1. Stormwater Calculations and Methodology
      1. _
        1. _
          1. _
    2. 8.2Existing Methodologies for Runoff Volume Calcu
    3. 8.2.1Runoff Curve Number Method…………………………………………..…..1
    4. 8.2.2Small Storm Hydrology Method…………………………………………….2
    5. 8.2.3Infiltration Models for Runoff Calculations………
    6. The Rational Method…………………………………………………………..3
    7. 8.3.2 SCS \(NRCS\) Unit Hydrograph Method………..……
    8. 8.4Computer Models……………………………………………………………………....4 8.4.1HEC Hydro
    9. 8.4.2 SCS/NRCS Models \(WIN TR-20 and WIN TR-
    10. 8.4.4 Storm Water Management Model \(SWMM\)…………‗

 
Pennsylvania Stormwater
Best Management Practices
Manual
Chapter 8
Stormwater Calculations and Methodology
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Chapter 8 Stormwater Calculations and Methodology
8.1
Introduction to Stormwater Methodologies……………………………………….1
8.2
Existing Methodologies for Runoff Volume Calculations and their
Limitations………………………………………………………………………………1
8.2.1 Runoff Curve Number Method…………………………………………..…..1
8.2.2 Small Storm Hydrology Method…………………………………………….2
8.2.3 Infiltration Models for Runoff Calculations……………………………….3
8.3
Existing Methodologies for Peak Rate/Hydrograph Estimations and their
Limitations………………………………………………………………………………3
8.3.1 The Rational Method…………………………………………………………..3
8.3.2 SCS (NRCS) Unit Hydrograph Method………..…………………………...4
8.4
Computer Models……………………………………………………………………....4
8.4.1 HEC Hydrologic Modeling System (HEC-HMS)…………………………..4
8.4.2 SCS/NRCS Models (WIN TR-20 and WIN TR-55)…………………..……..5
8.4.3 NRCS NEH 650 Engineering Field Handbook, Chapter 2 (EFH2) ….…5
8.4.4 Storm Water Management Model (SWMM)…………………………….…..5
8.5
Precipitation Data for Stormwater Calculations………………………………….6
8.6
Stormwater Quality Management…………………………………………………...7
8.6.1 Analysis of Water Quality Impacts from Developed Land……………..8
8.6.2 Analysis of Water Quality Benefits from BMPs…………………………10
8.6.3 Water Quality Analysis………………………………………………………12
8.7
Guidance for Stormwater Calculations for Volume Control Guideline 1 and
Volume Control Guideline 2………………………………………….……………..13
8.7.1 Stormwater Calculation Process………………………………………….14
8.7.1.1
For Volume Control Guideline 1 (Flowchart B)…….……14
8.7.1.2
For Volume Control Guideline 2 (Flowchart C)………….15
8.7.2 Water Quality Calculations (Flowchart D)………………………………..16
8.8
Non-structural BMP Credits………………………………………………………..17
8.9
References and Additional Sources………………………………………………45
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8.1
Introduction to Stormwater Methodologies
There have been many methodologies developed to estimate the total runoff volume, the peak
rate of runoff, and the runoff hydrograph from land surfaces under a variety of conditions. This
chapter describes some of the methods that are most widely used in Pennsylvania and
throughout the country. It is certainly not a complete list of procedures nor is it intended to
discourage the use of new and better methods as they become available.
There is a wide variety of both public and private domain computer models available for
performing stormwater calculations. The computer models use one or more calculation
methodologies to estimate runoff characteristics. The procedures most commonly used in
computer models are the same ones discussed below.
To facilitate a consistent and organized presentation of information throughout the state, assist
design engineers in meeting the recommended control guidelines, and help reviewers analyze
project data; a series of Worksheets is provided in this Chapter for design professionals to
complete and submit with their development applications.
8.2
Existing Methodologies for Runoff Volume Calculations and their
Limitations
8.2.1 Runoff Curve Number Method
The runoff curve number method, developed by the Soil Conservation Service (now the Natural
Resources Conservation Service), is perhaps the most commonly used tool for estimating runoff
volumes. In this method, runoff is calculated based on precipitation, curve number, watershed
storage, and initial abstraction. When rainfall is greater than the initial abstraction, runoff is
given by (NRCS, 1986):
Q
PI
S
a
a
=
(
−+
)
(
PI
)
2
I
=
02
.
S
where:Q
=
runoff (in.)
P
=
rainfall (in.)
I
a
=
initial abstraction (in.)
S
=
potential maximum retention after runoff begins (in.)
Initial abstraction (
I
a
) includes all losses before the start of surface runoff: depression storage,
interception, evaporation, and infiltration. I
a
can be highly variable but NRCS has found that it
can be empirically approximated by:
a
Therefore, the runoff equation becomes:
Q
PS
PS
=
+
(
. )
(
. )
02
08
2
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Finally, S is a function of the watershed soil and cover conditions as represented by the runoff
curve number (CN):
S
CN
=−
1000
10
Therefore, runoff can be calculated using only the curve number and rainfall. Curve numbers
are determined by land cover type, hydrologic condition, antecedent moisture condition (AMC),
and hydrologic soil group (HSG). Curve numbers for various land covers based on an average
AMC for annual floods and I
a
= 0.2S can be found in Urban Hydrology for Small Watersheds
(Soil Conservation Service, 1986) and various other references.
Often a single, area-weighted curve number is used to represent a watershed consisting of sub-
areas with different curve numbers. While this approach is acceptable if the curve numbers are
similar, if the difference in curve numbers is more than 5 the use of a weighted curve number
significantly reduces the estimated amount of runoff from the watershed. This is especially
problematic with pervious/impervious combinations: “combination of impervious areas with
pervious areas can imply a significant initial loss that may not take place.” (Soil Conservation
Service, 1986) Therefore, the runoff from different sub-areas should be calculated separately
and then combined or weighted appropriately. At a minimum, runoff from pervious and directly
connected impervious areas should be estimated separately for storms less than approximately
4 inches. (NJDEP, 2004)
The curve number method is less accurate for storms that generate less than 0.5 inches of
runoff and the Soil Conservation Service (1986) recommends using another procedure as a
check for these situations. For example, the storm depth that results in 0.5 inches of runoff
varies according to the CN; for impervious areas (CN of 98) it is a 0.7-inch storm, for “Open
space” in good condition on C soils (CN of 74) it is 2.3 inches, for Woods in good condition on B
soils (CN of 55) it is over 3.9 inches. An alternate method for calculating runoff from small
storms is described below.
8.2.2 Small Storm Hydrology Method (SSHM)
The Small Storm Hydrology Method was developed to estimate the runoff volume from urban
and suburban land uses for relatively small storm events. Other common procedures, such as
the runoff curve number method, are less accurate for small storms as described previously.
The CN methodology can significantly underestimate the runoff generated from smaller storm
events. (Claytor and Schueler, 1996 and Pitt, 2003) The SSHM is a straightforward procedure
in which runoff is calculated using volumetric runoff coefficients. The runoff coefficients, Rv, are
based on extensive field research from the Midwest, the Southeastern U.S., and Ontario,
Canada over a wide range of land uses and storm events. The coefficients have also been
tested and verified for numerous other U.S. locations. Runoff coefficients for individual land
uses generally vary with the rainfall amount – larger storms have higher coefficients.
Table 8.1
below lists SSHM runoff coefficients for seven land use scenarios for the 0.5 and 1.5 inch
storms.
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Table 8.1. Runoff Coefficients for the Small Storm Hydrology Method (adapted from Pitt, 2003)
Volumetric Runoff Coefficients, R
v
Impervious Areas
Pervious Areas
Flat Roofs/
Large Unpaved
Parking Areas
Pitched
Roofs
Large
Imperv.
Areas
Small
Imperv.
Areas and
Uncurbed
Roads
Sandy
Soils
(HSG A)
Silty Soils
(HSG B)
Clayey
Soils
(HSG C
& D)
0.5
0.75
0.94
0.97
0.62
0.02
0.09
0.17
1.5
0.88
0.99
0.99
0.77
0.05
0.15
0.24
Rainfall
(in.)
Runoff is simply calculated by multiplying the rainfall amount by the appropriate runoff
coefficient. Because the runoff relationship is linear for a given storm (unlike the curve number
method), a single weighted runoff coefficient can be used for an area consisting of multiple land
uses. Therefore, runoff is given by:
Q = P x R
v
where: Q
=
runoff (in.)
P
=
rainfall (in.)
R
v
=
area-weighted runoff coefficient
8.2.3 Infiltration Models for Runoff Calculations
Several computer packages offer the choice of using soil infiltration models as the basis of
runoff volume and rate calculations. Horton developed perhaps the best-known infiltration
equation – an empirical model that predicts an exponential decay in the infiltration capacity of
soil towards an equilibrium value as a storm progresses over time. (Horton, 1940) Green-Ampt
(1911) derived another equation describing infiltration based on physical soil parameters. As
the original model applied only to infiltration after surface saturation, Mein and Larson (1973)
expanded it to predict the infiltration that occurs up until saturation. (James et al., 2003) These
infiltration models estimate the amount of precipitation excess occurring over time – excess
must be transformed to runoff with other procedures to predict runoff volumes and hydrographs.
8.3
Existing Methodologies for Peak Rate/Hydrograph Estimations and their
Limitations
8.3.1 The Rational Method
The Rational Method has been used for over 100 years to estimate peak runoff rates from
relatively small, highly developed drainage areas (generally less than 200 acre drainage area).
The peak runoff rate from a given drainage area is given by:
Q
y
= C x I x A
where: Q
y
=
peak runoff rate (cubic feet per second)
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C
=
the runoff coefficient of the area (assumed to dimensionless)
I
=
the average rainfall intensity (in./hr) for a storm with a duration equal to
the time of concentration of the area
A
=
the size of the drainage area (acres)
The runoff coefficient is usually assumed to be dimensionless because one acre-inch per hour
is very close to one cubic foot per second (1 ac-in./hr = 1.008 cfs). Although it is a simple and
straightforward method, estimating both the time of concentration and the runoff coefficient
introduce considerable uncertainty in the calculated peak runoff rate. In addition, the method
was developed for relatively frequent events so the peak rate as calculated above should be
increased for more extreme events. (Viessman and Lewis, 2003) Because of these and other
serious deficiencies, the Rational Method should be used only to predict the peak runoff rate for
very small, highly impervious areas. (Linsley et. al, 1992)
The Rational Method, discussed in detail above, has been adapted to include estimations of
runoff hydrographs and volumes through the Modified Rational Method. Due to the limitations
of the Rational Method itself (see above) as well as assumptions in the Modified Rational
Method about the total storm duration, this method should not be used to calculate water
quality, infiltration, or capture volumes.
8.3.2 SCS (NRCS) Unit Hydrograph Method
In combination with the curve number method for calculating runoff depth, the National
Resource Soil Conservation Service (NRCS) also developed a system to estimate peak runoff
rates and runoff hydrographs using a dimensionless unit hydrograph derived from many natural
unit hydrographs from diverse watersheds throughout the country (NRCS Chapter 16, 1972).
