Prepared by:
Prince George's
County, Maryland
Department of
Environmental
Resources
Programs and
Planning Division
January 2000
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DEVELOPMENT HYDROLOCIC ANALYSIS
CONTENTS
LOW-IMPACT DEVELOPMENT HYDROLOGIC ANALYSIS 1
CHAPTER 1. INTRODUCTION 1
1.1 OBJECTIVES 1
1.2 KEY HYDROLOGIC PRINCIPLES 1
1.3 HYDROLOGIC ALTERATIONS TO SITE DEVELOPMENT 5
1.4 CONVENTIONAL STORMWATER MANAGEMENT 7
1.5. HYDROLOGIC COMPARISON BETWEEN CONVENTIONAL
AND LOW-IMPACT DEVELOPMENT APPROACHES.. 8
CHAPTER 2. LID HYDROLOGIC ANALYSIS COMPONENTS 11
CHAPTER 3. HYDROLOGIC EVALUATION 13
3.1 LOW-IMPACT DEVELOPMENT RUNOFF POTENTIAL 13
3.2 MAINTAINING THE PREDEVELOPMENT TIME OF
CONCENTRATION 17
3.3 MAINTAINING THE PREDEVELOPMENT RUNOFF VOLUME 18
3.4 POTENTIAL REQUIREMENT FOR ADDITIONAL
DETENTION STORAGE 20
CHAPTER 4. PROCESS AND COMPUTATIONAL PROCEDURE 22
4.1 INTRODUCTION 22
4.2 DATA COLLECTION 22
4.3 DETERMINING THE LID RUNOFF CURVE NUMBER 22
4.4 DEVELOPMENT OF THE TIME OF CONCENTRATION (TC) 25
4.5 LOW-IMPACT DEVELOPMENT STORMWATER MANAGEMENT
REQUIREMENTS 25
4.6 DETERMINATION OF DESIGN STORM EVENT 36
REFERENCES 39
APPENDICES
A. STORAGE VOLUME REQUIRED TO MAINTAIN THE PRE-DEVELOPMENT
RUNOFF VOLUME USING RETENTION STORAGE
B STORAGE VOLUME REQUIRED TO MAINTAIN THE PRE-DEVELOPMENT
PEAK RUNOFF RATE USING 100% RETENTION STORAGE
C STORAGE VOLUME REQUIRED TO MAINTAIN THE PRE-DEVELOPMENT
PEAK RUNOFF RATE USING 100% DETENTION STORAGE
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LID Hydrologic Analysis
Prince George's County, PER
FIGURES
Figure 1.1. Low-Impact Development Analysis Procedure 2
Figure 1.2. Runoff Variability with Increased Impervious Surfaces (FISRWG, 1998) 4
Figure 1.3. Groundwater in Local, Intermediate, or Regional Setting 5
Figure 1.4. Hydrologic Alterations Due to Site Development 6
Figure 1.5. Rainfall Frequency Distribution at National Airport 1980 to 1985 '.'....8
Figure 1.6. Comparison of the Hydrologic Response of Conventional and LID IMPs 9
Figure 3.1. Comparison of Land Covers Between Conventional and LID CNs 14
Figure 3.2. Effect of Low-Impact Development CN on the Postdevelopment
Hydrograph without Stormwater IMPs 16
Figure 3.3. Low-Impact Development Hydrograph That Has a Reduced CN and
Maintains the Tc Without Stormwater IMPs 18
Figure 3.4. Retention Storage Required to Maintain Peak Development Runoff Rate.... 20
Figure 3.5. Effect of Additional Detention Storage on LID Retention Practices 21
Figure 4.1. Approximate Geographic Boundaries for NRCS Rainfall Distributions 26
Figure 4.2. Procedure to Determine Percentage of Site Area Required for DVIPs to
Maintain Predevelopment Runoff Volume and Peak Runoff Rate 28
Figure 4.3. Comparison of Retention of Storage Volumes Required to Maintain Peak
Runoff Rate Using Retention and Detention 30
Figure 4.4. Storage Volume Required to Maintain Peak Runoff Rate 31
Figure 4.5. Comparison of Storage Volumes for Various Tcs.. 32
TABLES
Table 2.1. Low-Impact Development Techniques and Hydrologic Design
and Analysis Components 12
Table 3.1. Comparison of Conventional and LID Land Covers 14
Table 3.2. Low-Impact Development Planning Techniques to Reduce the
Postdevelopment Low-Impact Development CN 15
Table 3.3. Low-Impact Development Techniques to Maintain the Predevelopment
Time of Concentration 19
Table 4.1. Representative Lid Curve Numbers 23
Table 4.2. Representative Percentages of Site Required for Volume and
Peak Control 33
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Prince George's County, Maryland
LID Hydrologic Analysis
LOW-IMPACT DEVELOPMENT HYDROLOGIC ANALYSIS
CHAPTER 1. INTRODUCTION
LI OBJECTIVES
The purpose of this document is to provide low-impact development (LID) hydrologic
analysis and computational procedures used to determine low-impact development
stormwater management requirements. The hydrologic analysis presented is based on the
Soil Conservation Service (SCS) TR-55 hydrologic model (SCS.1986).
Design concepts are illustrated by the use of runoff hydrographs that represent
responses to both conventional and low-impact development. Low-impact development
site planning and integrated management practices (IMPs) are defined and categorized
into components of low-impact development objectives. Computational procedures for
determining IMP requirements are demonstrated through design examples.
The process for developing low-impact development hydrology is illustrated in Figure
1.1. This figure lists the sequential steps and the sections in the manual where the
methods to calculate or determine the specific requirements are provided.
1.2 KEY HYDROLOGIC PRINCIPLES
This section of the report provides an overview and general description of the key
hydrologic principles involved in low-impact development, and provides guidance on the
hydrologic analysis required for the design of low-impact development sites. The key
hydrologic principles that are described include: precipitation and design storm events,
rainfall abstractions, surface runoff, and groundwater recharge and flow.
Precipitation and Design Storm Events. Data for precipitation, including both snow
and rain, are used in site planning and stormwater design. Precipitation occurs as a series
of events characterized by different rainfall amount, intensity, and duration. Although
these events occur randomly, analysis of their distribution over a long period of time
indicates that the frequency of occurrence of a given storm event follows a statistical
pattern. This statistical analysis allows engineers and urban planners to further
characterize storm events based on their frequency of occurrence or return period. Storm
events of specific sizes can be identified to support evaluation of designs. Storms with 2-
and 10-year return periods are commonly used for subdivision, industrial, and commercial
development design.
The 1- and 2-year storm events are usually selected to protect receiving channels from
sedimentation and erosion. The 5- and 10-year storm events are selected for adequate
flow conveyance design and minor flooding considerations. The 100-year event is used to
define the limits of floodplains and for consideration of the impacts of major floods.
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LID Hydrologic Analysis
Prince George's County, PER
LID Hydrologic Analysis Procedure
LID Hydrologic
Analysis Process
Data Collection
_L
Calculate Existing Tc
Calculate Existing CN
Prepare Preliminary Layout
Calculate Proposed CN
Using LID Concepts
Calculate Proposed Tc
(Section 4.2)
(Section 4.4)
(Section 4.3)
(Section 4.3)
(Section 4.4)
Implement Additional
LID Tc Techniques
and Recalculate Tc
Yes
Legend
VQ Storage Volume Needed for
Water Quality Control
VR Storage Volume to Maintain CN
Using Retention Chart A
VR Storage Volume to Maintain
Peak Using 100% Retention
Chart B
VD Storage Volume to Maintain
Peak Using 100% Detention
Chart C
H Storage Volume for Hybrid
Design
H' Storage Volume for Hybrid
Design with Limited Retention
Determine Design Storm Event
Calculate Volume Required to
Maintain Existing CN Using Chart
Series A for Each Design Storm VR
Calculate the Storage Volume Area
Required for Quality Control VQ
(Section 4.6)
(Section 4.5
Step 1)
(Section 4.5
Step 2)
Select
'HigherValues of
VQ or VR for Storage
xVolume Required,
or
(Section 4.5
Step 2)
Hybrid Approach
Calculate Additional
Volume to Maintain Both
Predevelopment Peak and
Volume H Using VR,
VD ,VR
Calculate Volume Required to
Maintain Predevelopment Peak
Discharge Using Chart Series B for
Each Design Storm VRaj
(Section 4.5
Step 3)
(Section 4.5
Step 5)
Use Chart Series C to
Calculate VD
LID Final
Stormwater Design
Site Conditions
Accommodate
ofBMPsforV
Determine Storage Volume Area
That Is Acceptable for Retention
and Recalculate Storage Volume
to Maintain Peak H' using VR,
VD ,VR
(Section 4.5
Step 7)
Figure 1.1. Low-impact development analysis procedure
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Prince George's County, Maryland
LID Hydrologic Analysis
There are numerous excellent texts and handbooks that describe the use of rainfall data
to generate a "design storm" for the design of drainage systems (e.g., ASCE , 1994; Chow,
1964; SCS, 1985). For LID, a unique approach has been developed to determine the
design storm based on the basic philosophy of LID. This approach is described in Section
4.6.
