United States 

Environmental 

Protection Agency

Office of Water (4503T) 

Washington, DC 20460

EPA 841-B-09-001 

December 2009 

www.epa.gov/owow/nps/lid/section438

Technical Guidance on Implementing the 

Stormwater Runoff Requirements for 

Federal Projects under Section 438 of the 

Energy Independence and Security Act 

Section 438 Technical Guidance December 2009 

Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal 

Projects under Section 438 of the Energy Independence and Security Act 

INTRODUCTION

In December 2007, Congress enacted the Energy Independence and Security Act of 2007. 

Section 438 of that legislation establishes strict stormwater runoff requirements for federal 

development and redevelopment projects. The provision reads as follows: 

“Storm water runoff requirements for federal development projects. The 

sponsor of any development or redevelopment project involving a Federal facility 

with a footprint that exceeds 5,000 square feet shall use site planning, design, 

construction, and maintenance strategies for the property to maintain or restore, to 

the maximum extent technically feasible, the predevelopment hydrology of the 

property with regard to the temperature, rate, volume, and duration of flow.” 

The intent of Section 438 of the Energy Independence and Security Act of 2007 (EISA) is to 

require federal agencies to develop and redevelop applicable facilities in a manner that maintains 

or restores stormwater runoff to the maximum extent technically feasible. Until recently, 

stormwater programs established to address water quality objectives have been designed to 

control traditional pollutants that are commonly associated with municipal and industrial 

discharges, e.g., nutrients, sediment, and metals. Increases in runoff volume and peak discharge 

rates have been regulated through state and local flood control programs. Although these 

programs have merit, knowledge accumulated during the past 20 years has led stormwater 

experts to the conclusion that conventional approaches to control runoff are not fully adequate to 

protect the nation’s water resources (National Research Council, 2008). 

Implementation of Section 438 of the EISA can be achieved through the use of the green 

infrastructure/low impact development (GI/LID) infrastructure tools described in this guidance. 

The intention of the statute is to maintain or restore the pre-development site hydrology during 

the development or redevelopment process. To be more specific, this requirement is intended to 

ensure that receiving waters are not negatively impacted by changes in runoff temperature, 

volumes, durations and rates resulting from federal projects. It should also be noted that a 

performance-based approach was selected in lieu of a prescriptive requirement in order to 

provide site designers maximum flexibility in selecting control practices appropriate for the site. 

Section 14 of the Executive Order 13514, Federal Leadership in Environmental, Energy, 

and Economic Performance

On October 5, 2009, President Barack Obama signed Executive Order 13514, 

“Federal Leadership in Environmental, Energy, and Economic Performance.” 

Section 14 of the Executive Order provides: 

Stormwater Guidance for Federal Facilities. Within 60 days 

of the date of this order, the Environmental Protection 

Agency, in coordination with other Federal agencies as 

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Section 438 Technical Guidance December 2009 

appropriate, shall issue guidance on the implementation of 

section 438 of the Energy Independence and Security Act of 

2007 (42 U.S.C. 17094). 

This provision contains two significant elements. First, for the first time, EPA is formally 

assigned the responsibility to write and issue the Section 438 guidance, in coordination with 

other federal agencies. Second, it establishes a deadline for EPA to do so by December 5, 2009. 

Purpose and Organization of this Guidance 

The purpose of this document is to provide technical guidance and background information to 

assist federal agencies in implementing EISA Section 438. Each agency or department is 

responsible for ensuring compliance with EISA Section 438. The document contains guidance on 

how compliance with Section 438 can be achieved, measured and evaluated. In addition, 

information detailing the rationale for the stormwater management approach contained herein 

has been included. 

This document is intended solely as guidance. This document is not a regulation nor does it 

substitute for statutory provisions or regulations. This guidance does not impose any legally 

binding requirements on federal agencies and does not confer any legal rights or impose legal 

obligations upon any member of the public. This document does not create a cause of action 

against the EPA, other federal agencies, or the United States. 

The following information is presented within this document: 

Part I: Implementation Framework 

A. Background 

B. Benefits and outcomes of the new stormwater performance requirements 

C. Applicability and definitions 

D. Tools to implement the requirements of Section 438 

E. Calculating the 95th percentile rainfall event 

Part II: Case Studies on Capturing the 95th Percentile Storm Using Onsite Management 

Practices 

Case studies representing typical federal installations have been included. The case studies were 

selected to demonstrate the feasibility of providing adequate stormwater control for a range of 

site conditions and building designs. To the maximum extent technically feasible, each case 

study includes a description of a method that can be used to determine the design objectives of 

the project based on retaining the 95th percentile storm. Examples of onsite technologies and 

practices have also been provided. The case studies are intended to provide examples of 

modeling procedures that can be used to quantify treatment system performance and processes 

for assessing sites and determining appropriate control techniques to the maximum extent 

technically feasible. 

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Section 438 Technical Guidance December 2009 

Part I: Implementation Framework 

A. BACKGROUND

This section contains background on the causes and consequences of stormwater discharges, 

solutions that can be used to address the causes and consequences of stormwater discharges and 

how to implement those solutions to comply with Section 438 of EISA. 

Alterations to Natural Hydrology and the Impact on Stormwater Runoff 

In the natural, undisturbed environment rain that falls is quickly absorbed by trees, other 

vegetation, and the ground. Most rainfall that is not intercepted by leaves infiltrates into the 

ground or is returned to the atmosphere by the process of evapotranspiration. Very little rainfall 

becomes stormwater runoff in permeable soil, and runoff generally only occurs with larger 

precipitation events. Traditional development practices cover large areas of the ground with 

impervious surfaces such as roads, driveways, sidewalks, and buildings. Under developed 

conditions runoff occurs even during small precipitation events that would normally be absorbed 

by the soil and vegetation. The collective force of the increased runoff scours streambeds, erodes 

stream banks, and causes large quantities of sediment and other entrained pollutants to enter the 

water body each time it rains (Shaver, et al., 2007; Booth testimony, 2008). 

As watersheds are developed and impervious surfaces increase in area, the hydrology of the 

watersheds fundamentally changes over time which results in degraded aquatic ecosystems. In 

recognition of these problems, stormwater managers employed extended detention approaches to 

mitigate the impacts of increased peak runoff rates. However, wet ponds and similar practices 

are not fully adequate to protect downstream hydrology because of the following inherent 

limitations of these conventional practices (National Research Council, 2008; Shaver, et al., 

2007): 

 Poor peak control for small, frequently-occurring storms; 

 Negligible volume reduction; and 

 Increased duration of peak flow. 

Detention storage targets relatively large, infrequent storms, such as the two and 10-year/24-hour 

storms for peak flow rate control. As a result of this design limitation, flow rates from smaller, 

frequently-occurring storms typically exceed those that existed onsite before land development 

occurred and these increases in runoff volumes and velocities typically result in flows erosive to 

stream channel stability (Shaver, et al., 2007). Section 438 is intended to address the 

inadequacies of the historical detention approach to managing stormwater and promote more 

sustainable practices that have been selected to maintain or restore predevelopment site 

hydrology. 

A 2008 National Research Council report on urban stormwater confirmed that current 

stormwater control efforts are not fully adequate. Three of the report’s findings on stormwater 

management approaches are particularly relevant (National Research Council, 2008). 

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Section 438 Technical Guidance December 2009 

1. Individual controls on stormwater discharges are inadequate as the sole solution to 

stormwater in urban watersheds; 

2. Stormwater control measures such as product substitution, better site design, 

downspout disconnection, conservation of natural areas, and watershed and land-use 

planning can dramatically reduce the volume of runoff and pollutant load from new 

development; and 

3. Stormwater control measures that harvest, infiltrate, and evapotranspire stormwater are 

critical to reducing the volume and pollutant loading of small storms. 

Pre-development Hydrology. Courtesy of C. May, 

University of Washington.

Post-Development Hydrology. Courtesy of C. 

May, University of Washington.

Figure 1. Pre-Development and Post-Development Hydrology. (USDA). 

Figure 1 contains two sets of diagrams depicting the water balances at undeveloped and 

developed sites. Runoff patterns will vary based on factors such as geographic location, local 

meteorological conditions, vegetative cover and soils. The first set of figures represents 

conditions in the Pacific Northwest where storms have a long duration and low intensity, i.e., the 

volume of rain in an individual storm is small. The second set of figures from the U.S. 

Department of Agriculture represents a more generalized set of conditions, but was included to 

illustrate that heavily urbanized areas typically cause large increases in runoff. 

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Section 438 Technical Guidance December 2009 

Land cover changes that result from site development include increased imperviousness, soil 

compaction, loss of vegetation, and loss of natural drainage patterns, which result in increased 

runoff volumes and peak runoff rates. The cumulative impacts of the land cover changes result 

in alterations of the natural hydrology of a site, which disrupts the natural water balance and 

changes water flow paths. The consequences of these impacts include: 

1. Increased volume of runoff. With decreased area for infiltration and evapotranspiration 

due to development, a greater amount of rainfall is converted to overland runoff which 

results in larger stormwater discharges. 

2. Increased peak flow of runoff. Increased impervious surface area and higher connectivity 

of impervious surfaces and stormwater conveyance systems increase the flow rate of 

stormwater discharges and increase the energy and velocity of discharges into the stream 

channel. 

3. Increased duration of discharge. Detention systems generate greater flow volumes and 

rates. These prolonged higher discharge rates can undermine the stability of the stream 

channel and induce erosion, channel incision and bank cutting. 

4. Increased pollutant loadings. Impervious areas are a collection site for pollutants. When 

rainfall occurs these pollutants are mobilized and transported directly to stormwater 

conveyances and receiving streams via these impervious surfaces. 

5. Increased temperature of runoff. Impervious surfaces absorb and store heat and transfer it 

to stormwater runoff. Higher runoff temperatures may have deleterious effects on 

receiving streams. Detention basins magnify this problem by trapping and discharging 

runoff that is heated by solar radiation (Galli, 1991; Schueler and Helfrich, 1988). 

The resulting increases in volume, peak flow, and duration are illustrated in the hydrograph in 

Figure 2, which is a representation of a site’s stormwater discharge with respect to time. The 

hydrograph illustrates the impacts of development on runoff volume and timing of the runoff. 

Individual points on the curve represent the rate of stormwater discharge at a given time. The 

graph illustrates that development and corresponding changes in land cover result in greater 

discharge rates, greater volumes, and shorter discharge periods. In a natural condition, runoff 

rates are slower than those on developed sites and the discharges occur over a longer time period. 

The predevelopment peak discharge rate is also much lower than the post-development peak 

discharge rate due to attenuation and absorption by soils and vegetation. In the postdevelopment condition there is generally a much shorter time before runoff begins because of 

increased impervious surface area, a higher degree of connectivity of these areas and the loss of 

soils and vegetative cover that slow or reduce runoff. 

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Section 438 Technical Guidance December 2009 

t

Q

Post-Development Condition

Pre-Development Condition

Figure 2. Post-Development Hydrograph. 

(Q = volumetric flow rate; t = time) 

Figure 3. Stream Displaying the Effects of Stormwater Runoff and Channel Downcutting. 

The Solution: Preserving and Restoring Hydrology 

A new approach has evolved in recent years to eliminate or reduce the amount of water and 

pollutants that run off a site and ultimately are discharged into adjacent water bodies. 

6 
Section 438 Technical Guidance December 2009 

The fundamental principle is to employ systems and practices that use or mimic natural 

processes to: 1) infiltrate and recharge, 2) evapotranspire, and/or 3) harvest and use precipitation 

near to where it falls to earth. 

GI/LID practices include a wide variety of practices that utilize these mechanisms. These 

practices can be used at the site, neighborhood and watershed/regional scales. In this document 

the focus is on site-level practices, which is most consistent with the terms used in Section 438: 

“project,” “facility,” and “property.” Although these performance requirements apply at the 

project site-level, flexibility exists to utilize nearby areas or areas directly adjacent to the facility 

to manage the runoff, i.e., evapotranspirate, infiltrate or harvest and use. Where justifiable, it 

also may be appropriate to evapotranspirate, infiltrate or harvest and use an equivalent or greater 

amount of runoff offsite as long as the runoff is discharged or used in the same receiving 

subwatershed or watershed. 

The purpose of EISA Section 438 is to replicate the pre-development hydrology to protect and 

preserve both the water resources onsite and those downstream. For example, if prior to 

development, twenty five (25) percent of the annual rainfall runs directly into the stream and the 

remainder infiltrates into the ground or is evapotranspired into the air, then the post-development 

goal should be to limit runoff to twenty five (25) percent of the annual precipitation while 

maintaining the correct aquifer recharge rate. This has the benefit, in most cases, of delivering 

water to the stream at approximately the same rate, volume, duration and temperature as the 

stream had naturally evolved to receive prior to development. The result will be to eliminate or 

minimize the erosion of streambeds and streambanks, significantly reduce the delivery of many 

pollutants to water bodies, and retain historical 

instream temperatures. 

Restoring or maintaining pre-development hydrology 

has emerged as a control approach for several 

reasons. Most importantly, this approach is intended 

to directly address the root cause of impairment. 

Current control approaches have been selected in an 

attempt to control the symptoms (peak flow, and 

excess pollutants), but this strategy is not fully 

adequate because of the scale of the problem, the 

cumulative impacts of multiple developments and the 

need to manage both site and watershed level impacts. 

With current approaches, it is also difficult to 

adequately protect and improve water quality because 

the measures employed are not addressing the main 

problem which is a hydrologic imbalance. 

Designing facilities based on the goal of maintaining 

or restoring pre-development hydrology provides a 

site specific basis and an objective methodology with 

which to determine appropriate practices to protect 

the receiving environment. 

Figure 4. Parking lot bioswale and 

permeable pavers in Chicago.

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Section 438 Technical Guidance December 2009 

Using pre-development hydrology as the guiding control principal also allows the designer to 

consider climatic and geologic variability and tailor the solutions to the project location. Thus 

the need for a one size fits all approach is rendered unnecessary since the design objective is 

dictated by the pre-development site conditions and other technicalities of the project site and 

facility. Instead of prescribed approaches dictating discharge volumes or flow rates, site 

assessments of historical infiltration and runoff rates will inform the designer and provide the 

basis for a suitable design. The use of this approach will minimize compliance complications that 

may arise from prescriptive design approaches which do not account for the variability of 

precipitation frequencies, rainfall intensities and pre-development land cover and soil conditions 

that influence infiltration and runoff. 

