905R90111
United States
Environmental Protection
Agency
Region V
Water Division
Chicago, IL 60604
November 1990
4>EPA
Urban Targeting and
BMP Selection
Information and Guidance Manual for State
Nonpoint Source Program Staff Engineers a
Recycled Paper
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905R90111
Urban Targeting and
BMP Selection
An Information and Guidance Manual for State Nonpoint Source
Program Staff Engineers and Managers
Prepared by
Woodward-Clyde Consultants
Oakland, CA
Prepared/or
Region V, Water Division
Watershed Management Unit
U.S. Environmental Protection Agency
Chicago, IL
and
Office of Water Regulations and Standards
Office of Water Enforcement and Permits
U.S. Environmental Protection Agency
Washington, DC
Distributed by
The Terrene Institute
Washington, DC
November 1990
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TABLE OF CONTENTS
CHAPTER PAGE
1. INTRODUCTION 1
1.1 Background 1
1.2 Need for Targeting to Prioritize Efforts 1
1.3 Organization of Manual 1
2. URBAN NONPOINT SOURCE POLLUTANTS 3
2.1 Urban Runoff Hydrology 3
2.1.1 Regional Precipitation Characteristics 3
2.1.2 Estimating Runoff Volume 5
2.2 Urban Runoff Water Quality 7
2.2.1 Pollutants in Urban Runoff 7
2.2.2 CharacterizingUrban Runoff Water Quality 9
2.2.3 Sources of Urban Runoff Pollutants 10
2.3 Receiving Water Problems 14
2.3.1 Water Quantity Problems 14
2.3.2 Water Quality Problems 15
2.3.3 Examples of Urban Runoff Receiving Water Impacts 16
3. BMPs FOR CONTROL OF URBAN NPS 19
3.1 Types of Urban BMPs 19
3.2 Detention Basins 25
3.2.1 Design Features 25
3.2.2 Performance 27
3.2.3 Advantages and Limitations 27
3.3 Retention Devices 29
3.3.1 Design Features 30
3.3.2 Performance 30
3.3.3 Advantages and Limitations 30
3.4 Vegetative Controls 31
3.4.1 Design Features 31
3.4.2 Performance 32
3.4.3 Advantages and Limitations 32
3.5 Source Controls 32
3.5.1 Design Features 33
3.5.2 Performance 39
3.5.3 Advantages and Limitations 39
4. TARGETING TO PRIORITIZE URBAN AREAS FOR CONTROL 41
4.1 Introduction 41
4.2 Elements of the Targeting Procedure 41
4.3 Factors Used for Prioritization 42
4.4 Description of Targeting Procedure 45
4.5 Discussion 51
REFERENCES 53
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LIST OF TABLES
TABLE PAGE
2-1 TYPICAL VALUES OF ANNUAL STORM EVENT STATISTICS 4
2-2 WATER QUALITY CHARACTERISTICS OF URBAN RUNOFF 11
2-3 POTENTIAL SOURCES OF TOXIC AND HAZARDOUS
SUBSTANCES IN URBAN RUNOFF 13
3-1 RATIO OF BASIN VOLUME TO THE MEAN RUNOFF
VOLUME FOR DIFFERENT DESIGN RULES 27
3-2 TYPICAL PERCENT POLLUTANT REMOVED FOR DIFFERENT
RATIOS OF BASIN VOLUME TO MEAN RUNOFF VOLUME 28
3-3 TYPICAL PERCENT POLLUTANT REMOVAL
FOR RETENTION DEVICES 28
4-1 TYPICAL VALUES OF PERCENT IMPERVIOUS AREA
AND POLLUTANT CONCENTRATIONS 44
4-2 CHARACTERISTICS OF URBAN AREA AND
ESTIMATED RUNOFF CONCENTRATIONS 47
4-3 ESTIMATED TSS LOADS FOR THE TARGETED AREA 47
4-4 PRIORITIZATION ANALYSIS FOR URBAN AREA TARGETING 49
LIST OF FIGURES
FIGURE . PAGE
2-1 RAIN ZONES OF THE UNITED STATES 4
3-1 SCHEMATIC ILLUSTRATION OF VEGETATIVE BMPs 21
3-2 FEASIBLE BMP TYPES FOR DIFFERENT SIZES OF WATERSHED 22
v" 3-3 RESTRICTIONS FOR APPLICATION OF BMPs
-^ BASED ON SOIL PERMEABILITY 23
i
t2 3-4 OTHER COMMON RESTRICTIONS FOR BMPs 24
CJ
> 4-1 SCHEMATIC REPRESENTATION OF URBAN AREA 46
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A cknowledgements
The authors of this report were Mr. Eugene Driscoll and Dr. Peter Mangarella, both of Woodward-Clyde
Consultants. Mr. Thomas Davenport, the U.S. EPA Regional Nonpoint Source Coordinator, reviewed
the early versions of this report and provided a number of helpful comments and suggestions. We also
appreciate the comments and suggestions of Ms. Lynne Kolze, Mr. Kevin Weiss and Mr. Steve Dressing
of the U.S. EPA, and Mr. Gary Schaffer of the Northeastern Illinois Planning Commission.
The manual has been reviewed by the U.S. Environmental Protection Agency and approved
for publication. Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendations for use.
The manual was prepared by Woodward-Clyde Consultants under EPA Contract No. 68-C8-0034, Work
Assignment No. 0-1 from the Office of Water Regulations and Standards. This project was funded by the U.S.
Environmental Protection Agency Office of Water Enforcement and PermitsWater Permits Division and
Managed by Region V Watershed Management UnitWater Division. For copies of this publication, contact The
Terrene Institute, 1000 Connecticut Avenue, NW, Suite 300, Washington, DC 20036, (202) 833-3380.
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Foreword
Urban runoff is a major issue in most urban and suburban areas, because of the potential for urban
runoff to deliver pollutants to nearby receiving waters and cause flooding. Historically, urban runoff
abatement efforts have concentrated on the techniques to control the loss of property and lives due to
downstream flooding. The 1987 amendments to the Clean Water Act brought the issue of urban runoff
quality to the forefront when it established two programs to assist the States and municipalities in abate-
ment of urban runoff water quality problems: Section 319 (Nonpoint Source Control) and 402
(Stormwater Permitting).
In order for urban jurisdictions to focus limited financial and technical resources in an effective and
efficient manner they must be able to relate identified or suspected water quality problems to source
areas in an integrated approach. In many jurisdictions, sufficient resources may not be available imme-
diately to implement all of the urban runoff nonpoint source controls needed to correct the documented
water quality needs. Therefore, a staged implementation approach of a comprehensive management pro-
gram based upon technical economic and timing considerations is appropriate. In response to the State
and municipalities need for guidance on how to target controls, U.S. EPA, Region V has attempted to
provide a rational basis for ranking different areas that need control within a jurisdiction. This manual is
to assist State and local agency personnel in targeting areas within their jurisdiction. The Manual con-
solidates existing information and develops a noncomputerized methodology for targeting areas for con-
trol.
This manual is the first in a series of technical documents the U.S. EPA, Region V will cosponsor.
Our objective is to facilitate technology transfer and the exchange of information. Future technical
transfer efforts will include a stormwater handbook, a Great Lakes Nonpoint Source Symposium, a
workshop on monitoring the effectiveness of best management practice implementation, and an urban
nonpoint source control workshop.
Within each EPA Region, appropriate efforts are coordinated by the Nonpoint Source Coordinator.
Contact your regional coordinator for information on nonpoint source management activities in your
State, and for publications such as this manual. Comments or questions concerning this manual should
be forwarded to Tom Davenport, of my staff.
DALE S. BRYSON
Director, Water Division
U.S. Environmental Protection Agency
Region V
230 South Dearborn Street
Chicago, Illinois 60604
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CHAPTER 1
INTRODUCTION
1.1 BACKGROUND
There is general agreement as indicated in State 305(b) reports, nonpoint source (NFS)
Assessment Reports, and in the proposed Stormwater NPDES permitting requirements that
urban runoff can be a significant contributor to degraded water quality in receiving waters.
Management programs implemented by municipalities can prevent further deterioration,or
improve receiving water quality for existing urbanized areas. For newly developing areas,
such programs can protect existing water quality.
The focus on implementation of appropriate urban NFS management programs is at
the local level, as federal legislation and state stormwater management planning efforts
stipulate that a municipality is responsible for the quality and quantity of runoff within its
jurisdiction. Large and medium municipalities will be required to respond initially to these
issues in the context of the stormwater permit program. Other urban jurisdictions will face
these requirements after 1992. Independent of the implications for urban NFS management
associated with stormwater NPDES Permits, many municipal areas will be required to
develop management programs in response to initiatives instituted by individual States
1.2 NEED FOR TARGETING TO PRIORITIZE EFFORTS
All municipalities are encouraged to initiate action to implement some baseline
measures that will help to regulate pollutant discharges from stormwater runoff. There are
many management actions that can be considered that are low in cost, utilize existing
municipal activities, can be applied jurisdiction-wide, and/or address priority sources of
pollutants. Early action is particularly appropriate in the case of areas undergoing a high
growth rate with new areas coming under development, or older ones being redeveloped.
There are often unique opportunities in such situations, in terms of institutional and cost
factors, to address the NFS issues effectively.
Whether the impetus originates at the local level, or as a necessary response to Federal
or State mandated programs, it is likely that in many cases, after appropriate baseline
measures have been implemented, there will be a need to target specific parts of the overall
area for initial attention in continuing management efforts. In some situations, sufficient
resources may not be available to implement all of the controls that may be desirable. In
other cases, a staged approach to the implementation of a comprehensive management
program may be appropriate. For either of these possibilities, there is a need to target the
available resources and to prioritize the control program based on site specific conditions,
so that the greatest water quality benefit is realized for the resources expended.
1.3 ORGANIZATION OF MANUAL
This manual consolidates existing information and describes a methodology for
targeting urban areas for control. It is designed to assist State and local agency personnel
in targeting areas within their jurisdiction for priority in the development and
implementation of NFS management programs. The following topic areas related to NFS
pollutant discharges from urban areas are addressed:
1. The nature and characteristics of urban runoff, and the types of water quality
problems that are most likely to occur. (Chapter 2)
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2. The types of best management practices (BMP's) that are appropriate for control of
NPS pollutant loads from urban and developing areas, and guidance for their
selection. (Chapter 3)
3. A procedure for prioritizing urban areas for the application of controls beyond the
baseline measures initially applied on a jurisdiction-wide basis.. (Chapter 4)
This manual has a technical orientation with a level of detail appropriate for local and
state agency use for management program development or assessment. The information
and procedures presented are at a level of detail considered to be suitable for developing
local planning strategies for controlling the quality of urban runoff and associated receiving
water effects. The material presented does not provide comprehensive and exhaustive
treatment of technical aspects. For more detailed information, selected references are
provided.
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CHAPTER 2
URBAN NONPOINT SOURCE POLLUTANTS
Urban runoff quantity and quality are significantly affected by watershed
development and, in particular, drainage systems that are constructed for human safety,
health, and convenience. Urbanization alters the natural vegetation and natural infiltration
characteristics of the watershed, causing runoff from an urban area to have a much higher
surface flow component, a much smaller interflow component, and a somewhat reduced
baseflow component. Urbanization also can cause water quality problems because activities
associated with urbanization create sources of pollutants (e.g., automobile emissions) for
surface runoff. Thus urbanization tends to increase runoff and pollutant loadings to the
receiving water body. Specifically, some effects of urbanization are to:
increase peak discharges (typically by about two to five times)
increase runoff volume from a given storm (volume increases of 50% or more are
common)
decrease the watershed response time (the time of concentration)
reduce streamflow in dry weather periods (especially during prolonged dry spells;
urbanization can actually cause small headwater perennial streams to become
ephemeral)
increase runoff velocity during storms
increase the discharge of pollutants
significantly modify the type and nature of pollutants
The following sections discuss these effects and provide methods of estimating those
effects (runoff volume and water quality) that are required for the targeting methodology
presented in Chapter 4.
2.1 URBAN RUNOFF HYDROLOGY
2.1.1 Regional Precipitation Characteristics
Precipitation causes runoff which in turn mobilizes and transports pollutants from the
urban area to the receiving water. Storm event characteristics and patterns vary
considerably in different regions of the country, and can influence the nature and extent of
the receiving water impacts. For example, in the East and Southeast, short duration - high
intensity summer thunderstorms tend to increase the erosion potential compared to the
Pacific Northwest, where the prevailing rainfall pattern is one of longer duration - low
intensity events. Receiving water impacts are also affected by the characteristics of the
receiving water body. In streams and rivers, water quality effects are associated with the
individual storm events; whereas in the case of lakes and impoundments, the impact
produced is usually the result of the cumulative effect of NPS discharges over an extended
period of time. Regions with greater annual precipitation amounts will contribute higher
pollutant loadings to these waterbodies than areas with low annual precipitation.
A recent EPA study characterizes storm event properties that are useful for preliminary
planning assessments (Driscoll et al., 1989). Table 2-1 tabulates the statistics of a set of
storm event properties for various rain zones shown in Figure 2-1, based on the analysis of
rain gage data for the locations shown on the map.
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FIGURE 2-1. RAIN ZONES OF THE UNITED STATES.
TABLE 2-1. TYPICAL VALUES OF ANNUAL STORM EVENT STATISTICS FOR RAIN ZONES
RAIN ZONE
NORTHEAST
NORTH EAST COASTAL
MIOATLANTIC
CENTRAL
NORTH CENTRAL
SOUTHEAST
EASTQULF
EAST TEXAS
WEST TEXAS
SOUTHWEST
WEST INLAND
PACIFIC SOUTH
NORTHWEST INLAND
PACIFIC CENTRAL
PACIFIC NORTHWEST
Annual Statistics
No of Storms
Avg COV
70
63
62
68
55
65
66
41
30
20
14
19
31
32
71
0.13
0.12
0.13
0.14
0.16
0.15
0.17
022
027
0.30
0.36
0.36
023
OX
0.15
Precip
Avg COV
(m)
34.6
41.4
39.5
41.9
298
49.0
53.7
31.2
17.3
7.4
4.9
10.2
11.5
18.4
35.7
0.18
0.21
0.18
0.19
0.22
0.20
0.23
0.29
0.33
0.37
0.43
0.42
0.29
0.33
0.19
Independent Storm Event Statistics
Duration
Avg COV
(hrs)
11.2
11 7
10.1
9.2
9.5
8.7
64
8.0
7.4
7.8
9.4
11.6
104
13.7
15.9
081
077
084
0.85
0.63
0.92
1.05
097
0.98
0.68
0.75
0.78
0.82
0.80
0.80
Intensity
Avg COV
(in/rir)
0.067
0.071
0.092
0.097
0.087
0.122
0.178
0.137
0.121
0.079
0055
0.054
0.057
0.046
0.035
1.23
1.05
1.20
1.09
1.20
1.09
1.03
1.08
1.13
1.16
1.06
0.76
1.20
0.85
0.73
Volume
Avg COV
(in)
0.50
0.66
0.64
062
0.55
0.75
0.80
076
0.57
0.37
0.36
0.54
0.37
0.58
0.50
0.95
1.03
1.01
1.00
1 01
1.10
1.19
1.18
1.07
0.68
0.87
0.98
0.93
1.05
1.09
DELTA
Avg COV
(hr)
126
140
143
133
167
136
130
213
302
473
786
476
304
265
123
0.94
0.87
0.97
0.99
1.17
1.03
1.25
1.28
1.53
146
1.54
209
1.43
2.00
1 50
COV - Coefficient of Variation - Standard Deviation / Mean
DELTA - Interval between norm midpoints
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Specific information on the individual sites is provided in the referenced document.
