Prepared by:

Prince Georges

County, Maryland

Department of

Environmental

Resources

Programs and

Planning Division

July 1999

Low-Impact Development Hydrologic Analysis Low-Impact Development Hydrologic Analysis
	




	 

	




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
ii LID Hydrologic Analysis Prince George’s County, DER




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 IMPs 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





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
Prince George’s County, Maryland LID Hydrologic Analysis 1

LOW-IMPACT DEVELOPMENT HYDROLOGIC ANALYSIS

	




1.1 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.
2 LID Hydrologic Analysis Prince George’s County, DER

	
	
 !""!#$$
Prince George’s County, Maryland LID Hydrologic Analysis 3

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.
4 LID Hydrologic Analysis Prince George’s County, DER

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.

	
%
"&&'(!#)"$ $*&$+*,-.	//01

Groundwater Recharge. A considerable percentage of the rainfall abstraction

infiltrates into the soil and contributes to the groundwater. Groundwater may be part of a

local, intermediate, or regional water table, as illustrated in Figure 1.3. The local water

table is often connected to nearby streams, providing seepage to streams during dry

periods and maintaining base flow essential to the biological and habitat integrity of

streams. A significant reduction or loss of groundwater recharge can lead to a lowering of

the water table and a reduction of base flow in receiving streams during extended dry

weather periods. Headwater streams, with small contributing drainage areas, are

especially sensitive to localized changes in groundwater recharge and base flow.
Prince George’s County, Maryland LID Hydrologic Analysis 5

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.

Transpiratio n

Evapora tio n

Effluent

stream

Soil m oistu re m oving

dow n after a rain

Zon e of

aeratio n

Influent

stream

W ater table

Conf

Zo ne of saturation (grou ndw ater)

ining la yer

Capillary fringe

W e ll

Transpiration

	
2
-""!."."!*"

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 hydrologically 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,
6 LID Hydrologic Analysis Prince George’s County, DER

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 runoff’s

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

	
3
#!!"$* !"

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 hydrologic 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
Prince George’s County, Maryland LID Hydrologic Analysis 7

• 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
8 LID Hydrologic Analysis Prince George’s County, DER

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

	
4
"&!!5"#$(""!6	/07	/04

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
Prince George’s County, Maryland LID Hydrologic Analysis 9

(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. Lowimpact 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.

	
8
$"&)#!$"$&" ""!"

9$


• 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.
10 LID Hydrologic Analysis Prince George’s County, DER

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.
Prince George’s County, Maryland LID Hydrologic Analysis 11

%
:

-:**
9
*

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 1⁄2 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.
12 LID Hydrologic Analysis Prince George’s County, DER

• 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.

(!%
	
 !")"5$"#!

$"""!#$$""$

Low-Impact Development Technique

Low-Impact Hydrologic

Design and Analysis

Components

Flatten slopeIncrease flow pathIncrease sheet flowIncrease roughnessMinimize disturbanceFlatten slopes on swalesInfiltration swalesVegetative filter stripsConstricted PipesDisconnected imperviousareasReduce curb and gutterRain barrelsRooftop storageBioretentionRevegetationVegetation preservation

Lower Postdevelopment

CN

✔ ✔✔ ✔✔ ✔✔✔

Increase Tc ✔✔ ✔✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔

Retention ✔✔ ✔✔✔✔✔

Detention ✔ ✔ ✔✔
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 IMPs 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.
14 LID Hydrologic Analysis Prince George’s County, DER

2
	
$"&" $;"" ""!"$

(!2
	
$"&" ""!"" $

Conventional Land Covers

(TR-55 Assumptions) LID Land Covers

20% impervious

80% grass

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:
Prince George’s County, Maryland LID Hydrologic Analysis 15

(!2
%
 !"!""")"5$

)$ !" !"