As discussed below, the NRCS methodologies are available in several public domain computer
models including TR-55 (WinTR-55) computer model (2003), Technical Release 20 (TR-20);
Computer Program for Project Formulation Hydrology (1992), and in addition, the U.S. Army
Corp of Engineers’ Hydrologic Modeling System (HEC-HMS, 2003), EFH2 and the U.S. EPA’s
Storm Water Management Model (SWMM 5.0.003, 2004).
8.4
Computer Models
8.4.1 HEC Hydrologic Modeling System (HEC-HMS)
The U.S. Army Corp of Engineers’ Hydrologic Modeling System (HEC-HMS, 2003) supersedes
HEC-1 as “next-generation” rainfall-runoff simulation software. According to the Corp, HEC-
HMS “is a significant advancement over HEC-1 in terms of both computer science and
hydrologic engineering.” (U.S. ACE, 2001) HEC-HMS was designed for use in a “wide range of
geographic areas for solving the widest possible range of problems.” The model incorporates
several options for simulating precipitation excess (runoff curve number, Green & Ampt, etc.),
transforming precipitation excess to runoff (NRCS unit hydrograph, kinematic wave, etc.), and
routing runoff (continuity, lag, Muskingum-Cunge, modified Puls, kinematic wave). HEC-HMS
Version 2.2.2 (May 28, 2003) can be downloaded at no cost from:
http://www.hec.usace.army.mil/software/hec-hms/hechms-hechms.html.
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8.4.2 SCS/NRCS Models (WIN TR-20 and WIN TR-55)
“Technical Release No. 20: Computer Program for Project Formulation Hydrology (TR-20) is a
physically based watershed scale runoff event model” that “computes direct runoff and develops
hydrographs resulting from any synthetic or natural rainstorm.” (NRCS, 2004) Hydrographs
can then be routed through stream/channel reaches and reservoirs. TR-20 applies the
methodologies found in the Hydrology section of the National Engineering Handbook (NRCS,
1969-2001), specifically the runoff curve number method and the dimensionless unit
hydrograph. (NRCS, 1992) Version 2.04 was released in 1992 and can be downloaded at:
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models.html
. A Beta test version for Windows,
WinTR-20, was also released in 2004.
Technical Release 55 (TR-55) was originally published in 1975 as a simple procedure to
estimate runoff volume, peak rate, hydrographs, and storage volumes required for peak rate
control. (NRCS, 2002) TR-55 was released as a computer program in 1986 and work began
on a modernized Windows version in 1998. WinTR-55 generates hydrographs from urban and
agricultural areas and routes them downstream through channels and/or reservoirs. WinTR-55
uses the TR-20 model for all of its hydrograph procedures. (NRCS, 2002) WinTR-55 Version 1
was officially released in 2002 and can be downloaded at:
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models.html
.
8.4.3 NRCS NEH 650 Engineering Field Handbook, Chapter 2 (EFH2)
Peak discharge is determined by procedures contained in NRCS NEH 650 Engineering Field
Handbook, Chapter 2. Information needed to use this procedure include watershed
characteristics (drainage area, curve number, watershed length, watershed slope) and rainfall
amount and distribution.
The method applies when the:
-watershed is accurately represented by a single curve number between 40 and 98
-watershed area is between 1 and 2000 acres
-watershed hydraulic length is between 200 and 26000 feet
-average watershed slope is between 0.5 and 64 percent
-watershed requires no valley or reservoir routing
-urban land use within the watershed does not exceed 10%.
EFH2 Version 1.1.0 was released in March 2003 and can be downloaded at:
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models.html
Refer to NRCS Engineering Field Handbook, Chapter 2 for a complete discussion of the
methodology and its limitations.
8.4.4 Storm Water Management Model (SWMM)
The U.S. Environmental Protection Agency (2004) describes its model as:
“a dynamic rainfall-runoff simulation model used for single event or long-term (continuous)
simulation of runoff quantity and quality from primarily urban areas. The runoff component of
SWMM operates on a collection of subcatchment areas that receive precipitation and generate
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runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system
of pipes, channels, storage/treatment devices, pumps, and regulators.
SWMM was first developed in 1971 and has since undergone several major upgrades. It
continues to be widely used throughout the world for planning, analysis and design related to
storm water runoff, combined sewers, sanitary sewers, and other drainage systems in urban
areas, with many applications in non-urban areas as well. The current edition, Version 5, is a
complete re-write of the previous release. Running under Windows, SWMM 5 provides an
integrated environment for editing study area input data, running hydrologic, hydraulic and water
quality simulations, and viewing the results in a variety of formats.
SWMM is a powerful model capable of simulating areas consisting of a single, uniform
subcatchment to the drainage system of an entire city. Although typically not used to evaluate a
single development site, the recently released Version 5 is more user-friendly and should
promote an increase in use among design professionals.
Rainfall excess is calculated in SWMM by subtracting infiltration (based on Horton or Green &
Ampt) and/or evaporation from precipitation. Rainfall excess is converted to runoff by coupling
Manning’s equation with the continuity equation. (Rossman, 2004 and James et al., 2003) The
newest version of SWMM also incorporates the runoff curve number method for estimating
infiltration. (Rossman, 2004)
8.5
Precipitation Data for Stormwater Calculations
In 2004 the National Weather Service’s Hydrometeorological Design Studies Center published
updated precipitation estimates for much of the United States, including Pennsylvania. NOAA
Atlas 14 supercedes previous precipitation estimates such as Technical Memorandum NWS
Hydro 35 and Technical Papers 40 and 49 (TP-40 and TP-49) because the updates are based
on more recent and expanded data, current statistical techniques, and enhanced spatial
interpolation and mapping procedures. (Bonnin et al., 2003 and NWS, 2004) The
“Precipitation-Frequency Atlas of the United States,” NOAA Atlas 14, provides estimates of 2-
year through 1000-year storm events for durations ranging from 5 minutes to 60 days as shown
for Harrisburg in Table 8-2 (available online at http://hdsc.nws.noaa.gov/hdsc/pfds/). Users can
select precipitation estimates for Pennsylvania from over 300 observation sites, by entering
latitude/longitude coordinates, or by clicking on an interactive map on the Precipitation
Frequency Data Server. These new rainfall estimates are recommended for all applicable
stormwater calculations.
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Table 8.2
Harrisburg precipitation estimates.
Precipitation Frequency Estimates (inches)
ARI*
5
10
15
30
60
120
3
6
12
24
48
4
7
10
20
30
45
60
(years) min min min Min min min
hr
hr
hr
hr
hr
day
day
day
day
day
day
day
2
0.4
0.6
0.8
1.1
1.3
1.5
1.7
2.1
2.5
2.9
3.4 3.78 4.42 5.07 6.83 8.42 10.6
12.6
5
0.5
0.7
0.9
1.3
1.7
1.9
2.1
2.6 3.18 3.68
4.3 4.77 5.51 6.26 8.18
9.9 12.2
14.4
10
0.5
0.8
1
1.5
1.9
2.3
2.5
3.1 3.76 4.37
5 5.63 6.46 7.26 9.28 11.1 13.5
15.8
25
0.6
0.9
1.1
1.7
2.2
2.7
3
3.7 4.64 5.44
6.2 6.93 7.89 8.75 10.9 12.8 15.3
17.8
50
0.6
1
1.2
1.8
2.5
3.1
3.4
4.3 5.42 6.41
7.3 8.09 9.16
10 12.2 14.2 16.7
19.2
100
0.7
1
1.3
2
2.7
3.6
3.9
4.9 6.29 7.53
8.5 9.41 10.6 11.4 13.6 15.7 18.2
20.7
200
0.7
1.1
1.4
2.1
3
4
4.4
5.6 7.26 8.81
9.9 10.9 12.2
13 15.1 17.3 19.6
22.2
500
0.7
1.2
1.5
2.3
3.3
4.6
5.1
6.7 8.75 10.8
12 13.2 14.7 15.3 17.2 19.5 21.6
24.3
1000
8
1.2
1.5
2.5
3.6
5.2
5.7
7.5 10.1 12.7
14 15.3 16.8 17.4
19 21.3 23.2
25.8
00.12.0.24.0.1.1.1.1.14.5.1.3.2.5.2.2.2.6.3.2.3.7.4.1.5.9.6.1.8.11.10.-12.13.0.3010.15.0.1.0.1.0.1.0.2.7.2.1.3.20.3.1.4.18.4.2.5.10.6.2.8.15.9.2.12.12.14.3.0.13.0.4.1.13.1.5.1.11.2.6.2.15.3.6.3.10.4.7.21.9.19.8.1.8.14.10.13.Pus12.2.25.8.15.5.23.3.17.1.21.0.0.3.7.17.6.40.4.15.3.5.19.10.1.8.3.5.2.0.2.15.3.12.1.10.3.4.1.1.4.19.10.5.1.12.5.7.0.17.6.pf16.2.7.12.19.4.8.6.9.1.22.17.9.16.76.enns9642237876613621090574132586548803809301267987862733905546682885465928013289802529171482503406902219177639591998256229509523768896463689249089738372681316498317799466776042773212981221273258329413363660765889209442295734647135914277751458723026453161424275353683574707172541556421762678241635306190267677637293262479499063676026168825628721483523422422210383374962526907331519119873583571683256785643883458424321626923926800227356290457705923189652724156654663999117403857ylvania
8.6
Stormwater Quality Management
The purpose of this section is to ensure compliance with the water quality requirements for
stormwater runoff from developed sites. Unlike the approach for volume and rate control, which
considers the net change in hydrology resulting from land development, water quality evaluation
begins by assuming that the built site will generate pollutants from the new or disturbed
surfaces, and that the various BMPs can prevent or remove these pollutants from the resultant
runoff. As discussed in Chapter 2, reduction of Non-point Source (NPS) pollutants by
stormwater management is the primary issue of concern. If Control Guideline 1 or Control
Guideline 2 are met for volume reduction, then it follows that the first flush of NPS pollutants
have passed through one or more BMPs and the resultant runoff meets the water quality
criteria, except for solutes. There is no consideration of any transport of pollutants that might be
generated from the site before development, and the undisturbed portions of the site are to be
ignored as sources of NPS pollution.
The use of infiltration measures to meet water quality criteria as well as volume reduction has
one potential constraint; solutes, specifically nitrate, cannot be assumed to be sufficiently
reduced by infiltration alone. To further complicate the nitrate issue, it has been observed that
the concentration of nitrate in runoff remains fairly constant over the entire hydrograph, with
some reduction by dilution during the peak flow period. As a solute, this means that the nitrate
is dissolved in runoff throughout the rainfall process, and continues to move throughout the
entire storm. In effect, the “first flush” approach used for particulate-associated pollutants does
not apply, nor does the removal efficiency of the various BMP measures.