Storm events commonly used for evaluation of designs differ for the various climatic
regions of the United States Summaries of typical storm event characteristics (i.e.,
amount/intensity, duration, and return period) are provided in national maps in Technical
Paper 40 (Department of Commerce, 1963). In humid regions such as the Mid-Atlantic
states, the 2-year storm is approximately 3 inches of rainfall and the 10-year storm is
approximately 5 inches of rainfall. The 2-year storm has a 50 percent probability of
occurring in any given year, while the 10-year storm has a 10 percent probability of
occurring in any given year. In dry areas, such as portions of Colorado and New Mexico,
the 2-year storm is approximately 1.5 inches of rainfall and the 5-year storm is
approximately 2.0 inches of rainfall.
The rainfall time distributions vary throughout the geographic regions of the U.S.
They are Type I, Type IA, Type II, and Type III. These differences in the distributions
play a very important role in sizing the IMPs.
Rainfall Abstractions. Rainfall abstractions include the physical processes of
interception of rainfall by vegetation, evaporation from land surfaces and the upper soil
layers, transpiration by plants, infiltration of water into soil surfaces, and storage of water
in surface depressions. Although these processes can be evaluated individually, simplified
hydrologic modeling procedures typically consider the combined effect of the various
components of rainfall abstraction.
The rainfall abstraction can be estimated as a depth of water (inches) over the total
area of the site. This depth effectively represents the portion of rainfall that does not
contribute to surface runoff. The portion of rainfall that is not abstracted by interception,
infiltration, or depression storage is termed the excess rainfall or runoff.
The rainfall abstraction may change depending on the configuration of the site
development plan. Of particular concern is the change in impervious cover. Impervious
areas prevent infiltration of water into soil surfaces, effectively decreasing the rainfall
abstraction and increasing the resulting runoff. Postdevelopment conditions, characterized
by higher imperviousness, significantly decrease the overall rainfall abstraction, resulting
not only in higher excess surface runoff volume but also a rapid accumulation of rainwater
on land surfaces.
The LID approach attempts to match the predevelopment condition by compensating
for losses of rainfall abstraction through maintenance of infiltration potential,
evapotranspiration, and surface storage, as well as increased travel time to reduce rapid
concentration of excess runoff. Several planning considerations combined with
supplemental controls using LID integrated management practices can be used to
compensate for rainfall abstraction losses and changes in runoff concentration due to site
development.
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LID Hydrologic Analysis
Prince George's County, PER
Surface Runoff. The excess rainfall, or the portion of rainfall that is not abstracted by
interception, infiltration, or depression storage, becomes surface runoff. Under natural
and undeveloped conditions, surface runoff can range from 10 to 30 percent of the total
annual precipitation (Figure 1.2). Depending on the level of development and the site
planning methods used, the alteration of physical conditions can result in a significant
increase of surface runoff to over 50 percent of the overall precipitation. In addition,
enhancement of the site drainage to eliminate potential on-site flooding can also result in
increases in surface runoff. Alteration in site runoff characteristics can cause an increase
in the volume and frequency of runoff flows (discharge) and velocities that cause flooding,
accelerated erosion, and reduced groundwater recharge and contribute to degradation of
water quality and the ecological integrity of streams.
40% evapotranspirotion
10%
runoff
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Prince George's County, Maryland
LID Hydrologic Analysis
1.3 HYDROLOGIC ALTERATIONS TO SITE DEVELOPMENT
Climate coupled with the geological and vegetative features of a watershed produce a
unique hydrologic regime. Aquatic, wetland and riparian biota have evolved by adapting
to this unique regime (Cairns, 1993). Urban development changes this regime, resulting in
a new annual and seasonal hydrologic balance, causing frequency distribution changes of
peak flows, magnitude and duration of high flows, and magnitude and duration of low
flows.
Changes in the Existing Hydrologic Balance. Both the annual and seasonal water
balance can change dramatically as a result of development practices. These changes
include increases in surface runoff volume and decrease in evapotranspiration and
groundwater recharge rates. For example, eastern hardwood forests typically have an
annual water balance comprised of about 40% evapotranspiration, 50% subsurface flows
and less than 10% surface runoff volume. Development, depending on its size and location
in a watershed, alters the existing hydrologic balance by increasing surface flow volumes
up to 43%, reducing subsurface flows to 32%, and reducing evapotranspiration rates to
25%. All this results in major changes to the local hydrology.
moisture moving
down after a ram
Transpiration
Evaporation
Effluent
| stream
t
Con'f/r
Figure 1.3. Groundwater in Local, Intermediate, or Regional Setting
Changes in Frequency Distribution of High Flows. Increased stream flows due to
changes in surface topography result in more rapid drainage and increases in the amount
of hydro logically active areas within a watershed. Hydrologically active areas are areas
that produce runoff during precipitation events. These areas also increase in size, in
comparison to their predevelopment size, due to reductions in depression storage capacity
and in the retention capacity of the site's existing natural vegetation. Increases in
impervious ground covers also contribute to increasing volumes of runoff. These changes
coupled with shorter times of concentration result in sharp modifications to the shape of
the resulting hydrograph.
A hydrograph represents diagramatically the changes in stream flow over time and
during a storm event. As a site is developed, topography and land surfaces are modified,
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6 LID Hydrologic Analysis
Prince George's County, PER
with the resulting hydrograph reflecting decreases in base flow, higher and more rapid
peak discharge, and more runoff volume. As illustrated in Figure 1.4, development
generally results in stream discharges which increase rapidly and recede at rates much
greater than under natural conditions. Higher flow velocities also increase the runoffs
potential to erode and transport sediment and pollutants. The frequency of that peak flow
event also increases. In urbanized watersheds, extreme events, such as the frequency of the
bankfull flows, might be expected to occur 2 to 8 times per year compared to less than
once per year under natural condition.
Changes in Magnitude, Frequency, and Duration of Low Flows. Impervious surfaces
such as roads, rooftops, driveways, and sidewalks reduce infiltration, filtration, and
groundwater recharge. This can lower water tables, impacts flow to existing wetlands, and
reduce the water available for stream base flow. Similarly, decreases in the time of
concentration, or runoff travel time, reduces the time available for water to infiltrate. The
problem may be further compounded by the installation of shallow ground water drainage
systems to accommodate road or building construction. Lower recharge rates for
Q after development
Q
Q before development
Figure 1.4. Hydrologic Alterations Due to Site Development
groundwater in a watershed are generally reflected in lower stream base flows. Low rates
of recharge also extend low flow durations; particularly during prolonged droughts.
Conversely small storms which prior to development did not produce surface runoff now
frequently do so.
Typical alterations to the hydro logic regime as a result of development include, but
are not limited to, the following;
Increased runoff volume
Increased imperviousness
Increased flow frequency, duration, and peak runoff rate
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Prince George's County, Maryland
LID Hydrologic Analysis
Reduced infiltration (groundwater recharge)
Modification of the flow pattern
Faster time to peak, due to shorter Tc through storm drain systems
Loss of storage
1.4 CONVENTIONAL STORMWATER MANAGEMENT
Traditionally, the response of watersheds to urban development has been measured in
terms of changes in the flow regime, with management efforts focused on the prevention
of property damage from flooding as previously described. Stormwater management
efforts historically followed the design storm concept described earlier and focused almost
exclusively on runoff collection systems such as curbs and gutters, and pipe conveyance
systems which discharged directly to receiving water bodies. Stormwater quantity (peak
discharge rate) management was incorporated as IMPs to address concerns about
downstream flooding and stream bank erosion. Typically these IMPs, usually ponds or
detention basins were located at the lowest point of the site and at the end of the network
of inlets and pipes. This approach is often referred to as the "end of pipe" control
approach.
Stormwater Quantity. Stormwater quantity controls are set by states or local
government agencies to prevent site and downstream flooding and erosion. A typical
design criteria requires that "the post development peak discharge for a 2- and 10- year
frequency storm event be maintained at a level equal to or less than the respective 2-and
10-year predevelopment peak discharge rates, through the use of Stormwater management
structures that control the timing and rate of runoff." This requirement is based on the
design storm concept described earlier under in this section.
The selection of the 2-year return frequency storm is based on a belief that the 1.5- to
2-year storm dictates the shape and form of natural channels (Leopold, et al., 1964, 1968).
The selection of the 10-year storm is based on consideration of possible property damage
due to local flooding and stream bank erosion
It is now becoming increasingly recognized that this type of approach is insufficient
for a number of reasons:
It does not address the loss of storage volume provided by rainfall abstractions, and
consequently does not provide for groundwater recharge and maintenance of base
flow during low flow periods.
The 2/10 year storm policy does not adequately protect downstream channels form
accelerated erosion.
The inspection and maintenance costs of this approach are becoming an increasing
burden for local governments
Stormwater Quality. The second stage in Stormwater management was the recognition
that runoff from urban areas was more polluted than runoff from undeveloped areas and
was degrading the water quality of the receiving streams and other water bodies. For the
most part this problem was addressed by modifying and improving the end of pipe
approach to improve the pollutant removal effectiveness of these IMPs. Extended
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LID Hydrologic Analysis
Prince George's County, PER
detention, forebays, wetlands, permanent pools and numerous other design improvements
were introduced.
Also the concept of controlling the "first flush" was introduced. A "first flush" event is
defined as the first half inch of runoff from an impervious surface, and is expected to carry
with it most of the pollutant load associated with stormwater. In terms of a typical storm
hydrograph, the "first flush" represents a small portion of a storm's total discharge, but a
larger percentage of the total loading for a particular contaminant.
