EPA-600/2-77-051
May 1977
Environmental Protection Technology Series
. Environmental Research laboratory
•'• -""-" -•-••'
.;...-.
-45268 '•"'"''"
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application;of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in relatedifields.
The nine series are: .';
1. Environmental Health Effects Research
2. Environmental Protection Technology I
3. Ecological Research
4. Environmental Monitoring f
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development (
8. "Special" Reports
9. Miscellaneous Reports ;
This report has been assigned to the ENVIRONMENTAL PROTECTION ;TECH-
NOLOGY series. This series describes research performed to develop anB dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards. ;
5|T
it
This Qocument is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-051
May 1977
CATCHBASIN TECHNOLOGY OVERVIEW AND ASSESSMENT
by
John A. Lager, William G. Smith, and George Tchobanoglous
Metcalf & Eddy, Inc., Palo Alto, California 94303
in association with
Hydro-Research-Science, Santa Clara, California 95050
Contract No. 68-03-0274
Project Officer
Anthony N. Tafuri
Storm and Combined Sewer Section
Wastewater Research Division
Municipal Environmental Research Laboratory (Cincinnati)
Edison, New Jersey 08817
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental
Research Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the J.S.
Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation
for use. ;
11
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FOREWORD
The Environmental Protection Agency was created because of
increasing public and government concern about the dangers of
pollution to the health and welfare of the American people.
Noxious air, foul water, and spoiled land are tragic testimony to
the deterioration of our natural environment. The complexity of
that environment and the interplay between its components require
a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its
impact, and searching for solutions. The Municipal Environmental
Research Laboratory develops new and improved technology and
systems for the prevention, treatment, and management of
wastewater and solid and hazardous waste pollutant discharges
from municipal and community sources, for the preservation and
treatment of public drinking water supplies and to minimize the
adverse economic, social, health, and aesthetic effects of
pollution. This publication is one of the products of that
research, a most vital communications link between the researcher
and the user community.
The deleterious effects of storm sewer discharges and combined
sewer overflows upon the nation's waterways have become of
increasing concern in recent times. Efforts to alleviate the
problem depend in part upon the development of improved flow
attenuation and treatment devices.
This report describes the overview and assessment of current
catchbasin technology, the performance of catchbasin hydraulic
modeling analyses, an economic evaluation of alternative storm
and combined sewer designs, recent developments and continuing
program needs, and a recommended catchbasin design configuration.
Francis T. Mayo
Director
Municipal Environmental Research Laboratory
iii
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ABSTRACT
An overview and assessment of current catchbasin technology_has
been prepared to provide engineers and municipal managers with
technical and economic information on catchbasins and some
alternatives so that they can make intelligent, informed
decisions on runoff collection systems in light of pollution
control legislation, the municipality's financial status, and its
particular stormwater runoff characteristics.
Various catchbasin configurations and sizes were evaluated^for
hydraulic and pollutant removal efficiencies using hydraulic
modeling analyses.
Detailed study findings are presented in sections dealing with
(1) a state-of-the-art review, (2) a review of variables
affecting catchbasin efficiency, (3) hydraulic modeling analyses,
(4) an assessment of the role of catchbasins, (5) an economic
evaluation of alternative storm and combined sewer designs, and
(6) a review of recent developments and continuing program needs.
Detailed example problems of the evaluation of catchbasin
performance and economics are included.
A recommended catchbasin design configuration based upon
hydraulic performance and sediment capture efficiency is
presented.
This report was submitted in partial fulfillment of Contract
No. 68-03-0274 by Metcalf & Eddy, Inc., under the sponsorship of
the U.S. Environmental Protection Agency. This report covers the
period June 4, 1973 to November 30, 1976. Work was completed as
of December 1976.
IV
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CONTENTS '
Foreword ill
Abstract iv
Figures . vii
Tables ix
Abbreviations and Symbols ........ xi
Acknowledgment . xiii
1. Introduction „ 1
Purpose of Study „ 1
Report Format , 1
Data and Information Sources. 2
2. Conclusions . 3
State-of-the-Art „ 3
Hydraulic Modeling Analysis.. 4
Assessment 4
Economics ....» 6
3. Recommendations 7
4. State-of-the-Art Review „ 8
Background 8
Catchbasin Design „ 9
Operation and Maintenance..., 15
5. Review of Variables Affecting
Catchbasin Efficiency 27
Catchbasin Hydrology 27
Catchbasin Hydraulics „ 30
Pollutant Characteristics.... 33
Solids Washoff „ 39
6. Hydraulic Modeling Analyses.... 42
Objectives „ 42
Experimental Setup „ 43
Execution . 43
Results 49
Conclusions 70
7. Assessment 71
User Experience and Attitudes 71
Catchbasin Performance 72
Review of Alternatives 85
Assessment 88
8. Economic Evaluation 93
Economic Criteria 93
Cost Data and Information.... 94
Economic Analysis of Alternatives 96
Discussion 105
v
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Recent Developments and
Continuing Program Needs 107
Recent Developments 107
Case History Ill
Continuing Program Needs ........ 112
References and Bibliography.
Appendices
114
A. Glossary 123
B. Analysis of Catchbasin Survey Data...;...... 125
C. Foreign Language Bibliography 127
Conversion Factors,
128
VI
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FIGURES
Number
Paqe
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Representative catchbasin designs in
United States and Canada
Representative catchbasin designs in Europe. .......
Manual cleaning
Eduction cleaning
Vacuum cleaning
Typical old street cross-section
Comparison of inlets: intake capacity at 95%
capture of gutter flow: Manning's n = 0.013;
cross slope = 0.0417 ft/ft
Experimental setup
Model catchbasin.
Model components prior to assembly
Sieve analyses of test simulants
Typical discharge rating curve
General performance classifications
Photographic record - configuration 7
Photographic record - configuration 9
Photographic record - configuration 11
Photographic record - configuration 6
Influence of modifications on marginal basin
Recommended design
vii
12
14
20
21
22
29
32
44
45
46
50
51
53
54
56
57
58
59
60
63
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21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Discharge rating curve for recommended design
Photographic record - recommended basin
Sediment capture versus discharge
Photographic record - sediment capture
versus discharge .
Sediment capture versus accumulation
Sediment capture versus accumulated depth.
Photographic record - sediment accumulation
User experience and attitudes , 1973 „
Year use of catchbasins stopped
Relationship of flow into catchbasin and
Comparison of catchbasin and grit
chamber performance
Model basin performance versus flow
Prototype performance versus time
Conversion of catchbasin tp inlet detail...
Catchbasin cost versus storage capactiy
Shock flow reduction concept
63
64
66
57
68
68
69
73
74
75
78
79
80
86
95
109
Vlll
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TABLES
Number
Paqe
1 Catchbasin and Inlet Construction Standards, 1928 10
2 Catchbasin Volumes, 1973 „ n
3 Areal Distribution of Catchbasins, 1973 „ 15
4 Number of Cities Using Mechanical Means for
Cleaning Catchbasins, 1973 15
5 Representative Equipment Used 17
6 Catchbasin Debris Disposal Methods 23
7 Frequency of Catchbasin Cleaning in
Various North American Cities 25
8 Typical 5-Minute Rainfall Intensities 27
9 Typical Maximum Gutter Flows on Older City Streets 28
10 Typical Tributary Paved Areas to Catchbasins 30
11 Particle Size Distribution of Solids,
Selected City Composites 35
12 Street Solids Loading by Land Use. „ 35
13 Street Surface Simulant „ 35
14 Contaminant Characteristics and Quantity Summary 37
15 Fraction of Pollutant Associated with Each
Particle Size Range, Percent by Weight...
38
16 Analysis of Catchbasin Contents, City of
San Francisco, 1970
39
17 Representative Data on Sediment Yield 40
18 Model to Prototype Relationships 47
IX
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19 Principal Variables Tested.
48
20 Summary of Phase 1 -Program . 55
21 Percent Sediment Retained in Basin Versus Discharge.
65
22 Sediment Accumulation.
b'o
23 Aggregate Capture Efficiencies at Breakthrough.
70
24 Catchbasin Usage in Large U.S. Cities 71
25 Assumed Relationships Between Dry State
and Wet State Characteristics - 77
26 Comparison of Removals in Model and
Prototype Tests.
27 Test of "Dirty" Catchbasins,
28 Cost Data for Catchbasins and Inlets.
80
81
94
29 Catchbasin Cleaning Costs 95
30 Representative Sewer Cleaning Costs.
96
B-l Summary Data on Area Per Catchbasin for
Cities in the United States 126
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LIST OF ABBREVIATIONS AND SYMBOLS
ORGANIZATIONS
APWA
EPA
ABBREVIATIONS
American Public Works Association
Environmental Protection Agency
avg
BOD5
cfs
cm
cm/h
cm/s
COD
curb mi
d
diam
ENRCC
ft
ft2
ft3
ft/s
g
gal
gal/d
h
ha
in.
in./h
kg
kg/curb km
kg/km2.yr
km
km2
L
Ib
Ib/curb mi
lin ft
L/s
m
ni3
average
biochemical oxygen demand (5-day)
cubic feet per second
centimetre(s)
centimetres per hour
centimetres per second
chemical oxygen demand
curb mile
day(s)
diameter
Engineering News Record Construction Cost Index
foot (feet)
square foot (feet)
cubic foot (feet)
feet per second
gram(s)
gallon(s)
gallons per day
hour(s)
hectare
inch(es)
inches per hour
kilogram(s)
kilograms per curb kilometre
kilograms per square kilometre per year
kilometre(s)
square kilometre(s)
litre
pound(s)
pounds per curb mile
linear foot (feet)
litres per second
metre(s)
cubic metre
XI
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Mgal
Mgal/d
mg/L
mi
mi 2
min
mm
RRL
S.G.
tons/mi2-yr
Ydo
yd2
yd3
SYMBOLS
Dl
D2
Hi
H2
H3
HW
Mg
ffg
V
%
o
#
million gallon(s)
million gallons per day
milligrams per litre
mile(s)
square mile
minute
millimetre(s)
Road Research Laboratory
specific gravity
tons per square mile per year
yard(s)
square yard
cubic yard
barrel diameter
outlet pipe diameter
barrel height
barrel storage height
height from crown of outlet pipe to top of
inlet grating
discharge head (headwater) above invert
under discharge Q
geometric mean
standard deviation
micron
percent
degree
number
greater than
less than
greater than or equal to
equal to or less than
per
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ACKNOWLEDGMENTS
The successful completion of this study was dependent on the
cooperation and assistance of a number of individuals and
organizations. We are indebted particularly to Richard Field,
Chief of the Storm and Combined Sewer Section, Municipal Environ-
mental Research Laboratory—Cincinnati, EPA, Edison, New Jersey,
and Anthony N. Tafuri, Project Officer, for their guidance and
review of the work.
The hydraulic modeling was conducted under subcontract by Hydro-
Research-Science, Santa Clara, California, under the direction
of Dr. Alexander B. Rudavsky who also provided valuable consul-
tation during the course of the work. Section 6 of this report,
including the. figures and tables, is based upon a report pre-
pared by Dr. Rudavsky describing and providing an interpretation
of the hydraulic modeling and analyses.
The excellent case history and "real world" experience was
contributed by Harold C. Coffee, Jr., Senior Engineer, Division
of Sanitary Engineering, City and County of San Francisco,
Department of Public Works, and the several cooperating staffs
of the Bay Area communities.
This project was conducted under the supervision and direction
of John A. Lager, Project Director, and William G. Smith,
Project Manager. Portions of the report were.written by Michael
K. Mullis and George B. Otte. Project assistance, report
review, and example problem development were provided by Dr.
George Tchobanoglous, Professor, University of California at
Davis. Marcella S. Tennant served as technical editor.
Xlll
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SECTION 1
INTRODUCTION
Control of stormwater runoff is a problem of increasing
importance in the field of water quality management. Over the
past 70 years, there has been some extensive use of catchbasins
for coarse material removal from stormwater runoff. Yet
catchbasin effectiveness and economics have never been evaluated
in depth, even though ,jthe installation, operation, and
maintenance costs of a catchbasin system may be extremely high
relative to alternative methods. An informed decision cannot be
made about the need for a catchbasin system by a municipal
manager without the aid of such information.
PURPOSE OF STUDY
The purpose of this study is to provide municipal managers with
technical and economic information on catchbasins and some
alternatives so that they can make intelligent, informed
decisions on runoff collection systems in light of pollution
control legislation, the municipality's financial status, and
its particular stormwater runoff characteristics. An additional
purpose is to evaluate possible new devices to replace
catchbasins and to identify the need for additional study of
catchbasin performance.
REPORT FORMAT
In evaluating the use of catchbasins, consideration was given to
their hydraulic characteristics; performance with respect to the
removal of pollutants, street cleaning frequency, and catchbasin
cleaning programs; comparison with inlets and other
alternatives; and costs.
The detailed findings derived from this study are presented in
the six sections that follow, which deal with a state-of-the-art
review (Section 4); a review of variables affecting catchbasin
efficiency (Section 5); hydraulic modeling analyses (Section 6);
an assessment of the role of catchbasins (Section 7); an
economic evaluation of alternative storm and combined sewer
designs (Section 8); and a review of recent developments and
continuing program needs (Section 9). A glossary of terms is
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presented in Appendix A. The analysis of catchbasin survey data
reviewed during this study is presented in Appendix B.
DATA AND INFORMATION SOURCES
The data and information for this study were derived principally
from five sources: (1) a 1973 municipal survey of catchbasin
maintenance practices conducted by the American Public Works
Association (APWA) [102]; (2) an Environmental Protection Agency
(EPA) report, "Water Pollution Aspects of Street Surface
Contaminants," November 1972, by URS Research Company, San
Mateo, California [66]; (3) a comprehensive literature review of
both United States and European practice; (4) a series of
hydraulic modeling runs conducted specifically for this study by
Dr. A. B. Rudavsky, Hydro-Research-Science Company , (HRS), Santa
Clara, California; and (5) intetviews with selected
municipalities, equipment suppliers, and contractors.
Cited reports, studies, and other pertinent literature are
listed at the end of this report, following Section 9. Where
reference is made to this information in the text, the;
appropriate numbers are enclosed in brackets. A brief foreign
language bibliography is presented in Appendix C.
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SECTION 2
CONCLUSIONS
Conclusions derived from this investigation are as follows:
STATE-OF-THF-ART
1. Historically, the purpose of catchbasins was to
prevent sewer clogging by trapping coarse debris and
to prevent emanantion of odors from the sewer by
providing a water seal. The retention of solids is
achieved by providing a combination settling basin-
sump below the catchbasin outlet. A universal
standard design for catchbasins has not been
developed.
2. In U.S. regions with heavy winter snowfall, the area
drained by a single catchbasin generally is between
0.6 and 1.3 ha (1.55 and 3.75 acres). For all regions
in the United States, the typical drainage area varies
from 0.85 to 2.05 ha (2.15 to 5.05 acres).
3. There are four categories of catchbasin cleaning
methods: manual, eductor, bucket, and vacuum. Less
than 45 percent of the U.S. cities presently use
mechanical cleaning methods while more than 60 percent
of Canadian cities do.
4. Many of the problems presently associated with
catchbasins—blockage, odors, pollution source—are
directly related to inadequacies of the cleaning
program.
5. The required catchbasin cleaning frequency is a
function of several local parameters, such as sump
capacity, quantity of accumulated street solids,
antecedent dry period, meteorological conditions,
street cleaning methods and practices, surrounding
land use, topography, and the erodability of the soils
subject to washoff. While many of these factors are
subject to controls to optimize catchbasin system
efficiency, all too often the cleaning of catchbasins
is given a low priority until a major interruption in
service occurs.
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6. The median catchbasin cleaning frequency, as taken
from comprehensive national survey responses, was
reported as once per year in 1973 and twice per year
in 1956. Without comparing on a city-by-city basis
(especially the change, if any, in both street and
catchbasin cleaning practices), it appears that the
cleaning frequency has decreased on a nationwide
basis, even though the need, with rare exception, has
not abated.
HYDRAULIC MODELING ANALYSIS
1. Properly designed and maintained catchbasins can be
very efficient in removing medium to very coarse sands
(>1.0 mm diameter) from stormwater runoff. Further,
the removals remain high (65 to 90 percent) over a
wide range of flows and reduce to approximately 35
percent at maximum design inflow.
2. Removal efficiencies, as expected, are very sensitive
to particle size and specific gravity. Under the test
conditions examined, the removal of fine sands (0.25-
0.125 mm diameter) ranged from fair to poor with
increasing flow. Removals of very fine sand
(<0.125 mm diameter, S.G. of 2.65) and low specific
gravity material (gilsonite, S.G. of 1.06) were
negligible at 40 percent of maximum flow.
3. Storage basin depth is the primary control for
performance; efficiencies improve with increasing
depth.
4. The accumulation of sediment in catchbasins does not
appear to impair solids removal efficiencies until 40
to 50 percent of the storage depth is filled. Beyond
this depth, removals drop rapidly, even to the point
of negative values (washout exceeds sedimentation).
5. Of the standard modifications tested, hoods or traps
were found to increase the discharge head requirements
significantly. In the higher flow ranges, increased
scour currents were observed as the flow was diverted
downward by the obstruction of the outlet. By
comparison, curb openings or protrusions had
negligible effect.
ASSESSMENT
1. On the basis of a recent survey, there are
approximately 900,000 catchbasins in the United States
in cities with populations exceeding 100,;000 and an
estimated 850,000 additional catchbasins in sewered
areas of smaller communities.
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The practice of using catchbasins rather than inlets
in new construction continues to be strong (4:1);
however, there is a growing trend by the minority to
move positively away from using catchbasins.
Existing catchbasins exhibit mixed performance with
respect to pollution control. The trapped liquid
purged from catchbasins to the sewers during each
storm generally has a high pollution content that
contributes to the intensification of first-flush
loadings. Countering this negative impact is the
removal of pollutants associated with the solids
retained in, and subsequently cleaned from, the basin.
The collection of conclusive field data is hindered by
the prevailing poor conditions found in most basins
resulting from underfinanced and poorly monitored
cleaning programs.
Approximately 95 to 98 percent of the BOD5 load in the
liquid contained in a catchbasin prior to a storm will
be displaced to the sewer by a rainfall of as little
as 0.05 cm/h (0.02 in./h) lasting 4 hours. This is
approximately equivalent to the waste discharged by
one person in one day.
On an annual basis, the amount of material that would
be retained in a catchbasin is given in the following
tabulation:
Percentage of material retained in
catchbasin for individual storm
Probable % retained
Constituent
Total solids
Volatile solids
BOD5
COD
Kjeldahl. nitrogen
Nitrates
Phosphates
Total heavy metals
Total pestioideu
Worst
42, 1
15.2
15.5
7.5
.14.6
9.5
2.3
37.4
13,6
Best
75.0
25.5
26.6
14.1
27.4'
17.1
. 6.0
64.4
29.7
From a pollution abatement standpoint, the benefits of
catchbasins appear limited at this preliminary level
of analysis. For example, the net removal of BOD,-
from a well-designed and maintained system of
catchbasins based on conformance to observed data is
expected to be in the range of 5 to 10 percent of the
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applied load. A potential exception may be the
removal of heavy metals which, as tabulated above,
could be significant.
8. Catchbasin cleaning frequency should be adjusted to
limit the sediment buildup to 40 to 50 percent of the
sump capacity.
9. Decisions on the use or nonuse of catchbasins must be
viewed and implemented with a total system perspective.
The effectiveness of solids control and removal
practices impact each downstream element until final
removal of discharge is attained.
10. The principal alternatives to the use of catchbasins
involve replacement with inlets, sewer cleaning,
street cleaning, and the use of flow-attenuation
devices and off-line storage.
11. Catchbasins should be used only where there is a
solids transporting deficiency in the downstream
sewers or at specific sites where surface solids are
unusually abundant.
12. The advantages of converting existing catchbasins to
inlets, where solids transport is not a problem,
include (1) a probable reduction of the first-flush
pollutant load, (2) a reduction in required level of
maintenance, and (3) the opportunity to reallocate the
conserved labor.
ECONOMICS
The cost and required frequency of cleaning existing
sewers is the dominating economic consideration with
respect to converting catchbasins to inlets,,
On the basis of annual cost, it is generally more
economical to install inlets rather than catchbasins
in new developments in which separate storm drains are
to be used, provided that the required sewer cleaning
frequency when inlets are used is not less than one-
half that when catchbasins are used.
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SECTION 3
RECOMMENDATIONS
Recommendations derived from this investigation are as follows:
1. Studies should be undertaken to determine the impact
of best management practices in reducing solids and
other pollutant loads in surface runoff that must be
collected from urban areas and introduced to the sewer
through catchbasins.
2. Studies should be performed to evaluate the
effectiveness, through field scale demonstration, of
closely monitored patghbasin cleaning programs with
respect to impacts of cleaning frequency and
techniques on solids carryover, general pollution
abatement, and associated costs.
3. Studies should be conducted to determine the magnitude
of the problem of solids deposition within real sewer
systems and the extent to which this problem is
mitigated by properly designed and functioning
catchbagins. It should also be determined whether or
not the prime source of the deposited materials is the
surface, runoff introduced through catchbasins.
4. The cost effectiveness of converting catchbasins to
inlets should be evaluated in a major prototype0
demonstration study.
5. The field demonstration studies recommended above
shquld be carried out in a minimum of three to five
regionally representative 'urban areas. Regions recom-
mended are northeast, midwest, southern, and western
because of their differences in climate, hydrology,
and system characteristics. Selected catchments "should
range from 40 to 405 ha (100 to 1,000 acres).
6. Municipalities should keep systematic records of solids
buildup experience (rate and location) and removal '
costs in both catchbasins and sewers. Long-term docu-
mentation of the behavior of the real system is the
mo'st valuable input to cost-effective decision making.
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SECTION 4
STATE-OF-THE-ART REVIEW
In this state-of-the-art review of catchbasins, information is
presented on their historical development and function, design
(both American and European practices), and operation and
maintenance practices.
BACKGROUND
Definition
A catchbasin is defined as a chamber or well, usually built at
the curbline of a street, for the admission of surface water to a
sewer or subdrain, having at its base a sediment sump designed to
retain grit and detritus below the point of overflow. Because
some communities call any device that receives stormwater a
catchbasin, the distinction is made between those devices that
intentionally trap sediment and those that do not. In this
report, the device that traps sediment is called a ;catchbasin and
the device that does not is called an inlet.
The entrance to either the catchbasin or the inlet is through a
grate and/or a curb opening; or, in the case of a catchbasin not
connected directly to the street but supplied from one or more
inlets, the entrance is through an inlet pipe.
History and Function
Stormwater runoff in urban areas normally flows for a short
period of time in the gutter and is diverted by an inlet
structure leading to an underground conduit or open channel for
transportation to a receiving body of water. The underground
conduit, either a storm sewer or combined sewer, may be protected
from clogging by catchbasins built in conjunction with the
inlets.
Historically, the purpose of catchbasins was to prevent sewer
clogging by trapping coarse debris and to prevent odor emanations
from the sewers by providing a water seal. The prevention of
sewer clogging was especially important prior to the existence of
good quality street pavements. In areas where streets were
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partially or wholly unpaved, significant quantities of stone,
sand, manure, and other materials were washed into the sewer
system during periods of rainfall. Also, during the earlier
years of sewer construction, little attempt was made to maintain
self-cleaning velocities in sewers of at least 61 cm/s (2 ft/s)
[42].
The usefulness of catchbasins was considered marginal as far back
as 1900 [33]. Most modern texts generally agree and only provide
short disclaimers regarding the value of catchbasins, except
where deposition of large amounts of grit is expected in the
sewer without them [15, 87].
Despite the purported reduced need for catchbasins, they are
still used widely in many jurisdictions in many parts of the
country [102]. Thus, it appears that the contijpiued use of
catchbasins is a matter of custom rather than a well-defined
technical requirement.