The non-structural measures discussed in Chapter 4 offer very efficient preventive answers to
this issue, such as reduced fertilization, vegetative restoration and street sweeping. For the
land development projects that apply these various non-structural measures, the overall
pollutant load generated should be minimized for both particulates and solutes. If a project has
preserved and restored the woodland vegetation on portions of the tract as an integral part of
the development program, prevented compaction or restored permeability in disturbed soils, and
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kept to an absolute minimum the chemical maintenance required for new landscaping elements,
the pollutant load generated should be minimal, from a water quality perspective, and should not
warrant regulatory control. The determination of how successful a given site design is in
meeting water quality compliance with non-structural measures will be guided by the loading
data analysis described in this Chapter. The initial load estimate of NPS pollution generated by
the proposed building program will provide insight into the relative impact of different built
surfaces on ambient water quality in a watershed.
8.6.1 Analysis of Water Quality Impacts from Developed Land
Chapter 3 proposed criteria for three representative pollutants (Suspended Solids, Total
Phosphorus and Nitrate) in terms of percent reduction of the anticipated load produced from the
areas disturbed during construction. The specific values proposed for each pollutant are
intended to reflect the potential efficiencies of the various BMPs considered, as well as the
anticipated reduction required to sustain or restore water quality in receiving waters. The impact
of NPS pollution on surface water quality is well documented, but generally in terms of the
receiving water body. A reduction in ambient water quality in many major riverine, lacustrine
and estuarine systems has usually been associated with changes in land use within the
contributing drainage, and in some cases, specific pollutants have been identified as “key”
pollutants. A study of the Lake Erie drainage basin in the mid-1960’s focused on phosphorus as
the critical nutrient leading to trophic changes in the lake, and the resultant water quality
strategy reduced this nutrient from both point sources and land runoff. The pattern of lake and
estuary eutrophication has been repeated in countless water bodies across the US and
throughout the world, and in virtually every drainage catchment, phosphorus is the limiting
nutrient.
In the Chesapeake Bay drainage basin, which is largely provided by runoff from central
Pennsylvania, both phosphorus and nitrate are considered limiting nutrients. These pollutants
contribute to diminishing water quality and a loss of both habitat and species by enrichment of
the estuary waters. A major initiative has recently been undertaken by states in the
Chesapeake Bay drainage basin to significantly reduce both nutrients from wastewater effluent
at over 350 treatment facilities, a process that will require an investment of hundreds of millions
of dollars over the next decade (Chesapeake Bay Tributary Strategy, CEC, 8/12/04). In that
program, PA must reduce nitrate by 48.2 million pounds and total phosphorus by 1.98 million
pounds annually. Sediment has also played a major part in the reduction in water quality in the
Bay. Therefore a dual effort of reducing nutrients and sediment from the land runoff must be
included in any Bay recovery program, keeping in mind that the phosphorus is transported with
the colloid fraction of sediments.
Thus all three of the selected NPS criteria are appropriate for water quality management of
stormwater, not only in the Chesapeake bay drainage basin, but throughout the state. Again,
these pollutants serve as surrogates for a wide range of other pollutants that occur in lesser or
trace concentrations but also contribute to degraded water quality. Many of these other
pollutants are also solutes, and so the focus on nitrate serves a broader function.
Table 8.3 summarizes the concentration of representative pollutants, both particulate and
solute, that have been measured in the runoff from various built surfaces in a selected group of
studies. In the preparation of this BMP Manual, a larger body of literature has been reviewed
for comparative data, and is summarized in Appendix A. While this data is derived from
numerous sources, the studies referenced were performed on very different sites, and
measurement methods varied by investigator. The use of a value that represents the “mean”
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concentration of a pollutant in runoff is very dependent on the level of detail applied in the
development of this data. For the purposes of evaluating the water quality impacts of land
development and the benefits of a given BMP in reducing this pollution, the data were expanded
to consider variations in land cover type, and are shown in Table 8.3.
It is possible that a proposed development may not conform exactly to the land cover categories
shown in this Table. Independent sampling of representative stormwater chemistry from similar
sites can be prepared by a developer or other interested party, if desired. It is recommended
that any stormwater sampling be compiled by use of automated sampling equipment at flow
measurement stations, where the record of chemical variability during runoff incidents can be
gathered, and that the Department approves the program prior to initiation. These new
sampling data should allow the integration of hydrographs and chemographs to formulate mass
transport loads and develop flow-weighted concentrations for analysis and substitution in lieu of
Table 8.3 values.
In the absence of new sampling data prepared by a developer or other applicant, the values
shown in Table 8.3 will be applied to the volume of runoff estimated from new development for
completion of Worksheets. The concept of “Event Mean Concentration” was explained in
Chapter 2, and represents the anticipated average concentration of a given pollutant that could
be scoured from a given surface during a storm event of significant magnitude to produce
surface runoff. No specific rainfall amount is applied to this term, and the body of data from
which it is derived reflects very different hydrologic conditions.
LAND COVER CLASSIFICATION
Total
Suspended
Solids, EMC
(mg/l)
Total
Phosphorus,
EMC
(mg/l)
Nitrate-Nitrite
EMC
(mg/l as
N)
Forest
39
0.15
0.17
Meadow
47
0.19
0.3
Fertilized Planting Area
55
1.34
0.73
Native Planting Area
55
0.4
0.33
Lawn, Low-Input
180
0.4
0.44
Lawn, High-Input
180
2.22
1.46
Golf Course Fairway/Green
305
1.07
1.84
Grassed Athletic Field
200
1.07
1.01
Rooftop
21
0.13
0.32
High Traffic Street / Highway
261
0.4
0.83
Medium Traffic Street
113
0.33
0.58
Low Traffic / Residential Street
86
0.36
0.47
Res. Driveway, Play Courts, etc.
60
0.46
0.47
High Traffic Parking Lot
120
0.39
0.6
Low Traffic Parking Lot
58
0.15
0.39
POLLUTANT
Pervious Surfaces
Impervious
Surfaces
TABLE 8.3. EVENT MEAN CONCENTRATIONS (EMCs)
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8.6.2 Analysis of Water Quality Benefits from BMPs
Unlike the traditional approach to wastewater, the implementation of stormwater quality criteria
is intended to change development practices and land management concepts, rather than to
establish a series of treatment or pollutant removal methodologies. As a general rule, the
removal of pollutants, both particulate and dissolved, from stormwater is a difficult and inefficient
process. Because the rate of flow from a developed site, as well as the concentration of many
pollutants, varies greatly during a storm, the use of traditional wastewater “unit operation”
technologies is inappropriate. The intermittent nature of runoff also complicates the pollutant
removal process. NPS pollution is produced in concentrated “slugs” of runoff, and not contained
in a uniform flow that can be applied to a microbial based process in a medium or structure,
such as a sewage treatment plant. Finally, the form of NPS pollutant, particulate or solute,
determines the potential for removal by any physical BMP.
The BMPs described in detail in Chapters 5 and 6 represent a variety of measures that,
generally speaking, have not been broadly applied during the past twenty-five years for water
quality mitigation on land development projects throughout the state. A number of wet extended
detention basins have been built, as a variation on the conventional detention basin, but most of
these have not been subject to detailed monitoring that would quantify water quality benefits.
Infiltration BMPs have also seen limited application in PA, but again virtually none have had
thorough scientific monitoring measures included in their design. Several dozen porous
pavement systems have been built since 1981, largely in the southeast area of the state, but
even these systems have had little water quality monitoring data developed, simply because the
site owner declined to participate in and support such a program. Other infiltration measures,
including trenches, rain gardens and cisterns, have been built on a limited number of sites, but
these have also not been designed to provide sample collection from the unsaturated zone or
groundwater beneath the BMP. Thus the scientific basis for pollutant removal efficiency is
derived from other relevant literature, especially the soil sciences and agriculture.
The most complete record of pollutant removal efficiency for BMPs is based on surface
detention basins, as modified to include standing water, vegetation, multiple pond systems and
the like. While simple detention structures can provide significant reduction of Suspended
Solids, especially the larger particulate fraction, the NPS pollutant removal process is greatly
enhanced by these modifications. For the other BMPs, the evaluation process is largely a work
in progress. A review of the available literature, included in Appendix A, suggests a range of
benefits from BMPs, including their relative efficiency of pollutant reduction, removal or
prevention, as summarized in Appendix A.
The available water quality data demonstrates that the roof areas of structures will not
contribute a significant fraction of the total pollutant load, and can generally be ignored, since
much of the pollution washed from rooftops is comprised of atmospheric deposition. For “big
box” projects this may not necessarily be true because of the relative size and proportion, and
the potential loading analysis should guide the designer in this step. The estimate of NPS
pollution produced by a built site can be simplified by ignoring rooftop runoff and undisturbed
land areas as NPS sources. The analysis effectively limits the contributing surfaces to two
major categories; impervious pavements and chemically maintained landscapes. Both of these
types of surfaces can vary in their pollutant contribution, as illustrated by Table 8.3. In many if
not most new developments, the evaluation and reduction of pollutant impacts will focus on
these two types of sources.
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All infiltration BMPs shown in Table 8.4 assume the NPS pollutant removal efficiency for both
TSS and TP is 85%, although an efficiency of close to 100% is reasonable for all infiltrated
runoff. Any runoff greater than the design storms of Volume Control Guidelines 1 and 2
probably will overflow or bypass these BMPs, and so some NPS load during major storms will
discharge to surface waters. For the situation where an infiltration BMP is in close proximity to a
potable water supply source, the potential for contamination by solutes must be considered, and
additional BMPs applied if the site conditions warrant (e.g., groundwater concentration exceeds
10 mg/l).
Compliance with Volume Control Guidelines 1 and 2 requires the site plan to optimize runoff
capture, ideally with distributed BMPs. If they consist of a single measure or multiple measures
distributed across the site, the first question is the amount of total built surface that drains to one
or more BMP. This “capture efficiency” of the stormwater management system determines not
only hydraulic capacity of any given measure, but also how much of the site is controlled in
terms of pollutant containment. It is recognized that most site designs do not allow total capture
of all runoff, no matter how flat the parcel may be. Completion of the Worksheets for either
volume control guideline will result in a design capacity for the selected BMPs, which usually
can be aggregated by type for analysis of water quality impacts. That is, multiple small
measures such as rain gardens in a residential development can be treated as a single
measure in terms of pollutant reduction.
The removal efficiency of BMPs connected either in series or in parallel may be computed using
the two equations provided below. Figures 8-1 and 8-2 below illustrate BMPs connected in
series and in parallel.
r
1
r
2
r
3
Inflow
Outflow
Fig. 8-1
. BMPs Connected in Series
Equation for removal efficiency of BMPs in series:
Removal efficiency of BMP
i
Removal efficiency of n BMPs in series.
=
=
r
i
R
R =1-{(1- r
1
) + (1- r
2
) + (1- r
3
)}
1
=
1
(1
)
=
n
i
R
r
i
The removal efficiency R of the above three BMPS in series is,
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r
1
r
2
Inflow
Outflow
r
3
Fig. 8-2
. BMPs Connected in Parallel
Equation for removal efficiency of BMPs in connected in parallel:
r
Removal efficiency of BMP .