Designers and modelers discovered that the design storm approach used for peak
discharge control was not appropriate foe water quality control issues, since water quality
issues were related to the annual volume of runoff which consists of many small storms.
For example, the rainfall frequency distribution at National Airport, Arlington, VA, for the
period of 1908 to 1985 indicates that the average total annual precipitation is 38.40 inches
and storms of 1 inch or less account for 70% of the total annual precipitation (Figure 1.5).
In addition, if the first inch of the storm events greater than 1 inch are considered, the total
annual volume of 1 inch or less is in the range of 80 to 85%. These relationships are not
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10
Figure 1.5. Rainfall Frequency Distribution at National Airport 1980 to 1985
considered in the traditional design storm concept because that approach is based on
control of infrequent storms that are large enough to produce floods. However, this annual
rainfall distribution pattern becomes an important consideration in the selection of
appropriate rainfall conditions for low-impact development.
1.5. HYDROLOGIC COMPARISON BETWEEN CONVENTIONAL AND LOW-IMPACT
DEVELOPMENT APPROACHES
Conventional stormwater conveyance systems are designed to collect, convey, and
discharge runoff as efficiently as possible. Conventional stormwater management controls
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Prince George's County, Maryland
LID Hydrologic Analysis
(IMPs) are typically sited at the most downstream point of the entire site (end-of-pipe
control). The stormwater management requirement is usually to maintain the peak runoff
rates at predevelopment levels for a particular design storm event. Therefore, especially
where a stormwater management pond is constructed, the peak flow will not be fully
controlled for those storm events that are less severe than the design storm event. Low-
impact development approaches, on the other hand, will fully control these storm events.
This is a very important and significant difference between the two approaches. Figure
1.6 illustrates the hydrologic response of the runoff hydrograph to conventional IMPs.
Hydrograph 1 represents the response to a given storm of a site in a
predevelopment condition (i.e., woods, meadow). The hydrograph is defined by a
gradual rise and fall of the peak discharge and volume.
Hydrograph 2 represents a post development condition with conventional
stormwater IMPs, such as a detention pond. Although the peak runoff rate is
maintained at the predevelopment level, the hydrograph exhibits significant
increases in the runoff volume and duration of runoff from the predevelopment
condition.
Time
Figure 1.6. Comparison of the Hydrologic Response of Conventional and LID
IMPs.
Hydrograph 3 represents the response of post development condition that
incorporates low-impact development stormwater management. Low-impact
development uses undisturbed areas and on-lot and distributed retention storage to
reduce to reduce runoff volume. The peak runoff rate and volume remain the same
as the pre-development condition through the use of on-lot retention and/or
detention. The frequency and duration of the runoff rate are also much closer to the
existing condition than those typical of conventional IMPs.
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10 LID Hydrologic Analysis
Prince George's County, PER
The Distributed Control Approach. In comparison with conventional stormwater
management, the objective of low-impact development hydrologic design is to retain the
post development excess runoff volume is discrete units throughout the site to emulate the
predevelopment hydrologic regime. This is called a distributed control approach.
Management of both runoff volume and peak runoff rate is included in the design. The
approach is to manage runoff at the source rather than at the end of pipe. Preserving the
hydrologic regime of the predevelopment condition may require both structural and
nonstructural techniques to compensate for the hydrologic alterations of development.
The Hydrologically Functional Landscape. In low-impact development, the design
approach is to leave as many undisturbed areas as practical to reduce runoff volume and
runoff rates by maximizing infiltration capacity. Integrated stormwater management
controls or IMPs are then distributed throughout the site to compensate for the hydrologic
alterations of development. The approach of maintaining areas of high infiltration and low
runoff potential in combination with small, on-lot stormwater management facilities
creates a "hydrologically functional landscape." This functional landscape not only can
help maintain the predevelopment hydrologic regime but also enhance the aesthetic and
habitat value of the site.
Integrated Management Practices (IMPs). Low-impact development technology
employs microscale and distributed management techniques, called integrated
management practices (IMPs) to achieve desired post-development hydrologic conditions.
LID IMPs are used to satisfy the storage volume requirements described in Section 3.3.
They are the preferred method because they can maintain the predevelopment runoff
volume and can be integrated into the site design. The design goal is to locate IMPs at the
source or lot, ideally on level ground within individual lots of the development.
Management practices that are suited to low-impact development include:
bioretention facilities
dry wells
filter/buffer strips and other multifunctional landscape areas
grassed swales, bioretention swales, and wet swales
rain barrels
cisterns
infiltration trenches
More information on IMPs can be obtained in the publication titled, "Low-Impact
Development Design Strategies: An Integrated Design Approach," prepared by Prince
George's County, DM, May 1999.
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Prince George's County, Maryland
LID Hydrologic Analysis 11
CHAPTER 2. LID HYDROLOGIC ANALYSIS COMPONENTS
The low-impact development "functional landscape" emulates the predevelopment
temporary storage (detention) and infiltration (retention) functions of the site. This
functional landscape is designed to mimic the predevelopment hydrologic conditions
through runoff volume control, peak runoff rate control, flow frequency/duration control,
and water quality control.
Runoff Volume Control. The predevelopment volume is maintained by a combination
of minimizing the site disturbance from the predevelopment condition and then providing
distributed retention IMPs. Retention IMPs are structures that retain the runoff for the
design storm event.
Peak Runoff Rate Control. Low-impact development is designed to maintain the
predevelopment peak runoff discharge rate for the selected design storm events. This is
done by maintaining the predevelopment Tc and then using retention and/or detention
IMPs (e.g., rain gardens, open drainage systems, etc.) that are distributed throughout the
site. The goal is to use retention practices to control runoff volume and, if these retention
practices are not sufficient to control the peak runoff rate, to use additional detention
practices to control the peak runoff rate. Detention is temporary storage that releases
excess runoff at a controlled rate. The use of retention and detention to control the peak
runoff rate is defined as the hybrid approach.
Flow Frequency/Duration Control. Since low-impact development is designed to
emulate the predevelopment hydrologic regime through both volume and peak runoff rate
controls, the flow frequency and duration for the post development conditions will be
almost identical to those for the predevelopment conditions (see Figure 1.3.). The impacts
on the sediment and erosion and stream habitat potential at downstream reaches can then
be minimized.
Water Quality Control. Low-impact development is designed to provide water quality
treatment control for the first ฅ2 inch of runoff from impervious areas using retention
practices. Low-impact development also provides pollution prevention by modifying
human activities to reduce the introduction of pollutants into the environment.
The low-impact analysis and design approach focuses on the following hydrologic
analysis and design components:
Runoff Curve Number (CN). Minimizing change in post development hydrology
by reducing impervious areas and preserving more trees and meadows to reduce
the storage requirements to maintain the pre development runoff volume.
Time of Concentration (Tc). Maintaining the predevelopment Tc in order to
minimize the increase of the peak runoff rate after development by lengthening
flow paths and reducing the length of the runoff conveyance systems.
Retention. Providing retention storage for volume and peak control, as well as
water quality control, to maintain the same storage volume as the predevelopment
condition.
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12 LID Hydrologic Analysis
Prince George's County, PER
Detention. Providing additional detention storage, if required, to maintain the
same peak runoff rate and/or prevent flooding.
Table 2.1 provides a summary of low-impact techniques that affect these components.
Table 2.1. Low-Impact Development Techniques and Hydrologic
Design and Analysis Components
Low-Impact Hydrologic
Design and Analysis
Components
Lower Postdevelopment
CN
Increase Tc
Retention
Detention
Low-Impact Development Technique
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Prince George's County, Maryland
LID Hydrologic Analysis 13
CHAPTER 3. HYDROLOGIC EVALUATION
The purpose of the hydrologic evaluation is to determine stormwater management
requirements for low-impact development sites. The evaluation method is used to
determine the amount of retention and/or detention to control the runoff volume and peak
discharge rate. Appropriate detention and/or retention techniques are then selected to meet
these requirements.
3.1 LOW-IMPACT DEVELOPMENT RUNOFF POTENTIAL
Calculation of the low-impact development runoff potential is based on a detailed
evaluation of the existing and proposed land cover so that an accurate representation of the
potential for runoff can be obtained. This calculation requires the engineer to investigate
several key parameters associated with a low-impact development:
Land cover type
Percentage of and connectivity of impervious areas
Soils type and texture
Antecedent soil moisture conditions
A comparison of conventional and low-impact development runoff potential using the
SCS Curve Number (CN) approach is presented. The CN for conventional development
are based on the land cover assumptions and parameters shown in Table 2.2a of TR-55
(SCS, 1986 ). The low-impact development CN are based on a detailed evaluation of the
land cover and parameters listed above. As illustrated in Figure 3.1, customizing the CN
for a low-impact development site allows the developer/engineer to take advantage of and
get credit for such low-impact development site planning practices as the following:
Narrower driveways and roads (minimizing impervious areas)
Maximizing tree preservation or aforestation (tree planting)
Site fingerprinting (minimal disturbance)
Open drainage swales
Preservation of soils with high infiltration rates to reduce CN
Location of DVIPs on high infiltration soils.