The data presented is based on those storm events that produce storm volumes greater than
0.1 inch, because experience indicates that very small storms do not produce runoff. As a
result, the statistics shown are for runoff-producing storm events. For these events the
annual statistics show the average and year-to-year variability (expressed as the coefficient
of variation) of the number of storms per year and the annual precipitation volume. The
"event" statistics show the average and variability of the characteristics of individual
storms. These data indicate significant regional differences which must be taken into
account in NFS assessments. For example, as discussed earlier, note the four-fold
difference in average storm intensity between the Pacific Northwest (0.035 in/hr) and the
Southeast (0.122 in/hr).
2.1.2 Estimating Runoff Volume
Percent Imperviousness
An important element in targeting watersheds for control is the runoff volume from the
respective watersheds within a jurisdiction. The following describes various simplified
methods that may be used for estimating runoff volume.
The single most important factor in determining the quantity of runoff that will result
from a given storm event is the percent imperviousness of the land cover. Other factors
include soil infiltration properties, topography (which defines watershed slopes and
depression storage capacity), vegetative cover, and antecedent conditions.
Impervious areas include paved streets, sidewalks, driveways, parking areas,
rooftops, patios, decks, and similar man-made structures. Obviously, the extent of
imperviousness is a function of local development customs and zoning requirements such
as lot sizes, single- or multiple-level construction, preferences for garages, use of
alleyways, curb and gutter versus swale drainage, and similar factors. Such customs vary
widely across the country due to climate, land cost, and a host of other reasons. Even
within a given region or municipality, they are typically not uniform and may vary by land
use and, even within a given land use, by the age of the development or its location within
the city.
Given the importance of percent imperviousness in NFS assessments, it is strongly
recommended that the percent imperviousness for a given study area be determined from
site specific information. For example, the impervious area can be estimated from site
plans, maps, and aerial photographs. This method does involve some degree of uncertainty
(for example, rooftops may or may not drain to other impervious areas). An alternative or
complimentary method would treat the percent imperviousness as a "calibration factor" to
be determined utilizing a simple runoff model and rainfall and runoff records.
For preliminary screening purposes, prior to obtaining detailed site specific
information, initial estimates of percent imperviousness can be made based upon land use
category. While useful for larger urban areas where things tend to "average out," caution is
called for in inferring such values for small specific sites. Use local data where there is any
question.
The largest single land use in most metropolitan areas is residential, which typically
accounts for between 50% and 70% of the total area. Lacking any other information, it is
suggested that an initial estimate of 60% be used. If population data are available, an
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estimate of the percent imperviousness can be obtained using the following expression
(Shelley, 1988):
PI = 9(PD)°-5 (2-1)
where
PI = percent imperviousness [%]
PD = population density [persons/acre]
Note that the population density can be estimated by first determining the population
from census data or other sources and dividing this by the area of the residential land use.
Equation (2-1) is based upon extensive examination of all residential land use sites in the
NURP and USGS runoff data bases.
Commercial land use typically represents from 5% to 15% of the total urban area; a
value of 10% is suitable as a first approximation. The imperviousness of commercial areas
varies considerably, from around 50% for shopping malls with considerable landscaping
and associated undeveloped space to over 90% for central business districts. In the absence
of specific information, the use of 75% to 80% impervious area is suggested for
commercial land use.
Industrial land use will vary considerably with the region of the country and the nature
of the community, being rather high for older, heavily-industrialized communities. Typical
values range from 10% to 20% of the total urban area; with 15% recommended for use as a
first approximation. Here also, the percent imperviousness varies considerably, but often
is between 40% and 70%. A preliminary estimate of 55% to 60% impervious area is
suggested for use in lieu of site specific data.
The land use for the remaining portions of the urban area consists largely of open areas
such as parks, golf courses, cemeteries, and undeveloped land. Typically, this land use
category is around 15%, and this value is suggested as a preliminary estimate. Here also,
the percent imperviousness can vary widely (from 5% to over 40%); a value between 10%
to 20% is a reasonable first estimate.
The foregoing procedure can be used to arrive at a composite estimate of overall
percent imperviousness for an urban area, by developing a weighted average based upon
allocating the total urban area to the four major land uses just discussed and using the
preliminary percent imperviousness estimates as suggested. To illustrate, consider a
medium sized urban area with a population density of 9 persons per acre.
Land Use % of Total Area % Impervious Net Impervious Fraction
Residential 60% 27% 0.162
Commercial 10% 75% 0.075
Industrial 15% 55% 0.0825
Open/Other 15% 15% 0.0225
TOTAL IMPERVIOUSNESS 0.342
This value of 34.2% imperviousness is in the typical range of 30% to 35% for
moderate sized urban areas and is a reasonable first approximation.
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Runoff Coefficient
The runoff coefficient is the measure of the watershed response to rainfall events. It is
a dimensionless number obtained by dividing the total storm runoff by the total rainfall
volume.
Thus,
Rv = Qs/I (2-2)
where
Rv = the runoff coefficient [dimensionless]
Qs = runoff [inches]
I = rainfall [inches]
For a given site, the value for Rv will vary from event to event depending upon the
characteristics of the rainfall event (intensity, duration, etc.) as well as the antecedent
conditions. However, the variability can be statistically characterized from which mean
seasonal or annual values can be estimated. From site to site, the variability in Rv can
largely be attributed to differences in the percent imperviousness. Thus, a method is
needed for estimating Rv based upon the percent imperviousness. As a very simplistic first
approximation, the percent imperviousness can be used as the estimate of the runoff
coefficient, with a lower bound of around 0.1 and an upper bound of around 0.95. Thus,
in the example just cited, the runoff coefficient would be taken to be 0.342.
An alternative and preferred method for estimating the runoff coefficient from the
percent imperviousness utilizes the following regression equation (Shelley, 1988):
Rv = 0.050 + 0.009 (PI) (2-3)
where
PI is estimated from land use information (as in the above table) or from
population density (Equation 2-1).
The correlation coefficient for the regression is 0.71, indicating that over 70% of the
variance between Rv and percent impervious is explained by the regression. For the
example just discussed, the refined estimate of the runoff coefficient would be:
0.358 = [0.050 + (0.009X34.2)]
Given a rainfall event of a certain size, Rv is used to estimate the runoff volume that
will result. This is the general approach used to estimate runoff quantity. It must be
emphasized again that this approach only provides an estimate for the wet weather runoff
portion of the urban runoff flow. Dry weather and base flow contributions, to the extent
that they are present, must be determined from site-specific information.
2.2 URBAN RUNOFF WATER QUALITY
2.2.1 Pollutants in Urban Runoff
The net effect of urbanization is to increase pollutant runoff loads by at least an order
of magnitude over pre-development levels. The impact is felt not only on adjacent streams
and lakes, but also on downstream receiving waters. The following discussion identifies
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the principal types of pollutants found in urban runoff and describes their potential adverse
effects on receiving waters.
Sediment: Suspended sediment concentrations and mass loads are the highest of any
of the pollutant types discharged by urban runoff. Sediment has both short- and long-term
impacts on receiving waters. Among the immediate adverse impacts of high concentrations
of sediment are increased turbidity, reduced light penetration, reduced prey capture for
sight feeding predators, clogging of gills/filters offish and aquatic invertebrates, reduced
spawning and juvenile fish survival, and reduced angling success. Additional impacts
result after sediment is deposited in slower moving receiving waters and include
smothering of the benthic community, changes in the composition of the bottom substrate,
more rapid filling of small impoundments (necessitating more frequent dredging), and
reduction in aesthetic values. Sediment having a high organic or clay content is also an
efficient carrier of trace metals and toxicants. Once deposited, pollutants in these enriched
sediments can be remobilized under suitable environmental conditions, posing a risk to
benthic and other aquatic life.
Oxygen Demanding Substances: Decomposition of organic matter by
microorganisms depletes dissolved oxygen (DO) levels in receiving waters, especially
slower moving streams and lakes and estuaries. There are several measures of the degree
of potential DO depletion, the most common of which are the Biochemical Oxygen Demand
(BOD) test and the Chemical Oxygen Demand (COD) test. Both of these tests have
problems associated with their use in urban runoff, but it is clear that urban runoff can
severely depress DO levels after large storms.
Nutrients : The levels of phosphorus and nitrogen in urban runoff can lead to
accelerated eutrophication in downstream receiving waters. Generally, phosphorus is the
controlling nutrient in freshwater systems. The greatest risk of eutrophication is in urban
lakes and impoundments with long detention times (say two weeks or greater). Surface
algal scums, water discoloration, strong odors, depressed oxygen levels (as the bloom
decomposes), release of toxins, and reduced palatability to aquatic consumers are among
the problems encountered. High nutrient levels can also promote the growth of dense mats
of green algae that attach to rocks and cobbles in shallow, unshaded headwater streams.
Heavy metals: Heavy metals are of concern because of their toxic effects on aquatic
life and their potential to contaminate drinking water supplies. The heavy metals having the
highest concentrations in urban runoff are copper, lead, and zinc with cadmium a distant
fourth. However,when inappropriate connections between sanitary and storm sewers are
present, other heavy metals such as arsenic, beryllium, chromium, mercury, nickel,
selenium, and thallium can be found. A large fraction of the heavy metals in urban runoff
are adsorbed to particulates and thus are not readily available for biological uptake and
subsequent bioaccumulation. Also, the typical periods of exposure are those of urban
runoff events (typically under 8 hours), which are much shorter than the exposure periods
used in bioassay tests (typically 24 to 96 hours for toxicity testing). Nonetheless, it is
likely that the heavy metals in urban runoff are toxic to aquatic life in certain situations,
particularly for the more soluble metals such as copper and zinc. Compared to risks to
aquatic life, human health risks appear to be more remote.
Bacteria: Fecal coliform levels in urban runoff usually will exceed public health
standards for water contact recreation and shellfish harvesting. Furthermore, because
bacteria multiply faster during warm weather, it is not uncommon to find a twenty-fold
difference in bacterial levels between summer and winter in colder climates. The
substantial seasonal differences do not correspond with comparable variations in urban
activities, which suggests that in addition to temperature effects, sources of coliform
8
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unrelated to those traditionally associated with human health risk may be significant. Thus,
despite the high numbers of coliforms found in urban runoff, in the absence of
contamination from sanitary sewage, the health implications are unclear. The current
literature suggests that fecal coliform may not be useful in identifying health risks from
urban runoff pollution.
Oil and Grease : Oil and grease contain a wide variety of hydrocarbon compounds,
some of which (e.g., polynuclear aromatic hydrocarbons) are known to be toxic to aquatic
life at low concentrations. Hydrocarbons are often initially found as a rainbow colored film
or sheen on the water's surface. Other hydrocarbons, especially weathered crankcase oil,
appear in solution or in emulsion and have no sheen. However, hydrocarbons have a
strong affinity for sediment, and much of the hydrocarbon load eventually adsorbs to
particles and settles out. Hydrocarbons tend to accumulate rapidly in the bottom sediments
of lakes and estuaries, where they may persist for long periods of time and exert adverse
impacts on benthic organisms. The precise impacts of hydrocarbons on the aquatic
environment are not well understood. Bioassay data which do exist are largely confined to
laboratory exposure tests for specific hydrocarbon compounds. Remarkably few toxicity
tests have been performed to examine the effect of urban runoff hydrocarbon loads on
aquatic communities under the typical exposure conditions found in urban streams.
Other Pollutants : Other toxic chemicals are rarely found in urban runoff from
residential and commercial land use areas in concentrations that exceed current water quality
criteria. Pesticide concentrations in runoff from such areas, when they are found at all,
tend to be near their detection limits. However, it should be noted that there has been
relatively little sampling of runoff from industrial areas, where toxic compounds might be
expected to be more prevalent.
2.2.2 Characterizing Urban Runoff Water Quality
Pollutant concentrations in urban runoff vary considerably, both during the course of a
storm event as well as from event to event at a given site, from site to site within a given
city, and from city to city across the country. This variability is the natural result of high
variations in rainfall characteristics, differing watershed features that affect runoff quantity
and quality, and variable antecedent conditions. In many situations, where within-event
variability is not important, the event mean concentration or EMC (defined as the total
.constituent mass discharge divided by the total runoff volume) is frequently used as a
representative measure of pollutant concentration. The event mean concentration values at a
given site tend to be well represented by a lognormal probability distribution, and the
varying values for pollutant EMCs in runoff can be characterized by a mean (or median)
and coefficient of variation for the site.
The lognormal distribution has been used to characterize water quality because it has a
number of important benefits (WCC, 1989), including:
Concise summaries of highly variable data can be developed and the variability can
be quantified and dealt with appropriately.
Comparisons of results from different sites, events, etc. are convenient and more
easily understood.
Statements can be made about frequency of occurrence, i.e., one can express how
often values will exceed various magnitudes of interest.
A more useful and informative method of reporting data than the use of ranges is
provided; one that is less subject to misinterpretation.
-------
A framework is provided for examining the transferability of data in a quantitative
manner.