Little investigation has been conducted on the hydraulic
characteristics of flow within a catchbasin. The University of
Illinois conducted some investigations and concluded that
catchbasins are hydraulically inefficient [41] . in another study
of hydraulic characteristics by the APWA, it was found that for
all practical purposes complete mixing occurs within a catchbasin
[42]. This seems to fit in with the University of Illinois
studies which indicated that a catchbasin is a poor sedimentation
device because of its tendency to resuspend the solids in the
sludge deposits even at moderate inflow rates.
Attempts_by the University of Illinois group to improve settling
by baffling showed additional adverse effects. However, a more
recent study of catchbasins with large antecedent debris contents
indicated that only about 1 percent of the antecedent content
washed out [66]. It was concluded, however, that the material
flushed out as the initial slug would have a substantial
pollutional impact on the receiving waters. In the APWA study,
it was also concluded that catchbasins may be one of the most
important single sources of pollution from stormwater flows [42].
All of the studies concluded that catchbasins cannot efficiently
satisfy the competing objectives of good hydraulic
characteristics and solids retention.
CATCHBASIN DESIGN
Catchbasins serve two main purposes: to prevent sewer gases from
escaping through the inlet gratings and to prevent solid matter
from the street from entering the sewers. The trapping of sewer
gases is accomplished by water seals of different types. The
retention of solids is achieved by providing a sump or settling
basin in which the heavy solids settle to the bottom while the
light solids float on top. The water drains to the sewers
-------
through the inlet of a trap, which is generally a few inches
below the water surface. These basins are normally built under
ths inlet gratings or openings, either under the gutter or just
back of the curb. Occasionally, one catchbasin will serve two or
more standard inlets.
American Practice
In American practice, a standard catchbasin appears to be
nonexistent. There is some uniformity attempted within
individual cities which shows varying degrees of success. The
best source of catchbasin geometry yet located appeared in the
July 1928 issue of The American City [51]. Ninety-six American
cities in 28 states and the District of Columbia, and 4 cities in
Canada, provided data on catchbasin and inlet geometry, including
the number of units in use, average size, outlet location, and
storage capacity below the outlet. The data reflected a very
wide range overall, as shown in Table 1.
TABLE 1. CATCHBASIN AND INLET
CONSTRUCTION STANDARDS, 1928 [51]
Inlets
Catchbasin'
Equivalent diameter,
cm (in.)
Average
Range
Depth, cm (in.)
Average
Range
Outlet location above
bottom, cm (in.)
Average
Range
Storage capacity,
m3 (yd3)
Average
Range
76.2 (30)
15.2-137.2 (6-54)
115.6 (45.5)
40.6-160 (16-63)
121.9 (48) 182.9 (72)
45.7-182.9 (18-72) 91.4-304.8 (36-120)
99 (39)
45.7-213.4 (18-84)
1.11 (1.45)
0.21-3.8 ('0.28-5.0)
a. Median values: diameter, 121.9 cm (48 in.); depth, 198.1 cm
(78 in.); outlet above bottom, 76.2 cm (30 in.); capacity,
0.89 m3 (1.16 yd3).
From data derived in the 1973 APWA survey [102], it is
interesting to note that the storage capacity given in Table 2 is
essentially equal to that given in Table 1.
10
-------
TABLE 2. CATCHBASIN VOLUMES, 1973 [102]
Value
No. of cities in samplea
No. of catchbasins
Catchbasin storage
capacity, m3 (yd3)
Range
Average
Median
43
270,950
0.08-2.21 (0.11-2.96)
1.07 (1.44)
0.97 (1.30)
a. Random cities using catchbasins exclusively.
In American practice, the effectiveness of the water seal gas
trap is directly proportional to the antecedent dry period and
the corresponding evaporation rate. In addition, organics in the
catchbasin itself will decompose with time and contribute odors
similar to sewer gas even if the water seal has not evaporated
In climates supporting their existence, mosguitos will use the
trapped water as a breeding ground, creating an additional
nuisance.
American design standards for the interconnecting pipe between
the catchbasin and the combined or storm sewer are a minimum
flowing-full velocity of 91.4 cm/s (3 ft/s) and usually a maximum
surcharge in the catchbasin corresponding to a water surface of
30.5 to 45.7 cm (1 to 1.5 ft) below the top of the gutter curb.
Usually, the pipe is 30.5 to 38.1 cm (12 to 15 in.) in diameter,
depending on topography and design flow requirements.
Representative catchbasin designs in America are shown in Figure 1
European Practice
In Europe, where catchbasins are used, their sizes vary, except
ift Germany where they have been standardized. TWO types of
catchbasins are used: a simple depository type and another type
generally called a "selective" catchbasin in which a bucket sieve
or some other means is used to select and separate various solid
materials. The latter type varies greatly in different countries
and various cities. The,buckets provide an easy and rapid method
for cleaning by street cr'ews.
European catchbasins tend to be smaller in size, reflecting
closer spacing, i.e., smaller drainage areas per unit. Most
European cities are located on a relatively flat terrain with
long-duration, low-intensity, high-frequency rain patterns, and
11
-------
NEW YORK
SAN FRANCISCO
£k
in.
-2 ft - 0 IB.
ATLANTA
TORONTO
Figure 1. Representative catchbasin designs
in United States and Canada.
12
-------
most catchbasins do not include gas traps because of the frequent
flushing by storm runoff. Catchbasins are usually circular in
shape. A perforated removable bucket is used to catch large
objects, and the runoff flow is relied on to carry the smaller
material on into the sewer.
A grill is generally used for a top cover for every catchbasin
and various configurations are prevalent. Vertical openings in
the curb are used, but horizontal gratings are also prevalent.
At the present time, the recommendation is to use a "New York-
type" grating, where the grating is horizontal and overflow
openings are mounted into the curb. The vertical grating and the
vertical openings have the disadvantage of not being able to
catch the flow on steep sloping streets. On the other hand, the
horizontal grating very often gets obstructed by large pieces of
trash.
There is a definite tendency in most of the textbooks in Europe
to discount, or not recommend, depository type catchbasins,
because the material that accumulates with the water is subject
to fermentation odors and other problems of stagnation. When
depository type basins are required, a siphon modification, in
which a separation baffle is installed, is often used. The solid
material is left in one compartment, and the flow is basically
drained through the siphon and underneath the baffle.
In general, catchbasins are not used in Europe when a steep pipe
gradient is provided, but they are used where extremely flat*"
gradients prevail. Cleaning is generally accomplished by pumping
based on a suction arrangement. The most prevalent volumetric
capacity for a catchbasin sump is approximately 1.5 m^(2 yd3).'
Catchbasins that are representative of European practice are
shown in Figure 2.
Catchbasin Placement
Generally, the same spacing is used for catchbasins and inlets,
which means that wherever an inlet is placed, it could be
replaced with a catchbasin or the reverse. Frequently, it is
more economical to use fewer catchbasins and to"let one or more
inlets connect to a single catchbasin. Typical data on the areal
distribution of catchbasins in the United States are presented in
Table 3.
In the design of a storm drainage system, the designer initially
positions catchbasins or inlets at street intersections to
intercept the pavement runoff before it spreads across the street
and at the low points of vertical curves/ After the initial
positioning, the drainage system is designed and the spacing of
intermediate catchbasins is determined. The general procedure
for intermediate spacing is as follows; (1) establish the
allowable spread of water onto the roadway for the design storm;
13
-------
1 It - B In.-
DIN STANDARD
(GERMANY)
1 ft - 4 in.-'
I
REMOVABLE CAST-
IRON PERFORATED
BUCKET, TYPICAL
STRASBOURG
•1 ft - 6 In.
DIN STANDARD
(GERMANY)
1. ft » 8 in.
PARIS
Figure 2. Representative catchbasin
designs in Europe.
14
-------
TABLE 3. AREAL DISTRIBUTION
OF CATCHBASINS, 1973 [102]
Statistical measurea
ha (acres)/catohbasin
States with heavy
winter snowfall
Geometric mean (Mg) 0.63 (1.56)
Standard deviation (crg) 0.69 (1.71)
All states
Geometric mean 0.88 (2.17)
Standard deviation 1.17 (2.88)
a. See Appendix B.
(2) using the design storm and one of several methods for
computing runoff, such as the Rational Formula or the RRL Method,
establish the maximum spacing based on the allowable spread of
water, gutter and street cross-section, roughness of gutter
surface, and longitudinal slope; (3) using the flow calculated
for the maximum spacing, determine whether or not the inlet grate
and/or curb opening inlet is capable of intercepting this flow.
If it will not intercept this design flow, reduce the spacing of
the next inlet so that the amount of flow passing the first inlet
plus the flow accumulated between the first and second inlet does
not exceed the allowable gutter flow requirement.
OPERATION AND MAINTENANCE
Because the primary purpose of a catchbasin is to trap solids
that would otherwise enter the sewer and form deposits that, would
cause stoppages or otherwise impede the flow, it'is obvious that
the material that has been trapped must be removed if the
catchbasin is to perform in its designed manner. Gratings,
openings, traps, and outlets must also be kept free so that they
will not interfere with, or prevent, the flow of stormwater.
When catchbasins or inlets become clogged, stormwater backs up
and spreads over the pavement and adjacent areas, and serious"
property damage frequently results. The expenses and hazards
involved in cleaning clogged catchbasins during storm conditions
make a regular cleaning program an attractive alternative.
The following information is based primarily on the APWA text.
Street Cleaning Practice," published in 1959 [9].
The agency responsible for cleaning catchbasins and inlets
clearly should be the public works department. However,
assignment within the public works department varies from city to
city. Some cities consider catchbasins and inlets as
appurtenances to the sewer system and thus assign the cleaning
duty to the sewer maintenance division. Others consider
15
-------
catchbasins and inlets as part of the street system and assign
the cleaning to the street maintenance division. Still others
assign the cleaning to the street cleaning division._ In theory,
where cooperation among municipal officials exists, it makes
little difference which divisions perform the cleaning operation.
However, it is usually desirable for the street cleaning division
to also be responsible for the cleaning of catchbasins and
• inlets.
The following discus'sion presents details on catchbasin and inlet
cleaning methods and procedures, debris disposal, oiling, and
cleaning frequency.
Catchbasin Cleaning
There are four categories of cleaning methods: manual cleaning,
bucket cleaning, eductor cleaning, and vacuum cleaning. Many
cities use one or more of the methods. Data on the number of
cities using mechanical means for cleaning catchbasins are
reoorted in Table 4. Manual cleaning will always be required for
certain situations, but the major cleaning effort of any
catchbasin cleaning program should be based on modern,
pconomical, and efficient methods and machines. It is;
interesting to note from Table 4 that 44 percent of the U.S.
cities and 62- percent of the Canadian cities use mechanical
cleaning methods. Apparently the United States, more so than
Canada,, is just beginning to use mechanical cleaning methods,
while in Europe they have been used almost exclusively for many
years.
TABLE 4. NUMBER OF CITIES USING MECHANICAL
MEANS FOR CLEANING CATCHBASINS, 1973 [102]
No. of cities
United States • Canada
Total cities
responding to survey
Cities using catchbasins
Cities using mechanical
means for cleaning catchbasins
443
322
142a
43
42
26*
a. Of this total, 113 were vacuum, 20 were eductor,
and 9 were bucket machines.
b. Of this total, 17 were vacuum and 9 were eductor.
16
-------
Equipment and Crew Size —
Tble 5.
°f e
-------
flush the remaining silt along with the dirty water into the
sewer.
This method of manual cleaning is undesirable because the pile
of removed material on the pavement is usually unsightly and
odorous, it leaves the pavement dirty and unsanitary, and the
method itself is relatively slow and expensive (though low in
capital costs).
Another method commonly used for large sumps is pumping or _
bailing out the water and then having men enter the catchbasin
to shovel the contents into a bucket. This method eliminates
the messy step of piling the debris on the pavement and probably
results in better cleaning, but it requires the men to work in a
rather foul environment, is relatively expensive, and requires
the trucks to be absolutely watertight.
Some cities use hose flushing alone to clean catchbasins. A high-
or-ssure water jet breaks up the catchbasin contents and flushes
all the debris to the sewer. The method is relatively easy_and
inexpensive, but it defeats the purpose of the catchbasin, i.e.,
orevention of solids entering the sewer system. If inlets are
used in place of catchbasins, the flushing step coulct be
eliminated, for the most part, especially in the case, of sewers
sloped enough to provide self-scouring velocities.
In summary, manual cleaning should be limited to special cases
and to catchbasins too small for mechanical cleaning. It is
relatively expensive, inefficient, unsanitary (both for the
cleaning crew and the public), and in the case of hose flushing,
self-defeating.
Bucket Cleaning—
Bucket cleaning consists of lowering a standard or specially
designed orange oeel or clamshell bucket into the catcobasin,
lifting the full"bucket to the surface, and then discharging the
contents into a dump truck or hopper attached to the bucket and
crane machine. This method is effective for removing most of
the basin contents, but leaves behind material that cannot be
reached by the bucket. Also, much basin water is spilled on the
street surface and causes a nuisance of odor and aesthetics, as
in manual cleaning. By using special fabricated buckets, the
bucket method has been adapted to many basins that ordinarily
would reauire manual cleaning. A drawback to this metnod^as
with manual cleaning, is the large manpower requirement which
limits the frequency and quality of the cleaning.
Eductor Cleaning—
In the eductor method, the vacuum effect of an eductor is used
to draw up the catchbasin contents. The solids-water mixture is
18
-------
then separated by settling in the tank compartment of this unit.
The water is recycled to operate the eductor. This method can
be accompanied by a flushing procedure to facilitate solids
removal by breaking up the solids mass. The eductor usually will
not pass large debris, which may require manual cleaning for
removal. Though relatively high in"capital costs, the eductor
cleaning, method is a sound approach to catchbasin cleaning and
should be considered for new catchbasin designs or for
modifications of existing basins.
Vacuum Cleaning—
The vacuum cleaning method operates essentially the same as the
eductor method, except that an air blower is used to create the
vacuum, and air-solids-liquid separation is accomplished in thp
unit by gravity separation and baffles. The air is exhausted to
the atmosphere. Usually, larger pieces of debris can be removed
from the catchbasin with a vacuum unit than with an eductor
unit._ As with the eductor, the vacuum method is economical,
efficient, and does not create nuisances. Tt should be
considered one of the major catchbasin cleaning methods and
should be incorporated in concepts for new catchbasin designs
and for modification of existing catchbasins.
Inlet Cleaning
Inlet cleaning is very similar
are some small differences in
eauipment. Inlets are usually
as catchbasins (about once per
cleaned as frequently as every
usually quite rapid and often
inspection for large materials
to catchbasin cleaning, but there
cleaning frequency, crew size, and
cleaned with the name frequency
year), but occasionally they are
2 months. Inlet cleaning is
is little more than a visual
that could block the flow.
Typically, inlet cleaning requires only a two-man crew. The
work in most cases is performed manually. Occasionally, a
vacuum truck is used, but the time required to assemble and
disassemble the extension segments of the vacuum unit reduces
the time saving.
Summary of Specific Cleaning Procedures
The following step-by-step procedures 'for cleaning catchbasins
and inlets are typical of those used by the municipalities that
were interviewed.
Manual Cleaning
Catchbasins (approximate total time, 30 to 90 minutes)
1. Remove grating using grating lifter (Illustration 1, Figure 3).
2. Pump out excess water.
19
-------
3. Use shovels and clamshell shovels to remove accumulated solids
(Illustration 2, Figure 3). Solids can be placed directly in
truck or on pavement and then shoveled into truck.
4. Wash deposited solids not:placed in truck off street.
5. Refill catchbasin with water if desired to maintain effective
operation of trap.
6. Replace grating.
Inlets (approximate total time, 10 to 30 minutes) !
Steps 1, 3, and 6 above.
Illustration 1
City of Santa Clara, California.
Removing inlet grating with grating
lifter.
Illustration 2
Removing debris with clamshell
shovel.
Figure 3. Manual cleaning.
Eductor Cleaning
Catchbasins (approximate total time, 15 to 45 minutes). Before using
truck or after dumping trash, eductor must be filled with water
(Illustration 1, Figure 4).
1. Place truck in position so that catchbasin can be reached by
eductor hose.
2. Place safety cones.
3. Open grating.
4. Lift out floating debris and garbage onto pavement with rake.
5. Place eductor hose into catchbasin.
6. Turn on eductor, protecting nozzle with rake to assure that no solids
(cans or bottles) are sucked into hose (Illustration 2, Figure 4).
20
-------
7. Use pressure nozzle on pipe extension to clean corners and loosen
sludge for removal by eductor.
8. Stop eductor for removal of tin cans or bottles stuck in sediment.
9. Restart eductor to finish cleaning basin.
10. Stop eductor and allow water to flow into catchbasin to form water
seal on trap.
11. Remove eductor hose from catchbasin.
12. Load debris from pavement into truck.
13. Sweep or flush pavement clean.
14. Replace grating.
Inlets'- Eductors were not used for inlet cleaning.
Illustration 1
City of San Francisco, California.
Overall view of unit about to be
filled with water.
Illustration 2
Eduction nozzle in catchbasin with
man at left holding rake to keep
nozzle protected and man on right
using pressure jet to wash sides
and break up solids.
Illustration 3
Closer view of eduction nozzle in
bracket on truck (partially con-
cealed by main water supply hose).
Illustration 4
Closer view of eduction pump and
hose reel.
Figure 4. Eductor cleaning.
21
-------
Vacuum Cleaning
Catchbasins (approximate total time, 15 to 30 minutes).
1. .Place vacuum truck in position so that catchbasin can be reached with
vacuum hose.
2. Place safety cones and block wheel of truck.
3. Open grating. (Occasionally, pick used to open grating;is placed in
position to counterbalance grate and prevent accidents).
4. Set vacuum extension into catchbasin and connect hose (Illustration 1,
Figure 5). ;
5. Turn on vacuum unit and remove water and accumulated solids
(Illustrations 2'and 3, Figure 5).
6. Use bar to break up grit and scrape grit from corners (scraping end
of bar in foreground of Illustration' 1, Figure 5).
7. Wash down excess solids with pressure hose.
8. Disassemble vacuum extension unit.
9. Refill catchbasin with water if desired to maintain effectiveness of trap.
10. Replace grating.
Inlets (approximate total time, 15 minutes). Same procedures as above,
except omit Step 9. •
Illustration 1
City of Berkeley, California.
Connecting 6 ft extension to
flexible hose. Hand scraper in
foreground.
Illustration 2
Cleaning the catchbasin.
Illustration 3
Overall view of operation in
progress.
Figure 5. Vacuum cleaning.
22
-------
Debris Disposal
Catchbasin debris usually contains appreciable amounts of water
and offensive organic material. Depositing it on the pavement is
therefore objectionable aesthetically and sometimes creates a
traffic hazard (due to slippery pavement).
The debris is low in organic content on a mass basis, which
eliminates burning as a disposal method. Unless it is
immediately covered by a layer of soil, it is unusable for fill
material. This need for immediate removal from the pavement and
immediate use as fill material can present difficult coordination
problems. Consequently, the usual method of disposal is sanitary
landfill in spite of the appreciable water content. The
widespread use of sanitary landfill disposal is indicated by
results of the 1973 APWA survey [102] summarized in Table 6.
TABLE 6. CATCHBASIN DEBRIS
DISPOSAL METHODS [102]
No. of cities
United States Canada
Total cities
responding to survey
Disposal method
Landfill
Fill unimproved streets
Fill private property
Other
Not stated
443
338
11
26
23
45
43
38
0
1
2
2
Many cities have experienced difficulties, such as rapid slope
degradation or fill instability, when large amounts of the muck
are being disposed o'f in the sanitary landfill [9] . For these
reasons some municipalities have set aside separate sanitary
landfill sites for catchbasin debris. An important consideration
by administrators in evaluating the continued use of catchbasins
must be the availability of suitable disposal sites. Because of
the large quantities of material involved, up to a ton or more
per basin, long haul requirements may dominate the economics.
Oiling
The stagnant water in a trapped catchbasin is an ideal breeding
ground for mosquitos. Prevention measures are necessary,
particularly in warm climates, and usually consist of spraying
23
-------
the water surface with fuel oil or larvacides. The frequency of
application would logically be within 7 days (the normal
incubation cycle of mosquitos [103]) after each catchbasin
flushing event (either intentional or natural). Colder weather,
of course, lengthens the incubation cycle and thus the period
allowed between sprayings.
In the 1973 APWA survey, the annual frequency of oil or
larvacides spraying catchbasins for mosquito control was reported
as follows[102]:
No. of cities
responding
9
9
4
2
, 1
1
26
Annual
frequency
of spraying
1
2
3
4
6
12
The median frequency was 2 times per year. The two high annual
frequencies of 6 and 12 times per year occurred in two high-
income residential areas. Oiling of catchbasins was not reported
to be a widespread practice; 276 of 356 cities stated that they
do not spray their catchbasins [102]. Excessive use of oil or
larvacides is, of course, an indirect source of pollution to the
receiving waters and must be avoided.
Gleaning Frequency
As a minimum requirement, catchbasins must be cleaned often
enough to prevent debris from accumulating to such a depth that
the outlet to the sewer might become blocked, and this only
prevents plugging and subsequent street and basement flooding.
To achieve gross solids removal, the sump itself must be kept
clean so that storage capacity is provided. Otherwise, almooc
all of the solids may be forced through the trap into the sewer,
or the trap itself may become plugged, preventing the passage of
the stormwater.
The required frequency of catchbasin cleaning is a function of
several local parameters, such as sump capacity, quantity of
accumulated street solids, antecedent dry period, meteorological
conditions, street cleaning methods and practices, surrounding
land use, topography, and to some extent, the type of: surface
soil adjacent to the street. Many of these parameters are
subject to optimization to maximize catchbasin system efficiency,
because they are physical parameters. Unfortunately,, in reality,
24
-------
the most influential parameter in some cities is the human
element; i.e., many of the deficiencies of existing catchbasin
systems are the result of public apathy either through ignorance
or lack of concern on the part of citizens, responsible public
officials, cleaning crews, or a combination thereof. All too
often, the cleaning of catchbasins is given a low priority until
a major disaster occurs. A good example of this is the subway
flooding attributed to clogged catchbasins that occurred in New
York City on July 3, 1969, when the New York City Transit
Authority was forced to abandon service on its Pelham Line north
of 139th Street [108].
Another example of the low priority of catchbasin cleaning are
the data returns from the 1973 APWA survey [102]. The catchbasin
cleaning frequencies reported by 299 cities are summarized in
Table 7.
TABLE 7. FREQUENCY OF CATCHBASIN
CLEANING IN VARIOUS NORTH AMERICAN CITIES
Frequency
No. of cities
1973a 1956b
As needed
Once in 4 years
Once in 3 years
Once in 2 years
Twice in 3 years
1 time per year
1.5 times per year
2 times per year
2. 5 times per year
3 times per year
3.5 times per year
4 times per year
5 times per year
6 times per year
7-8 times per year
9 times per year
10-15 times per year
20-26 times per year
31 times per year
45 times per year
52 times per year
Total
Median annual frequency
Mean annual frequency
Mean of middle 80% of cities
a. Reference [102].
b. Reference [9] .
3
7
13
1
142
1
81
• • »
21
...
13
2
5
1
3
2
1
1
1
1
299
1
2.3
1.5
11
2
7
37
4
68
4
15
2
14
1
6
1
1
3
3
. 1
180
2
• • •
25
-------
The results of the 1973 survey are similar to results of a much
earlier survey (1956), also by the APWA [9], The reported
frequencies for this earlier survey are also summarissed in the
table. The median cleaning frequency was reported as once per
year for 1973 and twice per year for 1956. Without comparing on
a city-by-city basis (especially the change, if any, in both
street and catchbasin cleaning practices), it appears that the
cleaning frequency has decreased on a nationwide basis, even
though the need, with rare exception, has not abated,,
The data illustrate a crucial point about catchbasins—that many
of the problems associated are probably due to the inadequacies
of the cleaning programs. In the following sections,r performance
effectiveness will be related to design and maintenance
practices.