Table 3.1 illustrates a comparison of low-impact development CN land covers with
those of a conventional development CN, as found in Table 2.2a of TR-55 (SCS, 1986) for
a typical 1-acre lot. Figure 3.1 illustrates a comparison of conventional land covers, based
on the land covers in Table 2.2a of TR-55, with a low-impact development customized CN
for a 1-acre lot.
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14 LID Hydrologic Analysis
Prince George's County, PER
/
Lawn Area
i House
I Driveway
\ Cmb& Gutter
Detention Area
if Required
Conventional CN
for 1-Acre Lot
(Table 2.2a TR-55)
N.T.S.
Typical Low-Impact
Development Lot
N.T.S.
Figure 3.1. Comparison of Land Covers Between Conventional and LID CNs
Table 3.1. Comparison of Conventional and LID Land Covers
Conventional Land Covers
(TR-55 Assumptions)
20% impervious
80% grass
LID Land Covers
15% imperviousness
25% woods
60% grass
Table 3.2 provides a list of low-impact development site planning practices and their
relationship to the components of the low-impact development CN. Key low-impact
techniques that will reduce the post development CN, and corresponding runoff volumes,
are as follows:
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Prince George's County, Maryland
LID Hydrologic Analysis 15
Table 3.2. Low-Impact Development Planning Techniques to Reduce
the Postdevelopment Low-Impact Development CN
Suggested Options
Affecting Curve
Number
Land Cover Type
Percent of
Imperviousness
Hydrologic Soils Group
Hydrologic Condition
Disconnectivity of
Impervious Area
Storage and Infiltration
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Preservation of Infiltratable Soils: This approach includes site planning techniques
such as minimizing disturbance of soils, particularly vegetated areas, with high infiltration
rates (sandy and loamy soils), and placement of infrastructure and impervious areas such
as houses, roads, and buildings on more impermeable soils (silty and clayey soils). Care
must be taken when determining the suitability of soils for proposed construction
practices. Adequate geotechnical information is required for planning practices.
Preservation of Existing Natural Vegetation. Woods and other vegetated areas
provide many opportunities for storage and infiltration of runoff. By maintaining the
surface coverage to the greatest extent possible, the amount of compensatory storage for
IMPs is minimized. Vegetated areas can also be used to provide surface roughness,
thereby increasing the Tc. In addition, they function to filter out and uptake pollutants.
Minimization of Site Imperviousness. Reducing the amount of imperviousness on the
site will have a significant impact on the amount of compensatory IMP storage required
since there is almost a one-to-one corresponding relationship between rainfall and runoff
for impervious areas.
Disconnection of Site Imperviousness. Impervious areas are considered disconnected
if they do not connect to a storm drain system or other impervious areas through direct or
shallow concentrated flow. Directing impervious areas to sheet flow onto vegetated or
bioretention areas to allow infiltration results in a direct reduction in runoff and
corresponding storage volume requirements.
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16 LID Hydrologic Analysis
Prince George's County, PER
Creation of Transition Zones and Bioretention: Transition zones are vegetated areas
that can be used to store and infiltrate runoff from impervious areas before they discharge
from the site. These areas are located at the sheet or discharge points from graded and
impervious areas. These areas affect the land cover type calculations of the LID CN.
The use of these techniques will provide incentives in cost savings to the overall site
development and infrastructure. It will also reduce costs for stormwater permit fees,
inspection, and maintenance of the infrastructure as well as project based costs.
Figure 3.2. illustrates the hydrologic response using LID techniques to reduce the
impervious areas and increase the storage volume.
For hydrograph 1, refer to Figure 1.3 for description.
Hydrograph 2 represents the response of a post development condition with no
stormwater management IMPs. This hydrograph definition reflects a shorter time
of concentration (Tc), and an increase in total site imperviousness than that of the
predevelopment condition. The resultant hydrograph shows a decrease in the time
to reach the peak runoff rate, a significant increase in the peak runoff and
discharge rate and volume, and increased duration of the discharge volume.
Hydrograph 3 represents the resulting post development hydrograph using the low-
impact techniques to reduce impervious area and increase storage volume. There is
a reduction in both post development peak rate and volume.
Developed Condition without IMPs
Reduced Q peak
Developed Condition, with LID- CN
No Controls
Reduced Runoff Volume
Figure 3.2. Effect of Low-Impact Development CN on the Postdevelopment
Hydrograph without Stormwater IMPs
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Prince George's County, Maryland
LID Hydrologic Analysis 17
3.2. MAINTAINING THE PREDEVELOPMENT TIME OF CONCENTRATION
The low-impact development hydrologic evaluation requires that the post development
time of concentration (Tc) be maintained close to the predevelopment Tc. The travel time
(Tt) throughout individual lots and areas should be approximately the same so that the Tc
is representative of the drainage. This is critical because low-impact development is based
on a homogeneous land cover and distributed IMPs. To maintain the Tc, low-impact
developments use the following site planning techniques:
Maintaining predevelopment flow path length by dispersing and redirecting
flows, generally, through open swales and natural drainage patterns
Increasing surface roughness (e.g., reserving woodlands, using vegetated swales).
Detaining flows(e.g. open swales, rain gardens).
Minimize disturbance(minimizing compaction and changes to existing
vegetation).
Flattening grades in impacted areas.
Disconnecting impervious areas (e.g., eliminating curb/gutter and redirecting
downspouts).
Connecting pervious and vegetated areas (e.g., reforestation, aforestation, tree
planting).
To maintain predevelopment Tc, an iterative process that analyzes different
combinations of the above appropriate techniques may be required. These site planning
techniques are incorporated into the hydrologic analysis computations for post
development Tc to demonstrate an increase in post development Tc above conventional
techniques and a corresponding reduction in peak discharge rates.
Figure 3.3 illustrates the hydrologic response to maintaining equal predevelopment
and post-development Tc.
For hydrograph 1 refer to Figure 1.3.
For hydrograph 3 refer to Figure 3.2.
Hydrograph 4 represents the effects of the low-impact development techniques to
maintain the Tc. This effectively shifts the post peak runoff time to that of the
predevelopment condition and lowers the peak runoff rate.
The greatest gains for increasing the Tc in a small watershed can be accomplished by
increasing the Manning's roughness "n" for the initial surface flow at the top of the
watershed and increasing the flow path length for the most hydraulically distant point in
the drainage area. After the transition to shallow concentrated flow, additional gains in Tc
can be accomplished by:
Decreasing the slope
Increasing the flow length
Directing flow over pervious areas.
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18 LID Hydrologic Analysis
Prince George's County, PER
K Developed,
i in PN
no control. V .
i v
\^
Reduced Qpeak
Q
Developed, LID- CN, No control
Same Tc as existing condition
More Runoff Volume than the
predevelopment condition
Figure 3.3. Low-Impact Development Hydrograph That Has a Reduced CN
and Maintains the Tc Without Stormwater IMPs
In low-impact development sites, the amount of flow in closed channels (pipes) should
be minimized to the greatest extent possible. Swales and open channels should be
designed with the following features:
Increase surface roughness to retard velocity
Maximize sheet flow conditions
Use a network of wider and flatter channels to avoid fast-moving channel flow
Increase channel flow path
Reduce channel gradients to decrease velocity (minimum slope is 2-percent; 1
percent may be considered on a case by case basis).
The channel should flow over pervious soils whenever possible to increase
infiltration so that there is a reduction of runoff to maximize infiltration capacity
Table 3.3 identifies low-impact development techniques and volumes objectives to
maintain the predevelopment Tc.
3.3 MAINTAINING THE PREDEVELOPMENT RUNOFF VOLUME
After all the available and feasible options to reduce the runoff potential of a site
described have been deployed, and after ah1 the available techniques to maintain the Tc as
close as possible to predevelopment levels have been used, any additional reductions in
runoff volume must be accomplished through distributed on-site stormwater management
techniques. The goal is to select the appropriate combination of management techniques
that emulate the hydrologic functions of the predevelopment condition to maintain the
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Prince George's County, Maryland
LID Hydrologic Analysis 19
Table 3.3. Low-Impact Development Techniques to Maintain the
Predevelopment Time of Concentration
Low-Impact
Development
Objective
Minimize disturbance
Flatten grades
Reduce height of
slopes
Increase flow path
(divert and redirect)
Increase roughness "n"
Low Impact Development Technique
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existing runoff curve numbers and corresponding runoff volume. Low-impact
development sites use retention to accomplish this goal. These facilities must be sited on
individual lots throughout the site to provide volume controls at the source.
Retention storage allows for a reduction in the post development volume and the peak
runoff rate. The increased storage and infiltration capacity of IMPs allows the
predevelopment volume to be maintained. IMPs that maintain the predevelopment storage
volume include, but are not limited to the following:
Bioretention (rain garden)
Infiltration trenches
Vegetative Filter/Buffer
Rain barrels
As the retention storage volume of the low-impact development IMPs is increased,
there is a corresponding decrease in the peak runoff rate in addition to runoff volume
reduction. If sufficient amount of runoff is stored, the peak runoff rate may be reduced to a
level at or below the predevelopment runoff rate (see Figure 3.4). This storage may be all
that is necessary to control the peak runoff rate when there is a small change in runoff
curve number (CN) and storage volume. However, when there is a large change in CN, it
may be less practical to achieve flow control using volume control only.