The median EMC for a site (the site median or SMC) measures the central tendency or
the EMC for which half of the runoff events are higher and half are lower. The coefficient
of variation is a dimensionless measure of variability computed by dividing the standard
deviation by the mean. The appropriate statistic to employ for comparisons between
individual sites or groups of sites is the median value, because the median is less subject to
distortion by the one or a few very large values. However, for computations and analyses
that develop or use mass loads, (such as annual pollutant loads), the mean value is the
appropriate statistic. For a lognormal distribution, there is a defined relationship between
the mean and median values. Data summaries presenting median values (such as in the
NURP report) can be converted to mean values by the expression below. The mean is
calculated by multiplying the median by the square root of one plus the square of the
coefficient of variation, i.e.,
Mean = Median * [ 1+CV2]0-5 (2-4)
Based upon the results of considerable analysis of the NURP data, it was determined
that geographic location, land use category, rainfall characteristics, and other factors did not
adequately explain the site-to-site variability of site median concentrations. Therefore, the
SMC data for all urban sites were pooled and examined statistically. It was found that the
lognormal distribution was also an adequate representation of this pooled data set. The
results, given in Table 2-2, are based on the results obtained by the NURP study. Since
the targeting methodology presented later uses a comparison of pollutant mass loads,
approximate mean values for runoff concentrations have been listed. The mean of the
EMC values are listed for the median urban site in the NURP data set, along with the
values for the 10th and 90th percentile sites. That is, 10% of the urban sites are expected
to have a mean EMC concentration that is either lower or higher than the corresponding
percentile values.
The Table 2-2 results are presented to provide an indication of the range of the
concentrations of the indicated pollutants that may be present at an urban site. For
constituents that do not appear in Table 2-2, the NURP data base was too sparse to allow
the analytical treatment just described, and local estimates must be provided. Similarly, the
availability of local data can provide a basis for refining runoff concentration estimates.
The primary purpose for presenting this data is to provide background reference
information, and to convey a sense of the variability of urban runoff quality. In the
targeting methodology described in Chapter 4, this data has been used as a guide in
selecting reasonable approximate concentration levels. The actual concentration value
selected for the targeting analysis is unimportant, because no attempt is made to predict
actual loads. Only the relative differences between concentrations for different land use
types are a factor in the methodology.
2.2.3 Sources of Urban Runoff Pollutants
Erosion : Soil erosion can be an important source of pollutants in runoff from urban
areas, either because of stream bank erosion or as a result of the disturbance of land
surfaces. For example, initial clearing and grading operations during construction expose
much of the surface soils. Unless adequate erosion controls are installed and maintained at
the site, large quantities of sediment can be delivered to the stream channel, along with
attached soil nutrients, organic matter, and other adsorbed pollutants. Uncontrolled
10
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TABLE 2-2. WATER QUALITY CHARACTERISTICS OF URBAN RUNOFF
Constituent
TSS (mg/l)
BOD (mg/l)
COD (mg/l)
Tot. P (mg/l)
Sol. P (mg/l)
TKN (mg/l)
NO3-N (mg/l)
Tot. Cu (u.g/1)
Tot. Pb (u.g/1)
Tot. Zn (u.g/1)
Mean Concentration in Runoff
10th Precentile
Urban Site
35
6.5
40
0.18
0.10
0.95
0.40
15
60
80
For Median
Urban Site
125
12
80
0.41
0.15
2.00
0.90
40
165
210
90th Precentile
Urban Site
390
20
175
0.93
0.25
4.45
2.20
120
465
540
11
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construction site sediment loads on the order of 40 tons/acre/year and higher have been
reported (Novotny and Chesters, 1981; Wolman and Schick, 1967; Yorke and Herb, 1976
and 1978). Such loads are one to two orders of magnitude higher than from agricultural or
stabilized urban land uses respectively.
Atmospheric Deposition : A significant source of pollutants in urban areas is
atmospheric deposition in the form of both wetfall and dryfall. In almost all cases, some of
the pollutants from atmospheric sources are trapped and remain on the land surface, rather
than washing off in ordinary storm events. If source control of atmospheric deposition is
desired, control strategies based on automobile and industrial emission controls are
required.
Construction Materials : The various surfaces of the urban landscape are another
major source of pollutants. Metals, for example, are a common element in many urban
structures, such as flashing and shingles, gutters and downspouts, galvanized pipes, metal
plating, paints, wood preservatives, etc. Over time, these surfaces corrode, flake,
dissolve, decay, or are subject to leaching thereby allowing metals to be carried away in
urban runoff. The process is often exacerbated by the acidity of the rainfall.
Manufactured Products : The use of a variety of manufactured products represents a
source of some pollutants in urban runoff. For example, most copper found in urban
runoff originate from various anthropogenic sources including oxides and sulfates of
copper used for insecticides, algicides, and fungicides. Copper is frequently incorporated
into paints and wood preservatives to inhibit growth of algae and invertebrate organisms.
Copper salts are used in water supply systems for controlling biological growths. Primary
sources of copper in industrial wastewater are metal process pickling and plating baths.
Other sources include mine drainage, pulp and paper mills, fertilizer manufacturing,
petroleum refining, and certain rayon processes. Copper is used in the automobile industry
in brake linings, clutch facings, and certain tire compounds. Smelters may release copper
to the atmosphere which is eventually returned to surface waters. The variety of potential
sources for copper explains why it is almost invariably found in urban runoff at levels of
concern.
Many pollutants which derive from manufactured products are toxic or hazardous
materials. The more significant potential sources of some of these substances are given in
Table 2-3. Note that automobile use contributes significantly to many of these constituents.
Polycyclic aromatic hydrocarbons (PAHs), the most commonly detected toxic organic
compounds found in urban runoff (EPA, 1982), originate from oil and combustion
products. Phthalate esters, a relatively common toxic organic compounds, are derived
primarily from plastics. Pentachlorophenol, also frequently found, comes from wood
preservatives.
Plants and Animals: Other sources of pollutants that accumulate and subsequently
wash off urban surfaces include plant debris (leaves, etc.) and animal excrement which in
natural systems are recycled. For example, trees and shrubs deposit pollen and leaves
which, no longer able to be converted to humus on the forest floor, enter into urban runoff.
During the growing season, nutrients leach from tree leaves and stems during storms and
are quickly conveyed to the stream if the ground is saturated or the tree's drip line extends
over an impervious area.
Non-Stormwater Connections : An important potential source of toxic and other
pollutants in urban runoff is through non-stormwater discharges to stormwater drainage
systems. Inadvertent or deliberate discharges of sanitary sewage and industrial waters to
storm drains has been identified as a widespread and serious occurrence. The detection and
elimination of such discharges is a major focus of the NPDES Stormwater Permit program.
12
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TABLE 2-3. POTENTIAL SOURCES OF TOXIC AND HAZARDOUS SUBSTANCES IN URBAN RUNOFF
Heavy Metals
Copper
Lead
Zinc
Chromium
Halogenated Aliphatics
Methylene chloride
Methyl chloride
Phthalate Esters
Bis (2-ethyhexyl) phthalate
Butylbenzy! phthalate
Di-N-butyl phthalate
Polycyclic Aromatic Hydrocarbons
Chrysene
Phenanthrene
Pyrene
Other Volatiles
Benzene
Chloroform
Toluene
Pesticides and Phenols
Lindane (gamma-BHC)
Chlordane
Dieldrin
Pentachlorophenol
PCBs
AUTOMOBILE USE
metal corrosion
gasoline, batteries
metal corrosion
tires, road salt
metal corrosion
gasoline
gasoline, oil, grease
gasoline
gasoline, oil, asphalt
gasoline
formed from salt,
gasoline & asphalt
gasoline, asphalt
PESTICIDE USE
algicide
wood preservative
fumigant
fumigant
insecticide
wood preservative
insecticide
mosquito control
seed pretreatment
termite control
insecticide
wood preservative
INDUSTRIAL/OTHER USE
paint, wood preservative
electroplating
paint
paint, metal corrosion
paint, metal corrosion
electroplating
plastics, paint remover
solvent
refrigerant, solvent
plasticizer
plasticizer
plasticizer, printing inks
paper, stain, adhesive
wood/coal combustion
wood/coal combustion
solvent
solvent, formed from
chlorination
solvent
wood processing
paint
electrical, insulation
13
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As an illustrative example of how heavy metals can enter flows from industrial areas,
consider the use of heat exchangers in plating tanks at metal finishing and electro-plating
shops. It is not uncommon to have condensate return lines discharge directly to storm
drains. As long as the heat exchangers are intact, this operating condition has no impact on
discharge water quality. However, heat exchangers used in such applications typically
develop pin-hole leaks, and as a result, plating solutions with high heavy metal
concentrations can leak into the storm sewer system. In other instances, what is believed
by a manufacturing firm to be non-contact cooling water, and appropriate to discharge to a
storm drain, may in reality be a process cooling water with a significant concentration of
heavy metals and other pollutants.
Cross-connections delivering sanitary sewage to storm drains can, like industrial
contamination, occur in many different ways. There was a case reported where the sanitary
lines from a high rise building were tied into the storm drain rather than the sanitary sewer.
Locations with hydraulically overloaded sewage treatment plants or undersized sewers may
provide relief points that transfer excess flow into storm drains. This type of situation is
more likely in areas with aging sewer systems with excessive infiltration/inflow.
Accidental Spills : Another source of pollutants in urban runoff is accidental spills.
Here, virtually any pollutant can be found, depending upon the nature of the spill.
Deliberate dumping into storm sewers and catch basins (used crankcase oil is especially
common) is yet another common source. Leaking underground storage tanks, leachate
from sanitary landfills and hazardous waste treatment, storage, and disposal sites can also
contribute to pollutants in storm sewers.
This discussion of sources of urban runoff pollutants indicates the types of activities
that are believed to be the principal generators of urban runoff pollution. Once a particular
receiving water body or segment of concern has been identified, there is an obvious need to
identify the areas that contribute to it. As a rule of thumb, greater concentrations of
pollutants will be found in urban runoff from industrial areas and older parts of the city.
Given the role of the automobile in generating pollutants within the urban landscape, areas
of high automobile density can be expected to have increased levels of pollution. The need
to look for cross-connections and other illegal or inappropriate connections to separate
storm drains cannot be overemphasized.
2.3 RECEIVING WATER PROBLEMS
2.3.1 Water Quantity Problems
The two principal concerns relating to the quantity of stormwater runoff are the total
volume of runoff discharged, and the peak rates of flow that are produced. Problems
associated with runoff quantity include flooding and erosion/sedimentation impacts.
Historically, drainage has been the principal local-level concern regarding urban runoff.
Flooding concerns can be divided into two basic categories; nuisance flooding and major
flooding. Nuisance flooding (e.g., temporary ponding of water on streets, road closings,
minor basement flooding), rarely affects the entire urban populace and is seldom life-
threatening. Nonetheless, the concerns of affected citizens commonly requires that local
action be taken to minimize the recurrence of such events. Such mitigation activities are
usually locally determined, funded, and implemented because the affected public and
government decision makers perceive and concur that such flooding constitutes a problem.
Catastrophic flood events, on the other hand, have to be thought about differently for
several reasons:
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They typically affect the majority of the urban populace.
Mitigation measures often involve engineering improvements extending well
beyond local jurisdictions.
Mitigation measures often cost more than the local community can afford. In such
cases, water quantity problems are readily observable, the degree of damage as well
as the benefits of alternative flood control projects can be estimated. Thus,
decision makers face a relatively low risk in prescribing courses of action and
justifying the associated costs in light of the benefits. As will be discussed later,
decision making in the case of water quality concerns is less straightforward.
Erosion concerns may be the result of a relatively short-term condition produced by
disturbance of the land surface during construction activity, or a longer term condition of
stream bank erosion and scour and deposition of the stream bed produced by peak flow
rates. Although erosion and sedimentation are storm-event related, their resultant problems
are not exclusively either water "quantity" problems or water "quality" problems. When
sediment loads from undeveloped areas are discharged into receiving waters, the effects are
primarily physical and only secondarily chemical, because the mineral constituents which
make up the primary sediment load are relatively benign in most cases. Among the
physical problems in receiving waters subjected to increased sediment loads are:
Excess turbidity reduces light penetration, thereby interfering with sight feeding and
photosynthesis.
Paniculate matter clogs gills and filter systems in aquatic organisms, resulting, for
example, in retarded growth, systemic disfunction, or asphyxiation in extreme
cases.
Benthal deposition can bury bottom dwelling organisms, reduce habitat for
juveniles, and interfere with egg deposition and hatching.
degradation of general habitat.
Urbanization accelerates erosion through alteration of the land surface. Disturbing the
land cover, altering natural drainage patterns, and increasing imperviousness all increase
the quantity and rate of runoff, thereby increasing both flooding and erosion potential.
Furthermore, the sedimentation products that result from urban activities are generally not
as benign as the natural mineral sediments which result from soil erosion from undeveloped
'areas. Atmospheric deposition (associated with industrial and energy production activities)
and added surface particulates (resulting from tire wear, automobile exhaust, road surface
decomposition, and the like) are incorporated in the sediments discharged from urbanized
areas. Their effects on receiving waters tend to be more chemical than physical. This is
also true of natural mineral sediments that become contaminated by the adsorption of toxics
and other chemicals present in the urban environment.
2.3.2 Water Quality Problems
The following are three considerations in evaluating water quality problems in surface
waters subjected to urban runoff.
the nature of the designated beneficial uses , e.g., drinking water supply,
recreation, fisheries, wildlife and associated water quality objectives.
water quality characteristics, i.e., the physical, chemical, and biological data
resulting from analytical determinations made in the field or laboratory.
15
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ecological effects associated with a discharge, e.g., toxicity, carcinogenicity,
disease, eutrophication, altered vegetation and succession, reproductive disorders,
etc.
Water quality is what is typically measured, although laboratory and in situ toxicity
testing using representative aquatic species is a tool for addressing ecological effects.
Ecological effects determine how well a designated beneficial use is met. Although
Congress has established fishable and swimmable waters as a goal, water quality planning
activities will be more cost effective when the specific beneficial uses are supported by the
local community. This is because the determination of the most appropriate beneficial use
is strongly influenced by local attitudes, beliefs, needs and expectations. Unlike water
quantity problems, water quality problems tend to be elusive because their definition
involves subjective considerations. Also water quality problems are often not immediately
obvious and are less dramatic than floods. They also tend to vary markedly with locality
and geographic regions within the country.
For evaluating urban runoff impacts, three possible approaches for identifying the
presence of a problem can be considered.
1 Actual impairment or denial of a designated beneficial use;
2 Violation of a water quality criterion;
3 Local public perception and concern.
The first type of problem would be where a determination has been made that some
specific use, such as shellfishing, should be attained but that present aquatic tissue
contamination is such harvesting and eating the organisms poses a health risk causing
shellfishing to be banned for part or all of the year.