B. BENEFITS AND OUTCOMES OF THE NEW STORMWATER PERFORMANCE REQUIREMENTS

Implementation of these new stormwater performance requirements in EISA Section 438 

provides numerous environmental and economic benefits in addition to reducing the volume of 

stormwater runoff: 

Benefits to Water Resources: 

 Cleaner Water. The use of plants, soils and water 

harvesting and use practices can reduce stormwater 

runoff volumes and pollutant loadings and the 

frequency and magnitude of combined sewer 

overflows (volume and pollutant loading 

reductions). These practices are part of a larger set 

of practices called green infrastructure/low impact 

development. 

 Clean and Adequate Water Supplies. GI/LID 

approaches using soil based vegetated infiltration 

systems can be used to recharge ground water and 

maintain stream base flow. By recharging ground 

water aquifers, aquatic ecosystem health is 

maintained and base flows are increased which helps 

ensure more constant flows for drinking water 

withdrawals. Harvesting and reusing rainwater also 

reduces the need to use potable water for all uses 

and can reduce both the infrastructure and energy 

needed to treat and transport both drinking water and 

stormwater.

 Source Water Protection. GI/LID practices provide 

pollutant removal benefits, thereby providing some 

protection for both ground water and surface water sources of drinking water. In addition, 

GI/LID provides ground water recharge benefits. 

GI/LID approaches are a set of 

management approaches and 

technologies that utilize and/or 

mimic the natural hydrologic cycle 

processes of infiltration, 

evapotranspiration and use. GI/LID 

practices include green roofs, trees 

and tree boxes, rain gardens, 

vegetated swales, pocket wetlands, 

infiltration planters, porous and 

permeable pavements, vegetated 

median strips, reforestation and 

revegetation and protection of 

riparian buffers and floodplains. 

These practices can be used almost 

anywhere soil and vegetation can be 

worked into the urban or suburban 

landscape. They include 

decentralized harvesting approaches 

such as rain barrels and cisterns that 

can be used to capture and re-use 

rainfall for watering plants or 

flushing toilets. 

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Section 438 Technical Guidance December 2009 

Other Social and Environmental Benefits: 

 Cleaner Air. Trees and vegetation improve air quality by filtering many airborne pollutants 

and can help reduce the amount of respiratory illness (Vingarzan and Taylor, 2003). 

 Reduced Urban Temperatures. Summer city temperatures can average 10oF higher than 

nearby suburban temperatures (Casey Trees, 2007). High temperatures are also linked to 

higher ground level ozone concentrations. Vegetation creates shade, reduces the amount of 

heat absorbing materials and emits water vapor – all of which cool hot air (Grant, et al., 

2003). Reductions in impervious surface and the use of light colored pervious surfaces (e.g., 

permeable concrete) also can mitigate urban temperatures. 

 Moderate the Impacts of Climate Change. Climate change impacts and effects vary 

regionally, but GI/LID techniques can provide 

adaptation benefits for a wide array of 

circumstances. They can be used to conserve, 

harvest and use water, to recharge ground waters 

and to reduce surface water discharges that could 

contribute to flooding. In addition, there are 

mitigation benefits such as reduced energy 

demand and carbon sequestration by vegetation. 

 Increased Energy Efficiency. Green space helps 

lower ambient temperatures and, when 

incorporated on and around buildings, helps shade 

and insulate buildings from wide temperature 

swings, decreasing the energy needed for heating 

and cooling. Diverting stormwater from 

wastewater collection, conveyance and treatment 

systems can reduce the amount of energy needed 

to pump and treat the water. Energy efficiency not 

only reduces costs, but also reduces generation of 

greenhouse gases.

 Community Benefits. Trees and plants improve 

urban aesthetics and community livability by 

providing recreational and wildlife areas. Studies show that property values are higher when 

trees and other vegetation are present. Increased green space also has public health benefits 

and has been shown to reduce crime and the associated stresses of urban living. 

Figure 5. Rain water cistern.

C. APPLICABILITY AND DEFINITIONS

Applicability 

1. Who is a “Sponsor” of a project? 

Section 438 applies to the “sponsor of any development or redevelopment project involving a 

Federal facility . . .” Section 438 requires that the “sponsor . . . shall use . . . strategies for the 

property to maintain or restore . . . the predevelopment hydrology. . .” The “sponsor” should 

9 
Section 438 Technical Guidance December 2009 

generally be regarded as the federal department or agency that owns, operates, occupies or is the 

primary user of the facility and has initiated the development or redevelopment project. If the 

federal agency hires another entity to perform activities such as site construction or maintenance, 

the agency should nonetheless be regarded as the sponsor and be responsible to assure 

compliance with the requirements of Section 438. The agency sponsor is free to contract out 

various duties and responsibilities that are associated with achieving compliance. 

2. What is a “Federal facility”? 

Section 438 provides that its requirements apply to the “sponsor of any development or 

redevelopment project involving a Federal facility . . .” Section 401(8) of EISA states: “The 

term `Federal facility' means any building that is constructed, renovated, leased, or purchased in 

part or in whole for use by the Federal Government.”

3. What is a “footprint”? 

Section 438 applies to a federal facility “with a footprint that exceeds 5,000 square feet.” For the 

purposes of this guidance, any project involving a federal facility that disturbs 5,000 square feet 

or more of ground area is covered by this guidance. Existing facilities that have an overall 

footprint of 5,000 square feet or greater that disturb less than 5,000 square feet of land area as 

part of any single development or redevelopment project are not subject to Section 438 

requirements. Consistent with the purpose of Section 438 to preserve or restore pre-development 

hydrology, the term “footprint” includes all land areas that are disturbed as part of the project. 

4. What is “the property”? 

Section 438 provides that the project sponsor “shall use site planning, design, construction, and 

maintenance strategies for the property to maintain or restore, to the maximum extent 

technically feasible, the predevelopment hydrology of the property.” This clause has been 

interpreted to mean that the land surrounding the project site is available to implement the 

appropriate GI/LID practices where optimal. 

Although the performance requirements of EISA Section 438 apply only to the project footprint, 

the flexibility exists to utilize the entire federal property in implementing the stormwater 

strategies for the project. 

Definitions 

95th percentile rainfall event. The 95th percentile rainfall event represents a precipitation amount 

which 95 percent of all rainfall events for the period of record do not exceed. In more technical 

terms, the 95th percentile rainfall event is defined as the measured precipitation depth 

accumulated over a 24-hour period for the period of record that ranks as the 95th percentile 

rainfall depth based on the range of all daily event occurrences during this period. 

The 24-hour period is typically defined as 12:00:00 am to 11:59:59 pm. In general, at least a 20-

30 year period of rainfall record is recommended for such an analysis. This raw data is readily 

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Section 438 Technical Guidance December 2009 

available and collected by most airports across the county. Small rainfall events that are 0.1 of an 

inch or less are excluded from the percentile analysis because this rainfall generally does not 

result in any measureable runoff due to absorption, interception and evaporation by permeable, 

impermeable and vegetated surfaces. Many stormwater modelers and hydrologists typically 

exclude rainfall events that are 0.1 inch or less from calculations of rainfall events of any storm 

from their modeling analyses of rainfall event frequencies. See, for example, the Center for 

Watershed Protection's Urban Subwatershed Restoration Manual 3 (available at www.cwp.org). 

Federal facility. The term “federal facility” means any buildings that are constructed, renovated, 

leased, or purchased in part or in whole for use by the federal government as defined in section 

401(8) of the Energy Independence and Security Act. 

Development or re-development. For the purposes of this provision this term applies to any 

action that results in the alteration of the landscape during construction of buildings or other 

infrastructure such as parking lots, roads, etc, (e.g., grading, removal of vegetation, soil 

compaction, etc.) such that the changes affect runoff volumes, rates, temperature, and duration of 

flow. Examples of projects that would fall under “re-development” include structures or other 

infrastructure that are being reconstructed or replaced and the landscape is altered. Typical 

patching or resurfacing of parking lots or other travel areas would not fall under this requirement. 

D. TOOLS TO IMPLEMENT THE REQUIREMENTS OF SECTION 438

Section 438 of the Energy Independence and Security Act of 2007 reads as follows: 

Section 438. Storm water runoff requirements for federal development projects. 

The sponsor of any development or redevelopment project involving a Federal facility 

with a footprint that exceeds 5,000 square feet shall use site planning, design, 

construction, and maintenance strategies for the property to maintain or restore, to the 

maximum extent technically feasible, the predevelopment hydrology of the property with 

regard to the temperature, rate, volume, and duration of flow. 

The intention of EISA Section 438 is to preserve or restore the hydrology of the site during the 

development or redevelopment process. To be more specific, this requirement is intended to 

ensure that aquatic biota, stream channel stability, and historical aquifer recharge rates of 

receiving waters are not negatively impacted by changes in runoff temperature, volumes, 

durations and rates resulting from federal projects. A performance based approach was selected 

in lieu of a prescriptive requirement in order to provide site designers maximum flexibility in 

selecting control practices appropriate for the site. 

To meet these performance objectives, technically feasible stormwater control practices that are 

effective in reducing the volume of stormwater discharge should be used. To implement EISA 

Section 438, this guidance recommends that the federal facility use all known, available and 

reasonable methods of stormwater retention and/or use to the maximum extent technically 

feasible (METF). Tools to implement the requirements of Section 438 are described below and 

illustrated in Figure 8. 

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Section 438 Technical Guidance December 2009 

Establishing Section 438 Performance Design Objectives 

Described below are options site designers can use to comply with Section 438. There may be 

situations where Option 1 (retaining the 95th percentile rainfall event) is not protective enough to 

maintain or restore the predevelopment hydrology of the project (for example, in some 

headwater streams). In these cases, Option 2 (site-specific hydrologic analysis) could be used to 

determine the types of stormwater practices necessary to preserve predevelopment runoff 

conditions. Option 2 could also be used if predevelopment runoff conditions can be maintained 

by retaining less than the 95th percentile rainfall event. Because a performance based approach 

was selected in lieu of a prescriptive requirement in order to provide site designers maximum 

flexibility in selecting control practices appropriate for the site, Option 2 was provided in 

recognition that there are established methodologies that can be utilized to estimate the volume 

of infiltration and evapotranspiration based on site-specific hydrology and thus establish the 

predevelopment hydrology performance design objectives. 

Option 1: Retain the 95th Percentile Rainfall Event 

One approach to establishing the performance design objectives is to design, construct, and 

maintain stormwater management practices that manage rainfall onsite, and prevent the off-site 

discharge of the precipitation from all rainfall events less than or equal to the 95th percentile 

rainfall event to the maximum extent technically feasible (METF). This objective should be 

accomplished by the use of practices that infiltrate, evapotranspire and/or harvest and use 

rainwater. The 95th percentile rainfall event is the event whose precipitation total is greater than 

or equal to 95 percent of all storm events over a given period of record. For example, to 

determine what the 95th percentile storm event is in a specific location, all 24 hour storms that 

have recorded values over a 30 year period would be tabulated and a 95th percentile storm would 

be determined from this record, i.e., 5% of the storms would be greater than the number 

determined to be the 95th percentile storm. Thus the 95th percentile storm would be represented 

by a number such as 1.5 inches, and this would be the design storm (example 95th percentile 

storm events for selected cities are presented in Table 1). The designer would then select a 

system of practices, to the METF, that infiltrate, evapotranspire or harvest and use this volume 

multiplied by the total area of the facility/project footprint. Methods and data used to estimate 

the 95th percentile event are discussed in Part II of this document. 

For the purposes of this guidance, retaining all storms up to and including the 95th percentile 

storm event is analogous to maintaining or restoring the pre-development hydrology with respect 

to the volume, flow rate, duration and temperature of the runoff for most sites. This 95th

percentile approach was identified and recommended because this storm size represents the 

volume that appears to best represent the volume that is fully infiltrated in a natural condition 

and thus should be managed onsite to restore and maintain this pre-development hydrology for 

duration, rate and volume of stormwater flows. In general, only large storms generate significant 

runoff. In addition, this approach was identified because it employs natural treatment and flow 

attenuation methods that are presumed to have existed on the site before construction of 

infrastructure (e.g., building, roads, parking lots, driveways,) and is intended to infiltrate or 

evapotranspirate the full volume of the 95th percentile storm. Because this approach necessitates 

the use of practices that generally preclude extended detention, it will also typically address the 

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Section 438 Technical Guidance December 2009 

issue of maintaining predevelopment temperatures. However, in cases where there are discharges 

to cool water streams or other sensitive receiving waters, additional strategies may be needed to 

ensure that stormwater discharges do not result in greater thermal impacts than would occur in 

pre-development conditions (Schueler and Helfrich, 1988). 

Where technically feasible, the goal of Option 1 is that one hundred percent (100%) of the 

volume of water from storms less than or equal to the 95th percentile event over the footprint of 

the project should not be discharged to surface waters. In some cases, runoff can be harvested 

and used and ultimately may be discharged to surface waters or a sanitary treatment system; such 

direct or indirect discharges must be authorized or allowed by the regulatory authority. For 

example, if runoff is captured for nonpotable uses such as toilet flushing or other uses that are 

not irrigation related, these waters potentially could be discharged into the sanitary sewer system. 

Preferred mechanisms for retaining discharges from storms greater than the 95th percentile event 

are through overflow or diversion for the volume that exceeds the 95th percentile amount. 

Because standard underdrains typically discharge from smaller storms as well, underdrain 

designs, if employed, should ensure adequate retention capacity for the 95th percentile event 

volume. For structures such as roofs and paved surfaces that can increase the temperature of 

stormwater runoff, materials that minimize temperature increases (e.g., concrete vs. asphalt; 

vegetated roofs) should be considered and used as appropriate. 

Retaining 100 percent of all rainfall events equal to or less than the 95th percentile rainfall event 

was identified as Option 1 because small, frequently-occurring storms account for a large 

proportion of the annual precipitation volume, 

and the runoff from those storm events also 

significantly alters the discharge frequency, rate 

and temperature of the runoff. 

The runoff produced by these small storms and 

the initial portion of larger storms has a strong 

negative cumulative impact on receiving water 

hydrology and water quality. In areas that have 

been developed, runoff is generated from almost 

all storms, both small and large, due to the 

impervious surfaces associated with 

development and the loss of soils and vegetation. 

In contrast, natural or undeveloped areas 

discharge little or no runoff from small storms 

because the rain is absorbed by the landscape 

and vegetation. Studies have shown that 

increases in runoff event frequency, volume and 

rate can be diminished or eliminated through the 

use of GI/LID designs and practices, which infiltrate, evapotranspire and capture and use 

stormwater. 

Figure 6. Bioretention facility in Oregon.

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Section 438 Technical Guidance December 2009 

Option 1 was identified because it is a simplified approach to meet the intent of Section 438 in 

contrast to Option 2 which requires the designer to conduct a hydrologic analysis of the site 

based on site-specific conditions. 

Table 1. Example 95th Percentile Storm Events for Select U.S. Cities 

(adapted from Hirschman and Kosco, 2008). 

