are not described. Computer
or combination desktop-computer methods generally are needed.
Typically, the data requirements are long-term hourly rainfall
C-l
-------
records, overflow structure characteristics, treatment rates
and storage volumes, actual overflow quantity and quality
data, and varying details of drainage area characteristics,
streamflow data, etc. Continuous simulation models are used
to assess the magnitude of water quality problems and to
evaluate alternative solutions.
3. Single event simulation models are sophisticated computer
models that can be used to describe temporal and spatial
variations in runoff quantity and quality within a single
storm event. They require extensive and detailed data
specific to the watershed, including rainfall records and
runoff hydrographs, catchment and transport system deteils,
system maintenance information, and dry-weather and combined
flow characteristics. Results obtained with these models are
used in detailed planning and in the design of facilities.
Operational models, a fourth major stormwater management model category,
are not assessment models, but provide real-time control of sewerage
systems based on telemetered rainfall and sewer system data.
The categories of assessment models are illustrated in Figure C-l. The
categories tend to blend into one another on an ascending scale of
complexity and detail. More than 30 models are available. A number of
the more commonly used assessment models, listed by category in order of
increasing complexity are presented in Table C-l. The basic
capabilities of each model are also shown. More detailed and up-to-date
descriptions of capabilities and data requirements may be obtained from
the originator of each model, since most are constantly being refined
and expanded.
C-2
-------
POLLUTANT
MASS/fi
TOTAL YEARLY >.
POLLUTANT LOAD
(COMPARABLE TO A
CONTINUOUS LOAD)
1 TEAR
) PRELIMINARY ASSESSMENT MODEL
POLLUTANT
MASS/))
ACTUAL EVENT DISTRIBUTION OF LOAD
/-AVERAGE LOAD PER EVENT. I, .
-
1 YEAR
b) CONTINUOUS SIMULATION MODEL
POLLUTANT
MASS/h
ACTUAL DISTRIBUTION OF
LOAD WITHIN EVENT (FIRST FLUSH. ETC.?
^-AVERAGE LOAD II THIN SIN6LE EVENT
AVERAGE LOAD PER EVENT
SEVERAL
MOURS
C) SINGLE EVENT SIMULATION MODEL
Figure C-l. Assessment model categories.
C-3
-------
Table C-l. CHARACTERISTICS OF ASSESSMENT MODELS,
BY LEVEL, IN ORDER OF INCREASING COMPLEXITY [1]
Catchment
hydrology
Model
Originator Acronym
Level 1
Desktop
University SUM* L*wol 1
of Florida
Company
EPA-MERL »
Level 2
Continuous
catchment Inflows
ther flow
1
Hultfpli
Dry-wea
_ v
*
«C
r several hyetograp
t.
il
£. &
A
9
rrom Impervious are
rrtm pervious areas
*. *-
^ *^
ec BC
Sewer
hydraulics
*
I
W
E
Flow routing In sewers
Upstream and downstream ft
l
Surcharging and pressure f
Diversions
Pumping stations
Storage
Dry-weather quality
Naste«ater
quality
Stormwater quality
Quality routing
Sedimentation and scour
Quality reactions
VftStewater treatment
Miscellaneous
§
* §
i X
Receiving water quality si
Receiving water flow stmul
Can choose time Interval
Design computations
Applied to real problems
Computer program available
Metcalf 1 Eddy Simplified SWW X - - X X X X-X---XXXX-XX
Metcalf t Eddy MAC XX--XX----XXXXX--XXXX-XX
Corps Of STORM --.XXX-.-X-X-X.-X XX
Engineers
Kydrocomp HSP XXXXXXXX-X-XXXX-X-XXX-'x-
Dorsch Consult OQS XXX-XXXXXXXXXXX--XXXX-X-
Level 3
Single event
MIT Resource HITCAT XXXXXXX-XX-X X X - X -
Analysis
SOGREAH CAREOAS XX--XXXXXXXX--X---XXX-.X-
Kydrocomp HSP XXXXXXXX-X-XXX--X-XXX-X-
Battelle BMW XXX-XXX.XX-XXXX.-X-.XXXX
Northwest
EPA sum xxx-xxxxxxxxxxxxxxxxxxxx
Oorsch Consult WV-QQS XXX-XXXXXXXXXXX--XXXX-X-
Uater Resources STORHSEWER XXX>XXXXXXXXXXX-X-XXX-X-
Engineers
a. Proprietary.