Suggested Options

Affecting Curve

Number

Limit use of sidewalksReduce road length andwidthReduce driveway lengthand widthConserve naturalresources areasMinimize disturbancePreserve infiltratable soilsPreserve naturaldepression areasUse transition zonesUse vegetated swalesPreserve Vegetation

Land Cover Type ✔ ✔ ✔✔✔

Percent of

Imperviousness

✔✔ ✔ ✔

Hydrologic Soils Group ✔ ✔

Hydrologic Condition ✔ ✔✔

Disconnectivity of

Impervious Area

✔✔ ✔

Storage and Infiltration ✔ ✔

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.
16 LID Hydrologic Analysis Prince George’s County, DER

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 lowimpact techniques to reduce impervious area and increase storage volume. There is

a reduction in both post development peak rate and volume.

2
%
&&& !"")$ !"#)

)*9$
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.
18 LID Hydrologic Analysis Prince George’s County, DER

3

4

2
2
 !"#))$"

9""$),)*9$

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 all 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
Prince George’s County, Maryland LID Hydrologic Analysis 19

(!2
2
 !")"5$9"")

 !"&"""

Low Impact Development Technique

Low-Impact

Development

Objective

On-lot bioretentionWider and flatter swalesMaintain sheet flowClusters of trees and shrubsin flow pathProvide tree conservation/transition zonesMinimize storm drain pipesDisconnect impervious areasSave treesPreserve existing topographyLID drainage and infiltrationzones

Minimize disturbance ✔ ✔✔✔✔✔✔✔

Flatten grades ✔✔ ✔ ✔✔

Reduce height of

slopes

✔ ✔✔

Increase flow path

(divert and redirect)

✔✔✔ ✔✔✔

Increase roughness “n” ✔ ✔✔✔✔✔✔ ✔

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.
20 LID Hydrologic Analysis Prince George’s County, DER

2
3
""*59""< !"

"&&

In Figure 3.4., Hydrograph 5 represents the IMP inflow hydrograph for the postdevelopment condition for a site using low-impact development IMPs. Because of the

IMP retention storage, runoff is not released until the maximum retention storage volume

is exceeded. Line A represents the limit of retention storage. Hydrograph 6 is the outflow

hydrograph from the low-impact development retention IMP. The release begins at the

limit of retention storage, represented by line A. The storage maintains the

predevelopment volume and controls the peak runoff rate. For this situation, the falling

limb of the hydrograph represents a condition where the inflow (hydrograph 5) equals the

outflow (hydrograph 6).

3.4 POTENTIAL REQUIREMENT FOR ADDITIONAL DETENTION STORAGE

Even though the post-development Tc and CN are maintained at the predevelopment

level, in some cases additional detention storage is needed to maintain the predevelopment

peak runoff rate due to the spatial distribution of the retention storage provided (i.e.,

storage areas are not uniformly distributed throughout the site).

The amount of storage that maintains the predevelopment runoff volume might not be

sufficient to also maintain the predevelopment peak runoff rate. Therefore, additional onlot storage is required in the form of detention storage. Low-impact development

stormwater management techniques for providing detention storage include, but are not

limited to the following:

• Swales with check dams, restricted drainage pipe, and inlet entrances

• Wider swales

• Rain barrels

• Rooftop storage

• Diversion structures
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.

7

8

1

2
4
&&&"!""*"""$
22 LID Hydrologic Analysis Prince George’s County, DER

3

**
9



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 preand 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.
Prince George’s County, Maryland LID Hydrologic Analysis 23

TABLE 4.1. REPRESENTATIVE LID CURVE NUMBERS

Land Use/Cover Curve Number for Hydrologic Soils

Groups1

 A B C D

Impervious Area 98 98 98 98

Grass 39 61 74 80

Woods (fair condition) 36 60 73 79

Woods (good condition) 30 55 70 77

1

Figure 2.2a, TR-55 (SCS, 1986).

j

j j

c A A A

CN A CN A CN A

CN

+ +

+ + = ......

1 2

1 1 2 2 Eq. 4.1

Where:

CNc = composite curve number;

Aj = area of each land cover; and

CNj = curve number for each land cover.

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 postdevelopment 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.
24 LID Hydrologic Analysis Prince George’s County, DER

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.