Concentration of pollutant in flow i.
Rate of flow i passing through BMP .
Removal efficiency of n BMPs in parallel.
ii
i
1
=
=
=
=
=
i
i
i
ii
C
Q
R
CQ
(1
)
1
1
=−
=
n
n
i
C
i
Q
i
r
i
R
()
(
)
The removal efficiency R for the three BMPs shown in
Fig. 8-2
is,
11
22
33
11
1
22
2
33
3
CQ
CQ
CQ
C Q 1-r
C Q 1-r
CQ (1-r)
++
++
R
=
8.6.3 Water Quality Analysis
Confirmation that the BMP program has been successful in meeting the water quality criteria
assumes that either Volume Control Guideline 1 or 2 have been met, and that at least 90% of
the disturbed area is conveyed or mitigated by a BMP (Flow Chart D – page 40). Compliance
with the volume criteria assumes that the major portion of particulate pollutants have been
removed from runoff by one or more BMP, and so the only additional demonstration required for
compliance with water quality criteria is to confirm that one or more of the BMPs that are most
effective in solute reduction have been included in the stormwater management program.
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Worksheet 10 is a simple checklist of those measures, and is divided into two categories,
primary and secondary. Without performing a detailed loading analysis, the inclusion of a
combination of these measures should provide adequate demonstration that the site design has
considered this issue and incorporated the best feasible solution.
Worksheet 11 is intended for those sites where volume reduction cannot be met. This form
estimates the total pollutant load produced from all built surfaces, so that the designer can
appreciate the relative magnitude of the problem created by the proposed design. Where the
site design provides insufficient capture by BMPs, the designer should revisit the overall
program and apply additional measures to meet water quality criteria. That is, even if site
constraints prevent compliance with Volume Control Guideline 1 and 2, water quality criteria
should still be met.
In many site designs where NPS reduction is a concern, it is usually obvious that the greatest
pollutant impact is from two surfaces; impervious pavements and fertilized landscapes. As
designers focus attention on the uncontrolled runoff from streets and fertilized landscapes and
revisit the water quality impacts, the value of non-structural measures, including street sweeping
and the use of native plantings for landscape design, should become apparent.
Worksheets 12 and 13 indicate the uncontrolled load from built surfaces and gives credit for
load reduction and source omissions by using the full array of non-structural and structural
BMPs. It is likely that if compliance with Volume Control Guideline 1 and 2 is not feasible, no
additional structural measures can be included without major site plan redesign. That option is
not excluded, but if non-structural measures can be incorporated, then the answer is simple,
and additional structural measures may not be required. The designer can turn to land
management measures that can be incorporated in the finished building program without any
structural alterations. Clearly, it will require creative design to meet the recommended water
quality goals, but it is well within the capabilities of the BMPs described in this Manual.
8.7
Guidance for Stormwater Calculations for Volume Control Guideline 1 and
Volume Control Guideline 2
Stormwater management in Pennsylvania has historically focused on flow rate control for large
storm events. Stormwater management has traditionally required that there be no increase in
the rate of runoff from development as compared to the rate of runoff before development for
storm events ranging from the 2-year, 24-hour event to the 100-year, 24-hour event. The
Pennsylvania Stormwater Best Management Practices Manual
is recommending that
stormwater management be expanded to include:
• Rate of flow
• Volume of flow
• Groundwater recharge
• Water quality
• Stream channel protection
Volume Control Guideline 1 and Volume Control Guideline 2 provide recommended guidelines
to achieve these stormwater management elements.
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It should be noted that control of the rate of flow of stormwater runoff remains an important part
of stormwater management. This criteria is generally based on larger storm events of limited
frequency (i.e., the 1-year through the 100-year storm events).
By contrast, the additional elements of stormwater management – volume, groundwater
recharge, water quality, and stream channel protection – are based on the smaller, more
frequent storm events. Effective stormwater management includes rate control and the
additional elements of volume, groundwater recharge, water quality, and stream channel
protection.
Engineers and regulatory officials are familiar with the engineering methods and models used to
evaluate the rate of runoff for large storm events. There is general consistency in the
calculation methodologies used across the state, with the Cover Complex Method or the
Rational Method being the two most common methodologies applied to estimate rate of runoff.
To manage stormwater for volume, ground water recharge, quality, and channel protection,
additional or expanded analytical methods are needed. The following sections provide
guidance on recommended procedures and methodologies to improve stormwater
management, and include worksheets and flow charts intended to assist in this process.
8.7.1 Stormwater Calculation Process
Flow Chart A (page 31) is provided to guide the user in the first step of the stormwater
calculation process (
Stormwater Calculation Process Non-structural BMPs
).
Step 1
: Provide General Site information (Worksheet 1).
Step 2
: Identify sensitive natural resources, and if applicable, identify which areas will be
protected (Worksheet 2).
Step 3
: Incorporate Non-structural BMPs into the stormwater design. Quantify the
volume benefits of Non-structural BMPs (Worksheet 3).
Proceed to either Flow Chart B, Volume Control Guideline 1 or Flow Chart C, Volume Control
Guideline 2.
8.7.1.1 For Volume Control Guideline 1 (Flow Chart B)
Step 4
: Estimate the increased volume of runoff for the 2-Year storm event, using the
Cover Complex Curve Number method.
Combining Curve Numbers for land areas
proposed for development with Curve Numbers for areas unaffected by the
proposed development into a single weighted curve number is NOT acceptable.
Runoff volume should be calculated based on land use and soil types (Worksheet 4).
Step 5
: Design and incorporate Structural and Non-Structural BMPs that provide volume
control for the 2-Year volume increase indicated on Worksheet 4. Provide calculations
and documentation to support the volume estimate provided by BMPs. For Non-
structural BMPs, provide Non-structural BMP checklists to demonstrate that BMPs are
appropriate. Indicate the volume reduction provided by BMPs (Worksheet 5).
Note: if
the designer is unable to incorporate the 2-year volume increase after all feasible BMP
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options have been considered, the designer proceeds to Volume Control Guideline 2.
Step 6
: Determine if the site is exempt from peak rate calculations (Worksheet 6).
Step 7
: If the site is NOT exempt from peak rate calculations, provide detailed routing
analysis to demonstrate peak rate control for the 1-year through 100-year storm events.
This routing should consider the benefits of BMPs. Provide additional detention capacity
if needed.
Proceed to Flow Chart D, Water Quality Calculations
8.7.1.2 For Volume Control Guideline 2 (Flow Chart C)
This guideline integrates water quality, stream channel protection, and groundwater recharge
requirements into a simplified statement that can be implemented with relatively easy
computations. The guideline uses runoff depth rather than precipitation to compute required
capture volumes. The total capture volume of 2 inches corresponds roughly to the state-wide
average runoff produced by a 1-year 24-hour storm on an impervious surface. One-half of the
captured volume may be released slowly, one-fourth is recommended for reuse, and one-fourth
is recommended for groundwater recharge. These recommended values are based on a
generalized water budget analysis. During the development of watershed-based stormwater
management plans, the analysis can be re-computed to derive values that reflect local
watershed conditions more accurately (e.g. Act 167 plans). The generalized version of Volume
Control Guideline 2 is as follows:
Step 4
: Capture the first 2 inches of runoff from all contributing impervious surfaces.
The first 1-inch of runoff should be permanently removed and not be released to the
Surface Waters of the Commonwealth. The other 1inch of runoff should be detained.
Compute Runoff Volumes using
Worksheet 7
.
Step 5
: Design and incorporate Structural and Non-Structural BMPs that provide
permanent removal for the PRV and extended detention. The removal options for PRV
include reuse, evaporation, transpiration, and infiltration. Infiltration for the first 0.5 inch
is encouraged. Documentation to support the computations for volumes can be
provided using Worksheet 8. For Non-structural BMPs, checklists can be used to
demonstrate that selected BMPs are appropriate. Indicate the volume reduction
provided by BMPs on
Worksheet 8
.
Step 6
: Provide detailed routing analysis to demonstrate peak rate control for the 2-year
through 100-year storm events. This routing should consider the benefits of BMPs.
Proceed to Water Quality Calculations (Flow Chart D), Step 8.
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8.7.2
Water Quality Calculations (Flow Chart D)
Step 8
: Determine if the stormwater management design complies with either Volume
Control Guideline 1 or 2 . If volume compliance is achieved under either of these
guidelines, proceed to Step 9. If compliance is not achieved, proceed to Step 11.
Step 9
: Determine if at least 90% of the disturbed site area is controlled by a BMP
(maximum disturbed, uncontrolled area of 10%). To be considered “controlled” by a
BMP, the disturbed area must either drain to a structural BMP (or series of BMPs) or be
off-set by a preventive BMP, such as reduced imperviousness or landscape restoration.
If at least 90% of the disturbed area is controlled, proceed to Step 10; else proceed to
Step 12.
Step 10
: TSS and TP requirements are considered met. Demonstrate use of specific
nitrate prevention/reduction BMPs (Worksheet 10). If the required BMPs (2 primary or 4
secondary or 1 primary and 2 secondary) are proposed within the stormwater
management plan, then the water quality requirement for nitrate is achieved. If the
required BMPs are not proposed, proceed to Step 11.
Step 11
: If neither Control Guideline is met for volume control, demonstrate use of
specific BMPs for pollutant prevention (Worksheet 11).
Step 12
: Estimate pollutant load from disturbed areas of the site, excluding preventive
measures (if proposed). (Worksheet 12).
Step 13
: Calculate pollutant load reductions with the proposed structural BMPs
(
Worksheet 13
). If target load reductions are achieved for TSS, TP, and nitrate, then
the water quality requirements are met.
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8.8
Non-Structural BMP Credits
The use of Non-structural BMPs is an important part of a project’s stormwater management
system. However, the BMPs must be correctly implemented to be effective.
For the Non-Structural BMPs applied, use the appropriate checklists to demonstrate that BMPs
are applicable to project.
Worksheet 3 determines the amount of Volume credit or Peak Rate credit associated with Non-
structural BMPs.
The following BMPs are “self-crediting” in that the use of these BMPs automatically provides a
reduction in impervious area and a corresponding reduction in stormwater impacts.
Additionally, the use of these BMPs may be regulated by local ordinances. Local governments
and reviewing agencies are encouraged to promote the use of these BMPs where feasible:
BMP 5.5.1
Cluster Uses
BMP 5.5.2
Concentrate Uses through Smart Growth
BMP 5.7.1
Reduce Street Imperviousness
BMP 5.7.2
Reduce Parking Imperviousness
The following BMPs provide a quantitative runoff volume reduction:
BMP 5.4.1
Protect Sensitive/Special Value Features
BMP 5.4.2
Protect/Conserve/Enhance Riparian Areas
BMP 5.4.3
Protect/Utilize Natural Flow Pathways
BMP 5.6.1
Minimize Disturbed Area
BMP 5.6.2
Minimize Soil Compaction in Disturbed Areas
BMP 5.6.3
Re-Vegetate and Re-Forest Disturbed Areas
BMP 5.8.1
Rooftop Disconnection
BMP 5.8.2
Disconnection from Storm Sewers
References that support the quantitative BMP volume reduction are provided at the end of this
chapter.