26
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SECTION 5
REVIEW OF VARIABLES AFFECTING
CATCHBASIN EFFICIENCY
The principal variables that affect the performance of
catchbasins in removing pollutants found in stormwater are
reviewed in this section. These variables deal with
(1) catchbasin hydrology, (2) catchbasin hydraulics,
(3) pollutant characteristics, and (4) solids washoff.
CATCHBASIN HYDROLOGY
The hydrology of the drainage area tributary to the catchbasin is
important because the area contributes runoff water to the
catchbasin and thus affects the solids loading of the catchbasin.
The amount of runoff is controlled by the terrain and street
slopes, drainage area size and shape, distance to the catchbasin,
runoff coefficients, distribution of pervious and impervious
surfaces, lag time, storm intensity and duration, depression and
gutter storage, flow routing, and infiltration capacity.
Defining a typical runoff area hyetograoh and hydrograph for
universal application as an evaluation criterion for catchbasins
may be unrealistic. To illustrate this, the variation of
localized rainfall intensity extrapolated from the 1963 U.S.
Weather Bureau Rainfall-Frequency Atlas for various design storms
for three U.S. cities is reported in Table 8. It is apparent
that, to be realistic, evaluation should be based on known
hydrological data in a known runoff area.
TABLE 8. TYPICAL 5-MINUTS
RAINFALL INTENSITIES [106]
Recurre:
interv.
yr
10
5
2
1
nee
San
4.
3.
2.
2.
Intensity,
Francisco
29
48
68
01
(3.
(2.
(2.
(0.
1)
6)
0)
9)
in.
Chicago
8.15
6.63
5.10
3.83
(7.
(6.
(4.
(..
1)
1)
6)
-)
/h
Washington
9
a
6
4
.94
.07
.21
.66
(7.
<6.
(5.
(..
, D.C.
34)
4)
25)
..)
Note: Figures in parenthesis represent official
gage data.
cm/h = in./h x 2.54
27
-------
The area tributary to an inlet is usually dependent on the inlet
spacing. For a given rainfall intensity, inlet spacing is
dependent primarily on the longitudinal slope of the gutter and
the allowable spread of water on the traveled way. Using the
typical city street cross-section shown in Figure 6 and assuming
a maximum allowable water spread of 182.9 cm (6 ft), excluding
33.5 cm (1.1 ft) of gutter width, the depth of flow at the curb
would be 8.2 cm (0.27 ft). Based on the curb depth of 8.2 cm,
the maximum flows and corresponding velocities for various
longitudinal gutter slopes are shown in Table 9, as computed by
using a modified form of Manning's equation [68]:
where
Q
Z
n
So
T
d
Q = 0.56 (Z/n) S01/2 d8/3
rate of discharge, cfs
reciprocal of the gutter cross slope (T/d)
Manning's coefficient of channel roughness
longitudinal slope, ft/ft
top width of water surface, ft
depth of channel at deepest point, ft
(1)
The true Manning equation cannot be used without modification to
compute flow in triangular gutter sections because the hydraulic
radius does not adequately describe the gutter cross-section,
particularly when the top width T of water surface may be more
than 40 times the depth d at the curb. To compute gutter flow,
the Manning equation for an increment of width is integrated
across the width T using Equation 1. Equation 1 ignores the
resistance of the curb face, but this resistance is negligible
from a practical viewpoint, provided that the width of flow is at
least 10 times the depth at the curb face. Equation 1 gives a
discharge about 19 percent greater than the incorrect solution,
obtained by computing the discharge by the true Manning equation.
TABLE 9. TYPICAL MAXIMUM GUTTER
FLOWS ON OLDER CITY STREETS
Longitudinal
gutter slope, m/m
Q, L/s (cfs) V, cm/s (ft/s)
0.002
0.004
(practical minimum)
0.010
0.060
0.100
(practical maximum)
25.2 (0.89)
35.7 (1.26)
56.6 (2.0)
138.8 (4.9)
179.0 (6.32)
36.6 (1.20)
51.8 (1.70)
82.3 (2.70)
201.8 (6.62)
260.3 (8.54)
28
-------
DISTANCE FROM CURBS. FT
20 15 12 8
5.0
STREET CROSS-SECTION
0.1 0.2 . 0.3 0.4 0.5
CURB DEPTH. FT
AREA DEPTH CURVE
NOTE: CH= FT x 30.48
0.6
0.5
- 0.4
-0.3
- 0.2
- 0.1
0.0
Figure 6. Typical old street cross-section [68]
29
-------
The corresponding tributary paved areas for the cities in Table 8
can be determined using the Rational formula, ;
Q = CiA
(2)
where Q
C
i
A
maximum rate of runoff, cfs
runoff coefficient = 0.8 to 0.9 for common pavements
rainfall intensity corresponding to time of
concentration, generally taken as 5 minutes
area tributary to inlet, acres
Assuming a 5-year 5-minute storm intensity and a C value of
0.9, tributary paved areas are as given in Table 10. The
importance of knowing the tributary area, is that the pollutant
load entering a catchbasin is directly related. The nature of
this relationship is considered in a subsequent subsection.
TABLE 10. TYPICAL TRIBUTARY
PAVED AREAS TO CATCHBASINS
Longitudinal
gutter slope
m/m
0.
0.
0.
0.
0.
002
004
010
060
100
Area,
San Francisco
0.
0.
0.
1.
2.
28
40
64
56
02
(0.
(1.
(1.
(4.
(5.
74)
05)
68)
11)
31)
acres
Chicago
0.15
0.21
0.34
0.82
1.06
(0.
(1.
(1.
(4.
(5.
77)
08)
74)
20)
44)
Washington, D.C.
0.13
0.17
0,28
0.67
0.87
(0.
(0.
(0.
(2.
(2.
41)
53)
88)
09)
72)
Note: Figures in parenthesis indictee approximate total
tributary area, both paved and unpaved, to a
catchbasin [84, 42, 80].
ha = acres x 0.40
CATCHBASIN HYDRAULICS
The hydraulics of a catchbasin are defined and determined by the
geometric configuration. The standard basin is basically a
barrel 182.9 cm (6 ft) deep and 121.9 cm (4 ft) in diameter with
an open top covered by a grating and an outlet pipe mounted at
the side approximately 107 cm (3-1/2 ft) above the bottom. The
hydraulics of such a system are best defined by following the
flow from the top entrance through the intermittent storage in
the barrel to the outflow through the pipe outlet.
The entrance flow conditions vary from a simple drop inlet
condition to free surface, peripheral, weir-type overflow to
orifice flow entering a barrel. Obviously, the inflow oattern
30
-------
is modified by the grating, which tends to spread the flow over
the top and to direct the flow in the form of jets falling
between the grating bars. The approach flow conditions also play
an important role, especially if there is a considerable approach
velocity. With a diminishing velocity of approach, the inflow
into the catchbasin is more uniform, and the discharge into the
catchbasin is more uniform and more concentrated around the
periphery. Comparative data on the intake capacities of various
inlets are given in Figure 7.
The flow in the barrel of the catchbasin consists first of
filling the basin until the water surface reaches the invert of
the outlet pipe, at which time a Control of outflow is
established. Depending on the slope of the pipe and entrance
geometry, a discharge control is effected. Under these
conditions, two controls exist, and the flow through the basin
presents a miniature flood-routing phenomenon of inflow from the
top, temporary storage in the basin, and controlled outflow into
the outlet pipe.
The control at the outlet pipe is typical of discharge
characteristics through a closed conduit, generally defined in
hydraulics as culvert flow. Different regimes can be established
for such a flow, beginning with weir control, proceeding to
orifice control, and finally reaching full pipe flow. Once the
opening becomes submerged, the discharge capacity diminishes
(discharge to square root of head relationship for pressure flow,
as contrasted to discharges to headwater to 3/2 power
relationship for open channel flow). If the outlet end is
submerged, the capacity will depend on the hydraulic gradient
between the head in the barrel and the head at the end of the
outlet pipe.
ffihen the flow in the outlet pipe becomes pressure flow and the
catchbasin is full, the head differential between the surface on
the street and pressure gradient in the main sewer conduit
determines the flow conveyance and discharge. The two controls
merge into one, and the geometric configurations of the barrel
and the entrance into the pipe outlet become important only in
terms of the coefficient for minor losses.
Influence of Various Parameters on Hydraulics
The key parameters in controlling the flow through the basin are
bhe geometric configuration of the top entrance (see Figure 7),
the volumetric capacity of the catchbasin, and the elevation,
slope, and entrance geometry of the outlet conduit. By properly
changing these variables, the catchbasin system can be optimized
to make it hydraulically most efficient for whatever purposes are
intended.
31
-------
CURVE 0 CURB OPENING, NO DEPRESS ION. GRATE LENGTH, L-10 FT
CURVE 0 CURB OPENING. 2%-IN. DEPRESSION. L-10 FT
CURVE 0 CURB OPENING, 3-FT WIDE DEFLECTOR. L-8.33 FT
CURVE 0 GRATE, NO DEPRESSION. W-2.5 FT, L-2.5 FT
CURVE 0 GRATE. 2S-IN. DEPRESSION, W>2.5 FT, L-2.5 FT
CURVE 0 COMBINATION. NO DEPRESSION
CURVE 0 COMBINATION, 2%-IN. DEPRESSION
10
± 4
CO
3 4
INTAKE CAPACITY,
CFS
NOTE: CH= FT x 30.48
Figure 7. Comparison of inlets: intake capacity at
95% capture of gutter flow: Manning's
n = 0.013; cross slope = 0.0417 ft/ft [76]
32
-------
Control o£ the Flow of Solids
Control of the solids flow by an intentional retention or
sluicing of solid material through the catchbasin can be effected
by modifications in catchbasin geometry. Establishing a
controlled conveyance and detention of flow, such a design can be
developed by experimental means. By use of baffles, separate
compartments, or flow-controlled devices (like weirs, orifices,
or side weirs), a flow conveyance can be established so that the
flow pattern is effective for whatever action is intended in the
movement of solid material. The consideration of turbulence,
flow agitation, and other conditions plays an important part in
proper development of the necessary geometry.
Other methods for the conveyance or separation 6f solid material
include swirl chambers, spiral flow, flow around bends, and other
ways of exploiting some definite hydraulic characteristics.
POLLUTANT CHARACTERISTICS
Pollutants in stormwater can be divided into the four general
categories of floatable, dissolved, suspended, and settleable
material. Each category can be further subdivided into organic
and inorganic components.
Because this report is concerned primarily with catchbasin
performance, the pollutant sources of interest are limited to
street accumulations and to those pollutants generated in
catchbasins. Stormwater pollutants are of concern only for gross
comparisons, because collected stormwater contains pollutants
from other sources as well as catchbasins. Unfortunately, these
limitations also greatly reduce the amount of available data.
Although many studies have been performed on stormwater after
collection (for example, in combined or storm sewers), few
studies have been performed on catchbasin pollutants.
From a recent study that dealt principally with street surface
contaminants on a nationwide basis, the following applicable
conclusions were formed [66].
1. Runoff from street surfaces is generally highly
contaminated.
2. The major constituent of street surface contaminants is
inorganic, mineral-like matter, similar to common sand
and silt.
3. A great portion of the overall pollutional potential is
associated with the fine solids fraction of the street
surface contaminants.
33
-------
4. On the basis of specially conducted field studies,
catchbasins (as they are normally used) are reasonably
effective in removing coarse inorganic solids from
storm runoff (coarse sand and gravel) but ineffective
in removing fine solids and most organic matter.
Little information is available on the floatable portion or the
dissolved portion of street contaminants. However, the suspended
and settleable solids portion of street surface contaminants has
been studied with respect to particle size and distribution and
the distribution of organic, inorganic, and specific
pollutants [66]. The following qualifications justify
consideration of the suspended solids portion only:
1. The dissolved portion of runoff passes on into the
storm or combined sewer regardless of the type of
intermediate device, whether it is a catchbasin or
inlet. The relationship that may exist between
dissolved solids generated by street cleaning practices
and those occurring naturally has not been studied.
2. The floatable portion is almost impossible to
•characterize, as it varies from oil droplets to small
beach balls and does not seem to be a function of land
use classification. The only apparent quantity trait
for large floatables deposited on the street surface is
their proportionality to street cleaning practices.
3. In one study it was found that an average of 92 percent
(by weight) of the i-n situ street litter collected in
the sampling program passed through a 2,000 micron
screen (10 mesh) and was composed mainly of dust, dirt,
sand, and gravel [66].
Particle Size and Distribution
In a recently completed nationwide study of street surface
contaminants [66] , the contaminants usually found on typical
American streets were characterized with respect to particle
size; distributions for five cities are reported in Table 11.
Street solids loading by land use and as a function of the
distance from the curb are given in Table 12.
Using the data derived in this study, a street surface particle
size distribution simulant was developed for use in experimental
studies, as shown in Table 13.
34
-------
TABLE 11. PARTICLE SIZE DISTRIBUTION
OF SOLIDS, SELECTED CITY COMPOSITES [66]
Particle size
ranges
Distribution, %a
>4,800 1-i
2,000-4,800 y
840-2,000 y
246-840 y
104-246 y
43-104 y
30-43 V
14-30 y
4-14 y
< 4 y f
, r* _••*
Sand, %
43-4,800 y
Silt, %
4-43 y
Clay, %
<4 p
Sand, kg/curb km
(Ib/curb mi)
Silt, kg/curb km
(Ib/curb mi)
Clay, kg/curb km
(Ib/curb mi)
Milwaukee
12. 0
12.1
40.8
20.4
5..5
1.3
4.2
2.0
1.2
0. 5
92.1
7.4
0. 5
699 (2,480)
56 (200)
3.8 (13.5)
Bucyrus
10.1
7.3
20.9
15.5
20.3
13.3
7.9
4.7
74.1
25.9
288 (1,020)
100 (356)
Baltimore
17. 4
4.6
6.0
22.3
20.3
11.5
10.1
4.4
2.6
0. 9
82.1
17.1
0. 9
238 (845)
50 (176)
2..6 (9.3)
Atlanta
14.8
6.6
30.9
29.5
10.1
5.1
1.8
0.9
0.3
91.9
7.8
0. 3
111 (394)
9,5 (33,5)
0.4 (1.3)
Tulsa
37.1
9.4
16.7
17.1
12.0
3.7
3.0
0.9
0.1
92.3
7.6
0.1
85 (300)
8.5 (30)
0.1 (0.3)
Note: y = microns.
a. By weight unless otherwise noted.
35
-------
TABLE 12. STREET SOLIDS LOADING BY LAND USE . [66]
Use
Residential
Industrial
Commercial
Mean value
Quantity,
kg/curb km
(Ib/curb mi)
338 (1,200)
790 (2,800)
102 (360)
395 (1,400)
Range, kg/curb km
(Ib/curb mi)
9-l;946 (31-6,900)
68-3,384 (240-12,000)
17-338 (60-1,200)
Street location,
distance from curb,
cm (in.)
Solids loading
intensity,
% of total
0-15.2 (0-6) 78
• 15.2-30.5 (6-12) 10
30.5-101.6 (12-40) 9
101.6-243.8 (40-96) 1
243.8 .(96) to
centerline 2
TABLE 13. STREET SURFACE SIMULANT [6'6]
Particle Composition,
size, y % by weight Descriptiona
2,000
840-2,000
246-840
104-246
43-104
43
8
20
30
20
16
6
Very coarse sand
Coarse sand
Medium sand
Fine sand
Very fine sand
Coarse silt
a. Handbook of Applied Hydrology.
36
-------
Organic and Inorganic Pollutants
The quantities of various pollutants found on street surfaces are
summarized on a weighted mean basis in Table 14. The
distribution of various pollutants associated with a particle
size range is presented in Table 15. As can be seen, the very
fine silt-like material (less than 43 microns) accounts for only
5.9 percent of the total solids, but it accounts for about
25 percent of the oxygen demand and from 30 to 50 percent of the
algal nutrients. This concentration of pollutants in the very
fine material is important because the catchbasin does not
efficiently trap particles in this size range and thus allows a
large percentage of these pollutants to pass through.
TABLE 14. CONTAMINANT CHARACTERISTICS
AND QUANTITY SUMMARY [66]
Measured
constituents
Weighted mean for
all samples,
kg/curb km
(Ib/curb mi)
Total solids
Oxygen demand
BOD5
COD
Volatile
Algal nutrients
Phosphates
Nitrates
Kjeldahl nitrogen
Bacteriological
Total coliforms,
org/curb mia
Fecal coliforms,
org/curb mi
Heavy metals
Zinc
Copper
Lead
Nickel
Mercury
Chromium
Pesticides
p, p-DDD
p, p-DDT
Dieldrin
Polychlorinated biphenyls
395 (1,400)
3.8 (13.5)
27 (95)
28 (100)
0.3 (1.1)
0.026 (0.094)
0.62 (2.2)
99 x 109
9.6 x 109
0.18 (0.65)
0.06 (0.20)
0.16 (0.57)
0.01 (0.05)
0.02 (0,073)
0.03 (0.11)
19 (67) x 10"6
17 (61) x 10-6
6.8 (24) x 10~6
310 (1,100) x 10'6
The term "org" refers to the number of
coliform organisms observed.
37
-------
TABLE 15. FRACTION OF POLLUTANT ASSOCIATED WITH
EACH PARTICLE SIZE RANGE, PERCENT BY WEIGHT [66]
Particle size,
Constituent
Total solids
Volatile solids
BOD5
COD
Kjeldahl nitrogen
Nitrates
Phosphates
Total heavy metals
Total pesticides
>2,000
24.4
11.0
7.4
2.4
9.9
8.6
0
16.3
0
840-2,000
7.6
17.4
20.1
4.5
11.6
6.5
0.9
17.5
16.0
246-840
24.6
12.0
15.7
13.0
20.0
7.9
6.9
14.9
26.5
104-246
27.8
16.1
15.2
12.4
20.2
16.7
6.4
23.5
25.8
43-104
9.7
17.9
17.3
45.0
19.6
28.4
29.6
27
31
<43
5.9
25.6
24.3
22.7
18.7
31.9
56.2
. 8
.7
Catchbasin Loading Intensit
The principal factors affecting the loading intensity at any
given site include the following: surrounding land use, the
-elapsed time since streets were last cleaned (either
intentionally or by rainfall), local traffic volume and
character, street surface type and condition, public works
practices, and season of the year [66].
In addition to the street surface contaminants, other materials,
such as crankcase drainings, leaves, and grass clippings, are
frequently discarded into catchbasins. This additional loading
is highly variable, highly polluting, and difficult to estimate.
A survey of San Francisco catchbasins, which illustrates the wide
range of pollutant loading, is shown in Table 16.
Sediment Pollution
Although the sedimentation problem is primarily related to the '
runoff that enters streams directly rather than the runoff that
flows through the storm drainage system, it is obvious that self-
cleaning storm drains could contribute large quantities of
sediment to waterways. These sediments can damage biological
structures, bury organisms, and clog respiratory, feeding, and
digestive organs [66] . In addition, sediment can contribute to
flooding problems by raising stream beds and clogging drainage
structures. Increased water treatment costs are associated with
increased turbidity of the water. Decreased reservoir capacity
caused by sedimentation is an expense that can be quite large.
38
-------
TABLE 16. ANALYSIS OF CATCHBASIM CONTENTS,
CITY OP SAM FRANCISCO, 1970 [65]
Catchbasin
location
Plymouth
and Sadowa
7th and
Hooper
Yo Semite
40th and
Moraga
Mason and
O'Farrell
32nd and
Taraval
Haight and
Ashbury
Marina area
Montgomery
Street
Webster and
Turk
Lower Selby
Upper
Mission
First
COD
3,860
15,000
739
9,060
8,100
15.3
37,700
701
6,440
1,440
288
5,590
sampling series
BODs
190
430
11
40
130
5
1,500
100
390
44
6
50
Total N
10.9
33.2
1.8
16.1
29.7
0.5
1.4
7.0
18.8
14.0
1.4
12.0
, mg/L
Total P
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2 ,
<0.2
<0.2
<0.2
0.2
Second
COD
8,610
2,570
21,400
51,000
7,720
70.8
143,000
8,60Q
8,160
sampling series
BOD5
122
170
120
130
85
15
420
40
300
Total N
2.8
2.0
4.6
12.0
16.5
1.4
14.6
0.5
3.9
, mg/L
Total P
0.3
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
<0.2
Note: Both sampling series were conducted in
on an analysis of total basin contents
winter 1970. All values based
after complete mixing.
Increased sediment pollution is associated with construction and
urbanization; the pollution usually decreases after the
construction phase is completed. Predevelopraent background
sediment yields generally range from 7.0 x 104 to
17.5 x 104 kg/kfli2.yr (200 to 500 tons/mi2.yr) [105]. Sediment
yields for various locations and conditions of land use are sbovn
in Table 17.
SOLIDS WAS"OFF
Solids movement phenomena from the surface of the street to the
gutter, and then along the gutter to the inlet and into a
catchbasin, are mainly a function of the following
factors: rainfall intensity, longitudinal slope of street, cross
slope of street, antecedent dry period, land use, size and shape
of drainage area, type and condition of street surface, season"of
year, street sweeping program, size distribution and availability
of solids, and possibly others.
39
-------
TABLE) 17. REPRESENTATIVE DATA
ON SEDIMENT YIELD [105]
Location
Drainage area,
(mi2)
Sediment yield,
kg/km2-yr
(tons/mi2-yr)
Condition
Johns Hopkins
University, ,
Baltimore, Md. 0.0065 (0.0025) 48.9 x 10° (140,000) Construction site
Tributary Mineback ,
Run, Towson, Md. 0.081 (0.031) 27.9 x 10b (80,000) Commercial
Tributary,
Kensington, Md
Oregon Branch,
Cockeysville, Md.
0.24 (0.091) 8.38 x 106 (24,000) Housing subdivision
0.61 (0.236) 25.1 x 106 (72,000) Industrial park
On the basis of experimental studies, it has been concluded that:
1. The soluble fractions go into solution. The impacting
raindrops and the horizontal sheetflow provide good
mixing turbulence and a continuously replenished clean
"solvent."
2, Particulate matter (from sand size to colloidal size)
is,dislodged from its resting place by the impact of
falling drops. Once dislodged, even reasonably heavy
particles will be maintained in a state of pseudo-
susoension by the repeated impact of adjacent drops,
creating a reasonably high general level of
turbulence [66].
Various equations have been developed to represent the solids
washo££ phenomenon. Perhaps the most utilized is that developed
for the Storm Kster Management £-5oriel [81] .
£t the start of the rain, the amount of a particular
pollutant on surfaces which produce runoff (both impervious
and pervious) will be PQ , pounds per subarea. Assuming
that the pounds of pollutant washed off in any time
interval, dt , are proportional to the pounds remaining on
the ground, P , the first order differential -equation is;
dt
= kP
which integrates to
-kt
(3)
(4)
in which P - P equals the oounds washed away in the
tirae, t .
40 '
-------
In order to determine k , it was assumed that k would
vary in direct proportion to the rate of runoff, r ,
or k = br. To determine b it was assumed that a uniform
runoff of 0.5 inch per hour would wash away 90 percent of
the pollutant in one hour. This leads to the equation:
- P = P0(l-e-4-6rt)
(5)
where
r
t
Runoff rate (in./hr)
Time interval (hr)
The use of Eguation 5 is illustrated in Section 7. Modifications
to this equation by the University of Cincinnati [64] and UHS
Research Company [109] provide for use of alternate units and
site specific data. In the ??torm Water Management Model version,
an availability factor "A" of pollutants available for>ashoff is
used for site specific calibration.