City 

95th Percentile 

Event Rainfall 

Total (in) City 

95th Percentile 

Event Rainfall 

Total (in) 

Atlanta, GA 1.8 Kansas City, MO 1.7 

Baltimore, MD 1.6 Knoxville, TN 1.5 

Boston, MA 1.5 Louisville, KY 1.5 

Buffalo, NY 1.1 Minneapolis, MN 1.4 

Burlington, VT 1.1 New York, NY 1.7 

Charleston, WV 1.2 Salt Lake City, UT 0.8 

Coeur D’Alene, ID 0.7 Phoenix, AZ 1.0 

Cincinnati, OH 1.5 Portland, OR 1.0 

Columbus, OH 1.3 Seattle, WA 1.6 

Concord, NH 1.3 Washington, DC 1.7 

Denver, CO 1.1 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percentile

Rainfall Depth (inches)

95%

Figure 7. Rainfall Frequency Spectrum showing the 95th percentile rainfall event for Portland, OR 

(~1.0 inches) 

14 
Section 438 Technical Guidance December 2009 

Calculating the 95th Percentile Rainfall Event 

Section E of this guidance contains information on how to calculate the 95th percentile rainfall 

event for a specific area. A long-term record of daily rainfall amounts (ideally, at least 30 years) 

is needed to calculate the 95th percentile rainfall. 

Designers opting to use Option 1 need to do the following: 

1) calculate or verify the precipitation amount from the 95th percentile storm event (this number 

would be typically expressed in inches, e.g., 1.5”, and 

2) employ onsite stormwater management controls to the METF that infiltrate, evapotranspire 

or harvest and use the appropriate design volume. 

The 95th percentile event can be calculated by using the following procedures below 

(summarized from Hirschman and Kosco, 2008, Managing Stormwater in Your Community: A 

Guide for Building an Effective Post-Construction Program, Center for Watershed Protection): 

 Obtain a long-term rainfall record from a nearby weather station (daily precipitation is 

fine, but try to obtain at least 30 years of daily record). Long-term rainfall records can be 

obtained from many sources, including NOAA at 

http://cdo.ncdc.noaa.gov/pls/plclimprod/poemain.accessrouter?datasetabbv=SOD&countr

yabbv=&georegionabbv=. 

 Remove data for small rainfall events that are 0.1 inch or less and snowfall events that do 

not immediately melt from the data set. These events should be deleted since they do not 

typically cause runoff and could potentially cause the analyses of the 95th percentile 

storm runoff volume to be inaccurate. 

 Using a spreadsheet or simple statistical package, sort the rainfall events from highest to 

lowest. In the next column, calculate the percentage of rainfall events that are less than 

each ranked event (event number/total number of events). For example, if there were 

1,000 rainfall events and the highest rainfall event was a 4” event, then 999 events (or a 

percentile of 999/1000, or 99.9%) are less than the 4” rainfall event. 

 Use the rainfall event at 95% as the 95th percentile storm event. 

Option 2: Site-Specific Hydrologic Analysis 

Another approach to establishing the performance design objective is to design, construct, and 

maintain stormwater management practices that preserve the pre-development runoff conditions 

following construction. Option 2 allows the designer to conduct a site-specific hydrologic 

analysis to determine the pre-development runoff conditions instead of using the estimated 

volume approach of Option 1. Under Option 2, the pre-development hydrology would be 

determined based on site-specific conditions and local meteorology by using continuous 

simulation modeling techniques, published data, studies, or other established tools. If the 

designer elects to use Option 2, the designer would then identify the pre-development condition 

of the site and quantify the post-development runoff volume and peak flow discharges that are 

equivalent to pre-development conditions. The post-construction rate, volume, duration and 

15 
Section 438 Technical Guidance December 2009 

temperature of runoff should not exceed the pre-development rates and the predevelopment 

hydrology should be replicated through site design and other appropriate practices to the 

maximum extent technically feasible. These goals should be accomplished through the use of 

infiltration, evapotranspiration, and/or rainwater harvesting and use. Defensible and consistent 

hydrological assessment tools should be used and documented. Additional discussions of 

appropriate methodologies to use in assessing site hydrology have been included in the technical 

sections of this document. See, for example, the discussion of spreadsheet versions or curve 

numbers based on the Natural Resource Conservation Service Technical Release 55 (TR-55) 

Method in Appendix A of this document. 

Development 

The pre-development hydrologic condition of the site is the combination of runoff, infiltration 

and evapotranspiration rates and volumes that typically existed on the facility site before 

"development" on a greenfields site (meaning any construction of infrastructure on undeveloped 

land such as meadows or forests). In practice, determining the pre-development hydrology of a 

given site can be difficult if there is no suitable reference site. As a result, reference conditions 

for typical land cover types in the locality often are used to approximate what fraction of the 

precipitation ran off, soaked into the ground or was evaporated from the landscape. The use of 

reference conditions can be problematic if suitable data are not available or unique site 

conditions exist that do not fit within a typical land use cover type for the area, e.g., meadow or 

forest. In cases where suitable data from comparable conditions cannot be found or is otherwise 

inadequate to be used in conducting an Option 2 analysis for the specific area being considered 

for development or redevelopment, the project sponsor should use the Option 1 analytical 

framework. 

Re-development 

For re-development sites, existing site conditions and uses of the site can influence the amount of 

runoff that can be managed on site through infiltration, evapotransporation and harvest and use 

and thus the performance design objective. In these cases the design process in Figure 8 and 

Scenario 9 illustrate the decision processes that can be used. 

In the context of some re-development projects, fully restoring predevelopment hydrology can be 

difficult to achieve and Congress recognized this potential difficulty by including the METF 

language in the statute. In these cases, Congressional intent can be best carried out by using a 

systematic METF analysis to determine what practices can be implemented at the site to 

maintain or store the hydrologic condition of the site. Scenarios 1-8 provide examples of METF 

analyses that demonstrate that pre-development hydrology can be achieved. Scenario 9 provides 

an example of an METF analysis that demonstrates that pre-development hydrology cannot be 

fully achieved and illustrates the extent to which pre-development hydrology can be restored. 

Note: It should also be emphasized that the performance based approach in Option 1 is intended 

to be a surrogate for determining the pre-development reference condition and this standard is 

intended to be used in cases where it is more practical, cost effective, and/or expeditious than 

Option 1, or where it is difficult or infeasible to identify the relevant reference conditions for the 

site.

16 
Section 438 Technical Guidance December 2009 

Determination of Maximum Extent Technically Feasible 

Compliance with Section 438 requires that stormwater management measures are implemented 

to the maximum extent technically feasible (METF) to maintain or restore the pre-development 

hydrology conditions specifically with respect to temperature, rate, volume, and duration of flow. 

Performance or design goals based on the pre-development hydrology can be established by 

using options such as the following: Retention of the 95th percentile rainfall event (Option 1), or 

through a site-specific hydrologic analysis that estimates the volume of infiltration, 

evapotranspiration or onsite stormwater harvesting and use based on site-specific hydrologic 

conditions (Option 2). 

Technical Infeasibility 

For projects where technical infeasibility exists, the federal agency or department sponsoring the 

project should document and quantify that stormwater strategies, such as infiltration, 

evapotranspiration, and harvesting and use have been used to the METF, and that full 

employment of these types of controls are infeasible due to site constraints. Some western states 

place restrictions on harvesting and use due to water rights, however, these requirements do not 

necessarily preclude the sponsor of the project from implementing strategies such as infiltration 

and evapotranspiration. Documentation of technical infeasibility should include, but may not be 

limited to, engineering calculations, geologic reports, hydrologic analyses, and site maps. A 

determination that the performance design goals cannot be met on site should include analyses 

that rule out the use of an adequate combination of infiltration, evapotranspiration, and use 

measures. Examples of where site conditions may prevent the full employment of appropriate 

management techniques to the METF include a combination of: 

 The conditions on the site preclude the use of infiltration practices due to the presence of 

shallow bedrock, contaminated soils, near surface ground water or other factors such as 

underground facilities or utilities. 

 The design of the site precludes the use of soil amendments, plantings of vegetation or other 

designs that can be used to infiltrate and evapotranspirate runoff. 

 Water harvesting and use are not practical or possible because the volume of water used for 

irrigation, toilet flushing, industrial make-up water, wash-waters, etc. is not significant 

enough to warrant the design and use of water harvesting and use systems. 

 Modifications to an existing building to manage stormwater are not feasible due to structural 

or plumbing constraints or other factors as identified by the facility owner/operator. 

 Small project sites where the lot is too small to accommodate infiltration practices adequately 

sized to infiltrate the volume of runoff from impervious surfaces, 

 Soils that cannot be sufficiently amended to provide for the requisite infiltration rates, 

 Situations where site use is inconsistent with the capture and use of stormwater or other 

physical conditions on site that preclude the use of plants for evapotranspiration or 

bioinfiltration. 

 Retention and/or use of stormwater onsite or discharge of stormwater onsite via infiltration 

has a significant adverse effect on the site or the down gradient water balance of surface 

waters, ground waters or receiving watershed ecological processes. 

17 
Section 438 Technical Guidance December 2009 

 State and local requirements or permit requirements that prohibit water collection or make it 

technically infeasible to use certain GI/LID techniques. 

 Compliance with the Section 438 requirements would result in the retention and/or use of 

stormwater on the site such that an adverse water balance impact may occur to the receiving 

surface waterbody or ground water. 

Please note that a single one of these characteristics is very unlikely to preclude meeting the 

performance standard, but a combination of factors may. 

In cases where the facility has a defensible showing of technical infeasibility and can provide 

adequate documentation of site conditions or other factors that preclude full implementation of 

the performance design goal, the facility should still install stormwater practices to infiltrate, 

evapotranspire and/or harvest and use onsite the maximum amount of stormwater technically 

feasible. Note: Facilities must still comply with all other applicable federal, state and local 

requirements. 

18 
Section 438 Technical Guidance December 2009 

Figure 8. Section 438 Implementation Process 

19 
Section 438 Technical Guidance December 2009 

Documenting EISA Section 438 Implementation 

Each agency or department is responsible for ensuring compliance with Section 438. It is 

recommended that: 1) the final design and as-built drawings of each facility shall be reviewed 

by a registered professional engineer and 2) the agency or department develop and maintain 

documentation of the following design criteria for each project subject to Section 438: 

 Site evaluation and soils analysis 

 Calculations for the 95th percentile rainfall event or the pre-development runoff volumes 

and rates to identify the volume of stormwater requiring management 

 Documentation of modifications to the performance design objective based on technical 

constraints (site-specific METF determination) 

 The site design and stormwater management practices employed on the site 

 Design calculations for each stormwater management practice employed 

 The respective volume of stormwater managed by each practice and the system as a 

whole 

 Operations and maintenance protocols for the stormwater management system 

The information should provide the necessary documentation and detail to demonstrate 

compliance and operation of stormwater management practices for the entire site. 

Common Green Infrastructure/Low Impact Development Tools to Implement Section 438 

Although Congress did not prescribe specific practices to be used to implement Section 438 it 

can be inferred that one of the goals of the Act was to promote the use of innovative stormwater 

management approaches, designs and practices that better protect receiving water quality, flow 

regimes and provide other important environmental benefits. GI/LID are preferred practices, to 

be supplemented with or replaced with conventional controls when site specific conditions 

dictate. 

The GI/LID management approaches and technologies that federal agencies would typically use 

enhance and/or mimic the natural hydrologic cycle processes of infiltration, evapotranspiration, 

and use. Federal agencies can also use footprint reduction practices (e.g., building up instead of 

out) to reduce their stormwater impact. GI/LID approaches include biological systems and 

engineered systems. These include but are not necessarily limited to: 

 Rain gardens, bioretention, and infiltration planters 

 Porous pavements 

 Vegetated swales and bioswales 

 Green roofs 

 Trees and tree boxes 

 Pocket wetlands 

 Reforestation/revegetation using native plants 

 Protection and enhancement of riparian buffers and floodplains 

 Rainwater harvesting for use (e.g., irrigation, HVAC make-up, non-potable indoor uses). 

GI/LID practices are recommended to implement EISA Section 438 for the following reasons: 

 cost savings in many cases 

20 
Section 438 Technical Guidance December 2009 

21 

 overall environmental performance 

 pollutant loading reduction capability 

 pollution prevention focus 

 effectiveness in managing runoff 

volumes and rates 

 energy efficient and energy 

conservative 

 appropriate in a wide range of site 

condition and locations 

 appropriate for new development and 

redevelopment projects 

 appropriate at multiple scales of 

development, e.g., site, neighborhood, 

region 

For more information on specific GI/LID 

practices and how they function, visit: 

www.epa.gov/greeninfrastructure and 

www.epa.gov/nps/lid. 

Cost of Compliance 

The cost of complying with Section 438 may 

require the use of approaches and techniques 

that initially may be more costly to design and 

implement. It is anticipated that as the 

expertise of the implementing agency or department increases and the demand for GI/LID 

materials and equipment increases that the overall costs of the projects will be lower or 

equivalent to the costs of constructing conventional stormwater practices. Initial studies 

conducted by EPA and others suggest that the use of GI/LID practices can be cost competitive. 

Recent evaluations of GI/LID projects have identified opportunities for cost savings because of 

reduced infrastructure and site preparation demands. In addition, longer term studies have 

indicated that GI/LID practices are continuing to gain cost efficiency as they are adopted more 

widely and with greater frequency thus reducing overall implementation costs. 

In Reducing Stormwater Costs through LID Strategies and Practices (EPA 841-F-07-006, 

December 2007 - available for download at www.epa.gov/nps/lid), EPA examined 17 case 

studies in which conventional development costs were compared to GI/LID costs. In the great 

majority of cases, the GI/LID approach was between 15 and 80 percent less expensive than 

conventional control measures because implementation of GI/LID practices can offset costs of 

conventional construction and stormwater management approaches. Significant cost savings that 

were identified in the report include: 

 Elimination or reduction of detention ponds 

 Elimination or reductions of stormwater and CSO treatment and conveyance systems 

such as pipes, storage structures, stormwater treatment devices, and other related 

stormwater infrastructure 

 Narrower streets with reduced material demands 

Figure 9. Disconnected downspout 

discharging to planter box.
Section 438 Technical Guidance December 2009 

 Fewer square yards of sidewalks 

 Reduced land purchases for stormwater control structures 

In addition, other benefits were achieved through the use of GI/LID such as more beneficial uses 

of land previously dedicated to stormwater devices, increased livability and higher property 

values. 