No more than 25% of the Volume Reduction may be met through Non-Structural
BMP credits
.
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Criteria and Credits for BMP 5.4.1 Protect Sensitive/Special Value Features
To receive credit, the proposed areas:
Shall include natural areas of floodplains, mapped wetlands, mapped woodlands, and
natural slopes over 15% and 25%.
May include other areas of significant natural resources that the applicant demonstrates
are of special natural value.
Shall not be disturbed during project construction (i.e., cleared or graded) except for
temporary impacts associated with mitigation and reforestation efforts.
Utility
disturbance is discouraged and should be kept to a minimum.
Shall be protected by having the limits of disturbance clearly shown on all construction
drawings and delineated in the field.
Shall be located within an acceptable land preservation/protection agreement or other
enforceable instrument, such as a deed restriction, that ensures perpetual protection of
the proposed areas. The preservation agreement shall clearly specify how the natural
area shall be managed and boundaries will be marked with permanent survey markers.
Managed turf is not considered an acceptable form of vegetation management.
Shall be located on the development project.
CREDITS
Volume and Quality
Protected Area is not to be included in Runoff Volume calculation
Stormwater Management Area = (Total Area – Protected Area)
Peak Rate and Channel Protection
Runoff from the Protected Area may be excluded from Peak Rate calculations and
Channel Protection calculations for rate control, provided that the runoff from the
protected area is not conveyed to and/or through stormwater management control
structures. If necessary, runoff from Protected Areas should be directed around BMPs
and stormwater pipes and inlets by means of vegetated swales or low berms that direct
flow to natural drainage ways.
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Criteria and Credits for BMP 5.4.2 Protect/Conserve/Enhance Riparian Areas
To receive credit, the Riparian Buffer Protection areas:
Shall include a minimum width of 25 feet from each streambank for Zone 1. Smaller
widths do not receive credit.
Shall include a minimum width of 75 feet from each streambank for Zone 2. Smaller
widths do not receive credit.
Shall not be disturbed during project construction (i.e., cleared or graded) except for
temporary impacts associated with mitigation and afforestation efforts. Utility
disturbance is discouraged and should be kept to a minimum.
Areas disturbed for stream crossings (temporary or permanent) do not receive credit.
Shall be protected by having the limits of disturbance clearly shown on all construction
drawings and delineated in the field.
Shall be located within an acceptable land preservation/protection agreement or other
enforceable instrument, such as a deed restriction, that ensures perpetual protection of
the proposed areas. The preservation agreement shall clearly specify how the Riparian
Buffer shall be managed and boundaries will be marked with permanent survey markers.
Managed turf is not considered an acceptable form of vegetation management within
Zone 1 or Zone 2.
Zone 1 shall not be subject to point discharges for the entire length of Zone 1. Zone 2
shall not be subject to point discharges unless the use of a level spreader or similar
device is implemented.
Shall be located on the development project.
Forested Buffers are encouraged. See BMP 5.6.3 for Tree Planting Credit.
CREDITS
Volume and Quality
Protected Area in Zone 1 and/or Zone 2 is not to be included in Runoff Volume
calculation or Water Quality volume
Mitigation Area = (Total Area – Protected Area)
Peak Rate and Channel Protection
Runoff from the Protected Area may be excluded from Peak Rate calculations and
Channel Protection calculations for rate control, provided that the runoff from the
protected area is not conveyed to and/or through stormwater management control
structures. If necessary, runoff from Protected Areas should be directed around BMPs
and stormwater pipes and inlets by means of vegetated swales or low berms that direct
flow to natural drainage ways.
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Criteria and Credits for BMP 5.4.3 Protect/Utilize Natural Flow Pathways in Overall
Stormwater Planning and Design
To receive credit, the proposed natural Drainage Features:
Shall include natural swales and drainage pathways that existed prior to development
and that will receive runoff from developed areas, including intermittent drainage areas
and intermittent wetland depressions. Manmade drainage features are not included.
May use check dams, low berms, native vegetation, and limited grading to improve
natural drainage features.
Shall be designed to receive runoff such that flows after development are non-erosive.
Care must be taken to maintain the non-erosive conditions and natural systems should
not be overloaded.
Shall be protected from compaction or unintended disturbance during construction by
having the limits of disturbance clearly shown on all construction drawings and
delineated in the field.
Shall be noted on stormwater management plans as part of stormwater management
system and included in any municipal easement requirements for stormwater systems.
Such areas shall be noted on parcel deeds and protected from future encroachment or
disturbance by deed restrictions.
Shall be located on the development project.
May not include perennial streams.
Does not include Constructed Vegetated Swales and Vegetated Filter Strips
CREDITS
Volume and Quality
A Volume Reduction may be credited based upon the area of the Natural Drainage
Feature that is vegetated.
Volume Reduction (ft
3
) = Area x ¼-inch runoff
= Vegetated Area of Natural Drainage Feature (ft
2
) x ¼” / 12
Note: A greater volume credit may be requested by the applicant if calculations support
a greater numerical value to Minimizing Soil Compaction
.
Peak Rate and Channel Protection
The Peak Rate is reduced by a longer travel time of runoff through Natural Drainage
Features. The Time of Travel (Tt) after development may be considered the same as
the Tt before development for flows through Natural Drainage Features.
When calculating flow rates:
Tt
BEFORE
= Tt
AFTER
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Criteria and Credits for BMP 5.6.1 Minimize Total Disturbed Area - Grading
To receive credit, areas of Minimized Disturbance/Grading must meet the following criteria:
Area shall not be subject to grading or movement of existing soils.
Existing native vegetation in a healthy condition may not be removed.
Invasive non-native vegetation may be removed.
Pruning or other required maintenance of vegetation is permitted. Additional planting is
permitted.
Area shall be protected by having the limits of disturbance clearly shown on all
construction drawings and delineated in the field.
The area not subject to grading shall be clearly delineated on the Stormwater
Management Plan. If future grading or disturbance of this area occurs, subsequent
stormwater management must be provided to address disturbance.
Shall be located on the development project.
CREDITS
Volume and Quality
Protected Area is not to be included in Runoff Volume calculation or Water Quality
volume
Mitigation Area = (Total Area – Protected Area)
Peak Rate and Channel Protection
Runoff from the Protected Area (area not subject to grading) may be excluded from
Peak Rate calculations and Channel Protection calculations for rate control, provided
that the runoff from the protected area is not conveyed to and/or through stormwater
management control structures. If necessary, runoff from Protected Areas should be
directed around BMPs and stormwater pipes and inlets by means of vegetated swales or
low berms that direct flow to natural drainage ways.
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Criteria and Credits for BMP 5.6.2 Minimize Soil Compaction in Disturbed Areas
To receive credit, areas of Minimal Soil Compaction must meet the following criteria:
Area shall NOT be stripped of existing topsoil.
Area shall not be subject to excessive equipment movement. Vehicles movement,
storage, or equipment/material laydown shall not be permitted in areas of Minimized
Disturbance/Grading.
The area shall be protected by having the limits of disturbance and access clearly shown
on the Stormwater Management Plan, all construction drawings and delineated in the
field.
The use of soil amendments and additional topsoil is permitted. Light grading may be
done with tracked vehicles that prevent compaction.
Lawn and turf grass are acceptable uses. Planted Meadow is an encouraged use.
Area shall be located on the development project.
CREDITS
Volume and Quality
A Volume Reduction may be credited based upon the area of Minimal Soil Compaction.
For Lawn Areas:
Volume Reduction (ft
3
) = Area of Min. Soil Compaction (ft
2
) x ¼” / 12
For Meadow Areas
:
Volume Reduction (ft
3
) = Area of Min. Soil Compaction (ft
2
) x 1/3” / 12
Note: The applicant may request a greater volume credit if calculations support a greater
numerical value to Minimizing Soil Compaction.
Peak Rate and Channel Protection
The Peak Rate for flood protection and channel protection will be reduced by the
reduction in runoff volume provided above.
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Criteria and Credits for BMP 5.6.3 Re-Vegetate and Re-Forest Disturbed Areas, Using
Native Species
This BMP includes both Protection of Existing Trees and Re-forestation:
Part 1 Protect Existing Trees
To receive credit for protecting existing trees
NOT
located within Sensitive/Special Value areas,
the following criteria must be met:
Trees shall be protected by having the limits of disturbance clearly shown on all
construction drawings and delineated in the field.
Protection during construction shall entail minimizing disruption of the root system;
construction shall not encroach within a space measured 10 feet outside of the drip line
to the tree trunk.
Trees credited for stormwater management shall be clearly labeled on the construction
drawings and recorded on Record Plan for project.
Trees shall be maintained and protected for the life of the project (50 years) or until
redevelopment occurs.
No more than 25% of the runoff volume can be mitigated through the use of trees.
Pruning or other required maintenance of existing vegetation is permitted for safety
purposed only, unless near a building.
Escrow shall be provided for the replacement of any protected trees used for stormwater
credit that die within 5 years of construction. Dead trees shall be replaced within 6
months.
Shall be located on the development project.
Existing tree canopy must be within 100 feet of impervious surfaces to gain credit.
Only applies for trees outside Sensitive/Special Value areas.
Applies to existing trees of 4-inch caliper or larger. Non-native species are not
applicable.
CREDITS
Volume and Quality
A Volume Reduction may be credited based upon the existing tree canopy.
For Trees within 100 feet of impervious cover
:
Volume Reduction (ft
3
) = Existing Tree Canopy (ft
2
) x 1/2” / 12
Peak Rate and Channel Protection
The Peak Rate for flood protection and channel protection will be reduced by the
reduction in runoff volume provided above.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Part 2 Revegetate and Reforest
To receive credit for planting trees, the following criteria must be met:
Trees must be native species (see Appendix), minimum 2” caliper. Minimum tree height
is 6 feet.
Trees shall be adequately protected during construction.
Trees credited for stormwater management shall be clearly labeled on the construction
drawings and recorded on Record Plan for project.
Trees shall be maintained and protected for the life of the project (50 years) or until
redevelopment occurs.
No more than 25% of the runoff volume can be mitigated through the use of trees.
Escrow shall be provided for the replacement of any protected trees used for stormwater
credit that die within 5 years of construction. Dead trees shall be replaced within 6
months.
Shall be located on the development project.
May be applied for trees required under Street Tree or Landscaping requirements.
May be applied for trees planted as part of Riparian Buffer improvement.
Non-native species are not applicable.
CREDITS
Volume and Quality
A Volume Reduction may be credited based upon the existing tree canopy.