The second type of problem refers to violations of an applicable water quality criterion.
An example would be a case where some measure of water quality characteristics (e.g.,
trace metal concentration) exceeds recommended or mandatory levels for the receiving
water classification (e.g., EPA toxic criteria for aquatic life). This problem definition is
less exact than the preceeding1 problem definition in that the receiving water classification
may not be appropriate, the beneficial use may not be impaired or denied, and the water
quality criteria associated with that classification may or may not be overly conservative or
directly related to the desired use.
The third basis for problem identification involves public perception. This may be
expressed in a number of ways, such as telephone calls to public officials complaining
about receiving water color, odor, or general aesthetic appearance. Public perception of
receiving water problems is highly variable, further complicating this level of problem
definition.
/
The foregoing approaches provide a framework which permits water quality problems
associated with urban runoff/to be defined in a way that will assist in the formulation of a
management plan, the implementation of an effective control strategy, and establishing a
means of assessing its effectiveness.
2.3.3 Examples of Urban Runoff Receiving Water Impacts
Stormwater discharges into urban streams can dramatically change the character of a
stream as it passes through an urban area. Some examples of the nature of the problems
that can be produced, based on actual cases reported in the literature, are described below.
16
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These cases provide background on the types of things to look for in a local assessment of
the significance of urban NFS discharges.
In-stream monitoring of Village Creek in Birmingham, Alabama (Water Quality
Engineers, 1981) provides a classic example of stream degradation due to intense urban
development. At the stream's origin at Roebuck Springs, the creek has excellent physical
and chemical characteristics, supporting watercress and other vegetation. By the time the
stream passes under Vanderbilt Road it has turned grey-green and has an oily sheen and
contains significant debris. Further downstream at the western limits of Birmingham, the
creek is dark green, has a putrid odor and contains considerable oil and grease. At this
point the creek is often anaerobic and contains no fish or other biological life. This study
found that, on an annual basis, more than 90 percent of the copper loadings, more than 75
percent of the chromium and zinc loadings, and about 40 percent of the lead loadings
originated from urban runoff.
A study (Dong et al., 1979, and Southeastern Wisconsin Planning Commission,
1976) of the Menomonee River near Milwaukee, Wisconsin indicated that the upper, more
rural reaches of the river had an average of 40 times more fish than the lower, urbanized
reach. The urban segments of the river supported a significantly reduced and scattered fish
population and some segments were virtually devoid of even very pollution tolerant
species. These conditions are the combined result of higher concentrations of toxic
pollutants and poorer habitat conditions resulting from increased flow velocities and
channelization. Further, the watershed benthic community is in "poor" condition in the
urban area. The Menomonee study concluded that a relatively small degree of urbanization,
less than 20 percent, is sufficient to cause significant receiving water degradation.
Studies at other locations have produced results similar to those cited above.
Interestingly, toxic pollutants or long-term oxygen depletion has been found to cause more
serious receiving water problems than short-term, event-related oxygen depletion or other
concentration excursions. The accumulation of toxics in sediments and their subsequent
movement through the food chain is especially pronounced in urban receiving waters.
Studies on the Saddle River near Lodi, New Jersey (Wilber and Hunter, 1980) found
significant enrichment of heavy metals (two to seven times) in lower Saddle River
sediments (affected by urbanization) as compared to upper rural reaches. Similar results
were found in a stream near Champaign-Urbana, Dlinois (Rolfe and Reinhold, 1977),
where the upper two inches of sediment in an urban stream reach had much higher lead
concentrations (almost 400 ug/g) than sediments in the rural stream reaches. Species
diversity of plants and animals were found to be lower in urban streams as compared to
streams in rural areas. This impact is likely to be influenced by habitat and temperature
changes, as well as pollutant levels.
Long-term biological, chemical, and physical investigations of Coyote Creek near San
Jose, California (Pitt and Bozeman, 1982) revealed distinctive urban-rural differences in
the composition and relative abundance of aquatic biota. A comparison of urban Kelsey
Creek to rural Bear Creek near Bellevue, Washington (Pitt and Bissonette, 1984; Perkins,
1982; Scott et al. 1982) indicated significant interrelationships among the physical,
biological, and chemical characteristics. The urban creek was significantly degraded when
compared to the rural creek, but it still supported a limited and unhealthy salmon fishery.
Most fish in the urban creek had respiratory anomalies attributed to carcinogens in the water
associated with urban runoff. Although Kelsey Creek did not appear to be as polluted as
some of the urban creeks cited earlier, flooding caused by increased runoff from urban
development increased dramatically, with the result that the large amounts of toxic
pollutants discharged to the stream during wet weather, were diluted to very low
concentrations by the increased runoff volumes. The large flows that produced habitat
17
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problems diverted attention away from the long-term toxic accumulation potential and to the
physical effects of accelerated runoff. The dilemma here is that if the flows were to be
reduced in order to improve habitat conditions, the stream's assimilative capacity would
decline, and the effects of toxic pollutants could become even more pronounced.
As the foregoing examples indicate, urban runoff can produce both water quantity and
water quality problems, and the two are often interrelated. The water quantity problems
include but are not limited to flooding, streambank erosion, habitat impairment and altered
salinity. The water quality problems depend on (i) the type of receiving water involved
(stream, lake, etc.) and its characteristics, (ii) the beneficial use or uses to be protected, and
(iii) the specific pollutants involved. These in turn depend upon the intensity and the nature
of the activities in the urbanized watershed.
18
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CHAPTER 3
BMPs FOR CONTROL OF URBAN NFS
The term BMP, or Best Management Practice, has gained wide acceptance as a general
term designating any method for controlling the quantity and quality of stormwater runoff.
For the purposes of this manual, a "best management practice" (BMP) is considered to be
either (1) a practice (routine procedure) that reduces the pollutants available for transport by
the normal rainfall-runoff process, or (2) a device that reduces the amount of pollutants in
the runoff before it is discharged to a surface water body.
It is almost impossible technically and economically to completely eliminate NPS
pollutant discharges to a receiving water body. Realistic objectives of an urban stormwater
management program include : (1) to establish baseline controls to reduce pollutants in
stormwater runoff; (2) to establish controls for priority sources of pollutants; (3) to
sufficiently reduce pollutant levels to eliminate or mitigate an existing water quality
problem; and (4) to avoid the creation of a future problem where none exists now.
This chapter identifies and presents an overview of the different types of BMPs that
may be considered in the development of urban stormwater management plans. Sufficient
information is provided to support a planning level assessment of control options, but other
appropriate studies and reports should be reviewed for additional detail on design,
installation and operating aspects of specific BMPs. The chapter organization is structured
to address BMP types individually, but in practice, BMPs can and should be considered in
combination. Some examples include vegetated filter strips for pretreatment of inflows to
infiltration systems, and detention basins to reduce sediment loads to a wetland. The
specific characteristics of the site will determine the BMP types and combinations that are
most appropriate in each case.
Institutional aspects of the development of an effective urban stormwater management
program are not emphasized in this manual, but planning activities must include a
recognition of the need to develop an understanding of the issues at several levels of local
government, and provide support for the resolution of institutional issues. This may
involve the identification of the relationships between stormwater management plan features
and existing programs, plans and activities of City Managers, Planning Directors, and
Public Works Directors, whose departments and responsibilities will provide the
institutional framework for implementation of many of the important elements of a
stormwater management plan.
3.1 TYPES OF URBAN BMPs
Effective techniques for the control of nonpoint runoff pollutant discharges from urban
areas are identified below. These techniques are grouped into four categories, based on the
operating principle or the physical mechanism that reduces the amount of runoff pollutants
discharged to surface waters.
Detention basins - The term "detention" applies when the runoff is temporarily
stored and, apart from relatively minor incidental losses due to evaporation or
percolation, is subsequently discharged to a surface water. Control results from a
reduction in pollutant concentrations due to settling during the period the runoff is
detained.
19
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Retention devices - The term "retention" applies when runoff is permanently
captured so that it never discharges directly to a surface water. The usual
mechanism by which stormwater controls permanently "capture" surface runoff is
by infiltration. These techniques are often referred to as infiltration BMPs.
Vegetative controls - Vegetative controls provide contact between stormwater
runoff and vegetated areas and accomplish pollutant removal by a combination of
filtration, sedimentation and biological uptake that reduce pollutant concentrations,
and/or by a reduction in runoff volume due to infiltration or evapo-transpiration.
Figure 3-1 provides a schematic illustration of various types of vegetative BMPs
that can be considered in urban areas.
Source controls - Source Control techniques include any practice that either (1)
reduces the amounts of accumulated pollutants on the land surface available for
washoff by rainfall, or (2) regulates the amount of impervious area to reduce the
portion of rainfall that will appear as runoff, or (3) excludes inappropriate
discharges to storm drains.
There is no generic method by which these different control techniques can be ranked
either qualitatively or quantitatively. Site-specific conditions determine which practices are
best, and even whether a particular approach is appropriate. Key factors that influence the
suitability of a particular BMP include the following.
Drainage area served - The feasibility of a particular control measure depends
on the drainage area. There tend to be upper and/or lower bounds of the urban
drainage area that can be served with a particular control practice. These bounds are
based on design features and size requirements, as well as the operating
characteristics of the BMP. Figure 3-2 presents a number of BMPs and the
associated range of feasible drainage areas.
Soil permeability - The soil type, which effectively governs the long-term
percolation rate, is an important feature, which can limit the applicability of a
technique at a site. Figure 3-3 illustrates typical ranges of infiltration rates
associated with different soil types and their impact on the feasibility of different
BMPs.
Local acceptance - The acceptability of particular types of BMPs in different
urban areas may vary considerably and will influence selection.
Other restricting factors - In addition to the factors discussed above, Figure 3-
4 summarizes several other factors that typically limit the applicability of a control
practice.
Consideration of the factors discussed above will usually permit a planner to
significantly reduce the choice of control practices appropriate for a detailed evaluation.
The following sections discuss each of the BMP categories identified above. The
level of detail is limited to that considered appropriate for a planning-level assessment of the
general features of a NPS urban pollutant control program. An annotated list of selected
references is provided for a detailed evaluation. Key design features of the different control
techniques are identified, but it should be recognized that other variations are possible. The
discussion is limited to the size of a device and the density or intensity of application of a
source control practice, because these factors are most important at the planning or
20
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From: CONTROLLING URBAN RUNOFF: A Practical Manual for Planning and Designing Urban BMPs.
Source: Schueler (1987)
FIGURE 3-1. SCHEMATIC ILLUSTRATION OF VEGETATIVE BMPs.
21
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assessment stage. Many of the detailed design and implementation features are critical to
the ultimate success of a program, but are generally beyond the scope of a planning level
assessment. Similarly, the discussion of performance is limited to the range in removal
efficiency that can be achieved by a BMP type. Finally general benefits and common
limitations or constraints of the particular practice are discussed briefly.
3.2 DETENTION BASINS
The dominant treatment mechanism is the reduction of pollutant concentrations by
sedimentation, so that this practice is most effective for suspended solids and the fraction of
a pollutant associated with paniculate matter. For example, most of the lead that is present
in urban stormwater is present in paniculate form. The soluble fraction of total lead is
typically on the order of only about 10 percent, and as a result the removal efficiency for
lead in comparable to that for sediment. In contrast, as much as 40 or 50 percent of a
pollutant such as copper in runoff may be present in a dissolved form, and not susceptible
to removal by sedimentation.
Although the main benefit results from the reduction of pollutants concentrations in the
runoff, water quality impacts may also be reduced by the delayed release of stormwater
runoff volumes. The resulting reduction in peak discharge flows will tend to reduce stream
bank erosion and place less stress on the physical habitat. A slower release of stormwater
to a flowing stream may also result in lower concentrations of runoff pollutants in the
stream because of higher dilution in the stream.
Depending on the design of the inlet and outlet structures, detention basins can be
classified into the following three categories.
Dry ponds - These are basins with the outlet located at the bottom. They are
almost always dry, except infrequently and for relatively short periods following
larger storm events. The outlet size is restricted to limit the maximum flow rate.
Dry ponds are used for flood and erosion control and are not effective for water
quality control purposes. They may often be retrofitted to achieve water quality
control.
Wet ponds - These basins employ outlet structures designed to maintain a
permanent pool of water. They can provide high removal efficiencies for
particulates, and have also been observed to be effective in significantly reducing
soluble nitrogen and phosphorus concentrations by means of biological activity
such as algal growth in the pool of water.
Extended detention dry ponds - These basins employ an outlet structure that
will cause most storms to pond in the basin. Following a storm these basins drain
in about 24 to 40 hour and will be dry at all other times. The outlet structures may
be either perforated risers or subsurface drains. They provide a practical technique
for retrofitting dry ponds to obtain water quality benefits, and can provide
paniculate (and the associated pollutant) removal efficiency equivalent to that for
wet ponds.
3.2.1 Design Features
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Pollutant removal efficiency of an otherwise properly designed and maintained
detention basin may be influenced by seasonal factors such as algal growth, shoreline
vegetation, and ice formation. However, overall efficiency is determined principally by the
size of the basin (the available storage volume provided) relative to the amount of runoff it
receives during storm events. For any storm event, the volume of runoff will depend
primarily on the size of the contributing drainage area, and the proportion of impervious
area. The latter is influenced by land use. Since performance of a basin will vary with
storm size, pollutant removal estimates reflect the long-term average removal efficiency
over all storms.
A variety of basin sizing rules are in current use, depending on the experience and/or
preference of the jurisdiction. In some of the agencies that have been active in the
implementation of urban stormwater controls for a number of years, the sizing rules have
changed over time, or alternate rules have been adopted for different situations. There is no
generally accepted rule or standard for the size of a detention basin. Four commonly used
basin sizing rules are discussed below.
Design storm basis - Basin volume is set equal to the runoff produced by a
specified design storm. For example, the 1 year or the 2 year, 24 hour duration
storm event is sometimes used to specify the size of an extended detention basin
where a reduction of flooding and peak flow are important. The volume of rainfall
must be converted to the amount of runoff it will produce, and this will vary with
the land use distribution (percent impervious area) of the watershed.
First flush basis - Basin volume is designed to store 1/2 inch of runoff per
impervious acre of the contributing watershed. This is the most common rule, but
the same rule, using 1 inch, is sometimes used. This rule is attractive, because it is
simple to use and apply.