41
-------
,. SECTION 6
HYDRAULIC MODELING ANALYSES
In the preceding sections, the functions of catchbasins and
design and maintenance practices were identified, and the
principal variables believed to affect performance were reviewed
with respect to the removal of pollutants found in stormwater.
Through these studies it was observed that virtually no basic
documentation exists on the operational characteristics of
catchbasins. Specifically, no data were found relating
performance to basin geometry, flow, influent solids gradation,
and accumulated sediment within the basins. To fill this data
gap, controlled hydraulic modeling analyses were performed and
the results are presented in this section.
The following presentation is extracted and adapted ;from Hydro-
Research-Science Project Report No. HRS-039-75 "Catchbasin
Hydraulic Model Studies of Plow Conveyance and Pollution Control"
by Dr. Alexander B. Rudavsky, November 1975, performed under
subcontract to this study.
OBJECTIVES
The flow-through pattern in catchbasins involves a three-
dimensional flow, the configuration and complexity of which
depend on the shape of the structure and the peripheral flow
conditions. Such flows are complex and not subject to
computational analysis. To analyze the flow patterns in existing
catchbasins and to develop a design for future units, modeling
techniques are imperative. Also, an experimental approach
through model studies is required to assess the efficiency of
solids capture quantitatively.
The objectives of the adopted modeling program were to test and
document the following:
• Flow-through variations from 5.7 to 175.6.L/s (0.2 to
6.2 cfs), approximately 4 to 100 percent of maximum
expected basin inflows i
• Basin geometry variations in barrel diameter, outlet
pipe diameter, barrel height, and barrel storage height
(defined as height of outlet pipe invert above base)
42
-------
Outlet discharge controls, both open and trapped, for
conditions from free flow to complete submergence
Sediment capture as a function of gradation and
accumulated sediment
Performance associated with a recommended design
configuration
EXPERIMENTAL SETUP
The setup
Figure 8,
model, (2)
conditions
supportive
dimensions
1:2.72 or
simulated.
Components
for the catchbasin experimental program, as shown in
consisted of three main components: (1) the catchbasin
the peripheral simulation of inlet and outlet
, and (3) the auxiliary" appurtenances, including
machinery and storage basins. Model to prototype
were fixed at undistorted linear scale ratios of
1:3.40, depending on the prototype barrel diameter
The catchbasin model consisted of a multisectioned barrel with a
movable bottom and two interchangeable outlet pipes, as shown in
Figure_9. Bolted and pressure-tight connections provided the
flexibility of substituting and removing sections to meet the
full range of geometric configurations required. Each component
was constructed of transparent plastic, permitting direct
observation of the flow when illuminated.
The peripheral flow conditions were simulated by a partial
representation of the street, the inlet opening with a grating,
and the outlet pipe section, as shown in Figure 10. The street
inflow conditions were simulated simply by inclining the surface
platform 10 percent longitudinally and 20 percent transversely.
The square grating was movable so that the"bars could run
parallel to, or across, the gutter flow. The-outlet pipe was set
at an angle of 5.4° below horizontal to force a critical control
section at its entry.
.Auxiliary equipment included (1) upstream and downstream tanks,
(2) a sump to store water, (3) a centrifugal circulating water
pump, (4) a system of discharge.valves and butterflv regulating
valves, (5) a solids feed system and a trap basin to avoid
recirculating solids with-the water, and (6) a metering system
for measuring elevations, velocities, and discharges, 'in the
sediment capture portions of the testing, commercial grade sands
and ground sands, as well as a synthesized graded sand mixture,
were used.
43
-------
MANOMETER
SUMP
TRAP BASIN
& TANK,—
PARTIAL STREET
MODEL
. MODEL
GRATING
HEAD
TANK
OUTLET
PIPE
PLAN
POLLUTANT
SIMULANT
DISTRIBUTOR
MODEL
CATCHBASIN
ELEVATION
Figure 8. Experimental setup,
DISTRIBUTOR
CONTROL
44
-------
NOTE: DIMENSIONS SHOWN ARE IN
FEET CONVERTED TO PROTOTYPE
SCALE (SCALE 1 :2.72). TO
CONVERT TO cm MULTIPLY BY
30.48.
LEGEND
BARREL HEIGHT
BARREL STORAGE HEIGHT
HEIGHT FROM SOFFIT OF
OUTLET PIPE TO TOP OF
INLET GRATING
DISCHARGE HEAD (HEADWATER)
ABOVE INVERT UNDER DISCHARGE Q
BARREL DIAMETER
OUTLET PIPE DIAMETER
1 .06
^-
MOVABLE
BOTTOM
1.42
T
2.90
4;72
Figure 9. Model catchbasin,
45
"•\\
-------
Photograph 1
Upstream view of partial
street model with the
view of head basin and the
opening for grating
insert.
Photograph 2
Close-up view of
model grating.
Overall view of catch basin
barrel assembled.
Figure 10. Model components prior to assembly,
46
-------
Model Laws and Dimensional Analysis
The mathematical relationships between the model and the
prototype, based on the Froude law, are summarized in Table 18
These scale relationships were used to transfer quantitatively
the discharge, depth of flow, and velocities from the model to
the prototype. Unless otherwise designated or self-evident, only
prototype equivalents are presented.
TABLE 18. MODEL TO PROTOTYPE RELATIONSHIPS
Ratio of model Scale
Dimension to prototype relationships
Length Lr = j-S- 1:2.72 1:3.40
Area AE = (Lr)2 1:7.40 1:11.56'
Time Tr = (Lr)x/2 1:1.65 1:1.84
Velocity Vr = (Lr)'V2 1:1.65 1:1.84
Discharge Qr = (Lr)s/2 1:12.20 1:31.32
Roughness nr = (Lr)1/6 1:1.18 1:1.23'
Note: m = model; p = prototype; r = ratio
of model to prototype
Since complete dynamic similarity and accurate reproduction of
some prototype properties are not possible, some limitations must
be imposed on the model results:
• Measurements of discharge elevations and velocity can
be transferred without reservation.
• Since it is not feasible to reproduce the roughness of
a concrete surface in a plexiglass model of this scale,
some differences in conveyance efficiencies can result.
In this case, the differences are considered
negligible.
• Air entrainment cannot be modeled by the Froude law
alone, and there is now no acceptable method of
correlating air entrainment between the model and
prototype.
• Grain size dimensioning is based on settlement
velocities and subject to many practical limitations.
Thus, capture efficiencies are presented as a design
guide and not as precise research data.
Dimensional analysis techniques were used to identify and group
the significant variables.
47
-------
EXECUTION
The hydraulic modeling was carried out in four phases:
• Phase 1. An experimental analysis of flow conditions
in catchbasins representing current practice
• Phase 2. A selective repetition of Phase 1 tests with
standard inlet and outlet modifications
• Phase 3. A series of runs to evaluate sediment capture
• Phase 4. The development and verification of flow
conditions in the recommended catchbasin design
Prototype equivalents of variables used in the experimentation
are listed in Table 19. Complete tests were run in four physical
groupings (based on the ratio of barrel diameter to outlet
diameter), three barrel heights (long, medium, and short), and
two storage depths (deep and shallow), for a total of 24 discrete
configurations.
TABLE 19. PRINCIPAL VARIABLES TESTED
Variable
Discharge, L/s (cfs)
Barrel diameter,
cm (ft)
Barrel height,
cm (ft)
Barrel storage
height, cm (ft)
Exit pipe
diameter, cm (ft)
Qrtax =178
also Qsmal
(»iW =
max =
(Hi) min =
(H2)max -
(D2>max "
(D2>(nax =
.4
152
243
121
30.
38.
30.
Range
(6.3) Qdes = 35.4 (1.25)
14.2 (0.5) Qmin = 7-1 (0.25)
.4 (S.O) (Dl)min « 121.9 (4.0)
.8 (8.0) (HiJmedium = 182.9 (6.0)
.9 (4.0)
.9 (4.0) (50% of HI max)
5 (1.0) (25% of K! min)
1 (1.25)
5 (1.0)
Typically, the test procedure was as follows:
1. Set up components in selected configuration.
2. Apply maximum flow and observe approach conditions,
flow over grating, and flow conditions in the basin and
outlet.
3. Record headwater height (above the invert D2) and flow
patterns, including extensive photography.
48
-------
4.
5.
Trim to next lower flow and repeat until all desired
flows are covered.
Drain, change to next configuration, and repeat full
sequence.
In Phase 3, where solids were applied, only a minimum of
experimental setups were used because of the added long drying
and weight checking periods required. The range of materials
used was chosen from commercially available sand mixtures,
defined by their commercial designations as No. 20, No 30
No. 2, No. 57, and No. 84. Their respective sieve analyses are
shown in Figure 11 along- with the prototype gradation used by
Sartor and Boyd [66] .
A limited supportive program was executed to establish the
significance of discharge, pollutant load concentration, and test
duration to sediment capture results. Commercial No. 20 and
No. 30 sands were discharged with different concentrations
through a wide range of test durations. For example, No. 20 sand
was run separately at a constant feed rate for 25.5, 10.3, and
5.8 minutes for a single discharge. In all of these studies, the
retention characteristics appeared to be independent of the
concentration, and the deposition was directly proportional to
the length of run. Similar results were obtained using No 2
sand,, and it was concluded that the test durations could be
uniformly fixed at a nominal 5 minutes for the Phase 3 and
Phase 4 studies.
Note that in a typical 5-minute test under maximum flow
conditions, over 53,000 L (14,000 gal.) of water was circulated
tnrough the test unit carrying approximately 7,100 g (16 Ib) of
•simulant for a mean concentration of 133 mg/L. This is within
the typical range expected in surface runoff from streets.
RESULTS
In expressing the experimental results, somewhat detailed
descriptions are given for what may appear to the reader to be
rather obvious conclusions. The intent is to maximize the
benefits of this experimentation for potential future
investigations as well as to satisfy the immediate study
objectives. .
Phase 1 - Hydraulics
In Phase 1, all 24 configurations were tested, and a discharge
rating curve was constructed for each configuration. A
preliminary assessment was made as to which basins were
satisfactory, unsatisfactory, or marginal for sediment capture on
the basis of observed turbulence and flow patterns
49
-------
U.S. STANDARD SIEVE OPENINGS
100rrm-
90 - -
80 • -
t_ ...
a 70 •• •
._ 60 - •
S 50---
SE
ti.
40 - -
t— ^
SE
tu
S 30 -••
UJ
a.
20 ...
10 •- •
O'lll
i i [1 1
TT
i — rrm
TT
^
"Tffi
_^ u
L_S.jp]
— 0£\
1 ^
1 1 x
1 1^
1 r
I
L Jl
11
— Htfi
.... I J
5 1
•A 1 i
\L V
1 ^
¥^T
im
*
W n
i_| V)L\
V
-m
TOs
™ N
05 0.1*0.05 0.01
GRAIN SIZE.mm
O COMMERCIAL SAND GRADE #20
& COMMERCIAL SAND GRADE #30
D COMMERCIAL SAND GRADE #2
O NOMINAL GROUND SAND #57 MESH
O NOMINAL GROUND SAND #84 MESH
+ TYPICAL STREET CONTAMINANTS, SARTOR AND BOYD IM
-^- GRADED SAND'USED IN RECOMMENDED BASIN
Figure 11. Sieve analyses of test simulants.
Two flow pattern grouoings were evident: those influenced by the
exit conditions and those generated in the storage basin.
Exit Conditions —
As shown in Figure 12, the outlet pipe controlled the flow
through the following ranges, presented in the order of
increasing flow: (1) open channel flow, controlled through weir
control and directly related to critical depth at the outlet;
(2) orifice control flow, controlled by the share edges of the
entrance to the outlet pipe with subsequent open channel flow in
the pipe itself; (3) short tube control flow, controlled in the
outlet pipe with a short tube type of control of various lengths;
and (4) pipe control with pressure flow existing in the outlet
pipe and flowing completely full. Slug flow was also observed
where the flow in the pipe contained large bubbles and
represented unsteady flow conditions, tvith the exit pipe set
50
-------
very close to the grating at high discharges, the catchbasin
filled up, overflowed, and became totally submerged.
2.1
OPEN CHANNEL FLOW
SHORT TUBE AND
SLUG FLOW
0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1
Q/(gD25)1/2
Figure 12. Typical discharge rating curve.
As shown in Figure 12, the discharge-to-headwater relationship
can be directly associated to these flow conditions. For open
channel flow with critical depth control, the relationship of
discharge to headwater has an exponent of 3/2, indicating a large
discharge capacity. The discharge-to-headwater relationship for
pressure flow has an exponent of only 1/2, indicating a very
small discharge capacity. Although the discharge rating curve
for each catchbasin configuration is unique, all have the same
characteristic shape.
Storage Basin—
Flow patterns in the storage basin depend on its volume, depth,
and rate of discharge. The primary patterns are the jet
descending from the grating and an eddy pattern induced by that
jet in the storage basin. Distinct flow patterns were observed
in the experimental program, ranging from a plunging jet for very
large discharges inducing a macro eddy to a very weak descending
jet sequence being dissipated in the basin. When the storage
basin is shallow, the descending jet impinges upon the floor and
can go both toward and away from the outlet.
51
-------
The observed flow and control conditions are identified in
Figure 13. Since both the exit conditions and storage basin flow
patterns are unique to each basin configuration, the latter can
be classified, on the basis of experimental observation, into
three basic types: satisfactory, marginal, and unsatisfactory._
Typical flow conditions in each broad classification are shown in
Figure 14.
Summary—
A review of flow patterns controlled by exit conditions indicates
that all flows less than 50 percent of maximum were open channel
flows. This clearly indicates such a flow would be expected for
the majority of flow patterns. Of the 24 basins investigated and
summarized in Table 20, 8 showed satisfactory storage flow
conditions (i.e., conditions conducive to solids capture), 4 were
marginal, and 12 appeared unsatisfactory. Photographic
documentation is presented in Figures 15 through 18.
Nominal catchbasin depths of 183 to 244 cm (6 to 8 ft) with 50
percent or greater storage depths (H2/D2 >2.4) exhibit the best
flow conditions for solids capture. The shallow storage
configurations (H2/D2>1.5) invariably appeared unsatisfactory.
Phase 2 - Standard Modifications
In Phase 2 of the experimental program, the influence of standard
modifications to catchbasin inlet and outlet controls was
investigated. The first modification involved placing a hood
over the entrance to the outlet pipe, and the second involved the
addition of a curb protrusion above a portion of the grated
inlet. Four catchbasin configurations were tested, all chosen
from the marginal or unacceptable categories to magnify any
improvements in flow patterns.
In Configuration 11 (Table 20) a smell diameter, short height,
.but deep storage basin was tested first. The curb was moved out
15.2 cm (6 in.) into the gutter but was notched to fully expose
the inlet grating, thus simulating a combination grating and curb
inlet. The effect on the discharge rating curve was minimal,
even under very unstable conditions. Testing the same basin
without a protruding curb, but with a hood over the outlet to
tyoify common gas traps, produced a radical change in discharge
capacity and a substantially different rating curve. The
dramatic decrease in discharge capacity can be observed in
Figure 19. Investigation of the influence of the curb and hooded
outlet in combination again showed the dominance of the hood's
influence and the slight influence of the curb.
The tests were repeated for Configurations 23, 4, and 2 with
similar results. From Phase 2 it was concluded that hooded
entrances drastically changed the discharge rating curve,
52
-------
BASIN
DIMENSIONS,
FT
DISCHARGES
100«
6.3 CFS
75«
4.72 CFS
(1) 508
3.15 CFS
($>
1.58 CFS
HEAD vs
FLOWRATE
CURVES
H1 =9.24
H2=4.10
0, =4.01
WC
PJ
ME
f-
» v
'
HI = 6. 15
H2=3.12
1.06
WC
DISSIPATE
DISSIPATE
/
-------
SHORT TUBE CONTROL
PLUNGING JET WITH
MACRO EDDIES
PRESSURE FLOW
PLUNGING JET WITH
MACRO EDDIES
ACCEPTABLE BASIN - DEEP STORAGE
OVER FLOW
SLUG FLOW
IMPINGING JET WITH
MACRO EDDIES
MARGI HAL BASIN - MEDIUM STORAGE
WEIR CONTROL
DISINTEGRATING JET
ORIFICE CONTROL
PLUNGING JET DlSSIPATIED
UNACCEPTABLE BASIN - SHALLOW STORAGE
Figure 14. General performance classifications,
54
-------
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BASIH *
DIMENSIONS,
FT
8.3 CFS
75«
4.7 CFS
BOX
3.15 CFS
25% ,
1 .58 CFS
HEAD vs
FLOWRATE
CURVES
4.02
1.83
D2=1.08
(WITH HOOD ON)
IJ
0 .6
H, =4.02
H2=1.83
D =4.01
(WITH HOOD ON
AND CURB
MOVED OUT 2 In
PJ
ME
IJ
0.6
H1 = 4.02
H2=t.85
D = 4.01
WC
ME
PJ
LS.
NOTE O VALUES FLUCTUATE
LEGEND
TOTAL HEIGHT FROM BOTTOM
TO TOP OF GRATING
STORAGE BASIN HEIGHT
BARREL DIAMETER
EXIT PIPE DIAMETER
Q/(E°25)1/Z
VD2
ABBREVIATIONS
PJ PLUNGING JET
IJ IMPINGING JET
ME MACRO EDDIES
DC ORIFICE CONTROL
SF SLUG FLOW
WC WEIR CONTROL
PF PRESSURE CONTROL
OF OVER FLOW
NMWJ NO MAIN WATER JET
Figure 19. Influence of modifications on marginal basin.
60
-------
especially in the range of orifice and short tube control, and
drastically reduced, the discharge capacity. Compared to flows
under unhooded conditions, the influences of curb protrusions
seemed to be minor.
Phase 3 - Sediment Capture
In Phase 3, qualitative evaluation procedures were used to define
sediment retention for various conditions and these data were
used to develop an optimal design configuration. Generally, the
simulant approximated the medium-sized pollutant solids used in
other experimental programs. Where justified, supplemental tests
were run with finer or graded materials. Multiple flowrates were
attempted, but emphasis was placed in the middle ranges.
Initial Configuration—
Configuration 16 (large diameter, medium height, shallow storage)
was selected for the initial tests to set a base from which
improvements could be expected. The simulant was commercial
grade No. 30 sand. A maximum discharge of 232 L/s (8.2 cfs), 130
percent of expected maximum, plus simulant, was applied to a
clean basin. This resulted in a solids capture on only 3.4
percent by dry weight (i.e., 96.6 percent of the simulant sluiced
through the test unit and was recovered from the discharge sump).
The test was restarted using a flowrate of 152.9 L/s (5.4 cfs)
and observation of the retention characteristics showed that 77
percent of the material sluiced through and only 23 percent was
retained in the barrel. Short tube or orifice flow prevailed.
Next, the discharge was further reduced to 76.5 L/s "(2.7 cfs),
resulting in open channel, weir control, and the retained
material increased to 44 percent. Considering that the basic
configuration was in the unsatisfactory range, the retention
under the 76.5 L/s (2.7 cfs) flow was surprisingly good.
Seeking improvement, however, the storage basin depth was doubled
(H2/D2 = 1.74, Configuration 17). The overall depth was
increased from approximately 122 to 152 cm (4 to 5 ft), and the
same 76.5 L/s (2.7 cfs) flowrate was applied. The retention
jumped to 72 percent, indicating a marked advantage for deeper
basins, particularly in the storage zone.
Deep Basins—
The deepest basin geometry, Configuration 1, was attempted next,
holding the flowrate at 76.5 L/s (2.7 cfs). The retention showed
a further improvement to 80 percent, which appears optimal for
this flowrate. Then, the deepest 122 cm (4 ft) diameter basin,
Configuration 7, was tested at a discharge of 152.9 L/s (5.4
cfs), and the retention was a satisfactory 42 percent, nearly
61
-------
twice the efficiency of the shallow basin used in the initial
test.
Accumulation Impacts—
Using Configuration 1, a series of 10 consecutive runs were
executed in which the solids were left to accumulate in the
basin. Using No. 30 sand, the captured sediment increased rather
uniformly, with 73 percent or better sediment retained in each
run through the first five runs. At this point, corresponding to
a volumetric level approaching 0.4 H2 , the capture .efficiency
dropped off sharply and became erratic.
Fine Material—
Finer composition sands (specific gravity 2.65) were used in two
cases and mesh 100 gilsonite (specific gravity 1.06) was used in
one case. A mesh 250 material was also used. Gilsonite and mesh
250 material sluiced right through the system under a discharge
of 76.5 L/s (2.7 cfs). The fine sands results are , reported under
Phase 4.
Phase 4 - Recommended Design
From the studies conducted herein and the information from the
earlier sections, a simple recommended design evolved that is
appropriate for either 122 or 152 cm (4 or 5 ft) diameter basins,
as shown in Figure 20. The circular cross-section is preferred
from a cleaning and prefabrication viewpoint. The dimension from
the outlet pipe crown to the street or inlet grade is primarily a
structural consideration, as it contributes little to the
hydraulic performance. To be cost effective, the maximum depth
should be incorporated in the storage zone H~ and the putlet
pipe D2 should be sufficiently large to pass most flows under
open channel conditions.
In Phase 4, a complete series of rating and evaluation tests were
performed using the model in the recommended design
configuration.
hydraulic Performance—
The discharge rating curve for the recommended basin is plotted
on a dimensionless basis in Figure 21 with the identified flow
conditions. The corresponding photographic record is shown in
Figure 22. The discharge rating curve fixed the relationship
between discharge Q , discharge head above outlet pipe invert
8 , and outlet diameter
D,
assuming free discharge.
Open channel flow conditions/ are maintained in the outlet pipe
for flows up to approximately 50 percent of the design maximum.
62
-------
6.5D,
Figure 20. Recommended design,
2.0 r
1.6
1.2
0.8
0.4
0.0
Q-6.3
Q-4.7
Q-2 (SIMULANT ACCUMULATION
'0-1.58 STUDY)
'Q-1.25
'Q-0.50
'Q-0.25
NOTE: FLOWS SHOWN IN CFS. TO CONVERT
TO L/S, MULTIPLY BY 28.32.
D2= 32.3 cm (1.08 FT)
I i i
0.0 0.4 0.8 1.2
a/
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64
-------
Sediment Capture—
A graded solids simulant, shown in Figure 11, was used to test
the sediment capture characteristics of the recommended design as
a function of flow, particle size, and accumulated deposits in
the storage basin. For each test, a batch of simulant was
prepared with the following size-weight distribution. Double
batches were used in the accumulation test.
Size range, mm Weight, g (Ib)
>2.0
0.84 to 2.0
0.25 to 0.84
0.10 to 0.25
Total
364 (0.8)
909 (2.0)
1,364 (3.0)
909 (2.0)
3,546 (7.8)
With the exception of the accumulation test, the basin and setup
were cleaned between each run. The results of flow variation on
sediment capture in clean basins are shown in Table 21 and
Figures 23 and 24. while there is a loss in efficiency at higher
flows, a well-designed__baj3in is surprisingly tolerant of wide
flow variations with respect to heavy solids removal. For
example, a twenty-fivefold increase in flow reduced the net
removal efficiency only from 90 to 35 percent. However, in the
small particle size range (the most critical range with respect
to pollution load), the dropoff was much more dramatic: a
sixfold increase in flow reduced the removal efficiency from 68
to 14 percent. These results must be interpreted only as trends,
since replicate runs were not conducted and the specific gravity
for all size ranges was held at 2.65.