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20 LID Hydrologic Analysis
Prince George's County, PER
Provide storage
using retention
IMPs so that the
Predevelopment
Predevelopment
Peak Q
rate is |' w "
maintained mmmmfii^-
&
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Prince George's County, Maryland
LID Hydrologic Analysis 21
The effect of this additional detention storage is illustrated in Figure 3.5.
For hydrograph 1, refer to Figure 1.3.
Hydrograph 7 represents the response of a post-development condition that
incorporates low-impact development retention practices. The amount of
retention storage provided is not large enough to maintain the predevelopment
peak runoff discharge rate. Additional detention storage is required.
Hydrograph 8 illustrates the effect of providing additional detention storage to
reduce the post-development peak discharge rate to predevelopment conditions.
Provide additional detention storage
to reduce peak discharges to be equal
to that of the existing condition.
Figure 3.5. Effect of Additional Detention Storage on LID Retention Practices
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22 LID Hydrologic Analysis
Prince George's County, PER
CHAPTER 4. PROCESS AND COMPUTATIONAL PROCEDURE
4.1 INTRODUCTION
The hydrologic analysis of low-impact development is a sequential decision making
process that can be illustrated by the flow chart shown in Figure 1.1. Several iterations
may occur within each step until the appropriate approach to reduce stormwater impacts is
determined. The procedures for each step are given in the following section. Design charts
have been developed to determine the amount of storage required to maintain the existing
volume and peak runoff rates to satisfy county storm water management requirements
(Appendices A, B, and C).
4.2 DATA COLLECTION
The basic information used to develop the low-impact development site plan and used
to determine the Runoff Curve Number (CN) and Time of Concentration (Tc) for the pre-
and post-development condition is the same as conventional site plan and stormwater
management approaches.
4.3 DETERMINING THE LID RUNOFF CURVE NUMBER
The determination of the low-impact development CN requires a detailed evaluation of
each land cover within the development site. This will allow the designer to take full
advantage of the storage and infiltration characteristics of low-impact development site
planning to maintain the CN. This approach encourages the conservation of more
woodlands and the reduction of impervious area to minimize the needs of IMPs.
The steps for determining the low-impact development CN are as follows:
Step 1: Determine Percentage of Each Land Use/Cover.
In conventional site development, the engineer would refer to Figure 2.2.a of TR-55
(SCS, 1986) to select the CN that represents the proposed land use of the overall
development (i.e., residential, commercial) without checking the actual percentages of
impervious area, grass areas, etc. Because low-impact design emphasizes minimal site
disturbance (tree preservation, site fingerprinting, etc.), it is possible to retain much of the
pre-development land cover and CN.
Therefore, it is appropriate to analyze the site as discrete units to determine the CN.
Table 4.1 lists representative land cover CNs used to calculate the composite "custom"
low-impact development CN.
Step 2: Calculate Composite Custom CN.
The initial composite CN is calculated using a weighted approach based on individual
land covers without considering disconnectivity of the site imperviousness. This is done
using Equation 4.1. This weighted approach is illustrated in Example 4.1.
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Prince George's County, Maryland
LID Hydrologic Analysis
23
TABLE 4.1. REPRESENTATIVE LID CURVE NUMBERS
Land Use/Cover
Impervious Area
Grass
Woods (fair condition)
Woods (good condition)
Curve Number for Hydrologic Soils
Groups1
A
98
39
36
30
B
98
61
60
55
C
98
74
73
70
D
98
80
79
77
'Figure 2.2a, TR-55 (SCS, 1986).
CNC=-
A
Where:
CNc = composite curve number;
Aj = area of each land cover; and
CNj = curve number for each land cover.
Eq. 4.1
Overlays of SCS Hydrologic Soil Group (HSG) boundaries onto homogeneous land
cover areas are used to develop the low-impact development CN. What is unique about
the low-impact development custom-made CN technique is the way this overlaid
information is analyzed as small discrete units that represent the hydrologic condition,
rather than a conventional TR-55 approach that is based on a representative national
average. This is appropriate because of the emphasis on minimal disturbance and
retaining site areas that have potential for high storage and infiltration.
This approach provides an incentive to save more trees and maximize the use of HSG
A and B soils for recharge. Careful planning can result in significant reductions in post-
development runoff volume and corresponding stormwater management costs.
Step 3: Calculate low-impact development CN based on the connectivity of site
impervious area.
When the impervious areas are less than 30 percent of the site, the percentage of the
unconnected impervious areas within the watershed influences the calculation of the CN
(SCS, 1986). Disconnected impervious areas are impervious areas without any direct
connection to a drainage system or other impervious surface. For example, roof drains
from houses could be directed onto lawn areas where sheet flow occurs, instead of to a
swale or driveway. By increasing the ratio of disconnected impervious areas to pervious
areas on the site, the CN and resultant runoff volume can be reduced. Equation 4.2 is
used to calculate the CN for sites with less than 30 percent impervious area.
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24 LID Hydrologic Analysis
Prince George's County, PER
Land Use
Impervious (Directly Connected)
Impervious (Unconnected)
Open Space (Good Condition, Graded)
Woods (Fair Condition)
HSG
(1)
B
B
B
B
CN
(2)
98
98
61
55
%of
Site
(3)
5
10
60
25
Land
Coverage
(ft2)
(4)
2,178
4,356
26,136
10,890
Example 4.1: Detailed CN Calculation
Given:
One-acre residential lot
Conventional CN: 68 (From TR-55 Table 2.2a-Runoff curve numbers for urban areas (SCS,
1986) Table 2.2a assumes HSG B, 20% imperviousness with a CN of 98 and 80% open
space in good condition.
Custom-made LID CN: CN for individual land covers based on Table 2.2a. Assume 25% of
the site will be used for reforestation/landscaping (see Figure 3.1) HSG B.
Procedure:
Step 1: Determine percentage of each land cover occurring on site and the CN associated
with each land cover.
Step 2: Calculate composite custom CN (using Equation 4.1).
98x4,356 + 98x2,178 + 61x26,136 + 55x10,890
_
c~
CNC=65
43,560
Step 3: Calculate low-impact development CN based on the connectivity of the site
imperviousness (using Equation 4.2).
CNp =
61x26,136+55x10,890
37,026
CNf =592
10
*~Ts
ฃ = 0.67
0^=59.2+1jx (98-59.2)x (1-0.5 x 0.67)
0^=63.1(115663)
LID custom CN of 63 is less than conventional CN of 68 (predevelopment CN is 55).
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Prince George's County, Maryland
LID Hydrologic Analysis 25
CNC = CNp
*imp
Too"
Eq. 4.2
where:
R = ratio of unconnected impervious area to total impervious area;
CNC = composite CN;
CNP = composite pervious CN; and
Pimp = percent of impervious site area.
Example 4.1 uses steps 1 through 3 to compare the calculation of the curve number
using conventional and low-impact development techniques using the percentages of land
cover for a typical 1-acre residential lot from Figure 3.1.
4.4 DEVELOPMENT OF THE TIME OF CONCENTRATION (TC)
The pre- and post-development calculation of the Tc for low-impact development is
exactly the same as that described in the TR-55 (SCS, 1986) and NEH-4 (SCS, 1985)
manuals.
4.5 LOW-IMPACT DEVELOPMENT STORMWATER MANAGEMENT
REQUIREMENTS
Once the CN and Tc are determined for the pre- and post-development conditions, the
stormwater management storage volume requirements can be calculated. The low-impact
development objective is to maintain all the pre-development volume, pre-development
peak runoff rate, and frequency. The required storage volume is calculated using the
design charts in Appendices A, B, and C for different geographic regions in the nation.
As stated previously, the required storage volume is heavily dependent on the
intensity of rainfall (rainfall distribution). Since the intensity of rainfall varies
considerably over geographic regions in the nation, the National Resource Conservation
Service (NRCS) developed four synthetic 24-hour rainfall distributions (I, IA, II, and III)
from available National Weather Service (NWS) duration-frequency data and local storm
data. Type IA is the least intense and type II the most intense short duration rainfall.
Figure 4.1. shows approximate geographic boundaries for these four distributions.
The remaining low-impact development hydrologic analysis techniques are based on
the premise that the post-development Tc is the same as the pre-development condition. If
the post-development Tc does not equal the pre-development Tc, additional low-impact
development site design techniques must be implemented to maintain the Tc.
Three series of design charts are needed to determine the storage volume required to
control the increase in runoff volume and peak runoff rate using retention and detention
practices. The required storages shown in these design charts are presented as a depth in
hundredths of an inch (over the development site). Equation 4.3 is used to determine the
volume required for IMPs.
Volume = (depth obtained from the chart) x (development size) / 100
Eq. 4.3
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26 LID Hydrologic Analysis
Prince George's County, PER
Rilnf.lI
Distribution
Typซ I
Typ. IA
Typ. II
Typ* III
Figure 4.1. Approximate Geographic Boundaries for NRCS Rainfall Distributions.
It is recommended that 6-inch depth be the maximum depth for bioretention basins
used in low-impact development.
The amount, or depth, of exfiltration of the runoff by infiltration or by the process of
evapotranspiration is not included in the design charts. Reducing surface area
requirements through the consideration of these factors can be determined by using
Equation 4.4.
Volume of site area for IMPs = (initial volume) x (100 -x) I 100
Eq. 4.4
where: x = % of the storage volume infiltrated and/or reduced by evaporation or
transpiration. x% should be minimal (less than 10% is considered).