There are many different combinations of practices that can be employed at particular sites to 

achieve pre-development hydrology. In selecting the appropriate set of practices to be used at 

the site, project sponsors should consider a broad range of factors, including cost-effectiveness of 

particular combinations of practices as applied to the site, as well as the potential for ancillary 

cost savings or community benefits (e.g., elimination or reduction of infrastructure costs, or the 

creation of attractive green spaces). EPA encourages project sponsors to include these factors in 

the planning and design phases of their projects so as to maximize triple bottom-line (economic, 

environmental, and social) results. 

E. CALCULATING THE 95TH PERCENTILE RAINFALL EVENT

A long period of precipitation records, i.e., a minimum of 10 years of data, is needed to 

determine the 95th percentile rainfall event for a location. Thirty years or more of monitoring 

data are desirable to conduct an unbiased statistical analysis. The National Climatic Data Center 

(NCDC) provides long-term precipitation data for many locations of the United States. You can 

download climate data from their Web site (www.ncdc.noaa.gov) or by ordering compact discs 

(NOTE: The NCDC charges a fee for access to their precipitation data). Local airports, 

universities, water treatment plants, or other facilities might also maintain long-term 

precipitation records. Data reporting formats can vary based on the data sources. In general, 

each record should include the following basic information: 

 Location (monitoring station) 

 Recording time (usually the starting time of a time-step) 

 Total precipitation depth during the time-step 

In addition to the above information, a status flag is sometimes included to indicate data 

monitoring errors or anomalies. Typical NCDC flags include A (end accumulation), M (missing 

data), D (deleted data), or I (incomplete data). If there are no flags, the record has passed the 

quality control as prescribed by the NCDC and has been determined to be a valid data point. 

There are several data processing steps to determine the 95th percentile rainfall event using a 

spreadsheet. These steps are summarized below: 

1. Obtain a long-term 24-hr precipitation data set for a location of interest (i.e., from the 

NCDC website). 

2. Import the data into a spreadsheet. In MS Excel [Data / Import External Data / Import Data]

22 
Section 438 Technical Guidance December 2009 

3. Rearrange all of the daily precipitation records into one column if the original data set has 

multiple columns of daily precipitation records. 

4. Review the records to identify if there are early periods with a large number of flagged 

data points (e.g., erroneous data points). Select a long period of good recording data that 

represents, ideally, 30 years or more of data. Remove all of the extra data (if not using the 

entire dataset). 

5. Remove all flagged data points (i.e., erroneous data points) from the selected data set for 

further analysis. 

6. Remove small rainfall events (typically less than 0.1 inches), which may not contribute to 

rainfall runoff. These small events are categorized as depressional storage, which, in 

general, does not produce runoff from most sites. 

Note: Steps 4 through 6 can be processed by applying data sort, delete and 

re-sort spreadsheet functions. In MS Excel [Data / Sort] 

7. Calculate the 95th percentile rainfall amount by applying the PERCENTILE spreadsheet 

function at a cell. In MS Excel [=PERCENTILE(precipitation data range,95%)]

23 
Section 438 Technical Guidance December 2009 

Note: The PERCENTILE function returns the nth percentile of value in the entire 

precipitation data range. This function can be used to determine the 95th percentile storm 

event that captures all but the largest 5% of storms. 

8. The 95th percentile was calculated in the previous step. However, if the user would like 

to see this information represented graphically and get a relative sense of where 

individual storm percentiles fall in terms of rainfall depths, the following methodology 

can be used. Derive a table showing percentile versus rainfall depth to draw a curve as 

shown below. The PERCENTILE spreadsheet function can be used for each selected 

percent. It is recommended to include at least 6 points between 0% and 100% (several 

points should be between 80% and 100% to draw an accurate curve). 

Use the spreadsheet software to create of plot of rainfall depth versus percentile, as shown above. 

The 95th percentile storm event should correlate to the rainfall depth calculated in step 7, 

however the graph can be used to calculate rainfall depths at other percentiles (e.g., 50%, 90%). 

24 
Section 438 Technical Guidance December 2009 

Part II: Case Studies on Capturing the 95th Percentile Storm 

Using Onsite Management Practices 

INTRODUCTION

This section contains nine case studies that are intended to be representative of the range of 

projects that are subject to the requirements legislated in Section 438 of the Energy 

Independence and Security Act. The facility examples in the case studies were selected to 

illustrate project scenarios for differing geographic locations, site conditions, and project sizes 

and types. As noted in Part I, all projects with a footprint greater than 5,000 square feet must 

comply with the provisions of Section 438. What this means is that both new development and 

redevelopment projects should be designed to infiltrate, evapotranspirate, and/or harvest and use 

runoff to the maximum extent technically feasible (METF) to maintain or restore the predevelopment hydrology of the site. Scenarios 1-8 are examples of sites where it was technically 

feasible to design the stormwater management system to retain the 95th percentile storm onsite. 

Scenario 9, however, was provided as an example of an METF analysis where site constraints 

allowed the designers to retain only 75% of the 95th percentile storm. 

Given the site-specific nature of individual projects, the case study scenarios described herein do 

not include site specific design features such as runoff routing, specific site infiltration rates, the 

structural loading capacity of buildings, etc. in terms of stormwater practice selection. 

It should be noted that an example of Option 2, which requires a site-specific hydrologic 

analysis, has not been provided in this document because of the complexity of factors and the 

lack of general applicability such an analysis would have. 

Background 

Numerous approaches exist for determining the volume of runoff to be treated through 

stormwater management. Retaining stormwater runoff from all events up to and including the 

95th percentile rainfall event was identified as Option 1 because small, frequently-occurring 

storms account for a large proportion of the annual precipitation volume. Using GI/LID 

practices to retain both the runoff produced by small storms and the first part of larger storms can 

reduce the cumulative impacts of altered flow regimes on receiving water hydrology, e.g., 

channel degradation and diminished baseflow. For the purposes of this guidance, retaining all 

storms up to and including the 95th percentile storm event is analogous to maintaining or 

restoring the pre-development hydrology with respect to the volume, flow rate, duration and 

temperature of the runoff for most sites. 

Determination of the 95th Percentile Rainfall Event 

The 95th percentile rainfall rainfall event was determined using the long-term daily precipitation 

records from the National Climate Data Center (NCDC, 2007). By analyzing the frequency and 

rainfall depths from daily rainfall records over 24-hour periods, the 95th percentile storm event 

can be determined. From a frequency analysis viewpoint, the 95th percentile event is the storm 

event that is greater than or equal to 95% of all storms that occur within a given period of time. 

Regional climate conditions and precipitation vary across the U.S. Because of local values, it is 

essential that the implementing agency or department establish the 95th percentile storm event for 

25 
Section 438 Technical Guidance December 2009 

the project site since the control volume may vary depending on local weather patterns and 

conditions.

Onsite Stormwater Management Practice Determinations 

For the purposes of the case study scenarios, the following four categories of practices were 

selected as the most appropriate practices for implementing Section 438 requirements: 

bioretention, permeable pavements and pavers, cisterns, and green roofs. These practices were 

selected based on known performance data and cost. For each case study, the same hierarchy of 

selection criteria was used, i.e., the most cost effective practices were considered before other 

practices were considered. Bioretention practices were considered first because these systems 

generally have the lowest cost per unit of stormwater treated (Hathaway and Hunt, 2007). Thus, 

if the bioretention system could not be designed to adequately capture the desired runoff volume, 

permeable pavement and pavers, cisterns, and green roofs were considered in that order based on 

relative cost. In most cases a combination of practices was selected as part of an integrated 

treatment system. It should be noted that all treatment systems were designed to accomplish the 

goal of capturing the 95th percentile rainfall event onsite. Examples of onsite stormwater 

management practices selected for each site are presented in the results section. For the Boston, 

MA site, it was assumed that bioretention was not feasible in order to simulate a situation where 

space was severely limited; as a result, interlocking modular pavers were selected as the most 

cost-effective stormwater management to capture the requisite design volume. To further 

illustrate the range of site conditions designers may encounter, and how site conditions impact 

the selection of appropriate control options, Scenario #3 (Cincinnati, OH) was re-analyzed as 

Scenario #8. In Scenario #8, it was assumed that the site had clay soils and low infiltrative 

capacity. Given these site conditions, the range of potential control options was more limited 

and a combination of modular paving blocks, a green roof, and cisterns was ultimately selected 

based on cost and site suitability factors. 

For purposes of these modeling exercises, a number of assumptions were associated with each 

category of practice. These assumptions are not necessarily an endorsement of a particular 

design paradigm, but rather were used to keep a somewhat conservative cap on the scenarios in 

order to demonstrate the feasibility of the approach. For example, bioretention retrofits can and 

should often be located in prior impervious locations; however, in all modeled scenarios 

bioretention was restricted to currently landscaped areas. The assumptions were: 

 Bioretention areas: On-lot retention of stormwater through the use of vegetation, soils, 

and microbes to capture, treat and infiltrate runoff. 

It is assumed bioretention practices would be installed within currently landscaped 

pervious areas or that pervious areas would be created for bioretention cells. While 

termed bioretention, these systems are designed to provide infiltration as well as 

temporary storage. Bioretention areas would be designed to accept up to a depth of 10 

inches of water across the surface of the bioretention cell (see Appendix A). The 

conceptual design of this storage depth would occur within the media and/or could be 

included as ponded storage. Further design storage beyond the 10 inches would be 

acceptable (and encouraged) above the media on a site-by-site basis with ponded depth 

generally not to exceed 12 inches. 

26 
Section 438 Technical Guidance December 2009 

Uniform infiltration was assumed across the entire base of the bioretention cell. No 

additional media underneath the amended soils were included in the designs with 

infiltration rates in this layer governed by the in situ soils. Underdrains were not modeled 

directly but could be applied at the point of storage overflow such that no overflow 

occurs until the design depth of 10 inches is saturated. This approach was selected to 

maximize the storage and infiltration benefits of these systems. Designs utilizing 

underdrains at the base of the bioretention cell do not store the requisite volumes because 

the media is permeable and the underdrain conveys the runoff offsite through the 

underdrain before it can be infiltrated. Because standard underdrains typically discharge 

from smaller storms as well, underdrain designs, if employed, should ensure adequate 

retention capacity for the 95th percentile event volume. 

The bioretention footprint for modeling purposes was calculated as one uniform area that 

did not include side slopes. There is an expectation that actual bioretention cell 

construction would be distributed throughout the site with targeted locations based on 

hydrology (natural flow paths) and soils with greater infiltrative capacity. Side slopes 

may increase the surface excavation area required to accommodate the footprint and 

freeboard of these systems depending on the design or the bioretention system. 

 Porous/permeable pavement: Transportation surfaces constructed of asphalt, concrete 

or permeable pavers that are designed to infiltrate runoff. 

Infiltration was modeled for the entire porous pavement area with drainage pipes used 

only as overflow outlets. This design was chosen to maximize infiltration capabilities of 

the system. While many types of porous pavement systems can be used, modular block 

type pavers were generally applied in this design category under the assumption that they 

typically include sufficient volumetric storage in the media layer. [Note: Other types of 

porous pavement applications are available that support heavy loads and can be designed 

to temporarily store and infiltrate runoff beneath the surface of the pavement.] 

For these systems, an equivalent of 2 inches of design storage depth was assumed. This 

design depth could be achieved by specifying 10 inches of media depth that had 20% 

void space. Similarly, this could be achieved by designing six inches of media depth 

above the bottom surface, with specified media containing 33% void space. This 

alternative would have the overflow outlet at the 6 inch depth providing an equivalent 

water storage depth of 2 inches. 

The soils under the paver blocks may require or be subjected to some compaction for 

engineering stability. As a result, infiltration into underlying soils was modeled 

conservatively by applying the minimum infiltration rate for each soil type 

(see Appendix A). 

Generally, porous pavement is not recommended for high traffic areas or loading bays 

Because of this the scenarios assumed that only a percentage of total parking and road 

areas on a site can be converted to porous pavement. The assumed maximum percentage 

27 
Section 438 Technical Guidance December 2009 

applied in the scenarios was set at 60% of the total paved area. Guidance on porous 

pavements is available at: 

http://cfpub.epa.gov/npdes/greeninfrastructure/technology.cfm#permpavements

 Cistern: Containers or vessels that are used to store runoff for future use. 

Cisterns were modeled in cases where green roofs were not feasible or where it was 

necessary to include additional storage volume to meet the goal of onsite rainfall runoff 

capture. The sizes of cisterns would be calculated based on site-specific rainfall, sitespecific spatial and structural conditions, use opportunities and rates, and consideration of 

cost per volume of storage. For simplicity, cistern volume was reported as a total 

volume. This total volume could be subdivided into any number of cisterns to provide 

the total necessary storage but should be based on the impervious area and runoff 

quantities which will flow to the cistern. The most efficient cost per volume storage 

would need to be considered on a site-by-site basis (see Appendix A). 

 Green roof: Roof designed with light weight soil media and planted with vegetation. 

Frequently, green rooftop area is limited by structural capacity. In addition, other rooftop 

equipment may need to be accommodated in this space including HVAC systems and air 

handlers. For this reason, and to provide a somewhat conservative rate of application, it 

was assumed for these modeling analyses that up to 30% of a roof’s impervious area 

could be converted into a green roof. Green roof area was assumed to have 1 inch of total 

effective stormwater storage, i.e., a 2.5 inch media depth with 40% void space (see 

Appendix A).

General Approach 

Using site aerial photos, spatial analysis should be conducted to estimate the land cover types 

and areas for each site. The surface conditions of each site can be digitized using geographic 

information systems (GIS) techniques. Alternatively, computer-aided design (CAD) drawings 

can be used to estimate the surface area of each land cover type. The schematic in Figure 10 

illustrates the processes used for selecting and determining the overall size of stormwater 

management practices for each site. 

28 
Section 438 Technical Guidance December 2009 

Figure 10. Flow chart depicting the process for determining control measures 

using the 95th percentile, 24-hour, annual rainfall event. 

Stormwater 

Management Analysis & 

Design Process 

Collect spatial data from aerial 

photos (determine pervious 

and impervious areas) 

Collect historic rainfall 

data from nearest station

Determine the 95th percentile 

24-hour rainfall event

Estimate the current runoff 

Select onsite control measure options

Check whether control

measure options 

meet performance 

goals performance 

Determine the size(s) of control measure(s) 

Yes 

1. Select alternative control measures 

using METF analysis and site 

limitations to determine appropriate

runoff control measures if 

performance goals cannot be 

achieved 

and/or 

2. Exercise optional offsite runoff 

management approach and select 

appropriate control measures 

Determine location and size(s) of 

onsite or off-site control measures 

No 

Select control 

measure(s) to fit the site 

and confirm performance

Design and implement control measure(s) 

Yes 

No 

29 
Section 438 Technical Guidance December 2009 

The following steps provide more detailed information on acquiring and calculating the 

necessary data to complete the processes indicated in Figure 10. This methodology was used in 

the scenario analyses that follow. 