C-4
-------
MODEL SELECTION
A mathematical model may be a useful tool in analyzing stormwater
problems and possible solutions. The tool is most useful when carefully
-matched to the specific study needs. The important considerations are
the level of the assessment required to meet the study objectives, the
availability of the input data required, the usefulness of the model
output, and the overall cost of the model. The important criteria to be
considered when selecting a model are presented in Table C-2. The
criteria are based on a method developed by Systems Control, Inc., for
selection of water quality models [2].
In most cases, a sophisticated model should be employed only after its
use has been justified by results from a simpler one. The first step in
model selection is to perform a preliminary assessment of the problem.
The results can be used to assess whether the water quality problems are
storm-generated, or if other point and nonpoint discharges are the
source. A preliminary assessment will also help to identify the
pollutants and the time period of concern, the need for and appropriate
level of subsequent models, and the data requirements of the study.
If justified, a list of models that might be used is then assembled.
The models listed in Table C-l are a starting point. Additional models
may be identified through current publications. Adequate details of
model capabilities can be obtained from the literature so that a
preliminary screening can be performed. Models appropriate to the level
of the study and capable of simulating the pollutants and time period of
interest are given further consideration. A detailed description of the
capabilities of each of these models should be obtained from the model
originator. Each candidate model should then be evaluated according to
the criteria listed in Table C-2.
C-5
-------
Table C-2. MODEL SELECTION CRITERIA
Applicability
What features ere necessary to the analysis?
What features ire desirable?
What are the capabilities of the candidate models?
Accuracy
Whet level of accuracy is required?
What accuracy U justified by the available data?
How appropriate to the Individual situation are the representations and
assumptions of each model?
Usability
Is the available model documentation sufficient?
How usable are the output form and content?
How easily can data be changed to simulate alternative conditions?
How easily can the model be modified?
What are the capabilities of the planning staff to apply the model and
analyze results?
Cost
Model acquisition cost
Equipment requirements and costs
Data acquisition costa and time requirements
Manpower costs
C-6
-------
MODEL APPLICATION
For models beyond the preliminary assessment category, application may
be thought of as consisting of three steps: calibration, verification,
and analysis. In the calibration step, the known watershed
characteristics and hydrologic data for a selected set of storm events
are input and the unknown or uncertain model parameters adjusted so that
the model output corresponds to observed runoff and receiving water
responses. The model is verified when a significantly different set of
conditions is input, and the model again satisfactorily predicts
observed system behavior. If the model prediction for the subsequent
simulation is not satisfactory, then further calibration is necessary.
Data acquisition for calibration and verification usually represents a
major share of the cost of single-event and many continuous simulation
models. To ensure that the usefulness of model results justifies the
expenditure, consideration should be given to the parameters to be
monitored and to data collection techniques. An excellent guide to
monitoring and sampling for runoff data is Methodology for the Study of
Urban Storm Generated Pollution and Control, [3].
REFERENCES
1. U.S. Environmental Protection Agency. Urban Stormwater Management
and Technology: Update and Users Guide. EPA 600/8-77-014. 1977.
2. Grimsrud, G.T., E.J. Finnemore, and H.J. Owen. Evaluation of Water
Quality Models: A Management Guide for Planners. EPA 600/5-76-
004. 1976.
3. Envirex, Inc. Methodology for the Study of Urban Storm Generated
Pollution and Control. EPA 600/2-76-145. 1976.
C-7
-------
APPENDIX D
INFILTRATION MEASUREMENT TECHNIQUES
The infiltration rate value that is required in design of stormwater
"percolation facilities is the long-term acceptance rate of the entire
soil surface on the proposed site for the actual stormwater to be
applied. The value that can be measured is only a short-term
equilibrium acceptance rate for a number of particular areas within the
overall site. It is strongly recommended that hydraulic tests of any
type be conducted with the actual runoff whenever possible. Such
practice will provide valuable information relative to possible soil-
stormwater interactions that might create future operating problems. If
suitable runoff is not available at the site, the ionic composition of
the water used should be adjusted to correspond to that of the runoff.
Even this simple step may provide useful data on the swelling of
expansive clay minerals due to sodium exchange.
There are many potential techniques for measuring infiltration including
basin flooding, sprinkler infiltrometers, cylinder infiltrometers, and
lysimeters. The technique selected should reflect the actual method of
application being considered. For design of stormwater retention
facilities, the preferred techniques are basic flooding and cylinder
infiltrometers. The area of land and the volume of stormwater used
should be as large as practical.