Stormwater management is accomplished by selecting the appropriate IMP, or
combination of IMPs, to satisfy the surface area and volume requirements calculated from
using the design charts. The design charts to be used to evaluate these requirements are:
Chart Series A: Storage Volume Required to Maintain the Predevelopment
Runoff Volume Using Retention Storage (Appendix A).
Chart Series B: Storage Volume Required to Maintain the Predevelopment Peak
Runoff Rate Using 100% Retention (Appendix B).
Chart Series C: Storage Volume Required to Maintain the Predevelopment Peak
Runoff Rate Using 100% Detention (Appendix C).
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Prince George's County, Maryland
LID Hydrologic Analysis 27
These charts are based on the following general conditions:
The land uses for the development are relatively homogeneous throughout the
site.
The stormwater management measures are to be distributed evenly across the
development, to the greatest extent possible.
The design storm is based on 1-inch increments. Use linear interpolation for
determining intermediate values.
The procedure to determine the IMP requirements is outlined in Figure 4.2 and
described in the following sections.
Step 1: Determine storage volume required to maintain predevelopment volume or
CN using retention storage.
The post-development runoff volume generated as a result of the post-development
custom-made CN is compared to the predevelopment runoff volume to determine the
surface area required for volume control. Use Chart Series A: Storage Volume Required to
Maintain the Predevelopment Runoff Volume using Retention Storage. The procedure for
calculating the site area required for maintaining runoff volume is provided in Example
4.2. It should be noted that the practical and reasonable use of the site must be considered.
The IMPs must not restrict the use of the site.
The storage area, expressed is for runoff volume control only; additional storage may
be required for water quality control. The procedure to account for the first Va-inch of
runoff from impervious areas, which is the current water quality requirement, is found in
Step 2.
Step 2: Determine storage volume required for water quality control.
The surface area, expressed as a percentage of the site, is then compared to the
percentage of site area required for water quality control. The volume requirement for
stormwater management quality control is based on the requirement to treat the first Vz-
inch of runoff (approximately 1,800 cubic feet per acre) from impervious areas. This
volume is translated to a percent of the site area by assuming a storage depth of 6 inches.
The procedure for calculating the site area required for quality control is provided in
Example 4.3. The greater number, or percent, is used as the required storage volume to
maintain the CN.
- From the results of Example 4.3, 0.1" of storage is required for water quality using
retention; from Example 4.2, 0.35" of storage is required to maintain the runoff volume
using retention. Since the volume required to maintain the runoff volume is larger, in this
case 0.35" of storage over the site should be reserved for retention IMPs.
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28 LID Hydrologic Analysis
Prince George's County, PER
Stepl:
Determine storage volume required to maintain runoff volume or CN.
Use Chart Series A: Storage Volume Required to Maintain the Pre-
development Runoff Volume Using Retention Storage (Example 4.2)
Step 2:
Determine storage volume for water quality volume requirements.
Determine storage volume required for quality control IMPs. Use larger of
volumes to maintain CN (Step 1, Example 4.2) or water quality volume
(Example 4.3).
Step 3:
Determine storage volume required to maintain predevelopment peak runoff
: rate using 100% retention. Use Chart Series B: Storage Volume Required to
! Maintain the Predevelopment Peak Runoff Rate Using 100% Retention.
Step 4:
Determine whether additional detention storage is required to maintain
predevelopment peak runoff rate. Compare the results of Steps 1 and 2 to the
results of Step 3. If the storage volume in Steps 1 and 2 is determined to be greater
than that in Step 3, the storage volume required to maintain the predevelopment CN
also controls the peak runoff rate. No additional detention storage is needed. If the
storage volume in Step 1 is less than that in Step 3, additional detention storage is
required to maintain the pak runoff rate (Example 4.4).
Step 5 (use if additional detention storage is required):
Determine storage volume required to maintain predevelopment peak runoff
rate using 100% detention. Use Chart Series C: Storage Volume Required to
' Maintain the Predevelopment Peak Runoff Rate Using 100% Detention. This is used
\ in conjunction with Chart Series A and B to determine the hybrid volume in Step 6.
Step 6 (use if additional detention storage is required):
; Hybrid approach. Use results from Chart Series A, B, and C to determine storage
| volume to maintain both the predevelopment peak runoff rate and runoff volume.
: Refer to Equations 4.5 and 4.6 as found in Example 4.4.
Step 7 (use if additional detention storage is required):
Determine appropriate storage volume available for retention practices. If the
storage volume available for retention practices is less than the storage determined
in Step 3, recalculate the amount of IMP area required to maintain the peak runoff
rate while attenuating some volume using the procedure in Example 4.6 using
Equations 4.7 and 4.8.
Figure 4.2. Procedure to Determine Storage Volume Required for IMPs to Maintain Predevelopment
Runoff Volume and Peak Runoff Rate.
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Prince George's County, Maryland
LID Hydrologic Analysis 29
Example 4.2: Determine Site Area Required to Maintain Volume (CN) Using Chart Series A: Storage
Volume Required to Maintain the Predevelopment Runoff Volume Using Retention Storage
Given:
Site Area is 18 acres
Existing CN is 60
Proposed CN is 65
Design storm is 5 inches
Design depth of IMP is 6 inches
Solution: Use Chart Series A: Storage Volume Required to Maintain Runoff Volume or CN.
0.35" of storage over the site is required to maintain the runoff volume.
Therefore: if 6" design depth is used, 1.1 acres (18 acres x 0.35/6) of IMPs distributed evenly
throughout the site are required to maintain the runoff volume, or CN.
Additional Considerations:
1) Account for depths other than 6 inches:
Site of IMP Area =1.1 acres, if 6" depth is used
Depth of IMPs = 4"
Site of IMP Area = 1.1 x 6"/4"
Site of IMP Area = 1.65 acres
2) Account for infiltration and/or evapotranspiration (using Equation 4.4)
If 10% of the storage volume is infiltrated and/or reduced by evaporation and transpiration.
Site of IMP Area = (storage volume) x (100 - X) / 100
Site of IMP Area =1.1 x (100-10)7100
Area for IMP Storage =1.0 acre
Example 4.3: Calculation of Volume, or Site Area, for Water Quality Control
Given:
Site area is 18 acres
Impervious area is 3.6 acres (20%)
Depth of IMP is 6 inches
Solution:
Water quality requirement is for the first Vz inch of runoff from impervious areas
(18 acres x 20%) x 0.5" /18 acres = 0.1" storage for water quality
0.1" is less than 0.35 " (from example 4.2). Therefore, use storage for runoff volume control to meet
water quality requirement.
-------
30 LID Hydrologic Analysis
Prince George's County, PER
Step 3: Determine storage volume required to maintain peak stormwater runoff rate
using 100 percent retention.
The percentage of site area or amount of storage required to maintain the
predevelopment peak runoff rate is based on Chan Series B: Percentage of Site Area
Required to Maintain Predevelopment Peak Runoff Rate Using 100% Retention (Appendix
B). This chart is based on the relationship between storage volume, Vs / , and discharge,
/Vr
ฐ/n , to maintain the predevelopment peak runoff rate.
/&i
Where: Vs = volume of storage to maintain the predevelopment peak runoff rate using
100% retention;
Vr = post development peak runoff volume;
Qo = peak outflow discharge rate; and
Qi = peak inflow discharge rate.
The relationship for retention storage to control the peak runoff rate is similar to the
relationship for detention storage. Figure 4.3 is an illustration of the comparison of the
storage volume/discharge relationship for retention and detention. Curve A is the
relationship of storage volume to discharge to maintain the predevelopment peak runoff
rate using the detention relationship from Figure 6-1 of the TR-55 manual (SCS, 1986) for
a Type II 24-hour storm event. Curve B is the ratio of storage volume to discharge to
maintain the predevelopment peak runoff rate using 100 percent retention. Note that the
volume required to maintain the peak runoff rate using detention is less than the
requirement for retention. This is graphically demonstrated in Figure 4.4.
0,8
0.7
3 0.6
I
1 0.5
>- 0.3
0.1
0.2
0.4 0.5 0.6
Q, (Peak Outflow Discharge Rate)
Q, (Peak Inflow Discharge Rate)
Figure 4.3. Comparison of Retention of Storage Volumes Required to
Maintain Peak Runoff Rate Using Retention and Detention.
-------
Prince George's County, Maryland
LI DHydrologic Analysis 31
Q
Q peak for existing condition
Postdevelopment condition
with detention pond
Postdevelopment condition
with combination of detention
'\ storage and retention storage
Figure 4.4. Storage Volume Required to Maintain Peak Runoff Rate
For hydrograph 2, refer to Figure 3.2 for description.
For hydrograph 8, refer to Figure 3.5 for description.
Vi is the storage volume required to maintain the predevelopment peak
discharge ratio using 100% detention storage. The combination of Vi and V2 is the
storage volume required to maintain the predevelopment peak discharge rate
using 100% retention storage.
The following calculations apply to Design Chart Series B:
The Tc for the post-development condition is equal to the Tc for the
predevelopment condition. This equality can be achieved by techniques such as
maintaining sheet flow lengths, increasing surface roughness, decreasing the
amount and size of storm drain pipes, and decreasing open channel slopes.
Section 3.2 provides more details on these techniques.