Collecting spatial data for a site 

1. Collect an aerial orthophotograph for the desired site. 

2. Digitize land use/land cover conditions using GIS techniques. If CAD drawings of the 

site exist, they can be used to estimate land cover area (pervious, impervious). 

3. Categorize the digitized or planned land use/land cover based on surface hydrologic 

conditions, e.g., rooftop, pavement, and pervious/landscaped area. 

4. Estimate the size of each land use/land cover category (by polygon). 

Determining the 95th percentile, 24-hr rainfall event 

1. Obtain a long-term 24-hr precipitation data set for the location of interest (i.e., from the 

NCDC Web site or other source). 

2. Import the data into a spreadsheet. In MS Excel [Data / Import External Data / Import Data]

3. Rearrange all of the daily precipitation records into one column if the original data set has 

multiple columns of daily precipitation records. 

4. Remove all flagged data points (i.e., erroneous data points) from the selected data set for 

further analysis. 

5. Remove small rainfall events (typically less than 0.1 inches) that may not contribute to 

rainfall runoff. These small storms often produce little if any appreciable runoff from 

most sites and for modeling purposes are typically considered as volume captured in 

surface depression storage. 

6. Calculate the 95th percentile rainfall volume by applying the PERCENTILE spreadsheet 

function to a range of data cells. The PERCENTILE function returns the nth percentile 

value in the specified precipitation data range. This function can be used to determine the 

95th percentile storm event that captures all but the largest 5% of storms. In MS Excel

[PERCENTILE(precipitation data range,95%)]

Estimating Current Runoff and Placing onsite control measures to capture the 95th percentile 

rainfall event 

1. Collect spatial data for a site, e.g., rooftop, pavement, and pervious areas as above. 

2. Check soil type (USDA mapping, borings, or onsite testing) for the site to determine 

infiltration parameters. For this modeling, many of the assumptions that pertain to 

generalized soils groups and their infiltration properties come from the EPA Stormwater 

Management Model (SWMM 4.x) manual (see Appendix A). 

3. Determine the current runoff volume that would occur during a 24 hour period by 

applying the 95th percentile rainfall to the existing site conditions (land use and soil 

properties) as above using a hydrologic model (such as TR-55 or SWMM). For this 

analysis, it is assumed that the rainfall amount is distributed over a 24 hour period. 

Actual rainfall event duration (and intensity) was not considered for determining rainfall 

runoff (however, timing was considered when modeling infiltration). 

4. Determine flow paths so that management practice placements are in locations where 

flows can be intercepted and routed to practices. Because this is a site specific effort and 

may require detailed topographic information or further surveys this would be a task to be 

30 
Section 438 Technical Guidance December 2009 

completed onsite and therefore is not included as a part of the modeling scenario 

exercise. 

5. Select onsite control practices to capture the current 95th percentile runoff event; base the 

selection of appropriate options on site conditions, areas available for treatment options, 

and other factors such as site use and other constraints. 

Note: The steps above have been generalized for the purposes of this guidance. It is 

recommended that a qualified professional engineer determine or verify that stormwater 

management practices are sized, placed, and designed correctly. It should also be noted that the 

methodology to determine rainfall amount used a 24 hour time period based on daily records. 

Actual rainfall events may have occurred over shorter or longer time periods. Similarly, for 

modeling purposes, the 24 hour rainfall amount was distributed to pervious and impervious areas 

(and management practices) as a uniform event occurring during a 24-hour period. A large 

dataset (greater than 50 years) was used to reasonably represent rainfall depth on a daily bases. 

It stands to reason that more frequent, shorter duration precipitation events are better represented 

than less frequent, longer duration precipitation events. 

Scenarios 

Eight locations were selected for the 9 case studies as shown in Figure 11 and Table 2. Case 

study numbers 3 and 8 were both developed based on the Cincinnati, Ohio facility, although the 

site parameters were altered to represent differing site conditions and design constraints. Annual 

average rainfall depths for these locations range from 7.5 inches to 48.9 inches. Analyses of the 

95th percentile rainfall events for these locations produced rainfall depths that range from 1.00 

inch to 1.77 inches (Table 2). 

Figure 11. Locations for Analyzing Onsite Control Measures. 

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Section 438 Technical Guidance December 2009 

The government facilities in the 8 case study locations were selected because they represent 

generic sites from the major climatic regions of the U.S. These facilities also were selected 

because the sites have a range of site characteristics that can be used to illustrate different site 

designs and stormwater management options, e.g., pervious, roof, and pavement areas (Table 3). 

Site sizes ranged from 0.7 to 27 acres with percent site imperviousness area ranging from 47% to 

95% of the site. Aerial photos of the sites are included along with site specific rainfall runoff 

and soil results. 

Table 2. Summary of Rainfall Data for the Seven Locations. 

NCDC Daily Precipitation Data Rainfall Depth (inches) 

No Location Period of record Coverage Annual 

average 

95th percentile 

rainfall event 

1 Charleston, WV 1/1/1948 - 12/31/2006 (59 yrs) 99% 43.0 1.23 

2 Denver, CO 1/1/1948 - 12/31/2006 (59 yrs) 96% 15.2 1.07 

3 Cincinnati, OH 1/1/1948 - 12/31/2006 (59 yrs) 96% 36.5 1.45 

4 Portland, OR 1/1/1941 - 12/31/2006 (66 yrs) 98% 35.8 1.00 

5 Phoenix, AZ 1/1/1948 - 12/31/2006 (59 yrs) 99% 7.5 1.00 

6 Boston, MA 1/1/1920 - 12/31/2006 (87 yrs) 99% 41.9 1.52 

7 Atlanta, GA 1/1/1930 - 12/31/2006 (77 yrs) 100% 48.9 1.77 

8 Norfolk, VA 1/1/1957 - 12/31/2006 (50 yrs) 99% 45.4 1.68 

The results of the spatial analyses were summarized and divided into three land cover categories; 

rooftop, pavement, and pervious area, as shown in Table 3. 

Table 3. Summary of Land-use Determinations of the Study Sites. 

Facility Spatial Info (acres) No Location 

Rooftop Pavement Pervious Total 

Site 

Imperviousness 

1 Charleston, WV 0.1 0.4 0.2 0.7 73% 

2 Denver, CO 0.5 1.9 2.0 4.5 55% 

3 Cincinnati, OH 1.6 8.0 9.4 19 51% 

4 Portland, OR 8.8 16.9 1.3 27 95% 

5 Phoenix, AZ 0.2 0.7 1.1 2 47% 

6 Boston, MA 0.9 1.5 1.1 3.5 69% 

7 Atlanta, GA 3.9 10.8 6.2 21 70% 

8 Norfolk, VA 0.9 0.55 0.15 1.6 91% 

Methods for Determining Runoff Volume 

Direct Determination of Runoff Volume 

Runoff from each land cover was estimated using a simplified volumetric approach based on the 

following equation: 

 Runoff = Rainfall – Depression Storage – Infiltration Loss 

Again, this methodology does not consider routing of runoff; therefore slope is not considered 

when calculating on a volumetric basis. 

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Section 438 Technical Guidance December 2009 

Infiltration loss is calculated only in pervious areas (e.g., there is no infiltration in impervious 

areas). In this analysis, infiltration was estimated using Horton’s equation: 

 Ft = fmin + (fmax – fmin) e

 - k t

where, Ft= infiltration rate at time t (in/hr)

fmin = minimum or saturated infiltration rate (in/hr)

fmax = maximum or initial infiltration rate (in/hr)

k = infiltration rate decay factor (/hr) and

t = time (hr) measured from time runoff first discharged into infiltration area 

Infiltration loss for the 24-hr rainfall duration was estimated by the following equation with 

assumptions of a half hour ∆t and uniform rainfall distribution in time:

Infiltration Loss = ∑ (f ·∆t)

To more accurately describe the dynamic process of infiltration associated with Horton’s 

equation, infiltration loss was integrated over a 24-hour period using a half hour time step while 

applying the maximum and minimum infiltration rates (in/hr) with time using the appropriate 

soil decay factor. The results of this process are further illustrated in Appendix A. 

Once runoff from each land cover was estimated, the total runoff from a site can be obtained 

using an area-weighted calculation as shown below: 

Runoffsite ={(Runoffroof ×Aroof)+(Runoffpavement×Apavement)+(Runoffpervious×Apervious)}/Asite

Where Runoffsite = total runoff from the site (inches); Asite = site area (acres); Runoffroof = runoff 

from rooftop (inches); Aroof = rooftop area (acres); Runoffpavement = runoff from pavement area 

(inches); Apavement = pavement area (acres); Runoffpervious = runoff from pervious area (inches); 

and Apervious = pervious area (acres). 

An example demonstrating how to calculate runoff by applying the Direct Determination method 

is presented below using the Charleston, WV (Scenario #1) site condition presented in Tables 2 

and 3. 

Runoffroof = 95th Rainfall – Depression Storage 

 = 1.23 – 0.1 = 1.13 inches 

Runoffpavement = 95th Rainfall – Depression Storage 

 = 1.23 – 0.1 = 1.13 inches 

Runoffpervious = 95th Rainfall – Depression Storage – Infiltration Loss 

= 1.23 – 0.1 – 9.73 = 0 inches (i.e., no runoff because the result is a 

negative number)

33 
Section 438 Technical Guidance December 2009 

Runoffsite ={(Runoffroof ×Aroof)+(Runoffpavement ×Apavement)+(Runoffpervious×Apervious)}/Asite

 ={(1.13 ×0.10)+(1.13×0.41)+(0 ×0.19)}/0.7 = 0.82 inches 

Infiltration loss was estimated based on soil type B by applying the Horton equation as described 

above. Because the volume removed from surface runoff through infiltration was substantial, no 

runoff occurred from the pervious area. 

In cases where sites had limited physical space available for stormwater management, a series of 

practices was used (e.g., treatment train) to simulate the runoff and infiltrative behavior of the 

system. For example, if there was inadequate area and infiltrative capacity to infiltrate 100 

percent of the 95th percentile storm event within a bioretention system another onsite 

management practice was selected to manage the runoff that could provide the necessary 

capacity. In this manner, excess runoff was routed to another management practice in the series 

of treatment cells where possible. 

Two types of soils were considered for every site: hydrologic soil group B and C (except for 

scenario 8 in which hydrologic soil group D was used). Group B soils typically have between 10 

percent and 20 percent clay and 50 percent to 90 percent sand and either loamy sand or sandy 

loam textures with some loam, silt loam, silt, or sandy clay loam soil textures placed in this 

group if they are well aggregated, of low bulk density, or contain greater than 35 percent rock 

fragments. Group C soils typically have between 20 percent and 40 percent clay and less than 50 

percent sand and have loam, silt loam, sandy clay loam, clay loam, and silty clay loam soil 

textures with some clay, silty clay, or sandy clay textures placed in this group if they are well 

aggregated, of low bulk density, or contain greater than 35 percent rock fragments (USDANRCS, 2007). The application of these hydrologic soil groups was intended to give reasonable 

and somewhat conservative estimates of infiltration capacity. 

General hydrologic parameters in this analysis were assumed as follows (see Appendix A for 

citations of assumptions): 

 Depression storage (or initial abstraction) 

o Rooftop: 0.1 inches 

o Pavement: 0.1 inches 

o Pervious area: 0.2 inches 

 Horton Infiltration parameters 

o Hydrologic Soil Group B 

 Maximum infiltration rate: 5 in/hr 

 Minimum infiltration rate: 0.3 in/hr 

 Decay factor: 2 /hr 

o Hydrologic Soil Group C 

 Maximum infiltration rate: 3 in/hr 

 Minimum infiltration rate: 0.1 in/hr 

 Decay factor: 3.5 /hr 

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Section 438 Technical Guidance December 2009 

 Design storage assumptions of control measures 

o Bioretention: up to 10 inches (but variable based on balancing necessary storage 

volume, media depth for plant survivorship, and surface area limitations) 

o Green roof: 1 inch (2.5 inches deep media with 40% void space) 

o Porous pavement: 4 inches (10 inches deep media with 40% void space) 

Other Methods for Estimating Runoff Volume 

Runoff from a site after applying the 95th percentile storm can be estimated by using a number of 

empirical, statistical, or mathematical methods. Several methods were considered in this 

analysis. The Rational Method can be used to estimate peak discharge rates and the Modified 

Rational Method can be used to develop a runoff hydrograph. The NRCS TR-55 model can be 

used to predict runoff volume and peak discharge. TR-55 can also be used to develop a runoff 

hydrograph. The EPA Stormwater Management Model (SWMM) can be used to simulate 

rainfall-runoff, pollutant build-up and wash-off, transport-storage-treatment of stormwater flow 

and pollutants, backwater effects, etc. for a wide range of temporal and spatial scales. The 

SWMM model can be fit to model a small site with a distributed system. Hydrologic Simulation 

Program – Fortran (HSPF, USDA) is a watershed and land use based lumped model that can be 

used to compute the movement of water and pollutants when evaluating the effects of land use 

change, reservoir operations, water quality control options, flow diversions, etc. In general, 

regionally calibrated modeling parameters are incorporated into HSPF. QUALHYMO is a 

complete hydrologic and water quality model, which can be used to factor in snowmelt or soil 

moisture conditions or to simulate system behavior based on infiltration and ET, ground water 

storage tracking, baseflow and deep volumetric losses, and other variables. 

Many of the existing tools for analyzing distributed systems use some part or all of the principles 

or formulae of the modeling approaches highlighted above. For example, the Emoryville 

spreadsheet control measure model (Emoryville, CA) uses a runoff coefficient (i.e., Rational 

Method) for analyzing lot-level to neighborhood-scale control measure sizing. The Green 

Calculator (Center for Neighborhood Technologies) estimates the benefit of onsite GI/LID 

options on a neighborhood-scale by applying the curve numbers (i.e., TR-55) and the Modified 

Rational Method. The Northern Kentucky Spreadsheet Tool uses a TR-55 based approach for 

control measure sizing on neighborhood or site level spatial scales. The WWHM (Western 

Washington Hydrology Model) is a regionally calibrated HSPF model intended for use in sizing 

stormwater detention and water quality facilities to meet the Washington State Department of 

Ecology standards. WBM-QUALHYMO is a Canadian model used in conjunction with the 

Water Balance Model (WBM). This model can be used to continuously simulate stormwater 

storage routing, stream erosion, drainage area flow routing, and snowmelt runoff (and ultimately 

freeze-thaw). Table 4 contains a summary of these different methods based on generic modeling 

features. 

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Section 438 Technical Guidance December 2009 

Table 4. Potential Methods for Analyzing Control Measures. 