Mean storm volume basis - Basin volume is specified as a multiple of the mean
runoff volume of all storms. The value of mean runoff is determined by a statistical
analysis of the rainfall records. This method has the advantages of being able to
base the size on the desired level of performance, and to account for regional
rainfall characteristics. For example, the storm that produces 1/2 inch of runoff per
impervious acre is a more frequent event in the southeast, than it is in the midwest,
and there would be corresponding differences in the long-term pollutant removal
efficiencies for otherwise similarly sized basins in the two regions. For some
jurisdictions, this approach has been used (with local rainfall characteristics) to
determine the storage volume required to produce a particular performance level
(e.g., 70% TSS (Total Suspended Solids) reduction), and then translated to a
simple-to-apply sizing rule for everyday use.
Residence time basis - Basin volume is designed to provide a specified
residence time. Where this is used, long residence times (typically 14 days) are
used. This rule generally results in larger basins that provide higher levels of
reduction of most pollutants. However the principal objective is to enhance the
removal of soluble nutrients by improving conditions favorable for growth of algae
and aquatic plants.
A comparative evaluation of the above four approaches to determine basin size can be
obtained by the approximate ratio of the basin volume (VB) and the mean runoff volume
(VR). This requires an appropriate analysis of the rainfall record and the characteristics of
the contributing drainage area. For different regions of the country, the rainfall volume for
26
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the mean storm event ranges from about 0.2 to 0.5 inches. This can be taken as an
approximation of the runoff volume if we consider only the impervious acres. On this
basis, a basin with a VB/VR ratio of 1.0 would provide between 700 and 1800 cubic feet
of storage per impervious acre in the watershed. Note that the design volume of a basin is
directly proportional to the value of VB/VR. Approximate values of VB/VR for different
basin sizing rules are presented in Table 3-1.
TABLE 3-1
RATIO OF BASIN VOLUME TO THE MEAN RUNOFF VOLUME
FOR DIFFERENT DESIGN RULES
RULE CHARACTERISTIC VOLUME RATIO
VALUE (VB/VR)
First flush 1/2 inch per impervious acre 1 to 2
Mean storm volume 1 inch per impervious acre 2 to 4
Residence time 14 day residence time 4 to 5
Design storm 1 year storm 7 to 8
Design storm 2 year storm 8 to 9
Note in general the larger the basin volume, the greater the removal efficiency.
However basins with VB/VR ratios larger than 2.5 or 3 yield diminishing returns.
3.2.2 Performance
Depending on the size selected, wet ponds and extended detention ponds can reduce
suspended solid concentrations in stormwater runoff by 50 to 95 percent. Removal
efficiency for other pollutants is generally proportional to the pollutant fraction associated
with (adsorbed on to) the particulates. For screening level analysis, approximate removal
ranges that can be expected for detention basins are shown in Table 3-2. The performance
levels shown are estimates of the approximate order of the removal efficiency for different
pollutant types and basin sizes. Note that there are very limited data available on the
removal of bacteria. The high removal efficiencies shown in Table 3-2 may be deceptive,
because the water quality criteria levels are very low relative to the concentrations usually
present in stormwater.
3.2.3 Advantages and Limitations
Advantages:
Detention basins are effective runoff control devices, and there is an
appreciable body of experience that attests to their performance capabilities,
and provides a source of guidance for many important design details.
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They are suitable for relatively large drainage areas, and can be readily
incorporated into the overall plans for new developments.
Properly designed detention basins can enhance the value of the surrounding
property.
TABLE 3-2
TYPICAL PERCENT POLLUTANT REMOVED FOR DIFFERENT RATIOS OF BASIN
VOLUME TO MEAN RUNOFF VOLUME
POLLUTANT
Suspended solids
Organics (BOD, COD)
Total N and total P
Lead
Other heavy metals
Bacteria
PERCENT REMOVAL FOR INDICATED VB/VR
1 2.5 5 7.5
50-60
25-30
30-40
45-50
30-35
about
70-80
35-40
40-50
60-70
40-45
90 percent to
85-90
40-45
50-60
70-80
40-50
about 99 percent
90-95
45-50
60-70
80-90
45-60
TABLE 3-3
TYPICAL PERCENT POLLUTANT REMOVAL FOR RETENTION DEVICES
POLLUTANT
PERCENT REMOVAL FOR INDICATED SIZE
1/2 inch 1 inch 2 yr
per impervious acre from total area runoff vol
Suspended solids
Organics (BOD, COD)
Total N and total P
Heavy metals
Bacteria
75
70
45-55
75-80
75
90
80
55-70
85-90
90
99
90
60-75
95-99
98
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Existing dry ponds, previously installed for flow control, can often be
economically converted to serve as extended detention basins and provide
water quality control.
Limitations:
It is important to note that detention basins can become unsightly if routine
maintenance is not performed.
Removal of accumulated sediments will be required after 10 to 20 years of
service, and can be quite expensive.
The availability of sufficient land area at an appropriate location in the
watershed can be a problem.
Finally, it is usually difficult and often impossible to construct detention
ponds in an existing built-up area.
3.3 RETENTION DEVICES
Retention or infiltration devices enable a fraction of the runoff volume to percolate into
the ground, and hence reduce the discharge to a surface water body. Consequently, the
removal efficiency is the same for all pollutants, and is proportional to the percentage of the
total runoff volume that infiltrates. Many of the pollutants in urban runoff are effectively
trapped in the upper soil layers, and do not reach the subsurface aquifer. This filtration or
adsorption mechanism is particularly effective in the case of suspended solids, bacteria,
heavy metals and phosphorus. Note that some of the percolating runoff may reach the
surface water body, usually after a considerable delay, and after being "treated" by contact
with the soil. Retention devices can be classified into the following three categories.
Infiltration basins - These are relatively large open depressions, produced by
either natural site topography or by excavation, in which runoff is temporarily
stored while percolation occurs through the bottom or the sides. Outlet devices to
allow overflow of excess inflows are generally provided but are elevated so to
maximize the storage volume. Infiltration basins are normally designed so that any
stored runoff will percolate in no more than a day or two. Thus such basins are
generally dry.
Infiltration trenches and dry wells - The design of infiltration trenches and
dry wells is similar. The major difference is in the size and the configuration.
These are essentially excavated holes filled with coarse aggregate and then covered.
Dry wells are used primarily for roof drainage from residential and commercial sites.
Trenches or modifications of trenches serve larger drainage areas, and are
particularly applicable for streets and parking lots in commercial areas.
Porous pavement - The main practical application is for parking lots. Heavy
traffic and heavy loads that would tend to occur in most streets would compact the
surface and reduce the infiltration rate over time. Also, the vacuum sweeping to
remove fine sediments from the pavement, that is an important recommended
maintenance procedure, is most realistic for parking lot areas.
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3.3.1 Design Features
Key design factors that determine performance are the hydraulic conductivity of the
underlying soil and the size of the device relative to the contributing drainage area. In this
case, the size refers to the surface area available for percolation, and to the storage volume.
Examples of typical sizing rules that have been applied include the following:
Storage volume for 1/2 inch of runoff per impervious acre, or storage volume for 1
inch of runoff from the entire watershed. These rules are usually applied for
infiltration trenches. Generally, trenches are made relatively wide and shallow, and
percolation rates range from 0.5 to 1 inches per hour.
Storage volume equal to the volume of runoff from a 2 year storm. This sizing rule
is usually limited to infiltration basins, and makes assumptions comparable to the
preceeding rule.
Percolating area and storage volume may be determined by analyzing the rainfall
records and soil percolation rates for the site or area.
3.3.2 Performance
For retention basins, "treatment rate" can be thought of as the product of the
percolation rate and the available percolating area. Performance improves as the treatment
rate increases and efficiency can be enhanced by the amount of storage volume provided.
If large runoff volumes do not have time to drain between storms, basin performance may
decrease because the soil column does not dry out during the period between storms.
Depending on the size and the soil characteristics, infiltration devices are capable of
achieving removal efficiencies up to 99 percent. The removal of pollutants for different
sizes and designs in the Maryland-Northern Virginia area are listed in Table 3-3. Note that
the indicated performance can be expected to differ for areas with different rainfall and soil
types, but the indicated efficiencies are typical of infiltration BMPs.
3.3.3 Advantages and Limitations
Advantages:
Infiltration devices are capable of very high pollutant removals.
In many cases they can be built in developed areas.
In addition to water quality control, they also reduce stormwater runoff to
surface water bodies during and after storm events and provide desirable
subsurface recharge resulting in an increase in low, dry-weather stream
flows. This has the desirable effect of reducing flow variations in streams.
Limitations:
A variety of site specific factors (impermeable soils, high water table,
bedrock, etc) restrict the applicability of this type of BMPs.
Care during installation is necessary to prevent compaction of soil by
construction machinery, or the sealing of infiltration surfaces by sediment
generated during construction activities.
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Even during normal operating conditions, infiltration devices require
pretreatment (e.g., grass filter strips, geo-textile cloth) to reduce the amount
of coarse sediment reaching the infiltration surface.
3.4 VEGETATIVE CONTROLS
Vegetative BMPs include a variety of landscaping arrangements that serve to increase
the contact of rainfall and stormwater runoff with appropriate types of vegetation.
Vegetative control practices have the ability to reduce pollutant discharges by reducing the
quantity of runoff through enhanced infiltration, and to reduce concentrations through a
combination of filtration, sedimentation and biological uptake. The major types of
vegetative BMPs include the following.
Basin landscaping - Basin landscaping can be addressed during early
development of a watershed and can have a significant effect on the control of NFS
pollutants. The objectives of basin landscaping include but are not limited to
minimization of impervious surface area; protection and utilization of existing
wetlands; provision for green-belt buffers along stream banks; routing of runoff
flow through vegetated areas and away from erosion-prone steep slopes. Careful
selection of vegetation most suitable for site conditions has an important bearing on
physical appearance and the long-term performance of basin landscaping.
Wetlands - As part of site landscaping, it is possible to create new shallow marsh
wetlands specifically designed to operate as an urban runoff control measure. In
rare cases, there may be an existing wetland of appropriate type, size and location,
to warrant its consideration as a BMP for urban runoff, provided that the wetland is
not itself degraded as a result. In such cases, issues that will be difficult to resolve
with current knowledge, such as the potential of urban runoff flows or pollutants to
damage the existing wetland ecosystem, need to be addressed.
Grassed swales - Grassed swales are a shallow grass covered channel, rather
than a buried storm drain, that is used to convey stormwater. Grass channels are
mostly applicable in residential areas. They require shallow slopes, and soils that
drain well. Often grassed swales are used to provide "pretreatment" of runoff to
other controls, particularly infiltration devices.
Filter strips - These are similar in concept to grass swales, but are designed to
distribute runoff across the entire width and result in an overland sheet flow. These
strips should have relatively low slopes, adequate length, and should be planted
with erosion resistant plant species. They are often used as pretreatment for other
BMPs, for example, by being placed in the flow path between a parking lot and an
infiltration trench.
3.4.1 Design Features
Performance of vegetative controls is strongly influenced by the depth and velocity of
flow through or across the device (determined by slope and flow distribution), and by
contact time (determined by the length of the flow path). Soil with higher infiltration rates,
and the use of small check dams to produce temporary ponding of runoff improves
performance by enhancing the infiltration rates. Care in selecting plant species appropriate
for site specific conditions, and routine maintenance to maintain optimum height are
important maintenance requirements.
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3.4.2 Performance
The pollutant reduction capabilities of vegetative controls are not documented as well
for the other types of BMPs. Available information suggests that under favorable
conditions, vegetative controls can achieve moderate removals of particulates such as
sediment and heavy metals. They are generally not effective in reducing nutrients.
Many of the important design features are determined by physical characteristics of the
site, over which the planner or designer has little or no control. Thus, both the applicability
and the degree of performance that can be expected are highly site-specific.
3.4.3 Advantages and Limitations
Advantages:
The costs for vegetative controls tend to be lower than those for detention and
infiltration practices.
With appropriate planning and design, they can enhance the visual
attractiveness of a site.
Vegetative controls are usually most appropriate to provide pretreatment of
runoff in order to improve the operation and maintenance of other BMPs.
Limitations:
Vegetative controls are usually not adequate to serve as the only runoff control
practice for a site.
The overall pollutant reduction that can be obtained from vegetative practices
is usually limited, and depends to a substantial degree on the physical
characteristics of individual sites.
Seasonal differences in performance can be important. Removal effectiveness
for some pollutants can be markedly different during growing and dormant
periods.
Information on removal efficiencies for the range of conditions that might be
encountered is relatively limited.
3.5 SOURCE CONTROLS
This category of BMPs includes any practice that (a) reduces the amounts of
accumulated pollutants on the land surface available for washoff by rainfall, or (b) regulates
the amount of impervious area to reduce the amount of runoff, or (c) excludes inappropriate
discharges from storm drains.
Source controls address one or more of the above objectives. Depending on the basic
nature of a practice, it may apply at a local level or on an areawide basis. In most cases, a
management plan will incorporate an array of different source controls that are applicable
for the area. All source controls involve each of the following "implementation" aspects, to
a greater or lesser degree.
Education - Since many source control practices require either active public
participation, or general public acceptance, public education elements are an
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important feature. Developing a public understanding of the need for an action, the
benefit it can produce, and the pertinent details of its implementation, will be critical
to success, and will require a specific program element that addresses this
requirement.
Regulation - In many cases appropriate legal authority will have to be developed
and assigned to an appropriate agency. There may be a need for redefining roles or
establishing new agencies or departments. For example, an appropriate regulation
against a particular form of pollutant discharge and legal enforcement authority may
exist. If however, the enforcement authority resides in the Police Department, the
situation may fall so far down on the priorities dictated by the general mission of a
police agency, as to preclude any realistic expectation of active enforcement. This
is an example of one of the variety of issues that may not be apparent in a simple
listing of the elements of a particular NFS control action, and will have to be
resolved.
Guidance - For some source controls, specific formal technical guidance may
have to be developed and distributed to assure effective implementation. Examples
include details of erosion control practices, oil separators that may be required for
service stations, or detention facilities for new residential developments.
3.5.1 Design Features
There is no consistent way to characterize the salient design features of the variety of
different types of practices that can be included in the source control BMP category. An
important factor is the "application density". This generally (depending on the nature of the
particular practice) addresses how actively, frequently and/or thoroughly the practice is
pursued, and over how much of the total urban area it is applied. For example, the
frequency at which each catch basin is cleaned; the number of streets or parking areas that
are swept and how often the sweeper returns to a particular location are examples of
application density, and ultimately of how effective a source control practice will be in
reducing NPS pollutant loads from an overall urban area.