Land Use

HSG

(1)

CN

(2)

% of

Site

(3)

Land

Coverage

(ft2)

(4)

Impervious (Directly Connected) B 98 5 2,178

Impervious (Unconnected) B 98 10 4,356

Open Space (Good Condition, Graded) B 61 60 26,136

Woods (Fair Condition) B 55 25 10,890

Step 2: Calculate composite custom CN (using Equation 4.1).

43,560

98× 4,356 + 98× 2,178 + 61× 26,136 + 55×10,890 CNc =

= 65 CNc

Step 3: Calculate low-impact development CN based on the connectivity of the site

imperviousness (using Equation 4.2).

CN p = 61 26 136 55 10 890 × +×

37 026

, ,

,

CN p = 59 2.

R = 10

15

R = 0 67 .

( ) CN ( ) R

P

CN CN p

imp

c p × − × − × 













 = + 98 1 0.5

100

( )( ) 98 59.2 1 0.5 0.67

100

15 59.2 × − × − × 

 



 CNc = +

= 63.1(use 63) CNc

LID custom CN of 63 is less than conventional CN of 68 (predevelopment CN is 55).
Prince George’s County, Maryland LID Hydrologic Analysis 25

(98 ) (1 0.5 ) 100CN R

P

CN CN p

imp

c p × − × − 













 = + 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
26 LID Hydrologic Analysis Prince George’s County, DER

3
	
=-);"$&*"&!!$("$


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) / 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).

These charts are based on the following general conditions:
Prince George’s County, Maryland LID Hydrologic Analysis 27

• 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 1⁄2-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 1⁄2-

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.
28 LID Hydrologic Analysis Prince George’s County, DER

3
%
"*'!5&9$9"" !""&&

'!"<"&&

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)/100

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 1⁄2 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, DER

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 Chart 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, ∀

∀

s

r

, and discharge,

i

o

Q

Q , to maintain the predevelopment peak runoff rate.

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.

3
2
$"&""&*'!$5

9""<"&&$""""""

Prince George’s County, Maryland LID Hydrologic Analysis 31

8

3
3
*'!59""<"&&

• For hydrograph 2, refer to Figure 3.2 for description.

• For hydrograph 8, refer to Figure 3.5 for description.

∀1 is the storage volume required to maintain the predevelopment peak

discharge ratio using 100% detention storage. The combination of ∀1 and ∀2 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 postdevelopment Tcs are maintained.
32 LID Hydrologic Analysis Prince George’s County, DER

3
4
$"&*'!$&'$$


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

(!3
%
$" "$&*5&'!"<"!

Runoff Curve No. % of Area Needed

for IMP

 

Type of

24-Hour

Storm

Event

(1)

Existing

(2)

Proposed

(3)

Volume Control

Using 100%

Retention

Chart Series A

(4)

Peak Control

Using 100%

Retention

Chart Series B

(5)

Peak Control

Using 100%

Detention

Chart Series C

(6)

Hybrid

Design

(Eq. 4.6)

(7)

Percent of

Volume

Retention

for Hybrid

Design

(Eq. 4.5)

(8)

50

55

60

65

70

80

1.7

4.0

6.9

10.4

19.3

1.6

3.4

6.2

9.3

18.0

0.9

2.4

4.5

7.3

15.8

1.7

4.0

6.9

10.4

19.3

100

100

100

100

100

60

65

70

75

90

2.9

6.3

10.5

27.5

3.9

6.7

10.0

24.9

2.3

4.4

7.1

18.7

3.6

6.6

10.5

27.5

80

96

100

100

70

75

80

85

90

4.1

8.9

14.6

21.2

5.9

9.7

13.9

18.7

3.4

5.8

8.8

12.6

5.3

9.5

14.6

21.2

77

94

100

100

3"

75

80

85

90

4.8

10.5

17.1

7.5

11.8

16.6

4.2

7.0

10.2

6.6

11.4

17.1

73

91

100

50

55

60

65

70

80

4.8

10.1

16.0

22.4

36.7

6.9

11.1

15.6

20.6

32.8

4.0

6.9

10.4

14.5

23.9

6.3

10.9

16.0

22.4

36.7

77

93

100

100

100

60

65

70

75

90

5.9

12.3

19.1

42.9

9.5

14.6

19.8

37.2

5.3

8.4

12.0

25.3

8.3

13.9

19.6

42.9

71

88

97

100

70

75

80

85

90

6.9

14.3

22.2

30.7

13.2

18.9

24.5

30.5

7.2

10.7

14.3

18.2

10.9

17.4

23.8

30.7

63

82

93

100

5"