Before discussing the two techniques, it should be pointed out that the
standard U.S. Public Health Service (USPHS) percolation test used for
establishing the size of septic tank drain fields [1] is not recommended
except for very small subsurface disposal fields or beds. Comparative
field studies have shown that the percolation rate from the test hole is
always significantly higher than the infiltration rate as determined
from the double-cylinder (also called double ring) infiltrometer test.
The difference between the two techniques is of course related to the
D-l
-------
much higher percentage of lateral flow experienced with the standard
percolation test. The final rates measured at four locations on a
30 acre (12 ha) site using the two techniques are compared in Table D-l.
The lower coefficient of variation (defined as the standard deviation
divided by the mean value, C = o/M for the double-cylinder technique is
especially significant. A plausible interpretation is that the
measurement technique involved in inherently more precise than the
standard percolation test.
Table D-l. COMPARISON OF INFILTRATION MEASUREMENT USING
STANDARD USPHS PERCOLATION TEST AND DOUBLE-CYLINDER INFILTROMETER8
Equilibrium infiltration
rate, in./h
Location
1
2
3
4
Mean
Standard
deviation
Coefficient
of variation
Standard USPHS
percolation test
48.0
84.0
60.0
138.0
82.5
40.0
0.48
Double-cylinder
infiltrometer
9.0
10.8
14.4
12.0
11.6
2.3
0.20
a. Using sandy soil free of clay.
FLOODING BASIN TECHNIQUES
Where pilot basins have been used for determination of infiltration, the
2 2
plots have generally ranged from 10 ft (0.9 m ) to 0.25 acre (0.1 ha).
Larger plots are provided with a border arrangement for application of
the water. If the plots are filled by hose, a canvas or burlap sack
over the end of the hose will minimize disturbance of the soil [2].
Although basin tests are desirable, and should be used whenever
possible, there probably will not arise many opportunities to do so
because of the large volumes of water needed for measurements. - A sample
basin is shown in Figure D-l. In at least one known instance, pilot
D-2
-------
';' 'i^^t^^^r^'^^-'fel^Vrt''^'^;^^'
*;?<.<«*.*; viC'v 'A- «-'*:' -- ""''-iVinii ^T
**/i^^i:.i^:«hJv:sii,-%';-',iiL.i-t-iJ^ 7JL
Figure D-l. Flooding basin used for measuring infiltration.
D-3
-------
basins of large scale (5 to 8 acres or 2 to 3.2 ha) were used to
demonstrate feasibility of wastewater percolation and then were
Incorporated Into the larger full-scale system [3].
CYLINDER INFILTROMETERS
"A useful reference on cylinder infiltrometers is Haise et al [4]. The
basic technique, as currently practiced, is to drive or jack a metal
cylinder into the soil to a depth of about 6 in. (15 cm) to prevent
lateral or divergent flow of water from the ring. The cylinder should
be 6 to 14 in. (15 to 35 cm) in diameter and approximately 10 in.
(25 cm) in length. Divergent flow is further minimized by means of a
"buffer zone" surrounding the central ring. The buffer zone is commonly
provided by another cylinder 16 to 30 in. (40 to 75 cm) in diameter
driven to a depth of 2 to 4 in. (5 to 10 cm) and kept partially full of
water during the time of infiltration measurements from the inner ring.
Alternatively, a buffer zone may be provided by diking the area around
the intake cylinder with low (3 to 4 1n. or 7.5 to 10 cm) earthen dikes.
The quantity of water that might have to be supplied to the double-
cylinder system during a test can be substantial and might be considered
a limitation of the technique. For highly permeable soil, a 1,500 gal
(5,680 L) tank truck might be needed to hold a day's water supply for a
series of tests. The basic configuration of the equipment during a test
is shown in Figure D-2.
This technique is thought to produce data that are at least
representative of the vertical component of flow. In most soils, the
infiltration rate will decrease throughout the test and approach a
steady state value asymtotically. This may require as little as 20 to
30 minutes in some soils and several hours in others. The test cannot
be terminated until the steady state is attained or else the results are
meaningless.
D-4
-------
BUFFER POND
LEVEL
GAGE INDEX
ENGINEER'S SCALE
WELDING ROD
HOOK
WATER SURFACE
INTAKE CYLINDER
GROUND LEVEL
Figure D-2. Cylinder Infiltrometer in use,
D-5
-------
The following precautions concerning the cylinder infiltrometer test are
noted.
1. If a more restrictive layer is present below the intended
plane of infiltration and this layer is close enough to the
Intended plane to interfere, the infiltration cylinders should
be embedded into this layer to ensure a conservative estimate.