For Deciduous Trees:
Volume Reduction (ft
3
) = 6 ft
3
For EvergreenTrees:
Volume Reduction (ft
3
) = 10 ft
3
Peak Rate and Channel Protection
The Peak Rate for flood protection and channel protection will be reduced by the
reduction in runoff volume provided above.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Criteria and Credits for BMP 5.8.1 Rooftop Disconnection
To receive credit, Rooftop Disconnection Areas must meet the following criteria:
Roof leaders are directed to a pervious area where runoff can either infiltrate into the soil
or filter over it.
Shall be located on the development project.
The use of soil amendments and additional topsoil is permitted.
Lawn and turf grass are acceptable uses. Planted Meadow is an encouraged use.
Shall be noted on stormwater management plans as part of stormwater management
system and included in any municipal easement requirements for stormwater systems.
Rooftop cannot be within a designated hotspot.
Disconnection shall not cause basement seepage.
The contributing rooftop area to each disconnection point shall be 500 sf or less. For
greater areas, see BMP 6.20 Level Spreader.
The length of the disconnection shall be 75 feet or greater.
Dry wells, french drains, recharge gardens, infiltration trenches/beds, or other similar
storage devices may be utilized to compensate for areas with disconnection lengths less
than 75 feet. (Do not credit BMP 5.11)
In residential development applications, disconnections will only be credited for lot sizes
greater than 6000 sf.
The entire vegetated “disconnection” area shall have a maximum slope of 5%.
The disconnection must drain continuously through a vegetated swale or filter strip to the
property line or BMP.
Roof downspouts shall be at least 10 feet away from the nearest impervious surface to
discourage “re-connections”
For rooftops draining directly to a buffer, only the rooftop disconnection credit of the
buffer credit may be used, not both.
CREDITS
Volume and Quality
Volume Reduction (ft
3
) = Contributing Rooftop Area (ft
2
) x 1/4” / 12
Note: The applicant may request a greater volume credit if calculations support a greater
numerical value to Minimizing Soil Compaction.
Peak Rate and Channel Protection
The Peak Rate for flood protection and channel protection will be reduced by the
reduction in runoff volume provided above.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Criteria and Credits for BMP 5.8.2 Disconnection from Storm Sewers
To receive credit, the following must be met:
Runoff from the non-rooftop impervious cover shall be directed to pervious areas where
it is infiltrated into the soil.
May include Vegetated Swales as outlined in BMP 6.8.
May include check dams, low berms, native vegetation, and limited grading to improve
natural drainage features.
Shall be designed such that flows after development are non-erosive.
Shall be protected from compaction or unintended disturbance during construction by
having the limits of disturbance clearly shown on all construction drawings and
delineated in the field.
Shall be noted on stormwater management plans as part of stormwater management
system and included in any municipal easement requirements for stormwater systems.
Shall be located on the development project.
Runoff cannot originate from a designated hotspot.
The maximum contributing impervious flow path length shall be 75 feet.
The disconnection shall drain continuously through a vegetated swale or filter strip, or
planted area to the property line or BMP.
The length of the disconnection area must be at the least the length of the contributing
area.
The entire vegetated “disconnection” area shall have a maximum slope of 5%.
The contributing impervious area to any one discharge point shall not exceed 1000 ft
2
.
Disconnections are encouraged on relatively well-draining soils (HSG A & B).
If the site cannot meet the required disconnect length, a level-spreading device,
recharge garden, infiltration trench, or other storage device may be needed for
compensation.
CREDITS
Volume and Quality
Volume Reduction (ft
3
) = Contributing Impervious Area (ft
2
) x 1/4” / 12
Note: A greater volume credit may be requested by the applicant if calculations support
a greater numerical value to Minimizing Soil Compaction.
Peak Rate and Channel Protection
The Peak Rate for flood protection and channel protection will be reduced by the
reduction in runoff volume provided above.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Supporting Documentation
Natural Drainage Swales (BMP 5.4.3)
“Headwater streams and wetlands have a particularly important role to play in recharge. These
smallest upstream components of a river network have the largest surface area of soil in contact
with available water, thereby providing the greatest opportunity for recharge of groundwater.
Moreover, water level in headwater streams is often higher than the water table, allowing water
to flow through the channel bed and banks into soil and groundwater. Such situations occur
when water levels are high, such as during spring snowmelt or rainy seasons.” “Headwaters
can be intermittent streams that flow briefly when snow melts or after rain, but shrink in dry
times to become individual pools filled with water…wetlands are depressions in the ground that
hold water whether from rainwater, snowmelt, or groundwater welling up to the surface.”
The scientific Imperative for Defending Small streams and Wetlands Judy L. Meyer, PhD, et al,
American Rivers, September 2003
Trees (BMP 5.6.3)
“Besides taking in carbon dioxide and putting out oxygen, trees have an enormous impact on
temperature. As much as 90 percent of the solar energy is absorbed. Trees also cool by
transpiration, the evaporation of water from their leaves. A medium sized tree can move more
than 500 gallons of water into the air on a hot day, thereby reducing air temperature.”
The Natural Habitat Garden by Ken Druse with Margaret Roach, Timber Press 2004.
500 gal = 66.8 cf
Volume Credits (BMPs 5.4.3; 5.6.2; 5.8.2)
Protect natural drainage ways, avoiding compaction, and disconnecting impervious areas all
contribute to a reduction in the volume of runoff and the rate of runoff. The amount of reduction
will vary depending on the site-specific conditions, including soil type, cover, etc. The designer
may request additional volume credit by providing supporting calculations. The following table
compares the difference in runoff volume for protected versus disturbed area for three storm
events (1.5-inch, 2.7-inch, and 3.3-inch) for different soil types using the Cover Complex
Method.
For 1.5" Rainfall
A soil
B soil
C soil
D soil
Runoff Before
0
0.00
0.10
0.
Runoff After
0.00
0.07
0.26
0.41
Difference
0.00
0.07
0.16
0.1
23
8
For 2.7" Rainfall
A soil
B soil
C soil
D soil
Runoff Before
0
0
0.59
0.92
Runoff After
0.03
0.52
0.97
1.27
Difference
0.03
0.52
0.38
0.35
For 3.3" Rainfall
A soil
B soil
C soil
D soil
Runoff Before
0
0.38
0.94
1.
Runoff After
0.13
0.84
1.41
1.77
Difference
0.1
35
3
0.46
0.47
0.42
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Map Existing Conditions and Sensitive Natural
Resources
Determine applicable Non-Structural
BMPS
No
Yes
Recommended to use Flow
Chart B (Primary Control
Guideline 1 - CG 1)
Complete Worksheet 1
General Site Information
Complete Worksheet 2 to determine credits for
protecting sensitive Natural Resources
Complete Worksheet 3 for Non-Structural BMP
credit
Is the development site a Mining Area,
Urban Redevelopment Area, Brownfield
Area, or a small site with minimal
disturbance and imperviousness
Recommended to use Flow
Chart C (Primary Control
Guideline 2 - CG 2)
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Date:
Project Name:
Municipality:
County:
Total Area (acres):
Major River Basin:
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/default.htm#newtopics
Watershed:
Sub-Basin:
Nearest Surface Water(s) to Receive Runoff:
Chapter 93 - Designated Water Use:
http://www.pacode.com/secure/data/025/chapter93/chap93toc.html
Impaired according to Chapter 303(d) List?
Yes
http://www.dep.state.pa.us/dep/deputate/watermgt/wqp/wqstandards/303d-Report.htm
No
List Causes of Impairment:
Is project subject to, or part of:
Municipal Separate Storm Sewer System (MS4) Requirements?
Yes
No
Existing or planned drinking water supply?
Yes
No
If yes, distance from proposed discharge (miles):
Approved Act 167 Plan?
Yes
No
Existing River Conservation Plan?
Yes
http://www.dcnr.state.pa.us/brc/rivers/riversconservation/planningprojects/
No
Worksheet 1. General Site Information
INSTRUCTIONS: Fill out Worksheet 1 for each watershed
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagem
ent/Approved_1.html
http://www.dep.state.pa.us/dep/deputate/watermgt/wc/Subjects/StormwaterManagem
ent/GeneralPermits/default.htm
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
INSTRUCTIONS:
Steep Slopes, over 25%
Other:
Other:
TOTAL EXISTING:
Natural Drainage Ways
Steep Slopes, 15% - 25%
TOTAL AREA
(Ac.)
MAPPED?
yes/no/n/a
Floodplains
Riparian Areas
Wetlands
Woodlands
PROTECTED
AREA (Ac.)
EXISTING NATURAL
SENSITIVE RESOURCE
Worksheet 2. Sensitive Natural Resources
Waterbodies
1. Provide Sensitive Resources Map according to non-structural BMP 5.4.1 in
Chapter 5. This map should identify wetlands, woodlands, natural drainage ways,
steep slopes, and other sensitive natural areas.
2. Summarize the existing extent of each sensitive resource in the Existing
Sensitive Resources Table (below, using Acres). If none present, insert 0.
3. Summarize Total Protected Area as defined under BMPs in Chapter 5.
4. Do not count any area twice. For example, an area that is both a floodplain
and a wetland may only be considered once.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
363-0300-002 / December 30, 2006
Page 31 of 46
ft
3
* For use on Worksheet 5
TOTAL NON-STRUCTURAL VOLUME CREDIT*
1.1
Ac.
1.2
Ac.
3.1
Ac.
TOTAL
Ac.
Site Area
minus
Protected
Area
=
-=
3.1 Minimum Soil Compaction
Lawn
ft
2
x 1/4" x 1/12
=
ft
3
Meadow
ft
2
x 1/3" x 1/12
=
ft
3
3.3 Protect Existing Trees
For Trees within 100 feet of impervious area:
Tree Canopy
ft
2
x 1/2" x 1/12
=
ft
3
5.1 Disconnect Roof Leaders to Vegetated Areas
For runoff directed to areas protected under 5.8.1 and 5.8.2
Roof Area
ft
2
x 1/3" x 1/12
=
ft
3
For all other disconnected roof areas
Roof Area
ft
2
x 1/4" x 1/12
=
ft
3
5.2 Disconnect Non-Roof impervious to Vegetated Areas
For Runoff directed to areas protected under 5.8.1 and 5.8.2
Impervious Area
ft
2
x 1/3" x 1/12
=
ft
3
For all other disconnected roof areas
Impervious Area
ft
2
x 1/4" x 1/12
=
ft
3
Worksheet 3. Nonstructural BMP Credits
Area of Protected Sensitive/Special Value Features (see WS 2)
Area of Riparian Forest Buffer Protection
Area of Minimum Disturbance/Reduced Grading
PROTECTED AREA
Stormwater Management Area
This is the area that requires
stormwater management
VOLUME CREDITS

Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Se
e
"Guide
line
:
Volume
Cre
d
its
for De
te
ntion
Routing"
FLOW CHART B
Control Guideline 1 Process
Estimate Net Increase in Runoff Volume for
2-year/24 hour storm
Worksheet 4
Reduce Runoff Volume with Non-Structural
BMPs
Determine Structural BMPs
Determine Structural and Non-
Structural BMP Credits
Worksheet 5
Can 2-yr/24 hour volume
increase be managed with
structural and non-structural
Secondary Control
Guideline (CG 2) applies
Demonstrate Peak
Rate Mitigation
1-year to 100-year
Increase size and/or number of BMPs
Small Site Exemption
(Worksheet 6)
Model with Volume
Diversion
Model with Composite
BMPs
Model with Tt/Tc Adjustment
Yes
No
Other Method
Or
Or
Or
Or
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
in
acres
acres
acres
Existing Conditions:
Cover Type/Condition
Soil
CN
S
Ia
Q
Runoff
1
Runoff
Volume
2
Type
(0.2*S)
(in)
(ft
3
)
Woodland
Meadow
Impervious
TOTAL:
Developed Conditions:
Cover Type/Condition
Soil
CN
S
Ia
Q
Runoff
1
Runoff
Volume
2
Type
(0.2*S)
(in)
(ft
3
)
TOTAL:
2-Year Volume Increase (ft3):
2-Year Volume Increase = Developed Conditions Runoff Volume - Existing Conditions Runoff Volume
1. Runoff (in) = Q = (P - 0.2S)
2
/ (P+ 0.8S) where
P = 2-Year Rainfall (in)
S = (1000/ CN)-10
2. Runoff Volume (CF) = Q x Area x 1/12
Q = Runoff (in)
Area = Land use area (sq. ft)
Note: Runoff Volume must be calculated for EACH land use type/condition and HSGl.