TABLE 21. PERCENT SEDIMENT RETAILED
IN BASIN VERSUS DISCHARGE
Size of
simulant, mm
Q,
6.3 • 4.7
3.15
1.58
1.25
0.50
0.25
>2.0 75.20 83.24 90.17 96.12 96.34 98.98 99.44
0.84 to 2.0 50.03 57.93 78.62 93.19 96.00 98.88 99.33
0.25 to 0.34 33.04 26.41 56.85 '72.51 81.18 91.54 97.46
0.10 to 0.25 4.64 6.37 7.67 14,72 32.23 45.24 68.60
0.10 to 2.0 34.44 35.18 53.24 65.42 73.98 82.31 90.74
a. L/s
cfs X 28.32.
65
-------
100
50
•2.0 mm
100
- 2.0 ram
SO
- 0.25 mm
0.10.- 2.0 mm
1 2 3 4 5 G
BASIN INFLOW. CFS
1 2 3 4 5 li 7
BASIN INFLOW,. CFS
Figure 23. Sediment capture versus discharge.
In the tinal test, the simulant was allowed to accumulate in the
basin through a series of runs at a constant flowrate. The
results, as shown in Table 22 and Figures 25, 26, and 27, show
the removal efficiencies to be relatively unaffected until a
breakthrough point is reached, at which time they become erratic
and even negative. This breakthrough in the experimental test
occurred when the storage basin was filled to just over one-half
its depth. The cumulative percent retained by particle size at
the point of breakthrough is shown in Table 23.
TABLE 22. SEDIMENT
ACCUMULATION3
Event
Cumulative Depth, as
weight, lbb fraction of H2C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
IS
16
17
18
19
20
a.
b.
c.
11.4
22.3
33.3
44.1
55.1
66.3
77.6
88.2
98.6
108.4
116.9
126.1
135.1
143.4
151.3
159.1
162.4
167.9
170.4
167.0
Q = 56.6 L/s
g = Ib x 454.
H2 = distance
.04
.08
.12
.15
.19
.23
.27
.30
.34
.37
.40
.43
.46
.49
.52
.55
.56
.58
.59-
.57
(2.0 cfs).
from floor to
invert of outlet pipe
66
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-
. ^
0.50 1.0 1.5
PARTICLE SIZE, ram
2.0
180
160
2 140
E 120
ta
2 100
80
a
ui
= 60
<<
£ 40
20
0
BREAKTHROUGH
POINT
5 10 15 20
NUMBER OF EVENTS
NOTE: 1 EVENT CORRESPONDS TO THE ADDITION OF
7.092 E (15.6 Ib) GRADED SIMULANT.
Q-56.6 LA (2.0 cfs)
Figure 25. Sediment capture versus accumulation.
100
75
so
25
qey eii *7cu
, /OHn • OMn • /OH
MEAN SEDIMENT DEPTH
1.0H,
r APPROXIMATE
/ SURFACE
(PROFILE AT
BREAKTHROUGH)
MEAN DEPTH
- AFTER 19
EVENTS
— MEAN DEPTH
AFTER 9
EVENTS
SECTION THROUGH BASIN
Figure 26. Sediment capture versus accumulated depth.
68
-------
Photograph 40
Approach flow conditions during
simulant accumulation study.
Photograph 41
Simulant accumulation after
after 78 Ib
Photograph 42
Simulant accumulation
after 140 Ib
Photograph 43
Simulant accumulation
after 312 Ib
Graded Simulant retained' in Catchbasin During Accumulation Study
Q = 2 cfs
Note: Contour Elevation Numerals correspond to the following:
13 = 2.95 ft
12 = 2.72 ft
11 = 2.49 ft
10 = 2.27 ft
9 = 2.04 ft
8 = 1.81 ft
7 = 1.59 ft
6 = 1.36 ft
5 = 1.13 ft
4 = 0.90 ft
3 = 0.68 ft
2 = 0.45 ft
Figure 27. Photographic record - sediment accumulation,
69
-------
TABLE 23. AGGREGATE CAPTURE
EFFICIENCIES AT BREAKTHROUGH
Size, mm
2.0 0.84 to 2.0 0.25 to 0.84 0.10 to 0.25
Cumulative %
retained at
optimum event 90.11
75.43
47.77
10.Oa
a. Estimated. Direct measurement impossible because of
carryover of fines to sump and recycle system. 58.74%
measured in trap basin and tank.
CONCLUSIONS
The following conclusions are drawn from the hydraulic model
analysis:
1. Properly designed and maintained catchbasins can be
very efficient in removing medium to very coarse sands
from stormwater runoff. Further, the removals remain
high over a wide range of flows and reduce to
approximately 35 percent at maximum design inflow.
2. Removal efficiencies, as expected, are very sensitive
to particle size and specific gravity. Under the test
conditions examined, the removal of fine sands ranged
from fair to poor with increasing flow. Removals of
very fine sand and low specific gravity material
(gilsonite) were negligible at, 40 percent of maximum
flow.
Storage basin depth is the primary control for
performance; efficiencies improve with increasing
depth.
The accumulation of sediment in catchbasins does not
appear to impair solids removal efficiencies until 40
to 50 percent of the storage depth is filled. Beyond
this depth, removals drop rapidly, even to the point of
negative values (washout exceeds sedimentation).
5. Of the standard modifications tested, hoods or traps
were found to increase the discharge head, requirements
significantly. In the higher flow ranges, increased
scour currents were observed as the flow was diverted
downward by the obstruction of the outlet. By
comparison, curb openings or protrusions had negligible
effect.
3.
4.
70
-------
SECTION 7
ASSESSMENT
To assess the role of catchbasins, three questions are" of major
concern: When should a catchbasin be used? Should existing
basins be converted to inlets? When catchbasins are necessary,
how should they be designed and maintained?' The purpose of this
section is to identify user experience and attitudes, to review
the performance of existing catchbasins, to review some
alternatives, and to provide an overall assessment of the use of
catchbasins. The economic evaluation of the use of catchbasins
is considered in Section 8.
USER EXPERIENCE AND ATTITUDES
The 1973 member survey conducted by the APWA [102] discloses
some very interesting information on the role of catchbasins as
viewed by the users. Although, catchbasins are the subject of
much criticism and debate, they are still widely used, as shown
in Table 24.
By linear extrapolation of these sample results, there are
approximately 900,000 catchbasins in the United States in cities
of 100,000 and above and potentially 850,000 additional
catchbasins in sewered areas of smaller communities.
TABLE 24. CATCHBASIN USAGE IN LARGE U.S. CITIES
Combined system9 Separate system
No. Population • No. Population
Cities with population
greater than 100,000"
Reporting in survey^
Reporting in catchbasins
No. of catchbasins
62
31
18
18
30,787,781
9,905,897
5,028,190
203,847
91
49
20
20
24,180,093
16,949,878
5,989,376
' 189,163
a. 1974 Needs Survey
b. 1970 Census data.
c. 1973 APWA questionnaire responses [102].
71
-------
Further information drawn from the questionnaires, completed by
Public Works" Department representatives, are shown in Figures 28
and 29. The practice of using catchbasins in new construction
continues to be strong (4:1). However, a trend by the minority
to move positively away from such use appears to be, growing.
Clearly, the requirements and efficiency of cleaning catchbasins
is the major concern, and their effectiveness is viewed almost
exclusively from a solids removal viewpoint. Less than half of
the respondents' use traps in their catchbasins. Of those using
traps, the reasons given were to control odors, 65 percent; to
remove floating objects, 51 percent and other purposes,
32 percent.
CATCHBASIN PERFORMANCE
In the following discussion, catchbasin performance is
considered primarily from the point of view of hydraulics and
the removal of pollutants. Example problems dealing with the
computations involved in the evaluation of catchbasin
performance are presented. Odor production is also discussed.
Hydraulics
The hydraulic regime within the catchbasin is a function of inlet
configuration and location; drainage area size and runoff
coefficients; gutter longitudinal and street cross slopes;
rainfall intensity and duration? inlet and outlet conditions; and
internal geometry. Solids removal is affected by hydraulic
detention time and the degree of turbulence. As would be
expected, higher flow-through rates decrease the detention time
and increase the turbulence, which, in turn, decreases solids
removal efficiencies, particularly the finer sizes of solids.
For example, a flow of 14.2 L/s (0.5 cfs) through the standard
project catchbasin in a just-cleaned condition would have a
detention time of 1.8 minutes, and a flow of 2.8 L/s (0.1 cfs)
would have a detention time of 8.4 minutes. This range is
similar to that used in aerated grit chambers, as presented later
in the text. In general, the hydraulic regime in a catchbasin is
highly turbulent, and because of the accumulation of sediment,
extremely short detention times are the rule rather; than the
exception.
The techniques of reducing the turbulence and increasing
detention times apparently have not been used widely for design
criteria of catchbasins. Generally, within a city, a more-or-
less standard catchbasin is used with little regard to the size
of the drainage area or flow ranges to be expected. This results
in very diverse solids removal efficiencies.
Inlet grate configurations are usually limited to bars parallel
or perpendicular to the curb face with or without a vertical
opening in the curb face. From the standpoint of reducing grate
72
-------
QUESTIONNAIRE ITEM
ARE CATCHBASINS STILL BEING
USED IN NEW SEWER CONSTRUCTION
OR OLD SEWER RECONSTRUCTION?
NUMBER
RESPONDING
342
20
:;:;::::.•: NO;
RESPONSES, PERCENT
40 60
60
1 00
T I I
i
PERCENT OF CATCHBASINS:
ON COMBINED SYSTEMS?
0-10«
11 -89#
90-tOO»
ON SEPARATE SYSTEM?
0-10%
11-89%
90-100%
213
232
DO YOU BELIEVE CATCHBASINS ARE:
EFFECTIVE AS IS?
EFFECTIVE, BUT NEED
REDESIGN?
INEFFECTIVE AND SHOULD
BE ELIMINATED?
314
ARE CATCHBASINS TRAPPED?
DO YOU USE A DESIGN
OR DESIGN MODIFICATION TO:
ALLOW MORE EFFICIENT
SETTLING?
ALLOW CLEANING TO BE
DONE MORE RAPIDLY?
327
65
118
Figure 28. User experience and attitudes, 1973 [102]
73
-------
YEARS
1928-1932
1933-1937
1938-1942
1943-1947
1948-1952
1953-1957
1958-1962
1983-1987
1968-1972
NUMBER OF AFFIRMATIVE RESPONSES
8
8
9
i—i—i—i—r
T—i—i—r
10
T
11
T
12
T"
13
T
14
T
15
1
Figure 29. Year use of catchbasins stopped [102].
blinding, it is desirable to retain the vertical opening in the
curb face. Unfortunately, this vertical opening also invites
the use of the catchbasin as a garbage receptacle by the general
public. Most vertical openings in the curb face are limited to
12.7 cm (5 in.) because most curbs are only 15.2 cm (6 in.)
high.
From a hydraulic standpoint,•the greatest efficiency is obtained
by placing the inlet bar axis parallel to the flow, i.e., to the
curb face. Because this creates a hazard for bicyclists,
however, most inlet bars are placed perpendicular to the flow,
thereby reducing the hydraulic efficiency.
Source and Removal of Pollutants
Catchbasin performance with respect to pollution is mixed. The
trapped liquid that is purged from the basin to the collection
network during each storm generally has a high pollution content
and contributes to the intensification of the first-flush
loadings. Countering this negative impact is the removal of
pollutants associated with the solids retained in the basin and
subsequently cleaned out.
Liquid Fraction—
As street waste receptacles, the pollution content of the
retained liquid-solids mixture in catchbasins is both high and
variable [65, 120]. Normalizing the data presented in Table 16
by casting out the extremes and averaging, the characteristics
reduce to: COD, 6,400 rag/L; BOD, 110 mg/L; total nitrogen,
8 mg/L; and total phosphorus, 0.2 mg/L. For a. typical retained
volume of 545 L (144 gal), the approximate pollutant load (SOD,.)
74
-------
held in a basin computes to 82 g (0.18 Ib), or the equivalent
waste discharged by one person in one day.
The APWA estimated the way in which soluble pollutants in a
catchbasin at the start of a storm are flushed into a sewer
[42]. They experimented by adding 6.8 to 20.4 kg (15 to 45 Ib)
of sodium chloride dissolved in water to a catchbasin containing
1,336 L (353 gal). Water from a hydrant was discharged through
a hose and water meter to the gutter near the catchbasin.
Samples were taken from the effluent when various quantities of
water up to 6,378 L (1,685 gal) had been added to, "and passed
through, the catchbasin. The cumulative percent of salt
discharged as a function of gallons of liquid added is
illustrated in Figure 30.
100 r-
eo
3 60
40
20
EMPIRICAL CURVE Q81]
OBSERVED VALUES AT FLOWRATE OF:
1 cfm
4 cfm
7 cfn
o
+
•
200 400 600 600
LIQUID ADDED, GALLONS
1000
1200
Figure 30,
Relationship of flow into catchbasin and reduc-
tion of concentration on salt [adapted from 42],
An empirical formula developed to fit the curve in Figure 30
using BOD5 as the pollutant is given in Equation 6 [81].
R = 100
[l.O -
(6)
where R
x
V
percent of catchbasin BOD5 removed
cumulative inflow to catchbasin, gal
trapped volume of liquid in the basin before storm,
gal
75
-------
EXAMPLE PROBLEM 1: POLLUTION POTENTIAL OF DISPLACED LIQUID
CONTENTS OF CATCHBASINS
Determine the amount of pollution in terms of BODs released from catchbasins
for the specified conditions and relate this amount to the potential impact
on dry-weather treatment plant performance.
Specified Conditions
1. Average tributary area to catchbasin = 1.44 acres.
2. Volume of catchbasin sump =1.7 yd3, of which one-third is filled
with sediment.
3. Rainfall intensity = 0.02 in./h for 4 h.-
4. City population = 750,000.
5. Total number of catchbasins = 25,000.
Assumptions
1. BOD5 concentration in basin before storm = 110 mg/L.
2. Runoff coefficient = 0.50.
3. Equation 6 is applicable.
4. BOD5 in sewage =0.20 lb/capita-d.
Solution
Determine the pollution load in the basin prior to the storm.
Load = 110 mg/L x 1.7 yd3 x 764.6 L/yd3 x .67 = 95.8 g (0.21 Ib)
Determine the total runoff to the catchbasin. :
Runoff = 0.02 in./h x 0.50 x 1.44 acres
= 0.0144 acre-in./h = 0.0144 cfs
Volume = 0.-0144 cfs x 4 h x 3;600 s/h'x 7.48 gal/f3
= 1,551 gal
Determine the
% displaced
Dilution displaced from the basin using Equation 6.
-1,551
le pollution
= jl.Q - e1-
6 x 1.7 x 0.67 x 202
100
x 100
4.
I.O - e
= 98.5%
- 0,207 Ib/catchbasin
Determine the citywide release of pollution.
Release = 25,000 catchbasins x 0.207
= 5,171 Ib
5. Express the release of.pollution as. related to dry-weather plant loading.
Total plant loading = 750,000 people x 0.20
= 150,000 Ib/d
Equivalent reduction in plant performance on day of
storm = 5,171 Ib * 150,000 Ib = 3.4%.
Comment
The reported BOD concentrations measured in catchbasins are consistently
higher than concentrations normally found in running stormwater, fre-
quently by as high as 5:1. This may be accounted for by (1) the dumping
or flushing of waste material into basins between storms, (2) the
concentrating effect (treatment) of the runoff as it passes through the
small detention unit; (3) decomposition of the residual organic sediment
over time, and (4) evaporation.
76
-------
Sediment Fraction—
As determined in the hydraulic modeling studies, catchbasins can
be quite effective in removing medium-to-coarse sands and, to a
limited extent, they may also remove significant amounts of
smaller particles. Sartor and Boyd [66] have identified
pollutants in street surface contaminants associated by particle
fuZe,,in !rhe dry state- Thus, by assuming a fixed percentage of
the dry pollution remains with the particles in the wet and
turbulent confines of a catchbasin, solids removals can be
equated to pollution removals.
For first-cut assessments, ratios between particle-related
pollution in the dry and wet states have been assumed as shown
in Table 25.
TABLE 25. ASSUMED RELATIONSHIPS BETWEEN!
DRY STATE AND WET STATE CHARACTERISTICS
To convert dry
state value fora
To obtain wet
state value
Total solids and
heavy metals, multiply byb
Organics, nutrients,
bacteria, and
pesticides, multiply byb
o.5
a. As reported in Tables 14 and 15.
b. Pollution assumed adhering to
particles and not washed free.
These assumptions are consistent with the high pollution levels
found in catchbasin residuals and in pollution loadings
associated with the solids fraction removed in wastewater
treatment facilities.
Th-e performance of a catchbasin in capturing solids can be
related, at least qualitatively, to the performance of an aerated
grit chamber found in many sewage treatment plants. Solids
removal in both systems increases with increasing detention
times. The two systems differ in that the turbulence pattern in
a grit chamber, and therefore the size of particles that are kept
in suspension, can be more closely controlled. Otherwise, the
predicted performance of an aerated grit chamber and a catchbasin
at similar detention times follows the same pattern, as seen in
Figure 31. Data for catchbasin performance are from the'
experimental work presented in Section 6; data for an aerated
grit chamber are based on theoretical settling rates. A properly
77
-------
designed and operated air system would keep most particles below
a given size in suspension, as shown in the figure.
100
o
Of
Ul
a.
Ul
CO
Q
Ul
SO
0=
CO
LEGEND
A EXPERIMENTAL DATA,
STANDARD PROJECT CAKHBAS.IN
AT 3-HIN DETENTION TIME
© THEORETICAL REMOVAL.
AERATED GRIT CHAMBER
AT 3-MIN DETENTION TIME
BASED ON SOLIDS SPECIFIC
GRAVITY OF 2.B5
EFFECT OF IDEAL AIR SYSTEM
(ROLL VELOCITIES) IS TO KEEP
ALL PARTICLES ABOVE A SELECTED
SIZE-WEIGHT IN SUSPENSION.
SILT, VERY FINE SAND
* l-U :
FINE SAND
MEDIUM SAND
COAIRSE SAND
0.05
0.1
0. 5
I.O
PARTICLE DIAMETER, mm
Figure 31. Comparison of catchbasin
and grit chamber performance.
78
-------
The impact of flowrate and particle size on optimal catchbasin •
removals, as determined experimentally, is illustrated in Figure
32. Descriptions of the tests and other presentations of the
data are included in the preceding sections of this report.
While prototype field tests were beyond the scope of the present
study [note that 0.76 m3 (1 yd3) of simulant weighs 1,362 kg
(1.5 tons)], a few field tests were performed earlier by Sartor
and Boyd [66] from which comparisons can be drawn.
100
100
Q= 178.4 L/S
(6.3 CFS)
100
50
Q = 133.1 L/S
(4.7 CFS)
Q = 89.2 L/S
(3.15 CFS)
0 = 44.7 L/S
(1.58 CFS)
100
ui
ee
100 v
* 50
UJ
a:
Q =35. 4 L/S
(1.25 CFS)
0= 14. 2 L/S
(0.50 CFS)
Q = 7.1 L/S
(0.25 CFS)
PARTICLE SIZE, mm
LEGEND
Figure 32. Model basin performance versus flow.
In the first field test, a graded simulant was washed into a
clean catchbasin in San Francisco. The test was run at a set
flow condition 7.9 L/s (0.28 cfs), and the removal efficiencies
versus time for various size particles were determined as shown
in Figure 33. Note that virtually all of the particles larger
than 0.246 mm (fine sand) were removed and that virtually all of
the particles smaller than 0.10 mm (very fine sand and silts)
passed through. A comparison of removals achieved in the
modeling studies and this field test is shown in Table 26. The
comparisons are considered very good recognizing that the model
dimensions were optimized.
The low removals in the smaller size ranges are particularly
significant because most of the pollutants,, (e.g., 59.6 percent
volatiles, 56.8 percent BOD5, 80.1 percent COD, 77.0 percent
nitrates, 92.2 percent phosphates, and 43.4 percent total solids)
79
-------
100
0
LLJ
- 40 -
1 2 3 4 5 6 7 8
TIME SINCE FLUSHING BEGAN, MIN
9
I '
Figure 33. Prototype performance versus time [66]
TABLE 26. COMPARISON OF REMOVALS IN MODEL
AND PROTOTYPE TESTS
Size of
simulant,
mm
>3.0
0.84 to 2.0
0.25 to 0.84
0'. 10 to 0.25
<0.10
Model
Q = 7.1 L/s
(0.25. cfs)
99.4
99.3
97.5
68.6
• • • •
Removals ,
Fielda'b
Q = 7.9 L/s
(0.28 cfs)
97.8
91.1
82.3
51.1
12.6
%
Model '
Q = 14.2 ;L/s
.(0.50 cfs)\
99.0
98.9
91.5
45.2
• • • •
a. Value 10 minutes after flushing began.
b. From Sartor and Boyd [66].
are contained in the 0.25 mm diameter and finer size .particles
(see Table 15), which indicates that catchbasins may be
relatively ineffective in reducing pollution.
80
-------
Later in the field study, tests were run on "dirty" catchbasins
to see how much of the contents would be removed by various
flowrates of clean city water over a 40-minute time interval.
The test was described as follows [66]:
The catchbasins all had several thousand pounds of solids in
them, with a layer of water (and floating debris) up to the
outlet level. The water first discharged was very dirty,
composed primarily of this supernatant water, some of the
floating matter, plus particulate matter suspended by the
turbulence flow. Within a few minutes that water became
reasonably clear but still contained particulates. Even
after nearly an hour's flushing, the discharge contained
much particulate matter. At the end of an hour, inflow was
stopped and the volume of basin contents was measured.
The results are summarized in Table 27.
TABLE 27. TEST OF "DIRTY" CATCHBASINS [66]
Catchment
area,
Catchbasin ha (acres)
A 0.39
B 0.23
C 0.10
(0.96)
(0.57
(0.25)
Inflow rate,
L/s (cfs)
7.9
7.9
6.5
(0
(0
(0
.28)
.28)
-23)
Weight of
solids in
basin at outset,
kg (Ib)
929
1,162
1,580
(2,
(2,
(3,
047)
559)
481)
Equivalent
depth of
sediment3
0.75
0.94
1.27
H2
H2
H2
Solids
flushed from basin
during storm
Weight, Fraction,
kg (Ib) %
13.4
13.6
9.8
(29.6) 1.2
(30.0) 1.1
(21.6) 0.6
a. Based on standard configuration shown in Figure 1: H? = 121.9 cm (4.0 ft) D
= 83.8 cm (2.75 ft), dry unit weight = 1,846 kg/cm3 (115 Ib/ft3). 1
It can be concluded that, at normal rates of runoff, most of the
sediment material originally contained in catchbasins tends to
remain there.
Again referring to the model studies, under substantially higher
inflows—56.6 L/s .(2.0 cfs)—catchbasins were shown to be
ineffective for even coarse solids removals when the accumulated
sediment exceeded 50 to 60 percent of the storage basin depth
(0.5-0.6 Hg), but performance was relatively unaffected until
this breakthrough point was reached.
EXAMPLE PROBLEM 2: POLLUTION REMOVED BY SEDIMENTATION IN CATCHBASINS
Determine the amount of material that will enter a storm sewer for the
specified conditions and the amount of material.that will be removed as a
function of the number of times a catchbasin is cleaned each year.
81
-------
Specified Conditions
2.
3.
4.
For the area under consideration, the curb length per catchbasin
= 0.10 curb mile and the average tributary area to a catchbasin
— 1.44 acres.
The volume of the catchbasin sump =1.7 yd3.
Annual precipitation = 35.1 in.
Storm events per year =50.
Assumptions
1. Representative event duration = 5 h.
2. Unit weight of material retained in catchbasin = 110 lb/ft3.
3. Equation 3 is applicable, and runoff factors varying from
0.8 to 0.5 will be used for computation.
4. Maximum effective sump storage between cleanings = 50% of
sump volume.