2. The method of placement into the soil may be a serious
limitation. Disturbance of natural structural conditions
(shattering or compaction) may cause a large variation in
infiltration rates between replicated runs. Also the
interface between the soil and the metal cylinder may become a
seepage plane, resulting in abnormally high rates. In
cohesion!ess soils (sands and gravels), the poor bond between
the soil and the cylinder may allow seepage around the
cylinder and cause "piping." This can be observed easily and
corrected, usually by moving a short distance to a new
location and trying again. Variability of data caused by
cylinder placement can largely be overcome by leaving the
cylinders in place over an extended period during a series of
measurements [2].
Knowledge of the ratio of the total quantity of water infiltrated to the
quantity of water remaining directly beneath the cylinder is essential
if one is interested only in vertical water movements. If no correction
is made for lateral seepage, the measured infiltration rate in the
cylinder will be well in excess of the "real" rate [5]. Several
investigators have studied this problem of lateral seepage and have
offered suggestions for handling it [5, 6, 7,].
As pointed out by Van Schilfgaarde [8], measurements of hydraulic
conductivity on soil samples often show wide variations within -a
relatively small area. Hundred-fold differences are common on some
D-6
-------
sites. Assessing hydraulic capacity for a project site is especially
difficult because test plots may have adequate capacity when tested as
isolated portions, but may prove to have inadequate capacity after water
is applied to the total area for prolonged periods. Parizek has
observed that problem areas can be anticipated more readily by field
study following spring thaws or prolonged periods of heavy rainfall and
recharge [9]. Runoff, ponding, and near saturation conditions may be
observed for brief periods at sites where drainage problems are likely
to occur after extensive application begins.
Although far too few extensive tests have been made to gather meaningful
statistical data on the cylinder infiltrometer technique, one very
comprehensive study is available from which tentative conclusions can be
drawn. Burgy and Luthin reported on studies of three 40 by 90 ft (12.2
x 27.4 m) plots of Yolo silt loam characterized by the absence of
horizon development in the upper profile [10]. The plots were diked
with levees 2 ft (0.6 m) high. Each plot was flooded to a depth of
1.5 ft (0.5 m), and the time for the water to subside to a depth of
0.5 ft (0.15 m) was noted. The plots were then allowed to drain to the
approximate field capacity and a series of cylinder infiltrometer tests
357 totalwere made.
Test results from the three basins located on the same homogeneous field
were compared. In addition, test results from single-cylinder
infiltrometers with no buffer zone were compared with those from double-
cylinder infiltrometers. The inside cylinders had a 6 in. (15 cm)
diameter; the outside cylinders, where used, had a 12 in. (30 cm)
diameter.
For this particular soil, the presence of a buffer zone did not have a
significant effect on the measured rates. Consequently, all of the data
are summarized on one histogram in Figure D-3. The calculated mean of
the distribution shown is 6.2 in./h (15.7 cm/h). The standard-deviation
Is 5.1 in./h (12.9 cm/h).
D-7
1
-------
00
13
12
11
11
t
1
T
I
8
4
1
t
1
n
-
v
.
7
.
_i
Ul ^
r^
,y *: 'J
'&?, " i
-jV. ';t.-'
J -.>'
j;^-:-?^"^
< '
'
J1
\
'~'r\'- "»
:?-'.->/.
?;I--,'';{"
V' "* "'*"*'
Pii
11
n
X,
Lri ^^
2 4 8 8 10 12 14 IB IB 20 22 24 28 21 30 32 34 38 38 40 42 4449
AVERAGE INFILTRATION RATE, In./h
Figure D-3. Variability of infiltrometer test results on relatively homogeneous site.
-------
Burgy and Luthln suggest that the extreme high values, while not
erroneous, should be rejected in calculating the hydraulic capacity of
the site. Physical inspection revealed that these values were obtained
when the cylinders intersected gopher burrows or root tubes. Although
these phenomena had an effect on the infiltration rate, they should not
be included in the averaging process since they carried too much weight.
As a criterion for rejection, Burgy and Luthin suggest omitting all
values greater than three standard deviations from the mean value. They
further suggest an arbitrary selection of the mean and standard
deviation for this procedure based on one's best estimate of the
corrected values rather than the original calculations. From inspection
of the histogram, these values might be selected as about 5 in./h (12.7
cm/h) and 3.5 in./h (8.9 cm/h), respectively. Thus, all values greater
than 5.0 + 3(3.5), or 15.5 in./h (39.4 cm/h) are arbitrarily rejected:
a total of 12 of the 357 tests made (3.4%).