75

80

85

90

7.4

15.3

23.8

15.0

20.6

26.7

8.1

11.6

15.2

12.3

18.9

25.7

60

81

92

50

55

60

65

70

80

7.6

15.6

23.9

32.5

50.5

12.3

18.6

25.0

31.4

44.5

6.8

10.7

15.1

19.6

30.0

10.7

17.7

24.7

32.5

50.5

71

88

97

100

100

60

65

70

75

90

8.3

16.9

25.8

53.7

16.6

23.2

29.9

49.7

9.0

13.2

17.3

30.7

13.6

21.2

28.7

53.7

61

80

90

100

70

75

80

85

90

8.9

17.9

27.2

36.7

20.4

26.8

33.4

42.3

10.9

14.7

18.9

23.0

16.1

23.8

31.5

39.2

55

75

87

94

7"

75

80

85

90

9.1

18.4

27.9

22.1

28.6

35.3

11.5

15.6

19.8

17.1

25.1

32.9

53

73

85
34 LID Hydrologic Analysis Prince George’s County, DER

• 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 in 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, II and III) 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.

( 4 ( ) ) ( )

50

100 100 100

2

100

100 100

x D D R D R

R D

× −∀ + ∀ + × ∀ − ∀ × ∀

∀ − ∀ = Eq. 4.5

where

∀R = Storage Volume required to maintain predevelopment runoff

volume (Chart Series A)

∀R100 = Storage Volume required to maintain predevelopment peak runoff

rate using 100% retention (Chart Series B)

∀D100 = 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 = ∀R x (100 ÷ 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:

( ) χ = ( ) ( ) − ×− + +× − ×

50

62 31

31 31 4 62 31 35 2

. . . . .. .

χ = 68

 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.

H = × .35

100

68

H = 0 51 . "

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, DER

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 (∀R′) 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 (∀R’)

substitutes the storage volume required to maintain the runoff volume.

( 4 ( ) ) ( )

50

100 100 100

2

100

100 100

X′= D D R D

R D

∀ R′ × −∀ + ∀ + × ∀ − ∀ ×

∀ − ∀ Eq. 4.7

Where ∀R′ = storage volume acceptable for retention IMPs. The total storage with limited

retention storage is:

H′ = ∀R′ x (100 ÷χ ′) Eq. 4.8

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 II

• 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. (∀R′ = 0.35 x 0.50 = 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:

( ) ′ = ( ) − χ ×− + + × − ×

50

62 31

31 31 4 62 31 18 2

. . . . (. . ) .

χ ′ = 412%.

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′

H′ = × 018

100

412 . .

H′ = 0 43 . "

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, DER

factor of 1.5 account for the slower runoff release rate under the wooded predevelopment

condition.















 = × −10 1000 0.2

CNc

P Eq. 4.9

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 × (70)

CNc ≥ 56.5 ≈ 57 use 57

Step 2: Determine the amount of rainfall to initiate direct runoff using Equation 4.9.





 



 = × −10

57

1000 P 0.2

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

&"$

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.
40 LID Hydrologic Analysis Prince George’s County, DER

APPENDIX A

STORAGE VOLUME REQUIRED TO MAINTAIN

THE PRE-DEVELOPMENT

RUNOFF VOLUME USING RETENTION STORAGE
Prince George’s County, Maryland LID Hydrologic Analysis 41

APPENDIX B

STORAGE VOLUME REQUIRED TO MAINTAIN

THE PRE-DEVELOPMENT

PEAK RUNOFF RATE USING 100% RETENTION STORAGE
42 LID Hydrologic Analysis Prince George’s County, DER

APPENDIX C

STORAGE VOLUME REQUIRED TO MAINTAIN

THE PRE-DEVELOPMENT

PEAK RUNOFF RATE USING 100% DETENTION STORAGE