Model Considerations 

Rational 

Method TR-55 SWMM 

Direct 

Determination HSPF QUALHYMO 

Single Event Yes Yes Yes Yes Yes Yes Temporal 

scale Continuous 

Simulation No No Yes Possible Yes Yes 

Lot-level Yes Yesb Yes Yes No No 

Neighborhood Yes Yes Yes Yes Possible Possible Spatial 

scale 

Regional Yes Yesc Yes No Yes Yes 

Peak 

Discharge Yes Yes Yes No Yes Yes 

Runoff 

Volume Yes Yes Yes Yes Yes Yes 

Hydrograph Yesa Yes Yes No Yes Yes 

Outputs 

Water Quality No No Yes Possible Yes Yes 

a

 Modified Rational Method 

b

 No less than 1 acre. 

c

 No more than 25 square miles (up to 10 subareas). 

From the viewpoint of modeling both lot-level and neighborhood scale projects, the Rational 

Method, NRCS TR-55, SWMM, and Direct Determination approaches were selected for use in 

scenario analyses. Strength and weakness of these methods are presented below: 

Table 5. Comparison of approaches for determining runoff volume. 

Method Strengths Weaknesses 

Direct 

Determination 

 Methodology for runoff determination 

is same as SWMM 

 Models basic hydrologic processes 

directly (explicit) 

 Simple spreadsheet can be used 

 Direct application of Horton’s method may 

estimate higher infiltration loss, especially at 

the beginning of a storm 

 Does not consider flow routing 

Rational 

Method 

 Method is widely used 

 Simple to use and understand 

 Cannot directly model storage-oriented onsite 

control measures 

TR-55  Method is widely used 

 Simple to use and understand 

 May not be appropriate for estimating runoff 

from small storm events because depression 

storage is not well accounted for 

SWMM 

 Method is widely used 

 Can provide complete hydrologic and 

water quality process dynamics in 

stormwater analysis 

 Needs a number of site-specific modeling 

parameters 

 Generally requires more extensive experience 

and modeling skills 

Each method requires specific parameters for estimating runoff from a site. Runoff coefficients 

for the Rational Method are assumed to be 0.9 for rooftop and pavement areas, and 0.1 and 0.135 

for Group B and C soil pervious areas, respectively (Caltrans, 2003). The slope of the pervious 

area was assumed to be an average of 2%. Applying these runoff coefficients for each surface, 

the overall area-weighted runoff coefficient can be determined. 

36 
Section 438 Technical Guidance December 2009 

When applying the NRCS TR-55 method, Curve Numbers (CNs) should be determined for each 

drainage area. For rooftop and pavement areas the CN was assumed to be 98, and pervious area 

CN was determined on the basis of the hydrologic soil group and the status of grass cover 

condition. Curve numbers for pervious areas were assumed to be 61 and 74 for Group B and C 

soils, respectively, with an assumption of over 75% grass cover. The overall CN can be 

estimated by using an area-weighted calculation (USDA-SCS, 1986). 

In SWMM modeling, infiltration was modeled using Horton’s equation. The same infiltration 

parameters and depression storage values used in the direct determination method of runoff 

treatment volume described earlier were applied to the SWMM analyses. The average slope of 

the pervious area was again assumed to be 2%. The same uniform rainfall distribution and time 

step was applied for the SWMM model runs. 

Runoff Methodology Results 

Stormwater management practice sizes (and depth) were determined using the Direct 

Determination approach to capture the volume of runoff generated in a 95th percentile rainfall 

event at each location. Total acreage, impervious area, the 95th percentile rainfall event, the 

current expected runoff for the 95th percentile rainfall event, and the future runoff with 

stormwater management controls were reported for each site. Results were summarized for the 

two soil types (three soil types for scenarios #3 and #8 in Cincinnati). The spatial location of 

onsite control measures was also illustrated in the site aerial photo figures. Note that site 

practices were placed only on undeveloped or landscaped areas without regard for true flow 

paths or technical feasibility. It may be preferred to place practices in existing impervious areas, 

if possible. For the purposes of this modeling exercise, the least cost and most practical solutions 

were used, i.e., locating bioretention systems on undeveloped or landscaped areas. On an actual 

site, flow paths would be determined and berms and swales might be used to route runoff to 

areas that are most suitable for infiltration. In other cases, areas that are currently impervious 

could be modified to accept runoff, e.g., impermeable pavements removed and replaced by 

permeable, sidewalks could be redesigned to include sidewalk bioretention cells and streets 

could be designed with flow through or infiltration curb bumpouts/raingardens. 

To compare other approaches of runoff estimation, alternate methodologies were also employed 

for three scenarios. TR-55 was used for Scenario #1 (Atlanta), the Rational Method was applied 

to Scenario #2 (Denver), and the SWMM was run for Scenario #7 (Charleston). 

Although flood control is not the focus of this guidance, most localities have flood control 

requirements that will need to be considered in designing control measures to comply with 

Section 438. For flood control purposes, TR-55 was used to model the 10 year frequency design 

storm for each site under the assumption that all stormwater management practices were in place. 

The 10-year design storms were selected from the NRCS TR-55 Manual (USDA, 1986) for both 

the Eastern U.S. and the Western U.S. Precipitation Frequency Maps 

(www.wrcc.dri.edu/pcpnfreq.html). The 10-year frequency design storm was selected because it 

represents a common design standard used by state and local governments in order to manage 

peak rates of runoff and prevent flooding. 

37 
Section 438 Technical Guidance December 2009 

COST ESTIMATES FOR SELECTED SCENARIOS

Scenarios #2 and 7 include cost estimates comparing the capital costs for a design to comply 

with Section 438 (retention of the 95th percentile rainfall event) and capital costs for a traditional 

stormwater management design (e.g., typical curb and gutter, off-site pond for stormwater 

management). These costs are based on average unit costs to construct both traditional and 

GI/LID controls. 

38 
Section 438 Technical Guidance December 2009 

Scenario #1 - Charleston, WV

A 0.7-acre site with 73% impervious area was selected from Charleston, West Virginia (Figure 

12). If the 95th percentile rainfall event (1.23 inches) occurred on the existing site (i.e., with no 

control measures), 0.82 inches of runoff using the Direct Determination method would be 

generated and require management. The runoff from the 95th percentile rainfall event could be 

retained by the installation of bioretention systems totaling 0.03 acres if hydrologic soil group B 

is present, or 0.06 acres if hydrologic soil group C (Table 6) is the predominant soil type on the 

site. Assuming that bioretention practices are placed in areas that are currently pervious or 

landscaped, a total of 0.2 acres of pervious area would be available for the placement of 

bioretention systems. The effective design storage depth within the designated bioretention area 

was 8 inches. 

Figure 12. Actual Site and Onsite Control Measures (Charleston, WV) 

Table 6. Estimated Sizes of Onsite Control Measures for Scenario #1 (Charleston, WV) 

Total Area (acres) 0.7 

Estimated Imperviousness (%) 73% 

95th Percentile Rainfall Event (inches) 1.23 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.82 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Bioretention estimated by Direct Determination method (acres) 0.03 0.06 

Bioretention estimated by SWMM (acres) 0.03 0.05 

Off-site storage necessary to control the 10-yr With onsite controls 0.10 0.12 

event of 3.9 inches (acre-ft) Without onsite controls 0.16 0.17 

Note: The two hydrologic methods used (direct determination and SWMM) estimated similar 

bioretention sizes. 

39 
Section 438 Technical Guidance December 2009 

Scenario #2 - Denver, CO

A 4.5-acre site with 55% impervious area was selected from Denver, Colorado (Figure 13). If 

the 95th percentile rainfall event (1.07 inches) occurred on the existing site (i.e., with no control 

measures), 0.53 inches of runoff from the site would be generated and require management. The 

runoff from the 95th percentile rainfall event could be retained by the installation of bioretention 

systems totaling 0.16 acres if the hydrologic soil group B is present or 0.3 acres if hydrologic soil 

group C (Table 7) is the predominant soil type on the site. Assuming that bioretention practices 

are only placed in areas that are currently pervious or landscaped, a total of 2 acres of pervious 

area is available for the placement of bioretention systems. The design storage depth of media 

within the designated bioretention area was 6 inches. 

Figure 13. Actual Site and Onsite Control Measures (Denver, CO) 

Table 7. Estimated Sizes of Onsite Control Measures for Scenario #2 (Denver, CO) 

Total Area (acres) 4.5 

Estimated Imperviousness (%) 55% 

95th Percentile Rainfall Event (inches) 1.07 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.53 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Bioretention estimated by the Direct Determination method (acres) 0.16 0.3 

Bioretention estimated by Rational Method (acres) 0.16 0.28 

Off-site storage necessary to control the With onsite controls 0.35 0.52 

10-yr event of 3.2 inches (acre-ft) Without onsite controls 0.64 0.64 

40 
Section 438 Technical Guidance December 2009 

Cost estimates were also developed for this scenario (Table 8) to compare the costs of installing 

onsite control measures to retain the 95th percentile rainfall event versus the costs to install 

traditional stormwater management controls (e.g., curbs and gutters combined with off-site 

retention such as extended detention wet ponds). In a GI/LID scenario, the bioretention cell 

would occupy a specified area. This same area in a traditional design would be covered in turf 

since the pond would typically be offsite and not occupy the area planted in turf. Table 8 

includes this cost under the traditional column. Note: typical land development practices involve 

mass clearing and grading so little or no pre-existing vegetation is typically retained. It is also 

assumed that the use of GI/LID practices would require less underground infrastructure because 

the traditional design typically routes stormwater underground to an off-site pond via pipes or 

culverts while GI/LID practices are designed to manage runoff onsite and as close to its source as 

possible. They are also dispersed across the site and routing occurs through surface drainage via 

bioswales and overland flow. As a result GI/LID practices do not require as much or any hard or 

grey infrastructure. The cost estimates were developed for Hydrologic Soil Group B. 

Table 8. Estimated Costs for Scenario #2 (Denver, CO) 

Sizes of Onsite Control Practices 

Controls for 95th Percentile Event Traditional Stormwater Controls 

Rainfall depth (in) 1.07 

Bioretention (acres) 0.1 

Paver blocks (acres) 0 

Green roof (acres) 0 

Off-site Pond WQV (ac-ft) - 0.18 

10-Yr Fld Cntr (ac-ft) 0.15 0.14 

Total Off-Site Requirement (ac-ft) 0.15 0.32 

Land Area (assumes avg 3 ft depth) 0.05 0.11 

% of the site 2.8% 

Costs of Onsite Control Practices 

Biorention/alternative $32,495 $4,187 

Off-site Pond WQV (ac-ft) $14,833 

10-Yr Fld Cntr (ac-ft) $10,073 $9,527 

Infrastructure Pipe $8.990 $16,982 

Inlet $9.920 $14,880 

Land Area (assumes $300K/acre) $14,500 $31,500 

Sum $75,978 $91,909 

% difference from Traditional -17.3% 

41 
Section 438 Technical Guidance December 2009 

Scenario #3 - Cincinnati, OH

A 19-acre site with 51% impervious area was selected in Cincinnati, Ohio (Figure 14). If the 

95th percentile rainfall event (1.45 inches) occurred on the existing site (i.e., no control measures 

were in place), 0.68 inches of runoff from the site would be generated and require management. 

The runoff from the 95th percentile rainfall event could be retained by the installation of 

bioretention systems totaling 0.8 acres if the hydrologic soil group B is present or 1.3 acres if 

hydrologic soil group C (Table 9) is the predominant soil type on the site. Assuming that 

bioretention practices are only placed in areas that are currently pervious or landscaped, a total of 

9.4 acres of pervious area is available for the placement of bioretention systems. The design 

storage depth of media within the designated bioretention area was 8 inches. 

Figure 14. Actual Site and Onsite Control Measures (Cincinnati, OH) 

Table 9. Estimated Sizes of Onsite Control Measures for Scenario #3 (Cincinnati, OH) 

Total Area (acres) 19 

Estimated Imperviousness (%) 51% 

95th Percentile Rainfall Event (inches) 1.45 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.68 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Bioretention estimated by the Direct Determination (acres) 0.8 1.3 

Off-site storage necessary to control the 10-yr With onsite controls 2.42 3.24 

event of 4.2 inches (acre-ft) Without onsite controls 3.29 3.73 

42 
Section 438 Technical Guidance December 2009 

Scenario #4 - Portland, OR

A 27-acre site with 95% impervious area was selected in Portland, Oregon (Figure 15). If the 

95th percentile rainfall event (1.0 inches) occurred on the existing site (i.e., no control measures), 

0.86 inches of runoff would be generated and require management. This site has the greatest 

imperviousness among the 7 sites. 

Given these site conditions, there is not enough pervious area to manage the entire runoff volume 

discharged by the 95th percentile rainfall event with bioretention. As a result, other practices 

were evaluated and selected. The practices integrated into the design included a green roof, 

cisterns, and porous pavement. Based on the technical considerations of constructing and 

maintaining control measures at the site, it was assumed that approximately 30% of the available 

pervious area could be converted into bioretention cells; 20% of total rooftop area could be 

converted into green roofs; 40% of paved area could be converted into paver blocks; and 50,000 

gallons of total volume could be captured in cisterns for use on this urbanized site. Using this 

system of four different practices, all runoff for the 95th percentile rainfall event would be 

retained (Table 10). 

Figure 15. Actual Site and Onsite Control Measures (Portland, OR) 

43 
Section 438 Technical Guidance December 2009 

Table 10. Estimated Sizes of Onsite Control Measures for Scenario #4 (Portland, OR) 

Total Area (acres) 27 

Estimated Imperviousness (%) 95% 

95th percentile Rainfall Event (inches) 1.00 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.86 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Paver block area estimated by Direct Determination (acres) 1.4 3.5* 

Bioretention estimated by Direct Determination (acres) 0.4 

Green Roof estimated by Direct Determination (acres) 1.7 

Cistern volume estimated by Direct Determination (gallons) 50,000 

Off-site storage necessary to control the 10-yr With onsite controls 5.37 5.62 

event of 3.7 inches (acre-ft) Without onsite controls 7.70 7.71 

*The size of porous pavement area was increased because the other control options were maximized based on 

the site-specific design assumptions. 

A total of 1.3 acres of the site is pervious area or landscaped of which, 0.4 acres (30% of the 

pervious area) could be converted to bioretention cells that have a storage depth of 10 inches. Of 

the 8.8 acres of current rooftop area, 1.7 acres (20% of the rooftop area) could be retrofitted into 

green roof areas. Of the 16.9 acres of paved area, 1.4 acres (8% of the paved area) for hydrologic 

soil group B, or 3.5 acres (20% of the paved area) for hydrologic soil group C, of paver block 

systems could be implemented. One or more cisterns (as indicated in Figure 15) could be used to 

capture up to 50,000 gallons of runoff from rooftop areas. Note: The high percentage of 

imperviousness of the site (95%) requires that all infiltration designs be based on resident soil 

type and design volumes, or with adequate sub-bases or amended soils. 

