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

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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

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              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

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     NEW YORK
SAN  FRANCISCO
                                           £k
                                        in.
                                              -2 ft - 0 IB.
     ATLANTA
   TORONTO
Figure  1.   Representative  catchbasin designs
            in United States and Canada.
                         12

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 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

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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

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                    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

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 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

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  Equipment and  Crew Size —
  Tble 5.
                            °f  e
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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

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 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

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    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

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     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

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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

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 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

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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

-------
                            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 •- •

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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

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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

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                                                      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

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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

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            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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                            67

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         PARTICLE  SIZE, ram
                   2.0
                                            180

                                            160

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                                         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

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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

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                 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

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                             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

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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

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       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

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 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

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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

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 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

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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

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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

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               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

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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

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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.
                                  84

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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)
                                                 
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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,
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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.
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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
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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

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 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

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     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

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      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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                               116

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                         118

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                               121

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                               122

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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

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                             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

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                                 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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