Source controls that have broad general applicability are identified below, with
examples of some of the more important elements that are necessary for effective
implementation. The list is not exhaustive; local situations can be expected to suggest other
practices that are not included in this discussion. In addition, some of those that have been
included in the list may not be applicable in all areas.
A. Exclude Inappropriate Discharges to Storm Drains
Eliminate illicit or inappropriate connections - This is one of the more
important source controls. The proposed stormwater permit regulations emphasize
the detection and elimination of non-stormwater discharges to storm drainage
systems. Elements of such a measure include the following:
- Research, strengthen (if necessary), and enforce existing regulations which
give local jurisdictions the legal authority to eliminate cross-connections that
result in sanitary sewage or industrial wastewater entering the storm drainage
systems.
- Reevaluate previous decisions to allow certain relatively clean waters to be
discharged to the stormwater system.
- Ensure existing spill response measures consider impacts on water quality.
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- Develop and implement an aggressive field program to search for, detect and
control domestic, commercial or industrial cross-connections and illegal
dumping.
- Develop and implement an aggressive field program to search for, detect, and
control sanitary sewer leaks and areas where surcharging or overflows would
be most likely to occur.
Prevent rainfall and runoff from contacting potential contaminants -
This is a well established standard practice that has obvious benefit. It applies
primarily to industrial or commercial sites.
- Educate regarding the need to keep rainfall and runoff from contacting
potential contaminants. Describe typical examples of the problem and
practical solutions.
- Develop and implement regulations to require covers for outdoor storage areas
that contain contaminants. Keep runoff from passing over areas that contain
contaminants. Emphasize good housekeeping for open loading-unloading
areas.
- Develop and implement an aggressive field program to search for, detect and
correct situations where rainfall or runoff presently contact potential
contaminants.
Encourage proper use and disposal of materials by homeowners - The
contaminants addressed by this control activity include materials such as fertilizers,
pesticides and herbicides, oil and antifreeze, paints, and solvents. Specific actions
for preventing the discharge of household contaminants include the following.
- Educate regarding the proper storage and use of fertilizers, herbicides and
pesticides; application methods, rates and frequency appropriate for the area;
and the potential environmental damage that can be caused by these materials.
Identify alternative methods for controlling insects and weeds (e.g., physical
controls, biological controls, less toxic chemicals).
- Educate regarding the need to keep oils, paints and similar contaminants out
of storm drains; the potential environmental damage that can be caused by
these materials; and acceptable disposal methods.
- Develop and implement programs and set up receiving facilities and
procedures for specific pollutants such as crankcase oil, pesticide or paint
containers, and other potentially harmful chemicals. Recycle if possible. The
success of such a practice depends on the number and location (convenience)
of stations and the awareness of the community about the effect of pollutants
on the environment.
- Research, strengthen (if necessary), and enforce existing regulations which
give local jurisdictions the legal authority to prevent improper disposal of
pollutants into storm drainage systems.
- Label storm drain inlets and provide signs along the banks of drainage
channels and creeks explaining the environmental impacts of dumping wastes.
Develop and implement an aggressive field inspection program.
- Search for, detect and prevent dumping or routinely discharging pollutants
into storm sewers, drainage channels and urban streams.
34
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- Look for runoff or spill problems when conducting fire inspections.
B . Reduce Street and Land Surface Sources of Pollutants -
Control littering and improper waste disposal practices - In addition to
its pollution control benefits, an effective litter control program will improve the
general aesthetic appearance of the area. Because such programs have easily
achieved public acceptance, with visible effects, they can assist in developing
interest and acceptance of other BMPs where the relation between practice and
benefit may be less obvious. Specific actions might include the following.
- Educate regarding the NFS pollution impacts that result from littering and
improper waste disposal practices.
- Research, strengthen (if necessary), and enforce existing regulations which
give local jurisdictions the legal authority to control littering and the improper
disposal of potentially harmful or aesthetically objectionable materials.
- Provide litter bags for use in cars. Work with citizen action programs to
facilitate efforts to report littering incidents and illegal dumping.
- Develop and implement regularly scheduled cleanup days and corresponding
curbside collection of trash and household debris.
- Provide, collect and maintain an adequate number of litter receptacles in
strategic public areas, and during major public events.
- Coordinate with efforts (by others) to establish practical controls regarding
potentially harmful packaging of consumer products.
Control animal wastes - The specific practices considered should consider both
household pets and where appropriate, suburban livestock such as horses and
chickens.
- Educate regarding the need to clean up and properly dispose of pet wastes,
and where appropriate, the need for proper management of wastes from
suburban livestock and agricultural operations in the watershed.
- Provide informational signs and dispense doggie litter bags in parks and other
selected areas.
- Implement and enforce leash laws and pet waste cleanup ordinances in
selected public-use areas.
Improve the maintenance of major paved areas - Activities in this category
include both physical repairs to maintain pavement surfaces in good condition so
that pavement debris and degradation products are not washed into storm drains,
and street cleaning practices that remove litter and externally generated dust and
associated pollutants that accumulate on paved surfaces.
- Improve pavement repair and maintenance programs on streets and parking
areas (e.g., fill potholes, seal cracks, apply surface treatments).
- Develop and implement sufficiently intensive street sweeping programs for
strategic locations. For example, paved surfaces in central business districts,
shopping malls, major parking lots and industrial areas tend to produce more
concentrated surface sources of heavy metals, oil and similar contaminants.
35
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- Implement street parking regulations (e.g., alternate side parking days) where
necessary for effectiveness of street sweeping programs.
Institute programs to remove accumulations of litter and debris -
Floatables and accumulations of debris represent an important aesthetic problem for
urban streams in many areas.
- Sponsor periodic stream bank cleanup programs to remove accumulations of
litter and debris in urban streams or on their banks. Floatable materials often
accumulate behind roadway culverts. Encourage participation by suitable
community groups (e.g., Boy Scouts, etc.). Coordinate with Public Works
Departments for hauling and disposal of removed materials.
- Provide for routine sweeping of streets that border urban stream courses,
and/or other targeted areas , e.g., certain parking lots.
- Provide surveillance and enforce regulations against dumping.
Institute environmentally protective road maintenance practices -
Certain routine practices that are known to have adverse environmental effects
should be examined and modified to the extent possible.
- Calibrate road deicing spreader equipment to avoid excessive application
rates.
- Cover salt storage areas.
- Consider use of alternate materials in select areas.
- Consider use of low maintenance vegetation instead of herbicide use.
Control airborne pollutants - A significant source of many of the pollutants
present in urban stormwater runoff is the deposition of atmospheric particles that
originate from a variety of sources, on land surfaces in the urban area. Source
control activities that can address this situation include the following.
- Educate regarding the relationship between air pollution and NPS water
quality problems, and the need to coordinate with programs (by others) that
seek to reduce paniculate atmospheric emissions of pollutants from
individual, public, commercial and industrial sources.
- Educate regarding the potential benefits of reduced automobile use by various
means (e.g., ride sharing, carpooling, public transportation), and the
importance of frequent vehicle inspection and maintenance efforts to reduce
atmospheric emissions.
- Educate regarding the proper operation of fireplaces and wood burning stoves
to minimize the emissions of paniculate matter.
- Cooperate with public transportation agencies, public agency motorpools, and
public works departments to provide effective air pollution controls on
publicly owned vehicles and motorized equipment, and, where practical, on
the use of alternative clean-burning fuels.
C. Control Erosion -
Control erosion at construction sites - These actions suggested here are
directed at the control of erosion from land disturbed during construction, or the
prevention of eroded materials from leaving the site.
36
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- Educate architects, engineers, contractors, and public works personnel about
the need for and practical methods for erosion control, sediment control,
groundwater disposal, and site waste management and disposal.
- Develop and implement effective erosion and sediment control regulations,
and requirements for corresponding construction inspection programs. These
should apply to public-sector as well as private-sector construction programs.
- Develop and implement improved erosion and sediment control policies in the
environmental elements of all general Plans (develop and adopt General Plan
amendments, when needed).
- Adopt policies that require all CEQA compliance documents and all site
development plans to explicitly address the topics of erosion potential,
proposed erosion and sediment control plans, and enforceable mitigation
measures to minimize environmental impacts.
- Require contractors to post bonds to cover potential damages from erosion or
sediment deposition.
- Institute an active construction site inspection and enforcement program.
Control erosion of undeveloped land and park land - These efforts are
directed at the control of erosion from essentially undisturbed urban land areas, to
reduce potential adverse impacts on urban water bodies.
- Educate public works personnel and managers of parks and open-space lands
about the need for and practical methods for erosion control and sediment
control.
- Develop and implement programs to actively search for, identify, evaluate,
and prioritize erosion problems on undeveloped land, park land or open-space
urban land use areas.
- Develop and implement programs to work with landowners, tenants, and
public agencies to apply practical erosion and sediment control practices.
- Develop and implement practical programs for revegetating and otherwise
restoring eroding areas (e.g., areas damaged by fires, off-road vehicle use).
- Educate managers and users of park lands and open-space lands concerning
the need to restrict off-trail activities. Establish and enforce practical, site-
specific regulations to control harmful off-trail activities.
D. Implement Land Use Planning -
Implement Zoning regulations - Appropriate zoning ordinances may be used
in sensitive areas to provide for development patterns that are compatible with
control of NPS discharges and the protection of receiving waters.
- Zone to limit dwelling unit density and control the amount of on-site
pollutants generated and control the amount of runoff by limiting the
impervious surface area created.
- Restrict development adjacent to streambanks. Require vegetated buffer strips
along streambanks.
- Restrict development on sites with soils and slopes that are susceptible to
serious erosion.
37
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Limit the directly connected impervious area -
- Develop planning guidelines illustrating favorable development techniques.
- Use grass swales for drainage in preference to curbs and gutters and piped
drains, where feasible.
- Encourage use of cluster housing, buffer strips, open space, or other patterns
that reduce the quantity of runoff from the site.
- Avoid direct connection of roof leaders to drain pipes or paved surfaces.
Require physical controls for new developments -
- Require the installation of detention basins or infiltration devices as BMPs for
the control of the quality and/or quantity of runoff and for control of peak
flows on all new development sites.
- Develop specific guidelines for design and construction of these devices.
- Provide for the necessary supervision, inspection and enforcement of
regulations to insure compliance.
E. Other Control Measures -
Control oil and grease - Automobile operation and maintenance is the principal
source of oil and grease that can result in objectionable films and sheens on the
surface of receiving waters. Fractions that remain in solution may contribute toxic
contaminants. Food service facilities may contribute animal fats and greases (vs
hydrocarbon based) to runoff.
- Educate regarding the effective use of "housekeeping" practices, oil and
grease traps, the use of adsorbents and cleaning compounds for controlling oil
and grease at gas stations, automotive repair shops, parking areas,
commercial and industrial facilities, and food service facilities.
- Educate regarding the need to provide adequate and sufficiently frequent
vehicle inspection, and to maintain efforts to reduce leakage of oil, antifreeze,
hydraulic fluid, etc.
- Research, strengthen (if necessary), and enforce regulations which give local
jurisdictions the legal authority to require oil and grease controls in areas that
are significant sources (e.g., gas stations, automotive repair shops, parking
areas, commercial and industrial facilities, and food service facilities).
- Develop technical guidance that will facilitate efforts by responsible parties to
comply with regulations requiring oil and grease controls (e.g., oil traps, plate
separators, synthetic adsorbent material, grassed swales).
Control leaks from gasoline, fuel oil, and chemical storage tanks -
The actions listed can help to control pollutant contributions from leaking storage
tanks.
- Educate regarding the environmental impacts that result from leaks and spills
from gasoline, fuel oil, and chemical tanks, above and below ground.
- Coordinate with efforts (by others) to intensify the implementation of existing
regulations which call for improved design of new tanks,(e.g., double walls,
monitoring facilities); replacement of tanks over a specified age; self-
38
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monitoring programs; and implementation of a strategically focused spot-
check program to search for, identify, test, and control leaking storage tanks.
Intensify the maintenance and repair of stormwater drainage systems
These actions are directed at removing the pollutants that tend to be retained, and
accumulate at specific locations in the stormwater drainage system.
- Determine the effectiveness of increasing the frequency of cleaning out storm
sewer inlets, catchbasins, storm sewer pipes and drainage channels in areas
where sediments, debris, or floatable materials tend to accumulate. Develop
and implement improved programs where appropriate.
- Develop and implement an aggressive field program to search for, test,
remove, and properly dispose of sediment deposits in drainage channels and
streams, which contain relatively high concentrations of pollutants.
- Develop and implement a program which provides a means of recording the
observations of field inspection and maintenance personnel, so that this
information can be used to help locate the sources of pollutants.
Address indirect sources of sewage to stormwater drainage systems -
There are conditions or situations which make it possible for sanitary sewage to
contaminate stormwater other than by direct piping connections.
- Improve sanitary sewer maintenance where necessary to control excessive
exfiltration.
- Consider instituting a septic tank certification and inspection program.
3.5.2 Performance
While all of these practices will reduce water pollution, the current state of the art
precludes accurate estimates of the effect such practices may have on area-wide pollutant
loads or to problems in specific water bodies. There is considerable uncertainty associated
with the ability to quantify load reductions. In addition, even assuming performance levels
could be defined, estimating the extent to which the public at large applies a practice will
generally be very difficult.
3.5.3 Advantages and Limitations
Advantages:
Some source control actions will be very visible and will involve high level of
public awareness and involvement. They can help to generate a sense of
active community participation in an overall NFS control program, and may
help secure the implementation of other, less obvious, elements of a
management plan.
In addition to reducing pollutant discharges to water bodies, many will have
attendant aesthetic or cosmetic benefits.
Limitations:
Adoption (with or without enforcement) of the necessary ordinances may
create negative public reactions that may have an adverse effect on other areas
of the program.
39
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In most cases, there is no reliable way to estimate the effect of a particular
source control measure on the urban NFS pollutant loads.
Effectiveness of a practice depends on the degree to which it is applied and the
geographical extent of the application. Even with appropriate regulations in
place, there is no positive assurance of compliance to the extent desired.
Developing and assigning the necessary legal authority, and adding new
responsibilities to established public agencies whose budget, experience, and
priorities may not relate directly to NFS control may be difficult to resolve.