44 
Section 438 Technical Guidance December 2009 

Scenario #5 – Near Phoenix, AZ

A 2-acre site with 47% impervious area was selected near Phoenix, Arizona (Figure 16). If the 

95th percentile rainfall event (1.0 inches) occurred on the existing site (i.e., with no control 

measures), 0.42 inches of runoff would be generated and require management. The runoff from 

the 95th percentile rainfall event could be retained by installing bioretention systems totaling 0.06 

acres if the hydrologic soil group B is present or 0.1 acres if hydrologic soil group C (Table 11) 

is the predominant soil type on the site. Assuming that bioretention practices are only placed in 

areas that are currently pervious or landscaped, a total of 1.1 acres of pervious area is available 

for the placement of these practices. The design storage depth of media within the designated 

bioretention area was 6 inches. Note: If the design storage depth were increased to 10 inches, 

the off-site storage necessary for the 10-year event could be reduced to 0.03 acre-ft for type B 

soils and 0.08 acre-ft for type C soils. 

Figure 16. Actual Site and Onsite Control Measures (Phoenix, AZ) 

Table 11. Estimated Sizes of Onsite Control Measures for Scenario #5 (Phoenix, AZ) 

Total Area (acres) 2 

Estimated Imperviousness (%) 47% 

95th Percentile Rainfall Event (inches) 1.00 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.42 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Bioretention estimated by the Direct Determination (acres) 0.06 0.1 

Off-site storage necessary to control With onsite controls 0.05 0.12 

the 10-yr event of 2.4 inches (acre-ft) Without onsite controls 0.18 0.18 

45 
Section 438 Technical Guidance December 2009 

Scenario #6 - Boston, MA

A 3.5-acre site with 69% impervious area was selected in Boston, Massachusetts (Figure 17). If 

the 95th percentile rainfall event (1.52 inches) occurred on the existing site (i.e., with no control 

measures), 0.98 inches of runoff would be generated and require management. Given these site 

characteristics, there is adequate area to place appropriately sized bioretention cells to capture the 

95th percentile storm event. However, for the purposes of this analysis, unspecified conditions 

preclude the use of bioretention. As a result, a paver block system was selected as the best onsite 

control measure and the system was designed such that the necessary design parameters could be 

achieved by storing some of the volume in the paver media and by infiltrating the remainder of 

the volume. The runoff from the 95th percentile rainfall event could be retained by installing a 

paver block area totaling 0.4 and 0.8 acres assuming soil types B and C, respectively (Table 12). 

For the purposes of this case study, a total of 1.5 acres of parking lot was made available to 

accommodate the paver block system. The area retrofitted with paver blocks would primarily be 

dedicated for use as parking stalls. 

Figure 17. Actual Site and Onsite Control Measures (Boston, MA) 

Table 12. Estimated Sizes of Onsite Control Measures for Scenario #6 (Boston, MA) 

Total Area (acres) 3.5 

Estimated Imperviousness (%) 69% 

95th Percentile Rainfall Event (inches) 1.52 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.98 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Paver block area estimated by Direct Determination (acres) 0.4 0.8 

Off-site storage necessary to control With onsite controls 0.59 0.71 

10-yr event of 4.5 inches (acre-ft) Without onsite controls 0.89 0.96 

46 
Section 438 Technical Guidance December 2009 

Scenario #7 - Atlanta, GA

A 21-acre site with 70% impervious area was selected in Atlanta, Georgia (Figure 18). If the 95th

percentile rainfall event (1.77 inches) occurred on the existing site (i.e., with no control measures), 

1.17 inches of runoff would be generated and require management. The runoff from the 95th

percentile rainfall event could not be adequately retained solely with bioretention systems. Based 

on the technical considerations of constructing and maintaining control measures at the site, it was 

assumed that up to 15% of the pervious area could be converted into bioretention cells and up to 

40% of paved area could be converted into a paver block system. If the stormwater management 

techniques used on the site includes both bioretention and paver blocks as presented in Table 13, 

then all runoff for the 95th percentile rainfall event would be controlled. 

Figure 18. Actual Site and Onsite Control Measures (Atlanta, GA) 

Table 13. Estimated Sizes of Onsite Control Measures for Scenario #7 (Atlanta, GA) 

Total Area (acres) 21 

Estimated Imperviousness (%) 70% 

95th Percentile Rainfall Event (inches) 1.77 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 1.17 

Stormwater Management Area Required Hydrologic Soil Group 

 B C 

Bioretention estimated by the Direct Determination (acres) 0.9 

Paver block area estimated by the Direct Determination (acres) 0.9 3.2* 

Bioretention estimated by TR-55 0.8** 0.9 

Paver block area estimated by TR-55 0** 1.84 

Off-site storage necessary to control With onsite controls 5.85 6.62 

10-yr event of 6.0 inches (acre-ft) Without onsite controls 7.25 8.49 

*The size of porous pavement was increased because the bioretention already reached its 

maximum size based on the site-specific design assumptions. 

**Because TR-55 estimated smaller runoff in this scenario, bioretention can retain all of the 95th

percentile runoff if the site has soil group B. 

47 
Section 438 Technical Guidance December 2009 

For the example site in Atlanta, GA, areas of 1.8 acres for hydrologic soil group B, and 4.1 acres 

for hydrologic soil group C, would be required to manage the runoff discharged from a 95th

percentile rainfall event. Assuming that bioretention practices are only placed in areas that are 

currently pervious or landscaped, a total of 6.2 acres of pervious area is available for the 

placement of bioretention systems. The design storage depth of media within the designated 

bioretention area was 10 inches. Permeable pavement systems could be used to treat the 

remaining volume on the 10.8 acres of existing paved area. 

In applying the TR-55 model, the overall curve numbers for the site were 87 and 91 for Group B 

and C soils, respectively. TR-55 was used to estimate 0.73 inches of runoff for soil group B and 

0.97 inches for soil group C, which are smaller numbers than the 1.17 inches of runoff estimated 

by the Direct Determination method. As a result, the sizes of the onsite control measures 

designed using the TR-55 model were smaller than those designed using the Direct 

Determination method. Note: It is recommended that caution be exercised when using TR-55 to 

model storms less than 0.5 inches per event. See application of TR-55 in Table 5. 

Cost estimates were also developed for this scenario (Table 14) to compare the costs to install 

onsite control measures to retain the 95th percentile rainfall event, and costs to install traditional 

stormwater management controls (e.g., primarily curb and gutter with off-site retention). The 

cost estimates were developed for Hydrologic Soil Group B. 

Table 14. Estimated Costs for Scenario #7 (Atlanta, GA) 

Sizes of Onsite Control Practices 

Controls for 95th Percentile Event Traditional Stormwater Controls 

Rainfall depth (in) 1.77 

Bioretention (acres) 0.94 

Paver blocks (acres) 0.86 

Off-site Pond WQV (ac-ft) - 1.75 

10-Yr Fld Cntr (ac-ft) 0.84 0.0 

Total Off-Site Requirement (ac-ft) 0.84 1.75 

Land Area (assumes avg 3 ft depth) 0.28 0.58 

% of the site 8.5% 

Costs of Onsite Control Practices 

Biorention/alternative $232,923 $30,617 

Paver block/alternative $236,878 $88,409 

Off-site Pond WQV (ac-ft) $0 $72,888 

10-Yr Fld Cntr (ac-ft) $39,648 $0 

Infrastructure Pipe $54,827 $191,095 

Inlet $52,080 $79,360 

Land Area (assumes $300K/acre) $84,000 $175,000 

Sum $700,356 $637,368 

% difference from Traditional 9.9% 

48 
Section 438 Technical Guidance December 2009 

Scenario #8 - Cincinnati, OH

A 19-acre site with 51% impervious area was selected in Cincinnati, Ohio (Figure 19). If the 

95th percentile rainfall event (1.45 inches) occurred on the existing site (i.e., with no control 

measures), 0.68 inches of runoff would be generated and require management. The runoff from 

the 95th percentile rainfall event could be retained by the installation of bioretention systems 

totaling 0.8 acres if the hydrologic soil group B is present or 1.3 acres if hydrologic soil group C 

(Table 9) is the predominant soil type on the site. Assuming that bioretention practices are only 

placed in areas that are currently pervious or landscaped, a total of 9.4 acres of pervious area is 

available for the placement of bioretention systems. The design storage depth of media within 

the designated bioretention area was 8 inches. 

Scenario #8 represents an alternative to the Cincinnati, scenario in #3 (Figure 14). In this case, 

hydrologic soil group D was selected to represent the soil characteristics present for the entire 

site. Alternatively, simulations could have been run under the assumption that the use of 

infiltration practices were precluded by contaminated soils or high ground water tables. Under 

these site conditions, bioretention options are severely limited and cannot be used to adequately 

capture the entire 95th percentile storm event. As a result, options such as cisterns and green 

roofs were considered. In the absence of management practices, the 95th percentile rainfall event 

discharges 1.45 inches of stormwater and 0.53 inches of this runoff is captured by onsite 

depression storage. The difference, 0.92 inches of runoff, would then require capture and 

management. Based on the technical considerations of constructing and maintaining controls at 

the site, it was assumed that up to 20% of pervious area can be converted into bioretention areas; 

up to 30% of paved area can be converted into porous pavement; and up to 30% of the rooftop 

area can be converted into green roofs. Cisterns can be added to the system if additional storage 

volume is required. It should be noted that green roofs were selected lowest in the hierarchy of 

practices evaluated because of cost and potential structural issues associated with design and 

placement on existing buildings. By using the four onsite control options as presented in Table 

15, all runoff for the 95th percentile rainfall event would be retained. From a management 

perspective, it was assumed that the design storage depth within the designated bioretention area 

was 6 inches because of the low infiltration rates adopted for this scenario. 

49 
Section 438 Technical Guidance December 2009 

Figure 19. Actual Site and Onsite Control Measures (Cincinnati, OH) 

Table 15. Estimated Sizes of Onsite Control Measures for Scenario #8 (Cincinnati, OH) 

Total Area (acres) 19 

Estimated Imperviousness (%) 51% 

95th Percentile Rainfall Event (inches) 1.45 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 0.92 

Stormwater Management Applied Hydrologic Soil Group D 

Bioretention estimated by Direct Determination (acres) 1.9 

Paver block area estimated by Direct Determination (acres) 2.4 

Green Roof estimated by Direct Determination (acres) 0.5 

Cisterns estimated by Direct Determination (gallons) 13,000 

This site contains a total of 9.4 acres of pervious area, 8.0 acres of paved area, and 1.6 acres of 

rooftop area. If 1.9 acres (20%) of the pervious area were converted to bioretention cells; 2.4 

acres (30%) of parking lot converted to paver blocks; and 0.5 acres (30%) of rooftop area were 

retrofitted to green roof areas for this site, then 97% of stormwater runoff from the 95th percentile 

storm would be captured on site. By also adding one or more cisterns (as indicated in Figure 19), 

an additional 13,000 gallons could be captured, thus illustrating that 100% of the rainfall from 

the 95th percentile event can be managed onsite with GI/LID practices.

50 
Section 438 Technical Guidance December 2009 

Scenario #9 – Norfolk, VA

A 1.6 acre site with 91% impervious area was selected from Norfolk, Virginia. Table 16 

contains the land use categories for the site. Figure 20 depicts the site and associated facilities. 

Site specific factors based on an METF analysis allow management of 75 % of the 95th percentile 

storm onsite (1.27 inches). The remaining portion of the 95th percentile rainfall event (0.41 

inches would be discharged off of the site. 

Table 16. Land Use Determination After Redevelopment 

Land Use Acres Site Coverage Percent 

Building 0.90 56.3 

Parking 0.35 21.9 

Streets/Sidewalks 0.20 12.5 

Undeveloped 0.15 9.3 

Total 1.60 100% 

Figure 20. Proposed Redevelopment Scenario 

51 
Section 438 Technical Guidance December 2009 

Redevelopment Site 

Figure 21. Location of Facility (Norfolk, VA) 

Site conditions and intended uses limited the number of practices that were technically feasible 

to use onsite to manage runoff. For example, the use of a green roof was not feasible because the 

project includes the construction of an airplane hanger which lacks the structural strength to 

support a green roof. Cisterns were also not included in the set of suitable practices based on the 

analysis, which considered the number of people and amount of daily water use at the site, i.e., 

40 people x 3.5 toilet flushes per day would use only 280 gallons of runoff per day or 2,000 

gallons per week. Stormwater use for HVAC make-up would also be negligible based on the 

typical cooling system design. To put things in perspective, if the hanger rooftop covers the 

entire building footprint, 41,000 gallons of runoff would be generated from a 1.68 inch rainfall. 

Assuming a drawdown of 2,000 gallons per week based on toilet flushing, the users would only 

use 5% of the 95th percentile event. Because of the relatively large volume of water that would 

need to be collected and used, cisterns were not considered a feasible option to manage a 

significant volume of runoff at the site. 

However, site conditions did allow for the use of both permeable pavement and bioretention 

practices. Approximately 0.15 acres (6,500 sf) of the proposed site is undeveloped and available 

for bioretention. Based on Department of Defense facility requirements, ten percent of the 

parking area is designed with landscaping, usually around the perimeter and in landscaped 

islands. If this ten percent were designed as bioretention cells, then 0.035 acres of bioretention 

would be achieved. If bioretention cells were also placed in about 30% of the undeveloped area 

of the project, then an additional 0.045 acres of bioretention could be implemented. Note: not all 

undeveloped land was assumed to be available for bioretention because of conflicts with site 

52 
Section 438 Technical Guidance December 2009 

utilities, security and anti-terrorism requirements and slopes that limited the use of infiltration 

practices directly adjacent to the hanger. 

Figure 22. Actual Site and Onsite Control Measures (Norfolk, VA) 

Table 17. Estimated Sizes of Onsite Control Measures for Scenario #9 (Norfolk, VA) 

Total Area (acres) 1.6 

Estimated Imperviousness (%) 91% 

95th Percentile Rainfall Event (inches) 1.68 

Expected Runoff for the 95th Percentile Rainfall Event (inches) 1.50 

Stormwater Management Area Required Hydrologic Soil Group D 

 Porous Pavement estimated by Direct Determination method (acres) 0.21 

Bioretention estimated by Direct Determination method (acres) 0.08 

The bioretention cells were designed with an effective storage depth of 10 inches, which 

included a depth from media surface to outlet of 10 inches. In this case study, state regulations 

precluded the project from taking credit for the storage potential provided by the void space 

within the bioretention cell media. Similarly, approximately 0.55 acres of the proposed site is 

impervious due to parking lots, streets, and sidewalks. Due to manufacturer’s recommendations 

that permeable pavement materials not be used in applications subject to heavy loads and 

potential pollutant exposure the access roads and parking lot access isles were assumed to be 

constructed from conventional impervious concrete or asphalt. Thus 60% of the parking area 

(primarily parking stalls and sidewalks), which is about 38% of the entire paved area, is assumed 

to be suitable for paver blocks. A high water table at the site limited the modeled net storage 

depth under paver blocks placed in the parking areas and sidewalks to four inches. This storage 

was calculated using the assumption that the pavement sub-base of 12 inches would have a 

minimum void space of approximately 30%. 