IMPs are to be distributed evenly across the development cite.
If the Tc is equal for the predevelopment and post-development conditions, the peak
runoff rate is independent of Tc for retention and detention practices. The difference in
volume required to maintain the predevelopment peak runoff rate is practically the same
if the Tcs for the predevelopment and post-development conditions are the same. These
concepts are illustrated in Figure 4.5. In Figure 4.5, the difference in the required IMP
area between a Tc of 0.5 and a Tc of 2.0 is minimal if the predevelopment and post-
development Tcs are maintained.
-------
32 LID Hydrologic Analysis
Prince George's County, PER
I
CO
a
M
I
M
tn
I
I
70
60-
50-
40-
30-
20-
10
Tc = 0.5 hr.
Tc = 2.0 hr.
Tc = 0.5 hr.
Tc = 2.0hr
100% Retention
100% Detention
Rainfall = 7 inches
Depth of IMP = 6 inches
55
I
60
I
65
70
75
I
80
I
85
I
90
I
95
M M'iiilr: (III! Jii.!",!
Postdevelopment Curve Number
Figure 4.5. Comparison of Storage Volumes for Various Tcs.
Step 4: Determine whether additional detention storage is required to maintain the
predevelopment peak runoff rate.
The storage volume required to maintain the predevelopment runoff volume using
retention, as calculated in Step 1, might or might not be adequate to maintain both the
predevelopment volume and peak runoff rate. As the CNs diverge, the storage
requirement to maintain the volume is much greater than the storage volume required to
maintain the peak runoff rate. As the CNs converge, however, the storage required to
maintain the peak runoff rate is greater than that required to maintain the volume.
Additional detention storage will be required if the storage volume required to maintain
the runoff volume (determined in Step 1) is less than the storage volume required to
maintain the predevelopment peak runoff rate using 100 percent retention (determined in
Step 3).
The combination of retention and detention practices is defined as a hybrid approach.
The procedure for determining the storage volume required for the hybrid approach is
described in Step 5.
Table 4.2 illustrates the percentage of site area required for volume and peak control
for representative curve numbers. Using a 5-inch type II 24-hour storm event and 6"
design depth, with a predevelopment CN of 60, the following relationships exist:
For a post-development CN of 65, 5.9 percent of the site area (column 4) is
required for retention practices to maintain the predevelopment volume. To
maintain the predevelopment peak runoff rate (column 5), 9.5 percent of the site
is required. Therefore, additional detention storage or a hybrid approach
(calculated in column 7) is required.
-------
Prince George's County, Maryland
LID Hydrologic Analysis 33
Table 4.2. Representative Percentages of Site Required for Volume and Peak Control
Type of
24-Hour
Storm
Event
(1)
3"
5"
7"
Runoff Curve No.
Existing
(2)
50
60
70
75
50
60
70
75
50
60
70
75
Proposed
(3)
55
60
65
70
80
65
70
75
90
75
80
85
90
80
85
90
55
60
65
70
80
65
70
75
90
75
80
85
90
80
85
90
55
60
65
70
80
65
70
75
90
75
80
85
90
80
85
90
% of Area Needed
for IMP
Volume Control
Using 100%
Retention
Chart Series A
(4)
1.7
4.0
6.9
10.4
19.3
2.9
6.3
10.5
27.5
4.1
8.9
14.6
21.2
4.8
10.5
17.1
4.8
10.1
16.0
22.4
36.7
5.9
12.3
19.1
42.9
6.9
14.3
22.2
30.7
7.4
15.3
23.8
7.6
15.6
23.9
32.5
50.5
8.3
16.9
25.8
53.7
8.9
17.9
27.2
36.7
9.1
18.4
27.9
Peak" Control
Using 100%
Retention
Chart Series B
(5)
1.6
3.4
6.2
9.3
18.0
3.9
6.7
10.0
24.9
5.9
9.7
13.9
18.7
7.5
11.8
16.6
6.9
11.1
15.6
20.6
32.8
9.5
14.6
19.8
37.2
13.2
18.9
24.5
30.5
15.0
20.6
26.7
12.3
18.6
25.0
31.4
44.5
16.6
23.2
29.9
49.7
20.4
26.8
33.4
42.3
22.1
28.6
35.3
Peak Control
Using 100%
Detention
Chart Series C
(6)
0.9
2.4
4.5
7.3
15.8
2.3
4.4
7.1
18.7
3.4
5.8
8.8
12.6
4.2
7.0
10.2
4.0
6.9
10.4
14.5
23.9
5.3
8.4
12.0
25.3
7.2
10.7
14.3
18.2
8.1
11.6
15.2
6.8
10.7
15.1
19.6
30.0
9.0
13.2
17.3
30.7
10.9
14.7
18.9
23.0
11.5
15.6
19.8
Hybrid
Design
(Eq. 4.6)
(7)
1.7
4.0
6.9
10.4
19.3
3.6
6.6
10.5
27.5
5.3
9.-5
14.6
21.2
6.6
11.4
17.1
6.3
10.9
16.0
22.4
36.7
8.3
13.9
19.6
42.9
10.9
17.4
- 23.8
30.7
12.3
18.9
25.7
10.7
17.7
24.7
32.5
50.5
13.6
21.2
28.7
53.7
16.1
23.8
31.5
39.2
17.1
25.1
32.9
Percent of
Volume
Retention
for Hybrid
Design
(Eq. 4.5)
(8)
100
100
100
100
100
80
96
100
100
77
94
100
100
73
91
100
77
93
100
100
100
71
88
97
100
63
82
93
100
60
81
92
71
88
97
100
100
61
80
90
100
55
75
87
94
53
73
85
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34 LID Hydrologic Analysis
Prince George's County, PER
For a post-development CN of 90, 42.9 percent of the site area (column 4) is
required for retention practices to maintain the predevelopment volume. To
maintain the predevelopment peak runoff rate (column 5) 37.2 percent of the site
is required. Therefore, the storage required to maintain the runoff volume is also
adequate to maintain the peak runoff rate. However, 42.9 percent of the site for
IMPs is not a practical and reasonable use of the site. Refer to Step 7, hybrid
approach, for a more reasonable combination of retention and detention storage.
Step 5: Determine storage required to maintain predevelopment peak runoff rate
using 100 percent detention. (This step is required if additional detention storage is
needed.)
Chart Series C: Storage Volume Required to Maintain the Predevelopment Peak
Runoff Rate Using 100% Detention is used to determine the amount of site area to
maintain the peak runoff rate only. This information is needed to determine the amount of
detention storage required for hybrid design, or where site limitations prevent the use of
retention storage to maintain runoff volume. This includes sites that have severely limited
soils for infiltration or retention practices. The procedure to determine the site area is the
same as that of Step 3. Using Chart Series C, the following assumptions apply:
The Tc for the post-development condition is equal to the Tc for the
predevelopment condition.
The storage volume, expressed as a depth hi hundredths of an inch (over the
development site), is for peak flow control.
These charts are based on the relationship and calculations from Figure 6.1
(Approximate Detention Basin Routing for Rainfall Types I, IA, n and IE) in TR-55
(SCS, 1986).
Step 6: Use hybrid facility design (required for additional detention storage).
When the percentage of site area for peak control exceeds that for volume control as
determined in Step 3, a hybrid approach must be used. For example, a dry swale
(infiltration and retention) may incorporate additional detention storage. Equation 2.5 is
used to determine the ratio of retention to total storage. Equation 2.6 is then used to
determine the additional amount of site area, above the percentage of site required for
volume control, needed to maintain the predevelopment peak runoff rate.
50
VD100)
x(-VI,ioo+V2Dioo+4x (VMOO - VDIOO) x \/R
Eq. 4.5
where
VR = Storage Volume required to maintain predevelopment runoff
volume (Chart Series A)
VRloo = Storage Volume required to maintain predevelopment peak runoff
rate using 100% retention (Chart Series B)
VDloo = Storage Volume required to maintain predevelopment peak runoff
rate using 100% detention (Chart Series C)
x = Area ratio of retention storage to total storage
-------
Prince George's County, Maryland
LID Hydrologic Analysis 35
and the hybrid storage can be determined as:
H = VR x (lOO^x) Eq. 4.6
Equations 4.5 and 4.6 are based on the following assumptions:
x% of the total storage volume is the retention storage required to maintain the
predevelopment CN calculated from Chart Series A: Storage Volume Required
to Maintain Predevelopment Volume using Retention Storage.
There is a linear relationship between the storage volume required to maintain
the peak predevelopment runoff rate using 100% retention and 100% detention
(Chart Series B and C)
The procedure for calculating hybrid facilities size is shown in Example 4.4.
Example 4.4: Calculation of Additional Storage Above Volume Required to Maintain CN and
Maintain Predevelopment Peak Runoff Rate Using Hybrid Approach
Given:
5-inch Storm Event with Rainfall Distribution Type II
Existing CN = 60
Proposed CN = 65
Storage volume required to maintain volume (CN) using retention storage = 0.35" (from Chart
Series A)
Storage volume required to maintain peak runoff rate using 100% retention = 0.62" (from Chart
Series B)
Storage volume required to maintain peak runoff rate using 100% detention = 0.31" (from Chart
Series C)
Step 1: Solve for x (ratio of retention to total storage) using Equation 4.5:
50
X =
(62-3i)
'31
Therefore: 0.35" of storage needed for runoff volume control is 68% of the total volume needed
to maintain both the predevelopment volume and peak runoff rates.