40
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CHAPTER 4
TARGETING TO PRIORITIZE URBAN AREAS FOR CONTROL
4.1 INTRODUCTION
This chapter presents a ranking procedure for identifying the area (or areas) within an
urban jurisdiction where it is most appropriate to focus additional stormwater pollutant
control efforts, after jurisdiction-wide baseline controls and priority sources have been
addressed. Technical, economic and timing considerations usually require a staged
management effort in reaching water quality goals. The methodology ranks areas relative to
the other areas in the jurisdiction and is not intended to imply that areas receiving lower
ranking do not require controls.
A large urban jurisdiction will commonly involve a network of streams differing in
size, use and sensitivity to urban runoff, and which receive discharges from different parts of
the jurisdiction. A few of these may have greater local importance than others because of the
nature and visibility of the resource, or the type or level of the its use. In addition some
urban NPS control practices will generally apply to the entire jurisdiction (e.g., an anti-litter
ordinance), but in many cases the control choices will tend to be either site-specific or land
use-specific. For example appropriate pollutant control practices for industrial areas will
usually be quite different than those suitable for residential areas. Further, factors such as
topography, soil conditions, land availability and cost will often influence the selection of
controls even for similar land uses in different parts of the jurisdiction.
For large urban areas it may be necessary to develop a targeting procedure for
implementing stormwater controls for the following reasons:
Once jurisdiction-wide, baseline actions have been applied, the implementation
of additionsl stormwater controls over the entire urban jurisdiction, all at once is
not usually possible . Thus a phased approach will generally be necessary.
Development of a rational basis for ranking different areas within the jurisdiction
will be desireable.
Accounting for relevant site-specific attributes and documenting the decision
process is necessary. This will be particularly useful for describing the specific
targeting decisions to the public
Assist the urban jurisdiction in meeting the requirements of Sections 319 and 402
of the Clean Water Act.
4.2 ELEMENTS OF THE TARGETING PROCEDURE
The procedure described in this chapter provides a way to prioritize urban watersheds
so that a stormwater program can be sequentially implemented based on general and site-
specific considerations. The approach described assumes that appropriate jurisdiction-wide
measures have been identified and are being applied, and that any priority sources of
pollutants are independently addressed. The procedure for prioritizing additional efforts
consists of the following elements:
41
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1. Identify discrete watersheds within the overall urban area, and the corresponding
receiving stream segment. The necessary information could be developed from
topographic or drainage system maps.
2. Determine relevant physical characteristics and other attributes of each
watershed, and of the receiving stream segment.
3. Tabulate pertinent data in summary form.
4. Complete a "targeting table" by assigning a relative ranking value to each
attribute. Assign weights to the attributes to reflect their relative importance in the
local decision process.
5. Prioritize urban drainage areas in accordance with the resulting weighted sum of
ranks.
There is no standard or generally accepted set of attributes that are used for
establishing priorities for NFS control programs. A few attributes which are generally
important are presented in Section 4.3. Note that any additional attributes that may be
appropriate to consider for the local situation should be added to the procedure. The relative
importance of each factor is accounted for by assigning different weights to each factor.
4.3 FACTORS USED FOR PRIORITIZATION
The factors in the ranking process are discussed below.
Waterbodv Importance - This factor describes the general importance, and the
ability of the waterbody to support a variety of beneficial uses. Since aquatic life populations
and diversity as well as citizen use are likely to be greater for large streams (or lakes) with
appreciable amounts of water than for very small water bodies with low flows and small
surface areas, flow or watershed size can be used as a surrogate for waterbody importance.
Since measured flow data is not likely to be available at all points of interest, total drainage
area of the watershed upstream of the location of interest can be used for comparison
purposes. Absolute values of drainage area are thus assigned an appropriate rank in the 1 to
' 9 range. Note the larger the stream size (drainage area) is, the higher is the rank.
Type of Use - This factor is selected to provide a comparative measure of the
importance based on the primary type of beneficial use of the waterbody. The following
beneficial uses are suggested for use in this ranking factor. The list below indicates two
possible ways of ordering relative importance for local ranking.
BENEFICIAL USE
contact recreation aquatic life
non-contact recreation contact recreation
OR
aquatic b'fe non-contact recreation
LOW urban drainage urban drainage
42
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The order selected would be one that reflected local values and issues. Bear in mind
that the rank a particular use is assigned for this targeting exercise is not a statement of
concern for fish vs people or vice-versa. The rank values (1-9) assigned could be given a
wide difference where one or another had a high degree of local importance, and only slightly
different (or similar) values in cases where use types were of equal importance.
Status of Use - This factor accounts for the present status of beneficial uses of the
water body. There are different, but equally reasonable, rationales that can be applied to rank
the relative importance of the desired use.
It can be reasoned that an EXISTING use in fairly good condition (or a presently
good but merely threatened water), should have a lower priority than one for which the use is
present but IMPAIRED. The philosophy would be to give highest priority to rectifying
obvious problems, assuming that there would then be time for later actions to polish up the
only slightly impacted areas. This implies that the highest rank would be assigned to a use
whose degree of impairment is so great that it is effectively DENIED.
However, it is also reasonable to embrace a scheme that would assign higher ranks to
uses that are presently in good condition or are only somewhat IMPAIRED, than to the real
problem areas with DENIED uses. The philosophy here would be to give highest priority to
protecting what we have and correcting the small problems, and then tackle the difficult ones
further down 'the road. The' relative ease and cost of reclaiming the IMPAIRED use might be
such that demonstrable control program benefits would be achieved more rapidly, and there
might be a greater impact on health and safety (if there is, in fact, no actual use of the denied
resource).
Level of Use - Although the waterbody importance factor affects the level of use,
this independent factor is included to make an important additional distinction that recognizes
local factors. There will be cases where even a stream that would otherwise be considered
quite small, will have a disproportionately high level of use, e.g., a small stream that flows
through a park or recreational area and is popular with children for wading. The assigned
rank for this factor should be based on whether the level of use is low, moderate or high -
relative to the other waterbodies in the target area.
Pollutant Loads - The difference in annual pollutant load contributed by runoff
from the different watersheds can be used as a surrogate for the potential to cause water
quality problems that result in the impairment of a beneficial use. The procedure described
below for estimating these loads, can be used for a comparative evaluation of pollutant
sources for a screening type analysis.
Note that the procedure is not designed to estimate actual loads for the area. To do so
accurately would require considerable site-specific refinements in site characterization and
more elaborate analysis procedures. Nor is the procedure designed to provide an indication
of actual water quality impacts. Different analysis procedures would be required depending
on whether the water body in question was a stream, lake, estuary or coastal water body. In
addition, streams quality responds to the transient event-to-event inputs, much more than to
annual mass loads. The load estimates for the purpose of developing a relative rank, are
suitable for this purpose, but are not at all adequate for reliable pollutant loading estimates or
receiving water impact assessments.
43
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The pollutant mass load in urban runoff is determined by (i) the amount of rainfall (ii)
the total area of the watershed, and (iii) the distribution of land use types in the watershed.
The rainfall can be assumed to be the same in all parts of the urban jurisdiction. The land use
type influences the typical fraction of impervious surface area, which in turn determines how
much of the rain is converted to surface runoff. Land use also influences the average
concentration of a pollutant in the runoff.
The annual rainfall typical for the urban jurisdiction will normally be known.
However the exact value assigned will not be important because the comparison to be made is
one of relative differences between different watersheds in the jurisdiction. For the same
reason, it is not important for the pollutant load comparisons, whether or not the use affected
is largely seasonal in nature.and influenced by seasonal differences in rainfall. The
suggested procedure (used in the example later) is to base comparative loads on 1 inch of
rainfall. This is completely arbitrary, but it simplifies the arithmetic. Annual precipitation, or
precipitation volume in a critical season, could also be used.
The total area and the distribution of land use categories for each urban watershed
being considered, must be extracted from local maps. Table 4-1 provides typical values for
impervious fraction and pollutant concentration in urban runoff (in milligrams per liter), for
selected land use categories. It also lists values for runoff coefficient (Rv), which is
computed from the impervious fraction, and represents the fraction of the rainfall that
becomes surface runoff.
The values for the percentage of impervious area for different land use classifications
presented in Table 4-1, are based on data from EPA's NURP program, and are suitable for a
screening type analysis. However, they should be modified as appropriate if local
information is available, or if site-specific factors are likely to modify them. Note that the
NURP study was not designed to provide definitive information on the differences between
land use types, and provided no information on industrial runoff.
TABLE 4-1
TYPICAL VALUES OF PERCENT IMPERVIOUS AREA
AND POLLUTANT CONCENTRATIONS
POLLUTANT CONCENTRATION
(mg/1)
LAND USE
Open -Developing
Open- Park
Resid - LOW DENS
Resid - HIGH DENS
Commercial
Industrial
5
5
20
50
90
70
0.1
0.1
0.2
0.4
0.8
0.6
TSS
150
50
100
90
80
120
TP
0.800
0.800
0.600
0.400
0.200
0.200
O&G
0
0
5
10
15
20
Cll
0.010
0.010
0.030
0.040
0.050
0.050
A variation in concentration with land use is also shown in Table 4-1. This is based
on judgement concerning the relative differences that are likely to be associated with the
general differences in the land uses. The values shown are considered appropriate for an
analysis designed to estimate a relative comparison of mass loads. In particular, the values
44
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shown for industrial land use should be used with caution, because they are very sensitive to
the type of industrial activity.
Runoff concentrations are listed for only four pollutants. However, in this screening
analysis, these can be used as surrogates for other pollutants for which data may not be
readily available. The following relationships will generally apply. Total Suspended Solids
(TSS) can serve as a general surrogate for sediment deposition and siltation in streams and
lakes. Total Phosphorus (TP) can be used to reflect the discharge of nutrients and their
contribution to eutrophication potential. Oil and Grease (O&G) can be used as a surrogate
for the potential for problems associated with degraded aesthetic values. The heavy metal
Copper (Cu) can be used as a surrogate for potential toxic impacts on aquatic life.
The mass load (pounds) of a pollutant from runoff is computed by :
M = RainV * Rv * Area * Cone * 0.227 (4.1)
where:
M = mass load [pounds]
RainV = rainfall amount [inches]
Rv = runoff coefficient [unitless]
Area = drainage area [acres]
Cone = average concentration in runoff [mg/1]
0.227 = unit conversion factor
If the rainfall amount used is the annual rainfall, the load computed would be pounds
per year. However, since the analysis requires only comparative loads, a rainfall amount of
1 inch can be used for convenience. Loads should be computed for each land use within a
watershed, and then combined to provide the total watershed pollutant mass load to be used
for comparison. Rank values would be assigned to each watershed based on the range of
mass values computed.
Implementabilitv of Controls - This factor is designed to reflect the fact that an
effective management program may be easier to implement for certain watersheds than for
others. The prioritization scheme takes this into account. Differences may be based on
institutional factors, existing ordinances, or technical factors. For example, it will usually be
much easier to implement effective runoff controls for a newly developing area, than to
retrofit controls in an existing central business district. Similarly, control requirements may
be institutionally easier to implement for an industrial area than for scattered residential areas.
4.4 DESCRIPTION OF TARGETING PROCEDURE
A rank between 1 and 9 is assigned to each of the above factors to reflect increasing
value or importance of the selected factor. It is emphasized that in each case, the rank is not
an absolute measure of importance, but rather a comparative measure. Rank values are
assigned so that the higher the value, the higher will be the priority for action. Scale ranges
other than 1 to 9 could obviously be used as well. Very high ranges (e.g., 0 to 100) imply
an ability to make fine distinctions and gradations that will not usually be possible. A very
small range (e.g., 1 to 3) may represent the realistic level at which distinctions can be made in
a screening analysis of this type, but constrains the sensitivity of the comparisons. The range
of 1 to 9 is suggested as a reasonable compromise.
It is recognized that some factors will be more important than others. The relative
importance of different factors is accounted for by assigning different weights to each factor.
45
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This helps to avoid the choice of factors from forcing the results in a particular direction. The
assignment of weights also permits the emphasis of locally important considerations. For
example, if the planning body determined that it was most important to get some type of
implementation program started at an early stage, the "ability to implement" factor could be
given a high weight. In a case where the main concern is the violation of water quality
standards for fishable, swimmable waters, the relative weights would emphasize the "use
status" and "pollutant load" factors. It is not really important what the sum of the weights
add up to, since it is relative scores that matter.. However, it is suggested that factor weights
be assigned so that they add up to 100 (as in 100%). This will help in balancing the
assignment of relative importance to the different targeting factors, and help assure that
absolute scores that result from different trials are all on a common basis
It is important to bear in mind that there is a considerable subjective element involved
in the selection of factors, and in the assignment of ranks and relative weights. Presumably
these will be developed as a result of group discussions involving interested parties. The
methodology presented here provides a useful framework for balancing priorities and
forming a collective judgement.
The targeting procedure is best illustrated by an example. Figure 4-1 presents a
schematic illustration of a hypothetical urban jurisdiction. Watersheds and land use types are
delineated and their spatial relationship to important use locations are shown.
URBAN AREA
BOUNDARY
HIGH DENSITY^
RESIDENTIAL
FIGURE 4-1 SCHEMATIC REPRESENTATION OF URBAN AREA
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TABLE 4-2. CHARACTERISTICS OF URBAN AREA AND ESTIMATED RUNOFF CONCENTRATIONS.
LAND USE
INDUSTRIAL
COMMERCIAL
RESIDENTIAL - HD
RESIDENTIAL - LD
OPEN - DEVELOPING
OPEN - URBAN PARK
Rv
0.6
0.8
0.4
0.2
0.1
0.1
Avg CONC in Runoff mg/l
TSS
120
80
90
100
150
50
O&G
20
15
10
5
0
0
TP
0.20
0.20
0.40
0.60
0.80
0.80
Cu
0.05
0.05
0.04
0.03
0.01
0.01
DRAINAGE AREA IN ACRES
STREAM
A
0
10
100
200
0
0
STREAM
B
150
80
100
0
0
0
STREAM
C
0
110
50
200
150
50
URBAN
TOTAL
150
200
250
400
150
50
TOTAL URBAN AREA
UPSTREAM DRAINAGE AREA
TOTAL DRAINAGE AREA
310
600
910
330
0
330
560
20,000
20,560
1200
20,600
21,800
TABLE 4-3. ESTIMATED TSS LOADS FOR THE TARGETED AREA.