Solution
1. Determine the effective capacity of the catchbasin in pounds.
Capacity =1.7 yd3 x 27 ft3/yd3 x 110 lb/ft3 x 0.5
= 2,524 Ib per cleaning
2. Determine the amount of available material that can enter each
catchbasin for the following constituents: total solids, volatile
solids, BODs, COD, Kjeldahl nitrogen, nitrates, phosphates, and
total heavy metals. This is accomplished by multiplying the values
given in Table 14 by 0.10, which is the distance in curb miles that
is connected to each catchbasin. The results are given in the
following tabulation:
Material available for entry
into sewer with each storm
Value, lb/0.10
Constituent curb mile
Total solids
Volatile solids
BOD5
COD
Kjeldahl nitrogen
Nitrates
Phosphates
Total heavy metals
140
10
1.3
9.5
0.2
0.01
0.11
0.16
3. Compute the representative rainfall intensity in inches per hour.
Intensity = 35.1 4- (50 events x 5 h)
= 0.14 in./h
4. Using total solids as an example, determine the amount of maiterial that
actually enters the catchbasin using Equation 4.
PO
140
140
140
140
0
0
i
.14
.14
0.14
0
.14
A
0
0
0
0
.8
.7
.6
.5
0
0
0
0
r
.11
.10
.08
.07
t
5
5
5
5
Po - P
129
126
118
112
% removed
92
90
84
80
.0
.0
.1
.0
82
-------
5.
Determine the amount of total solids that enters each catchbasin annually
Assuming that the total solids available for removal are the same for each
storm, the amount that enters each catchbasin during a year for various
runoff factors is given in the following tabulation.
Total solids
A
0.8
0.7
0.6
0.5
Ib/storm
129
126
118
112
lb/yra
6,450
6,300
5,900
5,600
a. Ib/storm x 50
storms/yr.
6. Determine the average rate
of runoff factors!
tO *"" basin f°r the f °' l™ing range
High inflow = 0.11 in./h x 1.44 acres = 0.16 cfs
Low inflow =0.07 in./h. x. 1.44 acres = 0.10 cfs
n« ani°unt ?f material actually retained in the catchbasin. For
probable best conditions, use model data for the closest available flow
range (Table 21, Q = 0.25 cfs). For probable worst conditions ule
aggregate capture efficiencies at breakthrough (Table 23) Sfing the
oartioL ^ 6 " ±n "hlcl? the fraction °f particles associated with each
particle size range is given and Table 25 which accounts for dry to wet
state conversion, the amount of material entering the sewer that will be
removed in the catchbasin is given in the following tabulation-
Percentage of material retained in
Constituent
Total solids
Volatile solids
BOD5
COD
Kjeldahl nitrogen
Nitrates
Phosphates
Total heavy metals
Total pesticides
Probable 1
Worst
42. la
15.2
15.5
7.5
14.6
9.5
2.3
37.4
13.6
i retained
Best
75.0
25.5
26.6
14.1
27.4
17.1
6.0
64.4
29.7
a.
Total solids [24.4 x 0.9011 +7.6
x 0.7543 + 24.6 x 0.4777 + 27.8
x 0.100] x 1.0 = 42.1
83'
-------
Determine the amount of material that will be removed a;s a function of
the number of times a catchbasin is cleaned each year assuming best
removals. The amounts are summarized in the following tabulation:
Percentage of total amount of material entering catch-
Cleaning
Constituent
Total solids
Volatile solids
BODs
COD
Kjeldahl nitrogen
Nitrates
Phosphates
Total heavy metals
Total pesticides
0.
19
6
6
3
7
4
1
16
7
5
.6
.6
.9
.7
.1
.4
.6
.8
.7
1.
39,
13
13
7
14
8
3
33
15
frequency, times/yr
0
.la
.3
.9
.4
.3
.9
.1
.6
.5
2.
75
25
26
14
27
17
6
64
29
0
.0
.5
.6
.1
.4
.1
.0
.4
.7
3.
75
25
26
14
27
17
6
64
29
0
.0
.5
.6
.1
.4
.1
.0
.4
.7
4.
75
25
26
14
27
17
6
64
29
0
.0
.5
.6
.1
.4
.1
.0
.4,
.7
a. [(2,524 (see step 1) x 1.0 cleaning
frequency)7(6,450 (see step 5) x 0.750 (see
step 7)] x 100 = 52.2% of total solids xn
sediment removed which equals 52.2% x v.s.0%
(see step 7) = 39.1% of total solids entering
basin. ;
Comment
The percent retained values determined in step 7 of the solution
represent the probable ranges of material that could be removed in
the catchbasin assuming 100 percent efficiency, adequate cleaning,
and the use of the recommended or equivalent catchbasin design.
Odors
The use of a trap on a catchbasin can provide a water seal to
control odors. During a prolonged dry period, however, the water
evaporates and the seal is probably lost. In a survey of 725
catchbasins in San Francisco [65], it was found that more than 45
percent were too full of debris to determine whether a trap
existed'or not. Of the remaining 468 catchbasins, 30 percent had
no trap. Odors were observed in only 3 catchbasins. In the same
area, 38 percent of the catchbasins that had a trap had no seal.
In the catchbasins that had an odor (1 percent), the odor did not
appear to be related to the seal•condition.
Thus, it appears that the water seal trap incorporated in many
catchbasins is not necessary and, in fact, causes problems of
clogging and maintenance. The emission of sewer gas from a.
catchbasin does not seem to be a greater nuisance than the odors
generated within a septic catchbasin sump. The water seal trap
in a catchbasin should in many cases be eliminated.
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REVIEW OF ALTERNATIVES
The principal alternatives to the use of catchbasins involve
replacement with inlets, sewer cleaning, street cleaning, and the
use of flow-attenuation devices and off-line storage.
Standard Inlets
Catchbasins can be replaced by standard inlets without any
adverse effects where the sewers are laid at sufficient grade to
provide self-cleaning velocities. In this case, the catchbasin
replacement may provide a benefit by reducing the cost of
catchbasin cleaning.
In the San_Francisco Bay Area, both catchbasins and inlets are
currently in use. Catchbasins are used predominantly in older
sections of the communities, particularly where combined sewers
either are in use or were originally constructed but have now
been abandoned.
Inlets are being used exclusively in new construction throughout
the area. Catchbasins are being converted to inlets in some
municipalities, and inlets are being installed to replace
catchbasins when road reconstruction takes place. Catchbasins
are being eliminated even where combined sewers are still in use.
An example of a conversion standard used in San Francisco is
shown in Figure 34. To date, approximately 1,000 units have been
converted within the city.
Sewer Cleaning
If a catchbasin is replaced by a standard inlet and the sewer is
not laid at sufficient grade to provide self-cleaning velocities,
the effect will be increased sewer cleaning costs. These may be
offset, however, by the savings in catchbasin cleaning costs.
Such cost comparisons are considered in Section 8.
Maintenance personnel in cities with catchbasins have some strong
feelings with regard to the use of catchbasins and the conversion
of catchbasins into inlets. They feel that increased use of
inlets will cause restricted flow in storm sewers from sediment
buildup. This sediment buildup can be excessive during low-flow
periods. Yet, to date, among the municipalities interviewed, no
problems have developed, and there has been no need to drag or
extensively clean any storm sewers except for a few small lines—
15.2 or 20.3 cm (6 or 8 in.) diameter. Some of these lines have
plugged despite the existence of catchbasins on the particular
offending line.
85
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FRAME AND BRATIN8
PAVEMENT
CULVERT
REMOVE TRAP FLUSH
Wi'TH WALL
FILL WITH SLURRY
OR SAND
• '
- '
: 4*4
. | XAT CONTRACTOR'S
I j / OPTION
J L_S I
I
2 ft - 9 In.
3 In. CLASS
8-3,000-3/4
CONCRETE
I
NOTE:
1. EXISTING FACILITIES SHOWN AS DASHED LINES.
2. NEW WORK SHOWN AS SOLID LINES OR AS INDICATED.
SOURCE: MTV AND COUNTY OF SAN FRANCISCO.
DEPARTMENT OF PUBLIC WORKS.
BUREAU OF ENGINEERING.
Figure 34. Conversion of catchbasin to inlet detail.
86
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Street Cleaning
Street cleaning is not—but possibly should be—related to the
type of stormwater collection system in use. Street cleaning is
performed for mostly cosmetic or aesthetic reasons rather than to
prevent dirt and pollution from contaminating surface runoff.
Downtown and business districts are cleaned most frequently,
often daily, while residential areas are cleaned less frequently,
as few as four times per year.
To minimize sewer cleaning cost intensified by the absence of a
catchbasin, the street cleaning program may have to be stepped up
to reduce the amount of material entering the sewer. The costs
of increased street cleaning must be weighed against the costs of
sewer cleaning, with the added benefit of street cleaning's
reduction of pollutants evaluated as a benefit of increased
street cleaning.
In conjunction with street cleaning, the subject of public
education in keeping the streets clean and not using the storm
drain_inlets as receptacles for trash, garbage, and crankcase
drainings is worth emphasizing. As shown in the San Francisco
survey of catchbasin contents, the wide variability of the BODC
and COD values clearly indicates a contribution of other-than-
normal street surface contaminants.
Flow-Attenuation Devices and Onsite Storage
Some type of upstream flow-attenuation device or onsite storage
unit may eliminate the need for many catchbasins as well as
provide for increased treatment of combined flows by lowering
the peak discharge flow to the treatment plant. The reduction
in stormwater overflows would do much to decrease the pollution
load on receiving waters.
The objective of onsite storage of runoff is either to prevent
storm flow from reaching the drainage system or to change the
timing of the runoff by controlling the release rate. Retention
is the term for total containment, and detention is the common
term for delaying and controlling the release rate to smooth out
the peak flows. As a watershed undergoes urbanization/the
amount of impervious area increases, and the natural drainage
system is usually encroached upon by development. Greater
quantities of storm runoff are generated by the paved areas, yet
the flow must be transported by the limited drainage system.
The result is an unacceptable form of onsite storage where
basements, underpasses, and city streets serve as the
reservoirs.
One Alternative to solve the problem is to build a manmade
drainage system with concrete-lined channels or large-diameter
sewer pipe to carry the runoff out of the basin. However, this
87
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solution may be unacceptable either because it is too expensive
or because it compounds the problem by quickly moving large
quantities of water downstream arid flooding lower watersheds.
A second alternative is to control the runoff where it is
generated—including urbanized areas—by the controlled flooding
of off-highway cloverleafs and medians, parking areas,, park
lands, and roof tops—and hold it in retention or detention
ponds. Decreasing the flowrate may allow the natural drainage
system to serve the watershed or, at least, may require a less
extensive, less costly manmade system.
The impact on catchbasin usage is twofold: (1) if the natural
drainage system survives or is reinstituted, catchbasins are
unnecessary; and (2) if runoff rates—hence particulate carrying
velocities—are significantly reduced, more solids will remain
on the watershed instead of being flushed into the system, again
reducing the need for catchbasins.
Consolidation
Where catchbasins are necessary for either odor/floatable
control or sediment removal, their number may be reduced by
consolidating the flows of several inlets through a single
catchbasin before entering a drain or sewer. For example, at
street intersections there are typically three catchbasins to
intercept gutter flows without requiring any flow to pass across
a traveled way. If two were inlets and were connected to a
third which was a catchbasin, the same function would be
accomplished, but the cleaning requirement (setups) would be
less and the purged liquid pollution would be cut by 67 percent.
With less setups required, the frequency of cleaning could be
increased with potentially further benefits.
ASSESSMENT
Inherent Problems
Some of the problems associated with the use of catchbasins are:
o A single catchbasin configuration cannot perform
optimally for all possible flow conditions.
o The particle sizes most difficult to retain in a
catchbasin are the sizes associated with the highest
level of pollution.
9 Inlet grate hydraulic performance and bicycle safety
do not appear to be compatible.
9 Catchbasins can act as septic tanks and can generate
soluble BOD5 between storm events. This material is
88
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washed out of the catchbasin and contributes to the
"first flush" pollution load of the stormwater runoff.
• Catchbasin contents can generate obnoxious odors due
to the decomposition of the organic matter trapped by
the basin or dropped into the basin by people.
• Catchbasin cleaning and maintenance programs are
expensive and frequently inadequate.
• Failure to clean catchbasins regularly can render them
ineffective for any beneficial function, e.g., solids
removal, odor trap, or even unobstructed flow from
streets to collections.
Pollutant Contribution
As has been noted in the preceding discussion, catchbasin
performance is mixed with respect to pollution, even with proper
maintenance in effect. A perspective as to the potential
balance between good and bad is given in the following example.
EXAMPLE PROBLEM 3: ANNUAL POLLUTION ASSESSMENT OF CATCHBASIN PERFORMANCE
Given the conditions expressed in the preceding problems, determine the
aggregate effectiveness of the catchbasins over a period of years in terms
of BODs removed.
Specified Conditions
1.
2.
3.
4.
5.
Total number of catchbasins = 25,000.
Curb length per catch basin = 0.10 curb mile.
Annual precipitation = 35.1 in.
Catchbasins are cleaned twice a year.
The pollution load displaced from each basin is 0.21 Ib BOD,, for each
of 50 storms occurring in a year. 5
The runoff coefficient^ 50%.
Assumptions
1.
2.
The annual rainfall can be characterized as 50 equal 5-h storms.
BODs removal by sedimentation will total 26.6% of the applied load.
Solution
1. Determine the annual loss of BOD5 by liquid volume displacement.
BOD5 loss = 25,000 basins x 50 storms/yr x 0.21 Ib/basin per storm
=. 262,500 Ib/yr
2. Compute the BODs entering a catchbasin each storm (following
procedures of earlier example).
BOD5 entering = 1.3 Ib available x 0.80 removed from streets
=, 1.04 Ib
3. Determine the annual removal of BODs by sedimentation.
BODs removed = 25,000 basins x 50 storms x[1.04 Ib x 0.266]/basin per storm
= 345,800 Ib/yr ...'..
89
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4. Compute the annual net benefit.
Benefit = 345,800 Ib removed - 262,500 Ib lost
= 83,300 Ib removed
5. Express as a percent of the annual total applied load.
Beneficial removal = 83,300 (from step 4)
* [25,000 x 50 x 1.04 (steps 2 and 3)3 x 100
= 6.4% of applied load
Comment
The problem illustrates that from a pollution abatement standpoint the benefits of
catchbasins are limited. Of course, with the cleaning frequency of twice per year,
the liquid fraction pollution might average half the specified value, thereby in-
creasing the benefit; however, the gross improvement is still small (16. 5% versus 6. 4%) .
A comparison of the effectiveness in terms of ultimate BOD removed annually would
probably be more meaningful. However, no data could be found indicating the ulti-
mate BOD expected from urban surface runoff. The make-up of urban surface runoff is
sufficiently different from other wastes for which .ultimate BOD and reaction rate
constants are known, preventing meaningful extrapolation of existing data. Also,
there are no data available on the reactions that take place on material within the
sump of a catchbasin.
Toxic materials may cause a lag period in the BOD reaction or suppress the BOD
result, particularly the BODs' result. Since storm-generated discharges may contain
large amounts of heavy metals and other materials which are toxic to the biochemical
processes, the BOD determined may be lower than the actual oxygen demand.
It is also possible that septic conditions in catchbasins may result in the digestion
of large organic particles and refractories. This could make additional material
available in forms more conducive to biological degradation. Thus, it is possible
for not only the soluble BODs in the catchbasin to increase, but also for the
ultimate BOD to increase.
The BODs test is generally used to indicate the oxygen demand of the wastewater. A
first order equation is generally used, to describe the BOD progression with time;
namely,
_
y = L(l - 10 )
in which y = BOD, L = carbonaceous ultimate demand, k = reaction rate constant,
and t - time of sample incubation period in days. Using the above equation and a
series of BOD determinations at varying time intervals, the values of "L" and "k" can
be estimated. If different wastes have the same BODs but different rates of deoxygen-
ation, the ultimate demands can vary significantly.
The BOD20 (20-day BOD) test can be used to better estimate the ultimate oxygen demand
of a waste. The main advantage is that the importance of estimating the correct k
value is reduced. As the BOD approaches the ultimate value, the variation caused by
different k -values is lessened. In the BOD20 determination, a significant portion
of the nitrogenous demand is included and the effects of toxicity are less since the
organisms have had sufficient time to adapt to the environment.
Another test that can be used to evaluate the ultimate BOD and reaction rate constant
is the delta chemical oxygen demand (ACOD) test. The total oxygen demand (TOD) test,
a chemical rather than a biological test, can be used to estimate the ultimate BOD.
Wullschleger, et al. , have recommended that both the BODs and the TOD tests be used
as oxygen demand indicators [121] . The TOD test was chosen as the indicator of the
total potential oxygen demand and the BODs test satisfied the need for a comparative
and biochemical test.
Cleaning Programs
The literature sources, especially the textbook references,
indicate that the major problem associated with a catchbasin
installation is an inadequate cleaning program. The major
90
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obstacle appears to be the cost involved and the apparent lack
of concern of many public officials. The catchbasin cleaning
program is often neglected in favor of activities that are more
visible and thus may make a greater impression on the general
public [9] .
The allocation of manpower to inlet and catchbasin cleaning is a
problem confronted by all municipalities. Many municipalities
have learned that the key to a successful program is regular
maintenance. If these devices are not cleaned routinely, the
problem becomes uncontrollable very rapidly. The cleaning crews
must divide their time between a regularly scheduled cleaning
program and emergency repairs and cleanings required during wet
weather. Often, a job performed during an actual rainstorm must
be checked and completed after the storm has passed when more
time is available. This cycle of emergency repair and follow-up
can occupy most of a cleaning crew's time and thereby prevent
routine maintenance from being performed, hence becoming a self-
perpetuating and ever-increasing problem.
There is a tendency to modernize cleaning methods, but because
of the configuration of many of the older catchbasin designs,
many cities must continue to use less efficient hand and/or
bucket cleaning methods. Besides being inefficient in manpower
use, these methods do not provide as -adequate a cleaning as the
more modern eductor or vacuum cleaning methods.
Although it might be conceivable that a city with very few
catchbasins would find it uneconomical to invest in a modern
vacuum cleaning vehicle, the multiple-use features of- these
machines make this unlikely.
Ray Richards, City Engineer for Marion, Indiana (population
40,253), enthusiastically supported the mechanization of his
community's catchbasin cleaning effort in a recent Public Works
article [110]. The Marion program is described in the following
quotation taken from the article:
Part of our correctional program, started in July, 1974,
has involved planned preventive maintenance through the 254-
mile storm drain system. The worst sewer lines and
manholes have now been cleaned, or soon will be, by a
combination of jet cleaner and Elgin Jet-Eductor.
Complaints now average three per storm [—down from 25 or
more before program implementation].
Previously, we cleaned manholes and catchbasins using
manually-operated dip spoons and clamshell buckets.
Throwing material up and onto a dump truck slowed the work
so that the task took a two-man crew from 20 to 45 minutes
to complete. The Eductor does a better job and in less
time. Now it rarely takes two men more than 5 to 10 minutes
91
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per catch basin from the time they pull up to the site
until the work is done.
Cleanout is simplified, too. There are no extension tubes
for the men to attach; no accessory equipment and no hand
tools are needed. The Eductor and its suction nozzle are
all that is necessary to remove stones, bricks, leaves,
litter and muck.
Because no dump trucks are required to support the machine,
time is saved by the crew. And, of course, the equipment
and personnel needs for a catch basin cleaning job are
reduced. Also, we find that the men do not tire as they
did in the past.
The Elgin unit has helped us change our entire
program...Now, instead of rushing from one emergency to
another, we have established a planned preventive
maintenance schedule. Because the Eductor has made the job
so fast and effortless, we hope to clean catch basins in a
12-month cycle. Thus, we will get to each basin more
frequently, particularly those which are troublesome and
those located close to commercial and industrial
establishments. Cleanout on a regular basis has the
additional advantage of restoring full design flow to the
sewer lines. Our program now is one of action, not
reaction.
Conclusions
From the information thus far reviewed, the general response to
the questions raised at the beginning of this section is that
catchbasins should be used only where there is a solids
transporting deficiency in the downstream collection sewers and
drains, or at specific sites where available surface solids are
unusually abundant (such as beach areas, construction sites,
unstable embankments, etc.). The advantages to be considered in
the conversion of existing catchbasins to inlets, assuming these
criteria are satisfied, are (1) a direct reduction in the "first
flush" pollutant load, (2) a reduction in required maintenance,
and (3) the opportunity to reallocate the conserved labor.
Design criteria for new basins were given in the preceding
section, and the recommended cleaning frequency should be
adjusted to limit the sediment buildup to 40 to 50 percent of
the sump capacity.
Now let's look at the economics.
92
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SECTION 8
ECONOMIC EVALUATION
The economic evaluation of alternative storm and combined sewer
designs with respect to the use of catchbasins or inlets is
described and illustrated in this section. Economic criteria are
presented, along with basic cost information, an analysis of
alternatives, and a brief summary discussion.
ECONOMIC CRITERIA
To properly assess the economic feasibility of alternative storm
sewer installations, it is necessary to prepare detailed cost
estimates. Before such estimates can be prepared, however,
economic criteria must be selected to ensure that equivalent
costs are compared. For example, a true evaluation of
alternatives can be based on present worth or annual cost. In
general, annual cost comparisons are preferred because the
significance of the cost components is more easily understood.
For this reason, annual cost comparisons are used in this report.
Components of annual costs include operation, maintenance,
supervision, depreciation, and interest on borrowed capital.
Annual interest and depreciation, commonly referred to as "fixed
costs," are computed using the capital recovery method [107].
The recommended recovery period (also referred to as useful life)
for storm sewers will vary from 20 to 40 years. Often, short
return periods are used when future plans are uncertain,
especially with regard to regionalization. The current (November
1976) interest rate charged on borrowed money varies from 7 to
10 percent.
Because costs are changing so rapidly, both nationally and
locally, it is extremely important that any cost evaluation be
referenced to some index. One of the most common is the
Engineering News-Record Construction Cost (ENRCC) index. Other
important indexes include the EPA Sewer Cost and Treatment Plant
indexes. When possible, index values should also be adjusted to
reflect local costs, which may be higher or lower than the
national index. An ENRCC index of 2000 is used in this report.
The following formula can be used to adjust the reported costs to
another index value:
adjusted cost = (reported cost) (Valufnnn index)
-------
COST DATA AND INFORMATIONS?
To properly evaluate alternative plans involving :the use of
catchbasins or inlets, data must be available on catchbasin and
inlet construction costs, cleaning costs for catchbasins and
inlets, and sewer cleaning costs.
Catchbasin and Inlet Costs
After a drainage system has been designed, inlet facilities can
be constructed using either a standard inlet or a catchbasin
without affecting the design, since both devices have
practically the same maximum hydraulic capacity. Typical cost
data for catchbasins and inlets are presented in Table 28. The
reported costs will vary, depending on the size of the
catchbasin or standard inlet used by a particular city, but it
can be assumed that the construction cost of a typical
catchbasin will be about 20 to 40 percent more than the cost of
a standard inlet. Catchbasin costs are shown in Figure 35 as a
function of retained storage capacity.
TABLE 28. COST DATA FOR
CATCHBASINS AND INLETS
Catchbasins
Inlets
Range Avg Range Avg
Total installed
cost, $a 400-1,000 800 300-800 600
a. Based on an ENRCC index of 2000.
Catchbasin and Inlet Cleaning Costs ;
Catchbasin cleaning, when done adequately, is an expensive
aspect of catchbasin use. The operation and maintenance costs
of a catchbasin consist of (1) the catchbasin cleaning and
debris disposal costs, (2) maintenance costs of those items of
the catchbasin not found in a standard inlet, such as the trap
and sump, and (3) the operation and maintenance costs of the
catchbasin cleaning equipment prorated if used for other
purposes, such as leaf removal from gutters. Catchbasin
cleaning costs will vary, depending on the method used, the
required cleaning frequency, the amount of debris removed, and
debris disposal costs.