The use of a weighted CN value for volume calculations is not acceptable.
WORKSHEET 4 . CHANGE IN RUNOFF VOLUME FOR 2-YR STORM EVENT
PROJECT:
Drainage Area:
2-Year Rainfall:
Total Site Area:
Protected Site Area:
Managed Area:
Area
Area
(sf)
(ac)
Area
Area
(sf)
(ac)
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
PROJECT:
SUB-BASIN:
-
Proposed BMP
Area
Storage
Volume
(ft
2
)
(ft
3
)
6.4.1
Porous Pavement
6.4.2
Infiltration Basin
6.4.3
Infiltration Bed
6.4.4
Infiltration Trench
6.4.5
Rain Garden/Bioretention
6.4.6
Dry Well / Seepage Pit
6.4.7
Constructed Filter
6.4.8
Vegetated Swale
6.4.9
Vegetated Filter Strip
6.4.10
Berm
6.5.1
Vegetated Roof
6.5.2
Capture and Re-use
6.6.1
Constructed Wetlands
6.6.2
Wet Pond / Retention Basin
6.6.3
Dry Extended Detention Basin
6.6.4
Water Quality Filters
6.7.1
Riparian Buffer Restoration
6.7.2
Landscape Restoration / Reforestation
6.7.3
Soil Amendment
6.8.1
Level Spreader
6.8.2
Special Storage Areas
Other
Total Structural Volume (ft
3
):
Structural Volume Requirement (ft
3
):
DIFFERENCE
(Required Control Volume minus Non-structural Credit)
WORKSHEET 5 . STRUCTURAL BMP VOLUME CREDITS
Non-structural Volume Credit (ft
3
) -
from Worksheet 3
:
Required Control Volume (ft
3
) -
from Worksheet 4
:
Structural Volume Reqmt (ft
3
)
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
The 2-Year/24 Hour Runoff Volume increase must be met in BMPs designed in accordan
with Manual Standards
Total Site Impervious Area may not exceed
1 acre.
Maximum Development Area is
1
is
5 Acres
Maximum site impervious cover is 50%.
No more than 25% Volume Control can be in Non-structural BMPs
Infiltration BMPs must have an infiltration of at least 0.5 in/hr.
Site Area
Percent
Impervious
Total
Impervious
5 acre
20%
1 acre
2 acre
50%
1 acre
1 acre
50%
0.5 acre
0.5 acre
50%
0.25 acre
The following conditions must be met for exemption from peak rate analysis for small
sites under CG-1:
WORKSHEET 6 . SMALL SITE / SMALL IMPERVIOUS AREA
EXCEPTION FOR PEAK RATE MITIGATION CALCULATIONS
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
FLOW CHART C
Control Guideline 2 Process
Complete Worksheet 7 to estimate
2 inch of Runoff Capture Volume from all
impervious surfaces
Complete Worksheet 3
BMPs for Infiltration
and BMPs for Volume Reduction
Determine Structural BMPs
Adjust Design for Extended
Detention
Demonstrate Peak Rate
Calculate Flow Target
for 24-72 Hour Extended
Detention
Worksheet 9
Demonstrate Nitrate Pollution
Addressed
Worksheet 10
Model with Volume
Diversion
Model with Composite
BMPs
Model with Tt/Tc
Adjustment
Other Method
Or
Or
Or
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
WORKSHEET 7. CALCULATION OF RUNOFF VOLUMES (PRV and EDV) FOR
CG-2 ONLY
PROJECT:
DRAINAGE AREA:
Total Site Area:
acres
Protected Site Area:
acres
Managed Area:
acres
Total Impervious Area
acres
2 Inch Runoff - Multiply Total Impervious Area by 2 inch
Cover Type
Area
Runoff
Capture
Volume
(ac)
(ft
3
)
Roof
Pavement
Other Impervious
TOTAL:
1 Inch Rainfall -
Cover Type
Area
(sf)
Area
(ac)
Runoff
(in)
Runoff Volumes
(ft
3
)
TOTAL:
1. Total Runoff Capture Volume (ft
3
) =Total Impervious Area (ft
2
) x 2 inch x 1/12
2. PRV (ft
3
) = Total Impervious Area (ft
2
) x 1 inch x 1/12
3. EDV (ft
3
) = Total Impervious Area (ft
2
) x 1 inch x 1/12
Water quality volume requirements for land areas with existing cover consisting of meadow, brush,
wood-grass combination, or woods proposed for conversion to any other non-equivalent type of
pervious cover shall be sized for one-half (1/2) the volume required for impervious surfaces as
mentioned in this worksheet and calculated in items 1 through 3 above
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
PROJECT:
SUB-BASIN:
-
Proposed BMP*
Area
Storage
Volume
(ft
2
)
(ft
3
)
6.4.1
Porous Pavement
6.4.2
Infiltration Basin
6.4.3
Infiltration Bed
6.4.4
Infiltration Trench
6.4.5
Rain Garden/Bioretention
6.4.6
Dry Well / Seepage Pit
6.4.7
Constructed Filter
6.4.8
Vegetated Swale
6.4.9
Vegetated Filter Strip
6.4.10
Berm
6.5.1
Vegetated Roof
6.5.2
Capture and Re-use
6.6.1
Constructed Wetlands
6.6.2
Wet Pond / Retention Basin
6.6.3
Dry Extended Detention Basin
6.6.4
Water Quality Filters
6.7.1
Riparian Buffer Restoration
6.7.2
Landscape Restoration / Reforestation
6.7.3
Soil Amendment
6.8.1
Level Spreader
6.8.2
Special Storage Areas
Other
Total Structural Volume (ft
3
):
Structural Volume Requirement (ft
3
):
DIFFERENCE
(Required Control Volume minus Non-structural Credit)
WORKSHEET 8 . STRUCTURAL BMP VOLUME CREDITS
Non-structural Volume Credit (ft
3
) -
from Worksheet 3
:
Required Control Volume (ft
3
) -
from Worksheet 7
:
Structural Volume Reqmt (ft
3
)
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
3) Travel Time/ Time of Concentration Adjustment.
The use of widely-distributed,
volume-reducing BMPs can significantly increase the post-development runoff travel time
and therefore decrease the peak rate of discharge. The Delaware Urban Runoff
Management Model (DURMM) calculates the extended travel time through storage
elements, even at flooded depths, to adjust peak flow rates (Lucas, 2001). The extended
travel time is essentially the residence time of the storage elements, found by dividing the
total storage by the 2-year peak flow rate. This increased travel timecan be added to the
time of concentration of the area to account for the slowing effect of the volume-reducing
BMPs. This can reduce the amount of detention storage required for peak rate control.
4) Other Methods.
Other methods, such as adjusting runoff curve numbers based on the
runoff volume left after BMP application, or reducing net precipitation based on the volume
captured, can be applied as appropriate.
2) Composite BMPs.
For optimal stormwater management, this manual suggests widely
distributed BMPs for volume, rate, and quality control. This approach, however, can be very
cumbersome to evaluate in detail with common computer models. To facilitate modeling,
similar types of BMPs can be combined within the model. For modeling purposes, the
storage of the combined BMP is simply the sum of the BMP capacities that it represents. A
stage-storage-discharge relationship can be developed for the combined BMP based on the
configuration of the individual systems. The combined BMPs can then be routed normally
and the results submitted.
1) Volume Diversion.
Many computers models have components that allow a "diversion"
or "abstraction". The total volume reduction provided by the applicable structural and non-
structural BMPs can be diverted or abstracted from the modeled runoff before it is routed to
the detention system(s). This approach is very conservative because it does not give any
credut to the increased time of travel, ongoing infiltration, etc. associated with the BMPs.
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
Flow Chart D
Water Quality Process
Is 90% of the
disturbed area
controlled by a
BMP?
Show use of specific
nitrate prevention /
reduction BMPs
(Worksheet 10); TSS
and TP requirements
met
Does design
comply with CG 1
or CG 2
requirements for
volume control?