Because it is important to provide conservative design parameters for
this work, however, it is recommended that all values greater than two
standard deviations from the mean be rejected. For the example, this
results in the rejection of all values greater than 5.0 + 2(3.5), or 12
in./h (30.5 cm/h) from the average. A recomputation using this
criterion provides a mean of 5.1 in./h (12.5 cm/h) and a standard
deviation of 2.8 in./h (7.1 cm/h). This average value is within 16% of
the "true" mean value of 4.4 in./h (11.2 cm/h) as measured during
flooding tests of the entire plot.
The main question to be answered now is, how many individual tests must
be made to obtain an average that is within some given percent of the
true mean, say at the 90% confidence interval? The answer has been
provided by statisticians using the Student "T" distribution. Details
of the derivation are omitted here but can be found in most standard
texts on statistics.
D-9
9
-------
The results of two typical sets of computations are summarized in
Figures D-4 and D-5. The two sets of curves are for 90% and 95%
confidence intervals. The confidence interval and the desired precision
are, of course, basic choices that the engineer must make. A 90%
confidence in the measured mean, which is within 30% of the true mean,
may be sufficient for small sites where neighboring property is
"available for expansion if necessary. On the other hand, 95% confidence
that the measured value is within 10% of the true mean may be more
appropriate for larger sites or for sites where expansion will not be
easily accomplished once the project is constructed.
The coefficient of variation will have to be estimated from a few
preliminary tests because it is the main plotting parameter in these
figures. As an example, for the adjusted distribution of Burgy and
Luthin's data with a coefficient of variation estimated at 0.55, at
least 23 separate tests would be required to have 90% confidence that
the computed mean would be within 20% of the true mean value of
infiltration. Obviously, time and budget constraints must be considered
in making the confidence and accuracy determinations; 3 to 4 man-days of
work might be required to make 23 cylinder infiltrometer tests.
REFERENCES
1. Manual of Septic-Tank Practice. U.S. Public Health Service.
Publication No. 526. U.S. Gov't Printing Office, 1969.
2. Parr, J.F. and A.R. Bertrand. Water Infiltration Into Soils. In:
Advances in Agronomy. Norman, A.G. (ed.). New York, Academic
Press. 1960. pp 311-363.
3. Wallace, A.T., et al. Rapid Infiltration Disposal of Kraft Mill
Eflluent. In: Proceedings of the 30th Industrial Waste
Conference, Purdue University, Ind. 1975.
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4. Haise, H.R., et al. The Use of Cylinder Infiltrometers to
Determine the Intake Characteristics of Irrigated Soils. U.S.
Dept. of Agriculture, Agricultural Research Service. Publication
No. 41-7. 1956.
5. Hills, R.C. Lateral Flow Under Cylinder Infiltrometers: A
Graphical Correction Procedure. Journal of Hydrology. 13:153-162,
1971.
6. Swartzendruber, D. and T.C. Olson. Model Study of the Double-Ring
Infiltrometer as Affected by Depth of Wetting and Particle Size.
Soil Science. 92:219-225, April 1961.
7. Bouwer, H. Unsaturated Flow in Ground-water Hydraulics. In:
Proceedings of the American Society of Civil Engineers. 90:HY5:121-
141, 1964.
8. Van Schilfgaarde, J. Theory of Flow to Drains. In: Advances in
Hydroscience. Chow, V.T. (ed.). New York, Academic Press. 1970.
pp 43-103.
9. Parizek, R.R. Site Selection Criteria for Wastewater Disposal -
Soils and Hydrogeologic Considerations. In: Recycling Treated
Municipal Wastewater and Sludge Through Forest and Cropland.
Sopper, W.E. and L.T. Kardos (eds). Pennsylvania State University
Press. 1973. pp 95-147.
10. Burgy, R.H. and J.N. Luthin. A Test of the Single- and Double-Ring
Types of Infiltrometers. Trans. Amer. Geophysical Union. 37:189-
191, 1956.
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100
eo.
o 60
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c
- 40
o
M
£ 20
*>
in
»-
S 10
I
IM
2 6
105
20*
30*
1.1 9.1 t.3 t.4 fl.5 0.6
COEFFICIENT OF VARIATION
0.7
0.6
Figure D-4. Number of tests required for 90%
confidence that the calculated mean is
within stated percent of the true mean.
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1.1
1.2
«.J 1.4 t.B 1.1
COEFFICIENT IF VAIIATIIN
0.8
Figure D-5. Number of tests required for 95%
confidence that the calculated mean is
within stated percent of the true mean.
D-13
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