Typical costs for cleaning catchbasins by hand, with an eductor,
and by vacuum, are reported in Table 29 both for those regions
94
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£ 2.0
U)
•«
aa
1.5
1.0-
0.5
0.0
0.0
STORAGE CAPACITY (SUMP), yd*
Figure 35. Catchbasin cost versus storage capacity
TABLE 29. CATCHBASIN CLEANING COSTS
a,b
Manual cleaning
Statistical
measure0
Regions with
heavy winter
snowfall
Sample size
Geometric
mean , Mg
Standard
deviation, Og
National
Sample size
Geometric
mean , Mg
Standard
deviation, Og
$/catch-
basin
17
10.53
4.53
51
7.66
3.04
$/m3 ($/yd3)
10
9.08 (6.94)
10.10 (7.72)
37
18.86 (14.42)
11.18 (8.55)
Eductor cleaning
$/catch-
basin
5
3.23
3.38
10
5.92
3.30
$/m3 ($/yd3)
6
3.01 (2.30)
17.76 (13.58)
10
5.35 (4.09)
13.18 (10.08)
Vacuum
$/catch-
basin $/
26
4.94 9.
2.97 2.
51
7.99 11.
3.05 5.
cleaning
'm3 ($/yd3)
14
86 (7.54)
20 (1.68)
37
24 (8.59)
95 (4.55)
a
b. Data from APWA survey
c. See Appendix B.
(using breakdowns of survey data by state) with heavy winter
snowfall and all of the regions considered together. The cost
comparisons between cleaning methods appear reasonable; however,
the unit costs as a group appear low and should be verified
against local experience. In the case of hand cleaning,
95
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cleaning costs would be expected to be more expensive in regions
with heavy snowfall because of exposure. Cleaning costs with
eductor and vacuum systems in regions with heavy snowfall should
be lower because there are more catchbasins per unit area, and
the basins are usually cleaned more frequently. Geographic
location as related to the pollution load is also a factor.
Although there is little information or cost data available,
inlet cleaning costs must be considered in any analysis of
alternatives. On the basis of limited data, it appears that
cleaning costs for inlets are about $3.00 per inlet using a
vacuum system. The costs will vary with location and the design
of the inlet. ;
Sewer Cleaning Costs
Cleaning costs for sewers will vary with the size of the sewer
and amount of material to be removed. Representative sewer
cleaning costs based on the sewer size are reported in Table 30.
In view of the magnitude of the costs involved in cleaning
sewers of any type, accurate cost data must be obtained for
local conditions before preparing an economic evaluation of
alternatives where sewer cleaning costs will be a central issue.
TABLE 30. REPRESENTATIVE
SEWER CLEANING COSTS3
Cost
Sewer size
and type
$/cm diam
per- lin m
($/in. diam
per lin ft)
Diameter £122 cm
U48 in.)b
Storm 0.095 (0.075)
Combined 0.195 (0.15)
Diameter ^-122 cm
(>48 in.)
Storm 0.13 (0.10)
Combined0 0.26 (0.20)
a. Based on an ENRCC index of 2000.
b. Range $0.03 to $0.19 in. diam per lin
ft [111].
o. In Boston, 13,000 ft of 60 in; diam
combined sewer was cleaned for a total
cost of $11.50 per foot of sewer [112].
ECONOMIC ANALYSIS OF ALTERNATIVES
Because of the expense involved, sewer cleaning frequency is a
prime consideration in the installation of a catchbasin.
96
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Ultimately, the cost differential between the installation of
catchbasins and the installation of inlets can be defined as:
Acost = A installation cost + A sewer cleaning cost + A
catchbasin/inlet cleaning cost 4- A pollution costs
associated with use of catchbasins
The pollution cost term is composed of (1) cost savings
associated with grit or pollution load savings attributable to
the catchbasin cleaning program and (2) costs associated with
any pollution load attributable to the use of catchbasins.
These two costs are difficult to evaluate in most systems but
may be measurable in large systems. For practical purposes, the
decision on whether or not to' install a system with catchbasins
or inlets can be made by comparing (1) the annual costs for the
initial installation of catchbasins or inlets, (2) the yearly
cleaning costs, and (3) the equivalent annual costs for sewer
cleaning for each system. The actual computations involved in
the preparation of an economic evaluation of alternatives are
illustrated in the following examples. The first two examples
deal with the conversion of catchbasins to inlets in an existing
system. The third example deals with the question of whether to
install catchbasins or inlets in a new installation. The fourth
example illustrates the choice between the purchase of
additional equipment and investing in structural improvements.
EXAMPLE PROBLEM 4:
ECONOMIC EVALUATION OP CONVERTING CATCHBASINS TO INLETS
IN AN EXISTING STORM SEWER SYSTEM
Prepare an annual cost comparison between the continued operation of a storm
sewer with catchbasins and the same system if the catchbasins are converted
to inlets for return periods of 10, 20, 30, and 40 years.
Specified Conditions
1.
2.
Total number of catchbasins in storm sewer system
Storm sewer sizes, lengths, and volumes:
Diam, in. Length, ft Volume, £t3
140.
12
18
24
36
10,000
5,000
4,000
4,000
7,850
8,840
12,570
28,270
3.
4.
Storm sewers with catchbasins are cleaned once every 10 years.
Existing catchbasins are well designed, cleaned twice every year, and
achieve a 50 percent capture of the entering material.
Discussion
Separate storm sewer systems are traditionally designed to provide localized
flood relief at minimum cost. This frequently results in mixed systems of
natural channel, improved open channel, and enclosed conduit subsystems in
various combinations. As a result, street drainage, which may or may not be
routed through catchbasins, constitutes only a portion of the solids entry
to the system. In this example it is assumed that the solids deposited in the
enclosed conduit subsystem become cost effective to remove when the total
volume- of the enclosed conduits is reduced 10 percent [57,530 ft3 x 0.10
= 5,753 ft3j.
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In the specified case of storm sewers with catchbasins, this accumulation is
reached every 10 .years on the average, representing an annual accumulation of
of 575 ft3 per year, even though the storm sewers have been constructed with
"self-cleaning" velocities.
Under the modified conditions, catchbasins replaced with inlets, the accumulation
rate will be increased in proportion to the additional solids entering but not
carried through the system. "Assuming the catchbasins each had a sump volume
of 1.7 yd3 and were cleaned on the average when they were 40 percent full, the
total sediment removed per year per-basin was 36.7 ft3 [1.7 yd3 x 27 ft3/yd3
x 2 times per year x .40 full = 36.7 ft3] and for all catchbasins was 5,141 ft3
per year [140 basins x 36.7 ft3 = 5,141 ft3]. Because of the "self cleaning"
velocities most, say 90 percent, of this material would pass through the storm
sewer system. The remaining 10 percent, however, represents an additional
annual accumulation in the storm sewers of 514 ft3 per year; thus almost
doubling the accumulation rate from 575 ft3 per year to 1,089 ft3 per year and
shortening the time between sewer cleanings from 10 years to 5 years (see
Assumption 6 below).
Assumptions
1. Catchbasins and sewers have just been cleaned.
2. The cost of cleaning each catchbasin using a vacuum system
« $8 per cleaning (see Table 29).
3. Sewer cleaning costs are as specified in Table 30.
4. The cost of converting a catchbasin to an inlet = $200.
5. Each inlet will have to be cleaned once every 2 years at a cost
of $3 per inlet. . . j '
6. If the catchbasins are converted inlets, it is anticipated that
the sewers will have to be cleaned once every 5 years.
7. Interest rate = 8%.
8. Inflation rate for sewer cleaning costs =4%.
9. Catchbasin and inlet cleaning costs will increase by $0.50 and
$0.15 each year, respectively. These cost increases are consistent
with improvements in equipment which tend to decrease costs.
Solution
1. Determine the sewer cleaning costs at today's prices.
Pipe diam, Length,
in. ft
Cost, $
12
18
24
36
Total for
system
10,000
5,000
4,000
4,000
Per lin ft Total
0.90
1.35
1.80
2.70
9,000
6,750
7,200
10,800
33,750
2. Determine the future sewer cleaning costs taking into account
inflation and converting those costs to present worth.
Present worth
Time, yr Factora cost, $ Factor** Cost, $
33,750
27,955
23,137
19,159 :
15,861
13,137
10,879
9,003
7,454
Single payment compound amount factor at
4% for the period shown in years.
b. Single payment present worth factor at 8%
for the period shown in years.
0
5
10
15
20
25
30
35
40
1.000
1.217
1.480
1.801
2.191
2.666
3.243
3,946
4.801
33,750
41,074
49,950
60,784
73,946
89,977
109,451
133,177
162,034
1.0000
0.6806
0.4632
0.3152
0.2145
0.1460
0.0994
.0.0676
0.0460
98
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3.
Determine the present worth of the sewer cleaning costs for each
alternative plan for the various return periods, and convert those
costs to a uniform annual cost for those periods.
Alternative la
Alternative 2b
10
20
30
40
23,137
38,998d
49,877
57,331
0.14903
0.10185
0.08883
0.08386
3,448
3,972
4,431
4,808
51,092
86,112
110,128
126,585
4.
Period, Present Annual Present Annual
yr worth, $• Factor0 cost, $ worth, $ Factor0 cost, $
0.14903 7,614
0.10185 8,770
0.08883 ?,783
0.08386 10,615
a. Retain catchbasins.
b. Convert catchbasins to inlets.
c. Capital recovery factor at 8% for the period shown in years.
d. From Step 2 ($38,998 = $23,137 + $15,861).
Determine the initial cost of converting the catchbasins to inlets,
and convert this cost to a uniform annual cost.
Conversion cost = 140 x $200/conversion = $28,000.
Convert the initial cost to annual cost.
Period, yr Factor
10
20
30
40
0.14903 4,173
0.10185 2,852
0.08883 2,487
0.08386 2,348
5. Determine the annual cost of cleaning the catchbasins for the
various return periods.
Period, Base
vr cost, '•
b.
Gradient Gradient Annual
factor3 cost, $ cost.
2,782
3,226
3,527
3,720
Accounts for yearly incremental increase
in cost at i = 8% [107 pp 50-52] .
140 basins x $0.50 annual cost increase
x 3.87 x 2 cleanings/yr.
10
20
30
40
2,240
2,240
2,240
2,240
3.87
7.04
9.19
10.57
542b
986
1,287
1,480
6.
Determine the annual cost of cleaning the inlets for the various
return periods.
Period,
10
20
30
40
Base
cost, $
210
210
210
210
Gradient
factor
3.87
7.04.
9.19
10.57
Gradient
cost, $
41
74
96
111
Annual
cost, j
251
284
306
321
7. Prepare a summary of the annual costs for each alternative.
Annual cost, $
Alternative
1 - Retain
catchbasins
2 - Convert
catchbasins
to inlets
Return Catch-
period, Catchbasin Sewer basin Inlet
Yr conversion cleaning cleaning cleaning Total
10
20
30
40
10
20
30
40
__
— —
— —
"
4,173
2,852
2,487
2,348
3,449
3,972
4,431
4,808
7,614
8,770
9,783
10,615
2,782
3,226
3,527
3,720
__
__
__
—
251
284
306
321
6,230
7,198
7,958
8,528
12,038
11,906
12,576
13,284
99
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Comment
Prom the computational summary presented in Step 7 of the solution,, it can
be concluded that the cost and required frequency of cleaning the esxisting
sewers is the dominating economic consideration with respect to conversion
of catchbasins to inlets. For example, if the sewer cleaning frequency
were to remain the same after conversion (i.e./ sewer cleaning .costs would
be the same in each alternative), the economic advantage would .switch to
Alternative 2 by the 20th year.[$2,852 conversion + $3,972 sewer cleaning
+ $284 inlet cleaning = $7,108 which is less than $7,198].
EXAMPLE PROBLEM 5: RECONSIDERATION OP PROBLEM 4 WHERE SEWER CLEANING
CONCERNS ARE LIMITED TO A SMALL PORTION OP THE SYSTEM.
Repeat the annual cost comparison of Problem 4 assuming that 90 percent of
the specified storm sewer system is known to be free of solids sedimentation
problems.
Specified Conditions
1. Same as Problem 4, except that trouble spots with respect
to solids deposition are known and limited to 10 percent of the
pipe network.
Assumptions
1. The trouble spots are contiguous and cleaning unit costs remain
the 'same.
2. The storm sewer sizes and lengths requiring cleaning remain in
the same proportions as in Problem 4.
Solution
1. Repeat Step 7 of Problem 4, except reduce the sewer cleaning
costs to 10 percent of their previous value.
Annual cost, $
Alternative
1 — Retain
catchbasins
2 - Convert
catchbasins
to inlets
Return Catch-
period, Catchbasin Sewer basin Inlet
yr conversion cleaning cleaning cleaning Total
10
20
30
40
10
20
30
40
4,173
2,852
2,487
2,348
345
397
443
481
761
877
978
1,061
2,782
3,226
3,527
3,720
__
~
~
—
251 '
284
306
321
3,127
3,623
3,970
4,201
5,185
4,013
3,771
3,730
Comment ,
Knowledge and understanding of the operational characteristics of the
specific system under study is an essential input to the analysis for
proper decision-making.
EXAMPLE PROBLEM 6:
ECONOMIC EVALUATION OF INSTALLING CATCHBASINS
OR INLETS IN A NEW DEVELOPMENT
Prepare an economic comparison based on annual cost of the installation of
catchbasins and inlets in a proposed new development in which separate storm
sewers are to be used. Omit the storm sewer construction cost, as it will
be the same for both systems. Also, determine the sewer cleaning frequency
at which the annual costs for the two alternatives are essentially the same.
100
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Specified Conditions
1.
2.
3.
4.
5.
Development ,area = 520 acres.
Separate storm sewers are to be installed.
Return period for project = 36 years.
Interest rate = 8%.
Neglect inflation costs in economic analysis.
Assumptions
1. Catchbasin density = 0.46/acre.
2. Cost of cleaning each catchbasin using a vacuum system = $8
(see Table 29).
3. Cost of cleaning each inlet = $3.
4. Sewer cleaning costs as specified in Table 30.
5. Catchbasins will be cleaned twice per year.
6. Inlets will be cleaned once per year.
7. Cleaning of storm sewers with catchbasins will occur once
every 18 years.
8. Prepare computations assuming that the storm sewers with
inlets will have to be cleaned every 6, 9, 12, 15, and 18 years
(see discussion under Example Problem 4).
Solution
1. Total number of catchbasins required = 240 (520 acres x 0.46
catchbasins/acre).
2. Using four 130-acre units, a typical layout for the interceptor
storm sewers is presented below:
BO In. dim
84 In.
48 In. dlan
48 In-
3. The 7orresponding storm sewer pipe size distribution for each
130-acre parcel might be as follows:
Pipe diam,
in.
10
15
18
24
30
36
48
Length,
ft
530
4,450
880
3,100
1,030
1,200
1,900
The exact pipe size distribution- will vary with each location.
101
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4, Compute the cost of cleaning the storm sewers.
Pipe diam,
in.
10
1-5
18
24
30
36
48
60
84
•Total cost
Total
length, ft
2,120
17,800
3,520
12,400
4,120
4,800
7,600
2,380
1,190
Cost, $
Per ft
0.75
1.12
1.35
1.80
2.25
2.70
3.60
4.50
6.30
Total
1,590
19,936
4,752
22,320
9,270
12,960
27,360
10,710
7,497
116,395
5. Compute the present worth of future cleaning costs.
Time, yr Factor3 Cost, $
6
9
12
15
18
24
27
30
36
0.6302
0.5002
0.3971
0.3152
0.25'02
0.1577
0.1252
0.0994
0.0626
73,352
58,221
46,220
36,688
29,122
18,355
14,573
11,570
7,286
a. Single payment present
worth factor at'8% for
the period shown in years.
6. Determine the total present worth of future cleaning costs; and
convert them to annual costs.
alternative
Storm sewers with
catchbasins
Storm sewers with
inlets
Cleaning
interval,
yr
18
6
9
12
15
18
Total
present
worth, $
Factor3
36,408b .08535 3,107
185,905 .08535 15,867
109,202 .08535 9,320
71,861 .08535 6,133
48,258 .08535 4,119
36,408 .08535 3,107
7.
8.
9.
10.
a. Capital recovery factor at 8% for 3-year period.
b. Sum of present worths (Step 5) for 18th and 36th
year.
Determine the annual cost of installing catchbasins.
$800/catchbasin x 240 catchbasins x 0.08535 = $16,387/yr
Determine the annual cost of instafling inlets.
$600/inlet x 240 inlets x 0.08535 = $12,290/yr
Determine the annual cleaning cost for catchbasins. ,
240 catchbasins x 2 cleanings/yr x $8/catchbasin = $3,480/yr
Determine the annual cleaning cost for inlets.
240 inlets x 1 cleaning/yr x $3/inlet = $720/yr.
102
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11.
Prepare a summary of annual costs excluding storm sewer
construction costs, which will be the same for both systems.
Alternative
Storm sewers with
catchbasins
Cleaning
interval/
yr
18
Annual cost, $
Construction
16,387
Catchbasin or Sewer
inlet cleaning cleaning Total
3,840
3,107 23,334
Storm sewers with
inlets
6
9
12
15
18
12,290
12,290
12,290
12,290
12,290
720
720
720
720
720
15,867
9,320
6,133
4,119
3,107
28,877
22,330
19,143
17,129
16,117
12.
Determine the sewer cleaning frequency at which the costs for
the two systems are essentially the same. Based on the cost
xnformation presented in Step 11, the annual cost for' the two
systems will be about the same when the sewer cleaning frequency
for the system with inlets is approximately equal to 8.5 years
EXAMPLE PROBLEM 7:
ECONOMIC COMPARISON BETWEEN STRUCTURAL AND
NONSTRUCTURAL ALTERNATIVES
This example illustrates yet another option to be considered by city
administrators. Should a proposed capital investment be placed into
equipment that will improve the effectiveness of maintenance of the existing
system, or should a corresponding investment be used for structural modi-
fications that will reduce the need fo,r maintenance?
A community has 5,000 catchbasins that are presently cleaned once per year.
This cleaning frequency has proven to be inadequate and plans have been
proposed either to:
1. Double the cleaning frequency by the purchase and operation
of a new mechanical cleaner, or
2. Convert sufficient existing catchbasins to inlets to allow present
crews to clean the remaining catchbasins twice per year and each
inlet once every 2 years.
Which alternative will be more economically attractive over the next 20 years?
Specified Conditions
1. A new mechanical cleaner will cost $30,000, and with a crew it can
clean an average of 5,000 catchbasins per year. The useful life of
the cleaner is 10 years.
The average cost of cleaning a catchbasin is $8.00.
The average cost of cleaning an inlet is $3.00.
The cost to convert a catchbasin to an inlet is $200.
Interest rate = 8%.
Assumptions
1. Sewers are self-cleaning and will not be impacted by the conversion.
2. Neglect inflation costs in the economic analysis.
3. Neglect pollution control aspects. '
Solution
1.
Compute the existing cleaning capability in dollars.
5,000 catchbasins x 1 time/yr x $8/catchbasin = $40,000
103
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Determine the number of catchbasins that will have to be converted
to inlets to meet maintenance objectives with existing crews.
(a) No. catchbasins x $8 x 2 times/yr + No. inlets x $3 x 0.5 times/yr
- $40,000
(b) No. catchbasins + No. inlets = 5,000.
Solving (a) and (b) simultaneously,
No. catchbasins = 2,241
No. inlets = 5,000 - 2,241 - 2,759
= No. of catchbasins to be converted
Compute the capital cost of conversion, and express the amount as
annual cost over 20 years.
Capital cost - 2,759 x $200 - $551,800
Equivalent annual cost (capital recovery factor - 8% - 20 yr)
- 0.10185 x $551,800
= $56,200
Compute the present worth of purchasing one mechanical cleaner now
and a complete replacement unit 10 years from now, and express the
amount as annual cost over 20.years.
Capital cost - $30,000 + (single payment present worth
factor - 8% - 10 yr) x $30,000 = $30,000 + (0.4632) x $30,000
- $43,896
Equivalent annual cost = 0.10185 x $43,896 - $4,471.
Determine the annual cost for alternative (a).
5,000 x $8 x 2 times/yr -f $4,461 (from Step 4) = $84,471
Determine the annual cost for alternative (b).
$40,000 (from Step 2) + $56,200 (from Step 3) - $96,200'
Thus, the purchase of a mechanical cleaner would be more
economically attractive.
Comment
If inflation were a major consideration, as illustrated in Problem 4,
Assumption 9, or if the evaluation period were significantly longer,
the cost advantage could very well shift to the structural alternative.
The choice, however, is not exclusively economic as is shown in the
following example.
EXAMPLE 8: POLLUTION CONTROL AND OTHER COST CONSIDERATIONS
Given that the use of inlets in preference to catchbasins reduces surface
maintenance problems and costs, the questions remain as to what extent has the
cost merely been transferred to another maintenance area and how has overall
pollution control been effected? Compare the annual unit costs of removal of
sediment and pollution in terms of BODs for the following:
1. A separate storm sewer system with catchbasins
2. The same system without catchbasins
3. A conventional 10 Mgal/d activated sludge wastewater treatment facility
Specified Conditions ',
1. Criteria and assumptions of Problems 4 and 5 apply.
2. The activated sludge treatment plant removes' 90% of an average
influent BODs load of 200 mg/L.
Assumptions
1. The average annual capital and operation and maintenance costs of a
10 Mgal/d activated sludge plant are $950,100 and $283,200, respectively
[113].
104
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Within this plant the average annual capital and operation and maintenance
costs of the aerated grit chamber alone are $26,480 and $16,425,
respectively.
The average quantity of grit removed at the plant is 3.5 ft3 per million
gallons of wastewater.
Solution
1. For system No. 1, compute the average annual cost of removing solids
from catchbasins.
[$8.00 x 140 catchbasins x 2 cleanings per year] T 5,141 ft3 solids
removed = $0.44/ft3
Assuming a weight of 110 Ib/ft3, this is equivalent to $0.44/ft3
* 110 Ib/ft3 = $0.004/lb total solids removed.
2. For system No. 1, recompute the average annual cost in terms of BODs removed
[$8.00 x 140 catchbasins x 2 cleanings per year] -f [1.04 Ib/storm
x 50 storms x 140 basins x 0.064 removed (following procedures of
Problem 3)] = $4.81/lb BODs removed,
3. For system No. 2, compute the additional cost of removing street solids
from the storm sewer system assuming 10% by volume settles out in the pipes.
[$7,614 - $3,449 (annual cost chanae for,10-yr return period, Step 7,
Problem 4)] * 514 ft3 removed = $8.10/ft3.
Assuming a weight of 110 Ib/ft3, this is equivalent to $0.074/lb total
solids removed for conditions described in Problem 4 and $0.0074/lb total
solids removed for conditions described in Problem 5.
4. For system No. 2, the BODs removed is considered negligible.
5. For system No. 3, the average-annual cost of removing solids through the
aerated grit chamber is
[$*f'!!(Lcapital + $16'425 0&M3 * £3.5 ft3/Mgal x 10 Mgal/d x 365 d]
— 5-5.36/ft-J
6. For system No. 3, the average annual cost of removing BOD5 is
[$950,100 capital + $283,200 O&M] T l~200 mg/L x 0.90 removed x
mg/L~
Comment
In a combined sewer system, trapping and cleaning street solids from catchbasins,
if practiced effectively, could significantly reduce peak grit loadings on the
treatment plant headworks. This net cost savings, as well as reduced wear in
headworks pumps and screens should be considered when evaluating catchbasin
effectiveness. It should also be noted that in many combined systems, solids
^o^PJn "}S P^pe system mav be largely a dry-weather flow phenomenon, as a
result of reduced carrying velocities; thus, observation of the real system
^ =Vi0VS f "^essity. For pollution control benefits other than solids, the
impact of catchbasins is likely to be small, based on presently available data
DISCUSSION
The economic evaluations illustrated in this section emphasize
the importance of systematic and accurate recordkeeping in
catchbasin and inlet maintenance programs and in sewer cleaning.