Yes
No
No
Yes
Complete Worksheet 12
Pollutant Load Estimate
Complete Worksheet 13
Pollutant Load Reduction for
BMPs
Water Quality
Compliance
Yes
No
Show use of specific BMPs for
Pollutant Prevention
(Worksheet 11)
Water Quality
Compliance
Yes
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
PRIMARY BMPs FOR NITRATE:
YES NO
SECONDARY BMPs FOR NITRATE:
WORKSHEET 10. WATER QUALITY COMPLIANCE FOR NITRATE
Structural BMP 6.4.9 - Vegetated Filter Strip
Structural BMP 6.6.1 - Constructed Wetland
NS BMP 5.4.3 - Protect / Utilize Natural Drainage Features
NS BMP 5.6.2 - Minimize Soil Compaction
Structural BMP 6.4.5 - Rain Garden / Bioretention
Structural BMP 6.4.8 - Vegetated Swale
Structural BMP 6.7.1 - Riparian Buffer Restoration
Structural BMP 6.7.2 - Landscape Restoration
NS BMP 5.9.1 - Street Sweeping / Vacuuming
Structural BMP 6.7.3 - Soils Amendment/Restoration
Structural BMP 6.7.1 - Riparian Buffer Restoration
Structural BMP 6.7.2 - Landscape Restoration
NS BMP 5.4.1 - Protect Sensitive / Special Value Features
Does the site design incorporate the following BMPs to address nitrate pollution? A summary "yes"
rating is achieved if at least 2 Primary BMPs for nitrate are provided across the site or 4 secondary
BMPs for nitrate are provided across the site (or the
NS BMP 5.6.3 - Re-Vegetate / Re-Forest Disturbed Areas (Native Species)
NS BMP 5.4.2 - Protect / Conserve / Enhance Riparian Buffers
NS BMP 5.5.4 - Cluster Uses at Each Site
NS BMP 5.6.1 - Minimize Total Disturbed Area
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
BMPs FOR POLLUTANT PREVENTION:
YES NO
NS BMP 5.8.2 - Disconnection from Storm Sewers
NS BMP 5.9.1 - Street Sweeping
NS BMP 5.4.2 - Protect / Conserve / Enhance Riparian Buffers
NS BMP 5.5.1 - Cluster Uses at Each Site; Build on the Smallest Area Possible
NS BMP 5.7.1 - Reduce Street Imperviousness
NS BMP 5.7.2 - Reduce Parking Imperviousness
NS BMP 5.8.1 - Rooftop Disconnection
WORKSHEET 11. BMPS FOR POLLUTION PREVENTION
Structural BMP 6.7.3- Soils Amendment and Restoration
Structural BMP 6.7.1 - Riparian Buffer Restoration
Structural BMP 6.7.2- Landscape Restoration
NS BMP 5.4.1 - Protect Sensitive / Special Value Features
NS BMP 5.4.3 - Protect / Utilize Natural Flow Pathways in Overall Stormwater
Planning and Design
NS BMP 5.6.1 - Minimize Total Disturbed Area - Grading
NS BMP 5.6.3 - Re-Vegetate / Re-Forest Disturbed Areas (Native Species)
Does the site design incorporate the following BMPs to address nitrate pollution? A summary
"yes" rating is achieved if at least 2 BMPs are provided across the site. "Provided across the site"
is taken to mean that the specifications for that BMP set forward in Chapters 5 and 6 are satisfied.
NS BMP 5.6.2 - Minimize Soil Compaction in Disturbed Areas
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
TOTAL SITE AREA (AC)
TOTAL DISTURBED AREA (AC)
TOTAL DISTURBED AREAS:
LAND COVER CLASSIFICATION
TSS
EMC
(mg/l)
TP
EMC
(mg/l)
Nitrate-
Nitrite EMC
(mg/l as N)
COVER
(Acres)
RUNOFF
VOLUME
(AF)
TSS**
(LBS)
TP**
(LBS)
NO
3
(LBS)
Forest
39
0.15
0.17
Meadow
47
0.19
0.3
Fertilized Planting Area
55
1.34
0.73
Native Planting Area
55
0.40
0.33
Lawn, Low-Input
180
0.40
0.44
Lawn, High-Input
180
2.22
1.46
Golf Course Fairway/Green
305
1.07
1.84
Grassed Athletic Field
200
1.07
1.01
Rooftop
21
0.13
0.32
High Traffic Street / Highway
261
0.40
0.83
Medium Traffic Street
113
0.33
0.58
Low Traffic / Residential Street
86
0.36
0.47
Res. Driveway, Play Courts, etc.
60
0.46
0.47
High Traffic Parking Lot
120
0.39
0.60
Low Traffic Parking Lot
58
0.15
0.39
TOTAL LOAD
REQUIRED REDUCTION (%)
85%
85%
50%
REQUIRED REDUCTION (LBS)
* Pollutant Load = [EMC, mg/l] X [Volume, AF] X [2.7, Unit Conversion]
** TSS and TP calculations only required for projects not meeting CG1/CG2 or not controlling less than 90% of the disturbed area
WORKSHEET 12. WATER QUALITY ANALYSIS OF POLLUTANT LOADING FROM ALL
DISTURBED AREAS
Impervious
Surfaces
POLLUTANT LOAD
DISTURBED AREA
CONTROLLED BY BMPs (AC)
POLLUTANT
Pervious
Surfaces
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
BMP TYPE:
DISTURBED AREAS CONTROLLED BY THIS BMP TYPE:
LAND COVER CLASSIFICATION
TSS EMC
(mg/l)
TP EMC
(mg/l)
Nitrate-
Nitrite EMC
(mg/l as N)
COVER
(Acres)
RUNOFF
VOLUME
(AF)
TSS***
(LBS)
TP***
(LBS)
NO
3
(LBS)
Forest
39
0.15
0.17
Meadow
47
0.19
0.3
Fertilized Planting Area
55
1.34
0.73
Native Planting Area
55
0.40
0.33
Lawn, Low-Input
180
0.40
0.44
Lawn, High-Input
180
2.22
1.46
Golf Course Fairway/Green
305
1.07
1.84
Grassed Athletic Field
200
1.07
1.01
Rooftop
21
0.13
0.32
High Traffic Street / Highway
261
0.40
0.83
Medium Traffic Street
113
0.33
0.58
Low Traffic / Residential Street
86
0.36
0.47
Res. Driveway, Play Courts, etc.
60
0.46
0.47
High Traffic Parking Lot
120
0.39
0.60
Low Traffic Parking Lot
58
0.15
0.39
TOTAL LOAD TO THIS BMP TYPE
POLLUTANT REMOVAL EFFICIENCIES FROM TABLE 9-3 (%)
POLLUTANT REDUCTION ACHIEVED BY THIS BMP TYPE (LBS)
POLLUTANT REDUCTION ACHIEVED BY ALL BMP TYPES (LBS)
REQUIRED REDUCTION FROM WS12 (LBS)
** Pollutant Load = [EMC, mg/l] X [Volume, AF] X [2.7, Unit Conversion]
*** TSS and TP calculations only required for projects not meeting CG1/CG2 or not controlling less than 90% of the disturbed area
WORKSHEET 13. POLLUTANT REDUCTION THROUGH BMP APPLICATIONS*
* FILL THIS WORKSHEET OUT FOR EACH BMP TYPE WITH DIFFERENT POLLUTANT REMOVAL
EFFICIENCIES. SUM POLLUTANT REDUCTION ACHIEVED FOR ALL BMP TYPES ON FINAL SHEET.
Impervious
Surfaces
POLLUTANT LOAD**
DISTURBED AREA CONTROLLED
BY THIS BMP TYPE (AC)
POLLUTANT
Pervious
Surfaces
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
8.9 References and Additional Information Sources
Bonnin, G., Todd, D., Lin, B., Parzybok, T., Yekta, M., and Riley, D., 2003.
NOAA Atlas 14,
Precipitation-Frequency Atlas of the United States
, Volume 1. National Weather Service,
National Oceanic and Atmospheric Administration, Silver Spring, Md.
Claytor, R.A. and Schuler, T.R., 1996.
Design of Stormwater Filtering Systems,
Center for
Watershed Protection, Silver Spring, MD
Green, W.H. and Ampt, G.A., 1911. “Studies on Soil Physics, 1. The Flow of Air and Water
Through Soils,”
Journal of Agricultural Sciences
, Vol. 4, pp. 1124.
Horton, R.E., 1940.
An Approach toward a Physical Interpretation of Infiltration Capacity
. Soil
Science Society of America, Proceedings 4:399-417.
James, W., Huber W., Dickinson, R., Pitt, R., James, W.R., Roesner, L., and Aldrich, J., 2003.
User’s Guide to SWMM
. Computational Hydraulics International, Guelph, Canada.
Linsley, R., Franzini, J., Freyberg, D., and Tchobanoglous, G., 1992.
Water-Resources
Engineering
. 4
th
ed. Irwin McGraw-Hill, New York, NY.
Mein, R.G. and Larson, C.L., 1973. “Modeling Infiltration During a Steady Rain,”
Water
Resources Research
, Vol. 9, No. 2, pp. 334-394.
National Resources Conservation Service.
National Engineering Handbook
.
Part 630:
Hydrology, 1969-2001. Originally published as the
National Engineering Handbook
, Section
4: Hydrology. Available online at:
http://www.wcc.nrcs.usda.gov/hydro/hydro-techref-neh-630.html
.
National Resources Conservation Service, 2004.
National Water and Climate Center
“Hydraulics and Hydrology – Tools and Models” Website, U.S. Department of Agriculture.
Available online at: http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models.html
.
National Weather Service, Hydrometeorological Design Studies Center, 2004.
“Current
Precipitation Frequency Information and Publications” Website, National Oceanic and
Atmospheric
Administration.
Available
online
at:
http://www.nws.noaa.gov/ohd/hdsc/currentpf.htm
.
New Jersey Department of Environmental Protection, 2004.
New Jersey Stormwater Best
Management Practices Manual
.
Pitt, R., 2003.
The Source Loading and Management Model (WinSLAMM): Introduction and
Basic
Uses
.
Available
online
at:
http://unix.eng.ua.edu/~rpitt/SLAMMDETPOND/WinSlamm/Ch1/M1.html#_Introduction#_Intr
oduction.
Pitt, R. and Voorhees, J., 2000.
The Source Loading and Management Model (SLAMM): A
Water Quality Management Planning Model for Urban Stormwater Runoff
.” Available online
at: http://unix.eng.ua.edu/~rpitt/SLAMMDETPOND/WinSlamm/MainWINSLAMM_book.html
.
363-0300-002 / December 30, 2006
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 8
363-0300-002 / December 30, 2006
Page 46 of 46
Reese, S. and Lee, J., 1998. Summary of Groundwater Quality Monitoring Data (1985 - 1997)
from Pennsylvania’s Ambient and Fixed Station Network (FSN) Monitoring Program:
Selected Groundwater Basins in Southwestern, Southcentral and Southeastern
Pennsylvania, Bureau of Water Supply Management, PADEP.
Rossman, L., 2004.
Storm Water Management Model User’s Manual, Version 5.0.
National
Risk Management Research Laboratory, U.S. Environmental Protection Agency, Cincinnati,
OH. Available online at:
http://www.epa.gov/ednnrmrl/swmm/#A
.
Soil Conservation Service, 1986.
Urban Hydrology for Small Watersheds
. 2
nd
ed. Technical
Release
55,
U.S.
Department
of
Agriculture.
Available
online
at:
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html
.
Soil Conservation Service, 1992. TR-20 Computer Program for Project Formulation Hydrology.
U.S. Army Corp of Engineers, 2001.
Hydrologic Modeling System (HEC-HMS) User’s Manual
.
Version 2.1. Davis, CA.
U.S. Environmental Protection Agency, 2004.
Storm Water Management Model (SWMM)
Version 5.0.003
Website. Available online at:
http://www.epa.gov/ednnrmrl/swmm/#A
.
Viessman, W. and Lewis G, 2003.
Introduction to Hydrology
. 5
th
ed. Pearson Education, Inc.,
Upper Saddle River, NJ.
Woodward, D.E., Hawkins, R.H., Jiang, R., Hjelmfelt, A.T., Van Mullem, J.A., and Quan, D.Q.,
2003. “Runoff Curve Number Method: Examination of the Initial Abstraction Ratio,”
World
Water & Environmental Resources Congress, 2003: Proceeding of the Congress: June 23-
26, 2003, Philadelphia, PA
.

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