LAND USE CATEGORY
INDUSTRIAL
COMMERCIAL
RESIDENTIAL - HD
RESIDENTIAL - LD
OPEN - DEVELOPING
OPEN - URBAN PARK
TSS LOAD (pounds per inch of rain)
STREAM
A
0
145
817
908
0
0
STREAM
B
2452
1162
817
0
0
0
STREAM
C
0
1598
409
908
511
57
URBAN
TOTAL
2452
2906
2043
1816
511
57
WATERSHED TOTAL
WATERSHED RANK VALUE
1870
1.7
4431
4.1
3482
3.2
9784
9.0
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Table 4-2 presents a summary of the information that describes the urban area and the
runoff concentrations assigned for the different land uses in the urban area. The runoff
coefficient (Rv) is computed from the percentage of impervious area as described earlier in
Chapter 2. The percent impervious area must be estimated from appropriate local maps or
inspections, or from the approximate relationship described in Chapter 2.
Mass loading results for total suspended solids are presented in Table 4-3. These
loads are computed using Equation 4.1, based on one inch of rainfall, and using the assigned
areas, runoff coefficients and concentrations that were summarized in Table 4-2. The
pollutant load is computed for each land use category, in each watershed. Individual
watershed components are summed to provide a total for each stream. This table
organization permits a direct visual indication of the differences between stream totals, and
the contributions of the different land use types.
Each stream load is converted to a rank values at the bottom of Table 4-3, which will
become its assigned value for the pollutant load factor in the overall ranking procedure. The
rank values for any stream (X) is computed as its ratio to the load from the total urban
watershed.
rank X = rank MAX (load X / load MAX)
The overall urban total load (load MAX) is assigned a rank value of 9 (rank MAX) and the
other streams are assigned rank values on the basis of their ratio to the total.
To illustrate the use of the prioritization scheme, rank values have been assigned for
each of the targeting factors and entered as shown in Table 4-4. For this hypothetical case,
the considerations described below are assumed to be the result of a consensus reached in
planning discussions, and the basis for the rank values assigned . Bear in mind that there are
no "correct" or "incorrect" rules or rationales for assigning rank values, or weighing the
importance of different factors. There is a substantial subjective element involved, and the
rationale applied will vary with the local situation. For purposes of illustrating the
procedure, we assume that the determinations indicated below represent the collective
judgement of the local group involved in the targeting analysis.
Ranks for the stream importance factor, represented by stream size , are assigned
in proportion to the total drainage area (urban and upstream) providing flow to
the stream.
Beneficial use type ranks are based on providing a mid-range ranking value to
stream A, whose actual use is a habitat for aquatic life, with little or no direct
human contact. (Alternatively,if runoff quality impacts were not considered to
present a significant threat to human health, and protection of aquatic life was
considered the highest valued beneficial use, then a higher rank value would be
assigned to stream A.).
Stream B is assigned a low rank on the basis that, although water quality is the
poorest, the stream is viewed primarily as an urban drain. In effect, reclaiming this
stream is given a much lower priority than protecting or improving other streams in
the area.
Stream C, and the downstream combination of all the urban streams, are assigned a
high rank because of the active recreational use of the water at the city park and the
downstream recreational area. The high visibility and social value of these facilities
make the protection of their primary use an important issue.
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TABLE 4-4 PRIORITIZATION ANALYSIS FOR URBAN AREA TARGETING
URBAN
WATERSHED
WEIGHTS
WATERSHED A
WATERSHED B
WATERSHED C
TOTAL
URBAN WATERSHED
STREAM
SIZE
25
4
2
8
8
USE
TYPE
10
5
2
8
8
STATUS
10
7
2
2
5
LEVEL
5
4
1
6
8
POLLUTANT
LOAD
TSS
25
1.7
4.1
3.2
9.0
ABILITY
TO
IMPLEMENT
25
5
7
3
2
TARGET
SCORE
100
4.08
3.73
4.85
6.45
TARGET SCORE
WEIGHTED AVERAGE OF RANK POINTS
SUM (RANK SCORE ' WEIGHT)
SUM (WEIGHTS)
Beneficial use status ranks are based on the following reasoning.
Streams A's use is still a viable one, but is somewhat impaired, and it is felt that it
can be improved and protected by control of the runoff loads. It is assigned a high
rank compared to the others.
Streams B receives a low rank for two different reasons. Stream B has had poor
quality water for so long that the denial of any use other than conveyance of
drainage water has long been accepted. There is no current biota or human contact
to be protected. The part flowing through the city park is in a buried culvert.
Stream C is given a low rank for a completely different reason. The quality is still
good. The value of pollution prevention to avoid degradation is not being
dismissed, but rather a judgement is made that current status of the use is such that
the need for action on this stream is less pressing.
The combined total watershed flow below town is ranked higher than C. Quality is
somewhat poorer (because it is affected by the loads coming from A and B). A use
status rank between A and C is assigned.
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Use level ranks are a reflection of the relative number of people using or affected
by the different stream segments. The total urban watershed is assigned the
highest rank because the indicated recreational area there is popular and heavily
used.
Stream C is ranked slightly lower, but relatively high, because it is the main stream
flowing through the town, is visible from many areas, and protection of its aesthetic
appearance is considered desirable.
Stream A is next lower on the scale principally because it runs through a low
density residential area, and only a small portion of the local population comes in
contact with it
Stream B receives the lowest rank because few even see it and no one actually uses
it.
Pollutant load ranks are assigned in proportion to the loads from each area that
were developed on Table 4-2, as described earlier. TSS has been used as the
general indicator of runoff pollutants for illustrative purposes. The analysis
would be repeated using the other pollutants to determine whether the targeting
decision is affected by the choice, since each is reflective of a different type of
impact. For example, the concern stated above for protecting the aesthetic
quality in Stream C and at the recreational area would be best indicated by
evaluating the same type of comparison results using the Oil&Grease surrogate
pollutant.
The total watershed has the highest rank because it reflects the loads from all the
others.
Stream B has the second highest loading rank because of the high imperviousness
and concentrations of the industrial and commercial areas it contains. This is
followed by C which has a large area and significant commercial land use.
Moderately sized Stream A with essentially all residential use, has the lowest rank.
Ability to implement ranks assume that control would be easiest to apply in
watershed B, because a major part of the area is industrial. Watershed C and the
total urban watershed are given lower ranks than A because there is a greater area
to control. In practice, physical factors such as steep slopes, high groundwater,
rock, etc. would also influence the rank value assigned.
For the illustrative analysis, equal weights are assigned to the four factors. Since
beneficial use has three sub-categories, weights are assigned so that they total to 25 to be
consistent with the others.
On the basis of the site data and the series of value judgements discussed above, the
prioritization results shown by Table 4-4 suggests that pollutant controls be applied first to
watershed C, then to A, and finally to B. Although the "total urban area" has the second
highest score, it implies control of the entire urban area.
With the use of this targeting methodology, sensitivity analyses are easy to perform
and are recommended. In addition to repeating the analysis with other pollutants as
mentioned earlier, the ranks (or weights) can be modified to determine the sensitivity of the
targeting decision. For example, if all uses were considered equally important, and in about
the same condition (good or poor) then zero weights could be assigned to everything but
pollutant loads. In this case, the higher loads from watershed B would put it at the top of the
targeting list.
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4.5 DISCUSSION
There is no "magic formula" that can be applied that will "decide" for a user, the most
appropriate priority to assign for a general implementation program. The degree of
subjectivity in assigning both rank values and weights will be apparent as the targeting
procedure is applied. The principal value of a formal procedure for an analysis with this
unavoidable degree of subjectivity, is that it provides a way to document the decision
process. Equally useful is the fact that it provides a framework to organize the thought
process and provides a structure to the intermediate decision elements that determine the
relative importance of different factors in the local situation.
Some urban jurisdictions will have relatively unique features that cannot be
realistically reflected in a generalized procedure. In such a case, additional factors may be
added to the assessment.
Similarly, the procedure could be made more complex in terms of the estimation of
loads and relating them to the nature and severity or the water quality impact they cause. This
consideration was rejected in the interest of keeping the procedure simple, and because value
judgements on elements that are not subject to formal computation are also important in the
targeting process.
Some judgement is required in the interpretation of the computed pollutant loads.
They should not be used to infer either the presence or absence of an actual problem, which
will also depend on a number of other factors. The loads computed are based on an arbitrary
one inch of rainfall in an attempt to emphasize that they reflect only the comparative pollution
potential of the different areas.
The rank values developed for this factor, which are based on pollutant load
contributions from the different watersheds, will be quite different depending on the pollutant
selected for analysis. This can be inferred by inspecting the relative differences in runoff
concentrations listed in Table 4-2, for the different pollutants. For example it can be seen that
for Oil&Grease industrial and commercial runoff contributions are dramatically different than
residential inputs. They can be expected to make a greater contribution to aesthetic-type
problems. Residential lawns and vegetated undeveloped areas, on the other hand, will tend
to be more significant sources of nutrients.
The accuracy of the concentration relationships presented in Table 4-2 will obviously
influence the rank values developed for this factor. The ratios shown are reasonable, based
on interpretation of available data, but by no means absolute. This uncertainty is
compensated for by the fact that estimated loading comparisons are only one of a suite of
different factors that are used in establishing an overall targeting priority scheme. In cases
where the pollutant loading factor will have a dominant influence on the priorities, it will be
well to refine the Table 4-2 estimates with local data.
Pollutant loadings have virtually no effect on certain classes of problems commonly
associated with urbanization. Examples include habitat modification, loss of natural wetland
areas, and channelization of streambeds produced by physical site alterations. In such cases,
the pollutant loading factor would be assigned an appropriately low weight. Adding an
appropriate ranking factor that would reflect these issues (either an additional one, or as a
substitute for the pollutant load factor) should be considered.
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With regard to the weighting of the different ranking factors, note that there is no
absolute necessity to assign weights so that they add up to a total of 100 as suggested. Any
value would work equally well since we are comparing relative scores. Apart from
simplifying the arithmetic, the use of a value like 100 (or 1000) for a consistent total weight
will help a user keep in mind that is a relative weight (or relative degree of importance) that is
being assigned to a ranking factor. If the importance of a particular factor is increased by
giving it a greater weight, the emphasis is relative to and at the expense of the other factors,
one or more of which should be reduced accordingly. Thus, while the total weight matters
not at all for any single analysis, it is important to provide a consistent basis when the
analysis considers testing the relative importance of the different factors.
The targeting procedure presented in this manual does not address the follow-on steps
of determining what to do and where to start in the prioritized watershed. Such
determinations will be highly site-specific. Some background has been provided in Chapter
3, which will be helpful in getting started. Nor does it address other closely related issues
which should nevertheless receive adequate attention. A formal procedure is desireable for
tracking implementation progress and related water quality benefits that accrue. Progress in
the implementation of NFS control actions, and changes in the community through growth
and development will result in a changing situation, which in time may be sufficient to
warrant modifications to the prioritization scheme developed by the targeting analysis.
Periodic reassessment of management plans and updating of targeting results will be
important elements of an effective program.
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REFERENCES
Dong, A., G. Chesters, and G. V. Simsiman. "Dispersibility of Soils and Elemental
Composition of Soils, Sediments, and Dust and Dirt from the Menomonee River
Watershed," EPA-905/A-79-209F, U.S. Environmental Protection Agency; Chicago,
Illinois (1979).
Driscoll, E.D., G.E.Palhegyi, E.W.Strecker,,and P.E.Shelley. "Analysis of Storm Event
Characteristics for Selected Rainfall Gages Throughout the United States", Office of Water,
U.S. Environmental Protection Agency (1989).
Novotny, V. and G. Chesters. Handbook of Nonpoint Source Pollution: Sources and
Management, Van Nost and Reinhold Company; New York, New York (1981).
Perkins, M. A. "An Evaluation of In-Stream Ecological Effects Associated With Urban
Runoff to a Lowland Stream in Western Washington," Corvallis Environmental Research
Laboratory, U.S. Environmental Protection Agency; Corvallis, Oregon (July 1982).
Pitt, R. and P. Bissonette. "Bellevue Urban Runoff ProgramSummary Report," U.S.
Environmental Protection Agency and the Bellevue Surface Water Utility; Bellevue,
Washington (1984).
Pitt, R. and M. Bozeman. "Sources of Urban Runoff Pollution and Its Effects on an
Urban Creek," EPA-600/2-82-090, U.S. Environmental Protection Agency; Cincinnati,
Ohio (December 1982).
Rolfe, G. L. and K. A. Reinhold. "Environmental Contamination by Lead and Other
Heavy MetalsVolume I: Introduction and Summary," Institute for Environmental
Studies, University of Illinois; Champaign-Urbana, Illinois (July 1977).
Scott, J. B., C. R. Steward, and O. J. Stober. "Impacts of Urban Runoff on Fish
Populations in Kelsey Creek, Washington," Corvallis Environmental Research Laboratory,
U.S. Environmental Protection Agency; Corvallis, Oregon (May 1982).
Shelley, Phillip. Technical memorandum to S.A.I.C. dated 11/03/88.
Southeastern Wisconsin Planning Commission. "A Comprehensive Plan for the
Menomonee River Watershed," Planning Report No. 26; Waukesha, Wisconsin (1976).
U.S. Environmental Protection Agency. "NURP Priority Pollution Monitoring Program
Volume 1: Findings," Monitoring and Data Support Division, Office of Water, U.S.
Environmental Protection Agency; Washington, D.C. (1982).
Water Quality Engineers. "Village Creek; An Urban Runoff Sampling and Assessment
Report," Birmingham Regional Planning Commission; Birmingham, Alabama (January
1981).
Wilber, W. G. and J. V. Hunter. "The Influence of Urbanization on the Transport of
Heavy Metals in New Jersey Streams," Water Resources Institute, Rutgers University;
New Brunswick, New Jersey (February 1980).
Wolman, G. W. and A. P. Schick. "Effects of Construction on Fluvial Sediment, Urban
and Suburban Areas of Maryland," Water Resources Research, Vol. 3, No. 2, pp 451-464
(1967).
Woodward-Clyde Consultants. "A Probabilistic Methodology fore Analyzing Water
Quality Effect on Urban Runoff on Rivers and Streams." Office of Water, U.S.
Environmental Protection Agency (1989).
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REFERENCES (Concluded)
Yorke, T. H. and W. J. Herb. "Urban Area Sediment Yield; Effects of Construction Site
Conditions and Sediment Control Methods," Proceedings of the Third Federal Interagency
Sedimentation Conference, March 22-25, 1976; Denver, Colorado (1976).
Yorke, T. H. and W. J. Herb. "Effects of Urbanization and Streamflow and Sediment
Transportation in the Rock Creek and Anacostia River Basins, Montgomery County,
Maryland 1962-1974," U. S. Geological Survey Professional Paper #1003, U. S.
Geological Survey; Reston, Virginia (1978).
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