The approach discussed is basically one of how an alternative
course of action will prove to be economical in the long run, as
105
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compared to other possible actions. Contributing factors include
the time period under consideration, the interest rate, and the
anticipated inflationary or noninflationary trends.
The dominant cost factor for decision-makers appears to be sewer
cleaning. How will the required cleaning frequency change, and
which areas of the pipe network will be subjected to increased
deposition as a result of using inlets versus catchbasins? If
the differences are small, the use of inlets is favored.
Selected recent developments, a case history example, and
suggested continuing program needs are considered in the
following section.
106
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SECTION 9
RECENT DEVELOPMENTS AND CONTINUING PROGRAM NEEDS
As has_been documented in the previous sections, catchbasins
historically have been constructed solely as a reaction device.
That is, when solids deposition in sewers was found or suspected
to be a problem, catchbasins were installed to trap these solids
so that they could be removed at a more convenient location.
Recent thought, however, as now being evaluated through Public
Law 92-500, Section 208 Environmental Management Studies, is
directed at action rather than reactionary measures. Through
the adoption and implementation of best management practices,
perhaps we will no longer have to accept as a given condition
that gutter flows will be high in inorganic solids, thus closing
out the historic role for catchbasins. The purpose of this
section is to present briefly and review (1) some recent
developments in the design and operation of catchbasins, (2) a
case history example, and (3) some thoughts on continuing
program needs.
RECENT DEVELOPMENTS
Four aspects of recent developments in the design and operation
of catchbasins are described: source controls, shock flow
reduction, catchbasin modification, and system controls.
Source Controls
Best management practices are designed to remove or reduce the
problem at the source. High solids loadings in gutter flows
(oatchbasin feed water) are the result of two things; (1) a
high available supply of erodible material and (2) suspending
and carrying intensities of flow. Remove the supply (through
street sweeping, construction site controls, effective ground
covers, general good housekeeping, etc.) and reduce the rate of
flow (impounding, infiltration-percolation, selective flow
routing, check dams, grassed buffer strips, etc.), and you may
reduce or eliminate the problem. Because there has been too
little demonstration to date on controlled versus uncontrolled
broad test areas, the results that can be achieved,
unfortunately, remain ill-defined.
107
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Shock Flow Reduction
A system for reducing shock flows on storm drain systems has
been developed in Denmark and Norway over the past 15 years
[114]. The system basically consists of a storage basin with a
rate control orifice on the outlet pipe. Flow enters the basin
through the top grating and passes through a sediment trap
(optional) into the storage area. When a predetermined level is
reached in the basin, discharge begins. The orifice control
then regulates the discharge flow to a reduced amount as
compared to the inflow.
The sediment trap, located just under the inlet gratings, can be
obtained in different materials, depending on the desired size
of particle to be trapped. The trap is in the form of a bucket
or filter bag, both of which are reusable. The filter bag is
capable of retaining solids down to approximately 50 microns.
The bucket type is used to trap a much larger size;material.
Peak flow reductions to the sewer system (up to 95 percent have
been reported) can preclude the need for collection system
enlargement, and it is presumed that large quantities of
sediment will be retained in the basin or filter. Removal of
the sediment presents the same problems and opportunities as
with catchbasins.
This system is patented and is undergoing promotional marketing
in the United States and Canada at the present time. A
demonstration concept proposed for a United States, application
in Cleveland is shown in Figure 36. By retarding the street
runoff inflows to the collection system, preferential capacity
is given to roof and building drainage, thus hopefully reducing
basement flooding and overflows.
Catchbasin Modification
Existing catchbasins can be modified for one of two major
purposes. First, the function of trapping solids can be
eliminated by filling the sump of the catchbasin with concrete
or some other suitable material. Second, the catchbasin
geometry can be altered to effect better solids separation. In
addition to these major modifications, the catchbasin can be
modified by removing the water seal trap or by making the
catchbasin self-draining.
Filling the sump to eliminate the solids separation feature of a
catchbasin will allow the solids in the runoff to pass to the
sewer. Unless the sewer has adequate velocities to be
reasonably self-cleaning or the runoff contains very little
sediment, filling the sump should be viewed with caution because
this could greatly increase sewer cleaning costs, as has
previously been discussed. Designers should evaluate the sewers
108
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HYDRO-BRAKE REGULATOR IK MANHOLE
1ST1HG COMBINED SEWER SYSTEM
HYDRO-BRAKE CONCEPT FOR REHABILITATION
OF EX1STIIIG COMBINED SEWER SYSTEMS TO
ELIMINATE OVERFLOWS AND SURCHARGES.
UNITED STATES PROPOSAL.
Figure 36. Shock flow reduction concept [114].
to ensure that self-cleaning velocities are maintained before
recommending that catchbasin sumps be filled. A case history of
this approach is outlined later in this section.
Recommendations for optimal catchbasin geometry were presented
earlier. On the basis of the hydraulic model analyses, it is
concluded that supplemental baffling or extensive design
modifications would not be cost effective. The reason is that
present configurations effectively remove coarse solids if there
is proper maintenance, and selective removal of small particle
size and low specific gravity material (which constitutes the
maximum pollutant load) is impractical.
Removing the water seal trap is conditionally recommended on the
basis of the San Francisco catchbasin survey [65] in which it
was found that odor is not necessarily a result of not having a
trap^but probably is generated in most cases by septic
conditions in the catchbasin itself. The cleaning program for
catchbasins would be more efficient without the various types of
water seal traps, and the construction costs would be lower.
109
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The increased efficiency of the catchbasin,cleaning program
might lessen the chances of ddbr generation by preventing septic
conditions from occurring in the catchbasin.
In this same area of reducing septic conditions in the
catchbasins, providing a self-draining feature would help to
keep the catchbasin contents dry and could lessen the chance of
odor generation between cleanings. The problems associated with
the construction and maintenance of such a drainage feature,
however, appear to outweigh the benefits.
System Controls
Settling basins, flush tahks, and improved solids (swirl)
separators are potential system controls to augment or replace
catchbasins.
Settling Basins and Flush Tanks—
Conceptually, the objective to be achieved by replacing
catchbasins with settling basins is to reduce the cost and to
increase the effectiveness of stormwater solids separation
techniques. An undergound structure that would be large enough
to effectively trap the solids in the stormwater at peak
flowrates is envisioned. T&is basin would also attenuate the
storm flow reducing downstream carrying capacity requirements,
thus reducing combined sewer overflows in a similar manner to
that described under shock flow reduction. After the storm has
subsided, the liquid portion would continue to discharge to the
sewer and would eventually be treated at a wastewater treatment
facility.
In a study conducted by FMC Corporation for the EPA, it was
concluded that it was feasible to construct flush tanks in
conjunction with keeping combined sewer laterals clear of
sediment deposits from dry-weather buildup [104]. The principle
in the operation of a flush tank is the release of additional
water to the sewer to create a sufficient velocity in the sewer
to transport the sedimentary material. The same principle could
be used in the controlled cleansing of combined sewer trunklines
and storm drains. Either a flush tank or control gate bould
retard the storm flow until sufficient water was stored to
provide the required cleaning velocities, or it could release
water from its own supply and perhaps generate flushwaves in
sequence to periodically flush the storm drains.
Ideally, the benefits of shock flow reduction and system
flushing could be combined if the waters that are temporarily
held back contained minimum solids. This introduces a third
family of devices—the swirl and helical separators.
110
-------
Swirl and Helical Separators—
Swirl and helical separators rely on the centrifugal
acceleration caused by changing the direction of a stream of
water to separate the heavier solids from the overflow water [93,
94, 95, 96, 97, 115, 116, 117, 118, 119]. These devices have
been investigated for treating storm flows so that a concentrated
stream can be intercepted and sent to the wastewater treatment
plant, while the overflow water, which is relatively clean
compared to the normal combined sewer overflow, is allowed to
continue on to the receiving waters. In the foregoing
conceptual application, the overflow stream would be directed to
the flush tank to be released only after the downstream
collection system drained to near prestorm conditions.
Obviously, the complexity of such an approach precludes its
being assessed in the form of a general case.
CASE HISTORY
The City of San Francisco, Department of Public Works, has
embarked on a phased program to convert catchbasins to inlets in
a carefully selected and monitored manner [120]. This program,
initiated in 1969, has resulted in the conversion of nearly
1,000 units (out of a total of 25,000) to date, all associated
with scheduled street reconstruction and sewer projects.
Because the first-phase selection criteria require only
scheduled construction for other projects and the nondetection
of odors in the affected manholes, the units are located
randomly throughout the city.
Evaluation has included matching of odor complaints (recorded
with the Bureau of Water Pollution Control between 1967 and
1973) to the location of the units, a preliminary statistical
breakdown of the existing inlets with respect to factors
contributing to the generation of odors, comments from the
Health Department on the effects of public health and rodent
control, and comments from the bureau on the maintenance and
odor complaints.
Seven of 360 odor complaints over the 6-year study period were
in the vicinity of a converted unit. Thus, official complaints
in the vicinity of converted units are running at less than half
of the citywide rate.
The Health Department comments are particularly enlightening.
Eliminating the sumps is endorsed because standing water in
catchbasins provides a breeding ground for mosquitos; however,
the loss of the water trap creates a situation that may worsen
the rat problem [120]:
The main reason for concern appears to be the practice of
[the public] dumping garbage into catchbasins. The curb
111
-------
inlets provide a large opening that makes the dumping of
garbage convenient. This opening also allows rats to enter
the catchbasins to use the garbage as a food supply.
Furthermore, without the trap, rats in the sewer system
readily detect and have easy access to the garbage.
The present solution is to restrict the curb inlet openings (see
Figure 34). Based upon its experience to date, the city has
identified the following criteria with respect to proceeding
with the conversion of catchbasins to inlets in the next phase:
1. Does not create a public nuisance by providing a vent
for odors from the sewer main;
2. Does not contribute to public health problems by
continuing to be a convenient dump and becoming more
accessible as a food source for rats;
3. Minimizes the public nuisance and vehicular and
pedestrian traffic hazard of plugged catchbasins; and,
4. Minimizes the maintenance effort of cleaning
catchbasins; and, does not transfer the maintenance
problems to a more difficult situation of cleaning
culverts and sewer mains.
The city's program is continuing with a contract now being
prepared to convert 250 additional units.
CONTINUING PROGRAM NEEDS
To obtain the data and information required to further evaluate
the function and continued use of catchbasins or other devices,
continuing demonstration programs must be developed and
implemented. Proposed objectives and a discussion of some
recommended studies are presented in the following discussion.
Objectives
The overall objectives of continuing programs should be to
delineate clearly the following:
1. The impact of best management practices in reducing
solids and other pollutant loads in surface runoff
that must be collected from urban areas.
2. The effectiveness, through field scale demonstration,
of closely monitored catchbasin cleaning programs with
respect to impacts of cleaning frequency and
techniques on solids carryover, general pollution
abatement, and associated costs.
112
-------
3. The problem of solids deposition within real sewer
systems and the extent to which this problem is
mitigated by properly functioning catchbasins. Is
surface runoff introduced through catchbasins or
inlets the prime source of the deposit material or
merely a contributing source?
4. The cost effectiveness of converting catchbasins to
inlets in a major prototype demonstration.
Implementation
Implementation of these programs should be carried out in a
minimum of three to five regionally representative urban areas
using two similar catchments in each area (one for control and
one for demonstration) of, say, not less than 100 nor more than
1,000 acres. Desirable regions would.be northeast, midwest,
southern, and western because of their differences in climate,
hydrology, and system characteristics. The term of the
demonstrations would be from 1 to 2 years to cover full seasonal
impacts. Ten to 20 percent of the catchbasins in each
demonstration site would be monitored weekly on a fixed schedule
for sediment- accumulation or erosion, trap effectiveness,
quality characteristics of the retained flow after mixing, and
general observations as to the conditions of the catchment.
In addition, at least two catchbasins in each catchment should
be equipped and monitored (quantity and quality) through
sequential sampling of the basin influent and effluent during,
say, ten storm events.
The results would be compiled, related to hydrology, basin
condition, best management practice, cost, etc., and a
performance assessment given. Where appropriate, the
demonstration would include monitoring of sediment accumulations
within the downstream collection system. All maintenance
activities in the test catchments would be logged as to labor,
equipment, material, and costs, and an assessment as to the
transferability of results given.
113
-------
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Appendix A
GLOSSARY
CATCHBASIN - A chamber or well, usually built at the curb line of
a street, for the admission of surface water to a sewer or sub-
drain, having at its base a sediment sump designed to retain grit
and detritus below the point of overflow.
COMBINED SEWER - A sewer receiving both surface runoff and sewage.
CURB-OPENING INLET - Vertical opening in the face of a curb for
the admission of surface water.
DISSOLVED SOLIDS - The anhydrous residues of the dissolved con-
stituents in water which cannot normally be separated from the
water by laboratory filtering.
INLET - A structure that provides an entrance for surface water
into a drain which is located below ground. Does not have a sump
for trapping solids as in a catchbasin.
INLET.GRATE - Framework of bars over an inlet or catchbasin for
the admission of surface water.
LATERAL - A sewer which discharges into a branch or other sewer
and has no common sewer tributary to it.
SANITARY SEWER - A sewer which normally carries domestic sewage
and into which stormwater, surface water, and groundwater are
precluded, so far as possible, unless intentionally admitted.
SETTLEABLE SOLIDS - Suspended solids which will subside in
quiescent water or other liquid in a reasonable period. Such
period is commonly, though arbitrarily, taken as one hour.
SEWER - A pipe or conduit generally closed, but normally noti
flowing full, for carrying sewage and other waste liquids.
STORM SEWER - A sewer which carries stormwater and surface water,
street wash and other wash water, or drainage, but excludes
sewage and industrial wastes.
123
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SUSPENDED SOLIDS - Solids that either float on the surface of, or
are in suspension in, water or other liquids, and which are
largely removable by laboratory filtering.
TOTAL SOLIDS - The dissolved and undissolved mineral constituents
in water.
124
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Appendix B
ANALYSIS OF CATCHBASIN SURVEY DATA
The principal objective pursued in the analysis of experimental
or survey data is comprehension of its significance. Typically,
the approach followed when analyzing data related to a given
variable is to define this central tendency and dispersion. The
measures used most commonly for this purpose are the arithmetic
mean and the standard deviation. In general, these measures are
adequate so long as the data are more or less evenly distributed
above and below the mean. Unfortunately, this is often not the
case with certain types of experimental and survey data.
As an example, data dealing with catchbasins tend to be unevenly
distributed or skewed. The reason for this is that the more
extreme values tend to deviate beyond the mean to a greater
extent than do the values that are less than the mean. This can
be seen clearly in the sample data reported in Table B-l. Often,
when sample data are skewed, they can be analyzed using skewed-
probability paper or log-probability paper. For the data con-
sidered in this report, it was found that a geometric distribu-
tion was best. For a geometric distribution the mean, MCT / and
the standard deviation,
expressions:
crg , are computed using the following
(Z log x)/n
log Mg =
log ag =
log Xg = log x - log M
125
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TABLE B-l. SUMMARY DATA ON AREA PER
CATCHBASIN FOR CITIES IN THE UNITED STATES [102]
Incorporated
City city area, mi2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
36.7
18.9
18.4
45.4
13.3
29.9
34.3
60.3
22.4
50.2
29.4
26.1
83.9
19.0
41.8
95.2
316.9
131.5
Number of Area per
catchbasins catchbasin, acre
32,000
1,100
10,090
25,000
5,500
12,000
8,350
14,546
3,561
6,000
3,500
2,500
5,100
1,100
2.069
4,000
10,500
2,000
0.7
1.2
1.2 :
1.2
1.5
1.6
2.6
2.7
4.0
5.4
5.4
6.7 :
10.5
11.1
12.9
15.2
19.. 3
42.0
= 2.59 km2
acre = 0.4047 ha
126
-------
Appendix C
FOREIGN LANGUAGE BIBLIOGRAPHY
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Koch, P., Les Reseaux D'egouts, 2nd ed. Paris. Prance,
Dunod. 1962.
de Morais, M.E. Elementos Para Construcao de Coletores
de Esgoto. Reparticao de Aguas e Esgotos-Bul. (Sao Paulo,
Brazil) Vol. 10, No. 20., pp 24-46. April 1948.
Probst, E. Strassenablaeufe (Sinkkaesten) aus Beton.
Zement, Vol. 27, No. 25 and 26, pp 382-387, June 23,
1938, and pp 398-402, June 30, 1938.
Zur Normung Von Strassenbleaufen. Tonindustrie-Ztg,
Vol. 58, No. 176, pp 932-934. September 20, 1934.
Schulze, J. Das Ende des Geruchverschlasses am Strassen-
blauf. Gesundheits-Ingenieur, Vol 57, No. 30, pp 373-374
July 28, 1934.
Schulze, J. Zur Normung Von Strassenblaeufen.
Gesundheits-Ingenieur, Vol. 56, No. 31, pp 364-368.
August 5, 1933.
Burkhardt, K. Unmittelbare Strasseneinlauefe. Gesund-
heits-Ingenieur, Vol. 51, No. 51 and 52, pp 612-616,
December 17, 1932, and pp 626-628 December 24, 1932.
Schrader, F. Sinkkaesten Aus Steinzeug. Tonindustrie-
Ztg, Vol. 56, No. 51, pp 648-649. June 23, 1932.
Ringel, A. Die Bedentung des Unterdruckfoerdersystens
(absaugung fuer die Kanalentschlammung und die Sinkkaesten
reiningung). Gesundheits-Ingenieur, Vol. 53, No. 20,
pp 308-314. May 17, 1930.
Verbilligung der Schlammabfuhr aus den strassen sink-
kaesten in Halle. Gesundeits-Ingenieur, Vol. 52,
No. 33, pp 585-586. August 17, 1929.
127
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CONVERSION FACTORS
U.S. Customary to SI Metric
U.S. customary
acre
acre-foot
cubic foot
cubic feet per minute
cubic feet per second
cubic inch
cubic yard
degree Fahrenheit
feet per minute
feet per second
foot (feet)
gallon (*)
gallons per acre per day
gallons per capita per day
gallons per day
gallons per square foot
per day
gallons per minute
gallons per square foot
per minute
gallons per square foot
horsepower
inch(es)
Inches per hour
Billion gallons
million gallons per
acre per day
million gallons per day
mil*
parts per billion
parts per million
pound (s)
pounds per acre per day
pounds per day per acre
pounds por 1,000 cubic feet
pounds per million gallons
pounds per cubic foot
pounds per square foot
pounds per square inch
square foot
square inch
square mile
square yard
standard cubic feet
por minute
ton (short)
yard
Abbr.
acre
acre-ft
ft3
cfm
cfs
in. 3
yd3
•F
ft/min
ft/s
ft
gal
gal/acre -d
gal/capita-d
gal/d
gal/ft2.d
gal/min
gal/ft2.rain
gal/ft2
hp
in.
in./h
Mgal
Mgal/acre-d
Hgal/d
mile
ppb
ppra
Ib
Ib/acre-d
Ib/d-acre
lb/1,000 ft3
Ib/Mgal
Ib/ft3
Ib/ft2
lb/in.2
ft2
in. 2
ni2
yd2
std ftVmin
ton
yd
Multiplier
0.405
1,233.5
28.32
0.0283
28.32
16.39
0.0164
0.76S
764.6
0.55S CF-32)
0.00508
0.305
0.305
3.785
9.353
3.785
4.381 x ID"5
1.698 x 10-3
0.283
0.0631
2.445
0.679
40.743
0.746
2.54
2.54
3.785
3>78S.O
0.039
43.808
0.0438
1.609
0.001
1.0
0.454
453.6
0.112
1.121
16.077
0.120
16.02
4.882 X 10-4
0.0703
0.0929
6.452
2.590
0.836
1.699
0.907
0.914
Symbol
ha
n>3
L
m3/min
L/s
cm3
L
n>3
L
•c
m/a
m/s
m
L
L/ha.d
L/capita-d
L/s
m3/m2-h
m3/ha.min
L/S
m3/mz-h
L/m2..s
L/m2
kW
cm
cm/h
ML
E>3
m3/m2-h
L/s
m3/s
km
mg/L
mg/L
kg
g
g/m3-d
kg/ha- d
g/m3
mg/L
kg/m3
kg/cn\2
kg/cm2
m2
cm2
km2
D2
m3/h
Mg (or t)
m
SI metric unit
hectare
cubic metre
litre
cubic metres per minute
litres per second
cubic centimetre
litre
cubic metre
litre
degree Celsius
metres per second
metres per second
metre (s)
litre (s)
litres per hectare per day
litres per capita per day
litres per second
cubic metres per square
metre per hour
cubic metres per hectare
per- minute
litres per second
cubic metres per «quare
metre per hour
litres per square metre
per second
litres per square metre
kilowatts
centimetre
centimetres per hour
megalitres (litres x 10s)
cubic metres
cubic metres per square
metre per hour
litres per second
cubic metres per second
kilometre
milligrams per litre
milligrams per litre
kilogram
grams
grams per square metre
per day
kilograms per hectare
per day
grams per cubic metre
milligrams per litre
kilograms per cubic metre
kilograms per uquare
centimetre
kilograms per oquare
centimetre
square metre
square centimetre
square kilometre
square metre
cubic metres per hour
megagram (metric tonne)
metre
128
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-051
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
CATCHBASIN TECHNOLOGY OVERVIEW
AND ASSESSMENT
5. REPORT DATE
May 1977 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AU
John A Lager, William G. Smith, and
George Tchobanocrlous
8. PERFORMING ORGANIZATION REPORT NO.
rcnrwnivniMu ORGANIZATION NAME AND ADDRESS
Metcalf & Eddy, Inc., P.O.Box 10-046, Palo
Alto, California 94303 in Association with
Hydro-Research-Science, Santa Clara,
California 95050
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT' NO.
68-03-0274
12. SPONSORING AGENCY NAME AND ADDRESS Cin OH
Municipal Environmental Research Laboratory--
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Anthony N. Tafuri, 201/321-6679 (FTS 340-6679)
An overview and assessment of current catchbasin technology has been prepared to
provide engineers and municipal managers with technical and economic information
on catchbasins and some alternatives so that they can make intelligent, informed
decisions on runoff collection systema_in light of pollution control legislation,
the municipality's financial status, and its particular stormwater runoff
characteristics.
Various catchbasin configurations and sizes were evaluated for hydraulic and
pollutant removal efficiencies using hydraulic modeling analyses.
Detailed study findings are presented in sections dealing with (1) a state-of-the-
art review, (2) a review of variables affecting catchbasin efficiency, (3) hydraulic.
modeling analyses, (4) an assessment of the role of catchbasins, (5) an economic
evaluation of alternative storm and combined sewer designs, and (6) a review of
recent developments and continuing program needs. Detailed example problems of the
evaluation of catchbasin performance and economics are included.
A recommended catchbasin design configuration based upon hydraulic performance and
sediment capture efficiency is presented.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
*Catch Basins, *Combined Sewers,
*Storm Sewers, *Surface Waters,
Runoff, *Water Pollution, *Water
Quality
Catchbasin design,
economic assessment,
hydraulic models,
pollution'abatement,
storm runoff, urban
hydrology
13B
. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)'
UNCLASSIFIED
21. NO. OF PAGES
143
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
J
EPA Form 2220-1 (9-73)
129
U. S. GOVERNMENT PRINTING OFFICE: 1977-757-056/61)07 Region No. 5-11
-------
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