Step 2: Solve for the total area to maintain both the peak runoff rate and volume using Equation 4.6.
100
H =.35 x
68
H = 0.5 1"
Therefore , the difference between 0.35" and 0.51" is the additional detention area needed to
maintain peak discharge.
-------
36 LID Hydrologic Analysis
Prince George's County, PER
Step 7: Determine hybrid amount of IMP site area required to maintain peak runoff
rate with partial volume attenuation using hybrid design (required when retention
area is limited).
Site conditions, such as high percentage of site needed for retention storage, poor soil
infiltration rates, or physical constraints, can limit the amount of site area that can be used
for retention practices. For poor soil infiltration rates, bioretention is still an acceptable
alternative, but an underdrain system must be installed. In this case, the bioretention basin
is considered detention storage.
When this occurs, the site area available for retention IMPs is less than that required to
maintain the runoff volume, or CN. A variation of the hybrid approach is used to maintain
the peak runoff rate while attenuating as much of the increased runoff volume as possible.
First, the appropriate storage volume that is available for runoff volume control (VR') is
determined by the designer by analyzing the site constraints. Equation 4.7 is used to
determine the ratio of retention to total storage. Equation 4.8 is then used to determine the
total site IMP area in which the storage volume available for retention practices (VR')
substitutes the storage volume required to maintain the runoff volume.
50
(v*ioo - VDIOO)
V2D1oo+4x(Vfiloo-VZ)100)xVR'
. Eq. 4.7
Where VR' = storage volume acceptable for retention IMPs. The total storage with limited
retention storage is:
H' = VR'x(100*xO Eq. 4.i
where H' is hybrid area with a limited storage volume available for retention IMPs.
Example 4.5 illustrates this approach.
4.6 DETERMINATION OF DESIGN STORM EVENT
Conventional stormwater management runoff quantity control is generally based on
not exceeding the predevelopment peak runoff rate for the 2-year and 10-year 24-hour
Type II storm events. The amount of rainfall used to determine the runoff for the site is
derived from Technical Paper 40 (Department of Commerce, 1963). For Prince George's
County, these amounts are 3.3 and 5.3 inches, respectively. The 2-year storm event was
selected to protect receiving channels from sedimentation and erosion. The 10-year event
was selected for adequate flow conveyance considerations. In situations where there is
potential for flooding, the 100-year event is used.
The criteria used to select the design storm for low-impact development are based on
the goal of maintaining the predevelopment hydrologic conditions for the site. The
determination of the design storm begins with an evaluation of the predevelopment
condition. The hydrologic approach of low-impact development is to retain the same
-------
Prince George's County, Maryland
LID Hydrologic Analysis 37
Example 4.5: Calculation of Percentage of Site Area Required to Maintain the Peak Runoff Rate
Using the Hybrid Approach of Retention and Detention
Given:
5-inch storm event with rainfall distribution Type n
Existing CN = 60
Proposed CN = 65
Storage volume required to maintain volume (CN) = 0.35" (From Chart Series A)
Storage volume required to maintain peak runoff rate using 100% retention = 0.62" (from Chart
Series B)
Storage volume required to maintain peak runoff rate using 100% detention = 0.31" (from Chart
Series C)
Only half of the required site area is suitable for retention practices, remainder must incorporate
detention. (VR' =0.35x0.50 = 0.18")
Step 1: Determine appropriate amount of overall IMP area suitable for retention practices.
Half of area is appropriate (given above). Use Equation 2.7:
. 50 / / : \
X = (62_31)xr-31 + V-31 +4 x (-62 -.31) x .18)
ฃ' = 41.2%
Therefore, 0.18" of storage available for runoff volume control is 41.2% of the total volume needed
for maintaining the predevelopment peak runoff rate.
Step 2: Solve for the total area required to maintain the peak runoff rate using Equation 4.8.
Solve for H'
100
#' = 0.18 x
41.2
Therefore, totally 0.43" of the site is required to maintain the predevelopment peak runoff rate but
not the runoff volume. Of the 0.43" storage, 0.18" of the storage is required for retention volume.
amount of rainfall within the development site as that retained by woods, in good
condition, and then to gradually release the excess runoff as woodlands would release it.
By doing so, we can emulate, to the greatest extent practical, the predevelopment
hydrologic regime to protect watershed and natural habitats. Therefore, the
predevelopment condition of the low-impact development site is required to be woods in
good condition. This requirement is identical to the State of Maryland's definition of the
predevelopment condition. The CN for the predevelopment condition is to be determined
based on the land cover being woods in good condition and the existing HSG. The design
storm is to be the greater of the rainfall at which direct runoff begins from a woods in
good condition, with a modifying factor, or the 1-year 24-hour storm event. The rainfall at
which direct runoff begins is determined using Equation 4.9. The initial rainfall amount at
which direct runoff begins from a woodland is modified by multiplying this amount by a
-------
38 LID Hydrologic Analysis
Prince George's County, PER
factor of 1.5 account for the slower runoff release rate under the wooded predevelopment
condition.
lo Eq.4.9
.
CNC
where P is rainfall at which direct runoff begins.
A three-step process, illustrated in Example 4.6, is used to determine the design storm
event.
Step 1: Determine the predevelopment CN.
Use an existing land cover of woods in good condition overlaid over the hydrologic
soils group (HSG) to determine the composite site CN.
Step 2: Determine the amount of rainfall needed to initiate direct runoff.
Use Equation 4.9 to determine the amount of rainfall (P) needed to initiate direct
runoff.
Step 3: Account for variation in land cover.
Multiply the amount of rainfall (P) determined in Step 2 by a factor of 1.5.
Example 4.6 demonstrates this approach.
Example 4.6: Determination of Design Storm
Step 1 : Determine the predevelopment CN based on woods (good condition) and HSG.
Given: Site condition of 90% HSG soil type B and 10% HSG soil type C,
CNC = 0.9 (55) + 0.1 x (70)
CN0>56.5 = 57 use 57
Step 2: Determine the amount of rainfall to initiate direct runoff using Equation 4.9.
57
P = 1 .5 inches
Step 3: Multiply the amount of rainfall by a factor of 1 .5.
Design rainfall = P x 1 .5
Design rainfall = 1 .5 inches x 1 .5
Design rainfall = 2.25 inches
-------
Prince George's County, Maryland
LID Hydrologic Analysis 39
References
American Society of Civil Engineers (ASCE). 1994. Design and Construction of
Urban Stormwater Management Systems. ASCE Manuals and Reports of Engineering
Practice, No.77. Prepared by the Urban Water Resources Research Council of the
American Society of Civil Engineers and the Water Environment Federation, Reston, VA.
Cairns, J. 1993. Ecological Restoration: Replenishing our National Global Ecological
Capital. Nature Conservation 3: Reconstruction of Fragmented Ecosystems: Ed. By D.A.
Saunders, R.J. Hobbs, and P.E. Eherlich. Surrey Beatty & Sons.
Chow, V.T. 1964. Handbook of Applied Hydrology. McGraw-Hill, Inc., New York.
Department of Commerce. 1963. Rainfall Frequency Atlas of the United States for
Durations from 30 minutes to 24 hours and Return Periods from 1 to 100 Years. Technical
Paper 40. U.S. Department of Commerce, Washington, D.C.
Federal Interagency Stream Restoration Working Group (FISRWG). 1998. Stream
Corridor Restoration: Principles, processes, and Practices. PB98-158348LUW.
Leopold , L.B. 1968. Hydrology for Urban Land Planning: A Guidebook on the
Hydrologic Effects of Land Use. U.S. Geological Survey Circular 554.
Leopold, L.B., M.G. Wolman, and J.P. Miller. 1964. Fluvial Processes in
Geomorphology. Dover Publications, Inc., Mineola, New York.
Maidment, D.R. 1993. Handbook of Hydrology. McGraw-Hill, Inc. New York.
Maryland Department of the Environment (MDE). 1998. Maryland Stormwater
Prince George's County, Maryland. 1997. Low-Impact Development Design Manual,
Department of Environmental Resources, Prince George's County, Maryland.
SCS. 1986. Urban Hydrology for Small Watersheds. Technical Release 55, US
Department of Agriculture, Soil Conservation Service, Engineering Division, Washington,
DC.
SCS. 1985. National Engineering Handbook. Section 4, Hydrology (NEH-4). Soil
Conservation Service, US Department of Agriculture, Washington, DC.
-------
-------
APPENDICES
-------
-------
LID Hydrologic Analysis
Prince George's County, DER
APPENDIX A
STORAGE VOLUME REQUIRED TO MAINTAIN
THE PRE-DEVELOPMENT
RUNOFF VOLUME USING RETENTION STORAGE
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-------
Prince George's County, Maryland
LID Hydrologic Analysis
APPENDIX B
STORAGE VOLUME REQUIRED TO MAINTAIN
THE PRE-DEVELOPMENT
PEAK RUNOFF RATE USING 100% RETENTION STORAGE
TYPE 1 24-HOUR STORM CHART SERIES
TYPE 2 24-HOUR STORM CHART SERIES
TYPE la 24-HOUR STORM CHART SERIES
TYPE 3 24-HOUR STORM CHART SERIES
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