53 
Section 438 Technical Guidance December 2009 

COMPARISON OF THE RUNOFF ESTIMATION METHODS

As illustrated in each of the case studies above, runoff of the 95th percentile storm was estimated 

in order to size onsite control measures. These estimates were produced by applying four 

different methods: the Direct Determination method, the Rational Method, the NRCS TR-55, and 

the EPA SWMM. The results comparing each of these methods for scenarios 1-7 are presented 

in Table 18. 

Table 18. Comparison of the estimated runoff (unit: inches) 

Method 

Direct 

Determination Rational Method TR-55 SWMM 

Soil Groups B C B C B C B C 

1 Charleston, WV 0.82 0.82 0.83 0.84 0.36 0.53 0.82 0.83 

2 Denver, CO 0.53 0.53 0.57 0.59 0.12 0.26 0.53 0.53 

3 Cincinnati, OH 0.68 0.68 0.73 0.76 0.26 0.46 

4 Portland, OR 0.86 0.86 0.86 0.86 0.63 0.71 

5 Phoenix, AZ 0.42 0.42 0.46 0.48 0.06 0.17 

6 Boston, MA 0.98 0.98 0.99 1.00 0.51 0.70 

7 Atlanta, GA 1.17 1.17 1.17 1.19 0.73 0.97 1.19 1.23 

As shown in the above table, the estimated runoff results from direct determination, the Rational 

Method, and SWMM are relatively similar. Runoff volumes using TR-55 are lower than the 

other estimates. SWMM modeling results using NRCS 24-hour rainfall distributions were nearly 

identical to the results based on uniform distribution. 

Table 19. Applicability of the methods for analyzing onsite control measures 

Purpose 

Direct 

Determination Rational Method TR-55* SWMM 

Planning Tool Applicable Applicable Applicable Applicable 

Preliminary Design Applicable Applicable Applicable Applicable 

Detailed Design Not applicable Not applicable Not applicable Applicable 

Actual Assessment (Long-term) Not applicable Not applicable Not applicable Applicable 

Water Quality Not applicable Not applicable Not applicable Applicable 

*Use with caution when applying this method for small storms 

CONCLUSIONS

Although sites varied in terms of climate and soil conditions, in most of the scenarios selected, 

the 95th percentile storm event could be managed onsite with GI/LID systems. There are other 

infiltration, evapotranspiration and capture and use stormwater management options available 

than those used in these analyses. These options provide site managers additional flexibility to 

choose appropriate systems and practices to manage site runoff. 

54 
Section 438 Technical Guidance December 2009 

REFERENCES

Booth, Derek Direct Testimony. 2008. Pollution Control Hearings Board for the State of 

Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County Public 

Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish County; Clark 

County; and Pacificorp and Puget Sound Energy, Appellants, vs. Department of Ecology, 

Respondent, and King County; City of Seattle; Port of Tacoma, and Washington State 

Department of Transportation, Intervenors, August 2008. 

California Department of Transportation (Caltrans). 2003. Caltrans Storm Water Quality 

Handbooks. California Department of Transportation. 

Casey Trees. 2007. The Case for Trees – Relief from Summer Heat, 

www.caseytrees.org/resources/casefortrees.html. 

Galli, John. 1991. Thermal Impacts Associated with Urbanization and Stormwater Best 

Management Practices in Maryland, Anacostia Restoration Team for the Maryland Department 

of the Environment, Washington, DC. 

Grant, Gary, Luke Engleback and Barry Nicholson. 2003. Green Roofs: Their Existing Status 

and Potential for Conserving Biodiversity in Urban Areas, Report Number 498, English Nature 

Research Reports. 

Hathaway, J., and W.F. Hunt. 2007. Stormwater BMP Costs. North Carolina Department of 

Environment and Natural Resources. 

www.bae.ncsu.edu/stormwater/PublicationFiles/DSWC.BMPcosts.2007.pdf. 

Hirschman, David and John Kosco. 2008. Managing Stormwater in Your Community: A Guide 

for Building an Effective Post-Construction Program, Center for Watershed Protection, 

www.cwp.org/postconstruction. 

Holz, Thomas Written Direct Testimony. 2008. Pollution Control Hearings Board for the State 

of Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County Public 

Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish County; Clark 

County; and Pacificorp and Puget Sound Energy, Appellants, vs. Department of Ecology, 

Respondent, and King County; City of Seattle; Port of Tacoma, and Washington State 

Department of Transportation, Intervenors, August 2008. 

Horner, Richard Direct Testimony. 2008. Pollution Control Hearings Board for the State of 

Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County Public 

Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish County; Clark 

County; and Pacificorp and Puget Sound Energy, Appellants, vs. Department of Ecology, 

Respondent, and King County; City of Seattle; Port of Tacoma, and Washington State 

Department of Transportation, Intervenors, August 2008. 

55 
Section 438 Technical Guidance December 2009 

National Climatic Data Center (NCDC). 2007. NCDC precipitation data, CD-ROM, National 

Climatic Data Center. 

National Research Council. 2008. Urban Stormwater Management in the United States, The 

National Academies Press, Washington, DC. 

Shaver, E., R. Horner, J. Skupien, C. May, and G. Ridley. 2007. Fundamentals of Urban Runoff 

Management: Technical and Institutional Issues – 2nd Edition, North American Lake 

Management Society, Madison, WI. 

Schueler, T. and M. Helfrich. 1988. “Design of Wet Extended Detention Pond Systems,” Design 

of Urban Runoff Controls, L. Roesner and B. Urbonas eds., American Society of Civil 

Engineers, New York, NY. 

Schueler, T and H. Holland. 2000. The Practice of Watershed Protection: Techniques for 

Protecting our Nation’s Streams, Lakes, Rivers, and Estuaries. Center for Watershed Protection, 

Ellicott City, MD. 

U.S. Department of Agriculture, Natural Resources Conservation Service. 2007. National 

Engineering Handbook, title 210–VI. Part 630, chapter 7. Washington, DC. 

http://directives.sc.egov.usda.gov/. 

U.S. Department of Agriculture, Soil Conservation Service. 1986. Urban Hydrology for Small 

Watersheds. Technical Release No. 55. Second Edition. Washington, D.C. 

U.S. Environmental Protection Agency, Managing Wet Weather with Green Infrastructure, 

www.epa.gov/greeninfrastructure. 

Vingarzan and Taylor. 2003. Trend Analysis of Ground Level Ozone in the Greater Vancouver/ 

Fraser Valley Area of British Columbia, Environment Canada – Aquatic and Atmospheric 

Sciences Division. 

Wisconsin DNR. 2008. Impact of Redevelopment on TSS Loads, Runoff Management, available 

at http://www.dnr.state.wi.us/runoff/pdf/rules/nr151/Impact_of_RedevTSSLoads_021308.pdf.

56 
Section 438 Technical Guidance December 2009 

APPENDIX A: Runoff Methodology Parameter Assumptions 

Runoff from each land cover was estimated by the following equation: 

 Runoff = Rainfall – Depression Storage – Infiltration Loss (1) 

Depression Storage 

Reference depression storage (inches) 

Reference Impervious Pervious 

1 0.05 - 0.1 0.1 - 0.3 

2 0.01 - 0.11 0.02 - 0.6 

3 0.1 0.2 

1. ASCE, (1992). Design & Construction of Urban Stormwater Management Systems. New York, NY. 

2. Marsaleck, J., Jimenez-Cisreros, B., Karamouz, M., Malmquist, P-R., Goldenfum, J., and Chocat, B. 

(2007). Urban Water Cycle Processes and Interactions. Urban Water Series, UNESCO-IHP, Tyler & 

Francis. 

3. Walesh, S. G. (1989).Urban Surface Water Management. John Wiley & Sons, Inc. 

Based on the above reference data, depression storage (or initial abstraction, the rainfall required 

for the initiation of runoff) to the direct determination method was assumed as follows: 

 Rooftop: 0.1 inches 

 Pavement: 0.1 inches 

 Pervious area: 0.2 inches 

Infiltration 

Infiltration loss occurs only in pervious areas. In this analysis, infiltration was estimated by 

Horton’s equation: 

Ft = fmin + (fmax – fmin) e

 - k t (2) 

where, Ft = infiltration rate at time t (in/hr),

fmin = minimum or saturated infiltration rate (in/hr),

fmax = maximum or initial infiltration rate (in/hr),

k = infiltration rate decay factor (/hr), and

t = time (hr) measured from time runoff first discharged into infiltration area 

Reference infiltration parameters 

Maximum infiltration rate (in.hr), fmax

Infiltration Partially dried out with Dry soils with 

(in/hr) No vegetation Dense vegetation No vegetation Dense vegetation 

Sandy 2.5 5 5 10 

Loam 1.5 3 3 6 

Clay 0.5 1 1 2 

Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

Athens, GA.

57 
Section 438 Technical Guidance December 2009 

Minimum infiltration rate (in/hr), fmin

Hydrologic 

Soil Group Infiltration (in/hr) 

A 0.45 - 0.30 

B 0.30 - 0.15 

C 0.15 - 0.05 

D 0.05 - 0 

A: well drained sandy; D: poorly drained clay 

Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

Athens, GA.

Decay coefficient, k 

Soils k (sec-1) k (hr-1) 

0.00056 2 

0.00083 3 

0.00115 4 

Sandy 

Clay 0.00139 5 

Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

Athens, GA.

Based on the above reference data, infiltration parameters to the direct determination method 

were assumed as follows: 

 Hydrologic Soil Group B 

 Maximum infiltration rate: 5 in/hr 

 Minimum infiltration rate: 0.3 in/hr 

 Decay factor: 2 /hr 

 Hydrologic Soil Group C 

 Maximum infiltration rate: 3 in/hr 

 Minimum infiltration rate: 0.1 in/hr 

 Decay factor: 3.5 /hr 

 Hydrologic Soil Group D 

 Maximum infiltration rate: 1 in/hr 

 Minimum infiltration rate: 0.02 in/hr 

 Decay factor: 5 /hr 

Infiltration loss for the 24-hr rainfall duration was estimated by the following equations with 

assumptions of a half hour ∆t:

Infiltration Loss at the nth time-step = (f ·∆t) = })2/){( 1 tff  nn  (3)

Integrated Infiltration Loss for 24 hours = ∑ (f ·∆t) (4)

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Section 438 Technical Guidance December 2009 

Integrating infiltration loss during 24 hours with a half hour ∆t 

Infiltration rate (in/hr) a Infiltration volume (inches) b timestep t (hr) Soil B Soil C Soil D Soil B Soil C Soil D 

0 0 5 3 1 0 0 0 

1 0.5 2.03 0.60 0.100 1.757 0.901 0.275 

2 1 0.94 0.19 0.027 0.741 0.198 0.032 

3 1.5 0.53 0.12 0.021 0.368 0.076 0.012 

4 2 0.39 0.10 0.02 0.230 0.054 0.01 

5 2.5 0.33 0.1 0.02 0.179 0.05 0.01 

6 3 0.31 0.1 0.02 0.161 0.05 0.01 

7 3.5 0.30 0.1 0.02 0.154 0.05 0.01 

8 4 0.3 0.1 0.02 0.15 0.05 0.01 

9 4.5 0.3 0.1 0.02 0.15 0.05 0.01 

10 5 0.3 0.1 0.02 0.15 0.05 0.01 

11 5.5 0.3 0.1 0.02 0.15 0.05 0.01 

12 6 0.3 0.1 0.02 0.15 0.05 0.01 

13 6.5 0.3 0.1 0.02 0.15 0.05 0.01 

14 7 0.3 0.1 0.02 0.15 0.05 0.01 

15 7.5 0.3 0.1 0.02 0.15 0.05 0.01 

16 8 0.3 0.1 0.02 0.15 0.05 0.01 

17 8.5 0.3 0.1 0.02 0.15 0.05 0.01 

18 9 0.3 0.1 0.02 0.15 0.05 0.01 

19 9.5 0.3 0.1 0.02 0.15 0.05 0.01 

20 10 0.3 0.1 0.02 0.15 0.05 0.01 

21 10.5 0.3 0.1 0.02 0.15 0.05 0.01 

22 11 0.3 0.1 0.02 0.15 0.05 0.01 

23 11.5 0.3 0.1 0.02 0.15 0.05 0.01 

24 12 0.3 0.1 0.02 0.15 0.05 0.01 

25 12.5 0.3 0.1 0.02 0.15 0.05 0.01 

26 13 0.3 0.1 0.02 0.15 0.05 0.01 

27 13.5 0.3 0.1 0.02 0.15 0.05 0.01 

28 14 0.3 0.1 0.02 0.15 0.05 0.01 

29 14.5 0.3 0.1 0.02 0.15 0.05 0.01 

30 15 0.3 0.1 0.02 0.15 0.05 0.01 

31 15.5 0.3 0.1 0.02 0.15 0.05 0.01 

32 16 0.3 0.1 0.02 0.15 0.05 0.01 

33 16.5 0.3 0.1 0.02 0.15 0.05 0.01 

34 17 0.3 0.1 0.02 0.15 0.05 0.01 

35 17.5 0.3 0.1 0.02 0.15 0.05 0.01 

36 18 0.3 0.1 0.02 0.15 0.05 0.01 

37 18.5 0.3 0.1 0.02 0.15 0.05 0.01 

38 19 0.3 0.1 0.02 0.15 0.05 0.01 

39 19.5 0.3 0.1 0.02 0.15 0.05 0.01 

40 20 0.3 0.1 0.02 0.15 0.05 0.01 

41 20.5 0.3 0.1 0.02 0.15 0.05 0.01 

42 21 0.3 0.1 0.02 0.15 0.05 0.01 

43 21.5 0.3 0.1 0.02 0.15 0.05 0.01 

44 22 0.3 0.1 0.02 0.15 0.05 0.01 

59 
Section 438 Technical Guidance December 2009 

45 22.5 0.3 0.1 0.02 0.15 0.05 0.01 

46 23 0.3 0.1 0.02 0.15 0.05 0.01 

47 23.5 0.3 0.1 0.02 0.15 0.05 0.01 

48 24 0.3 0.1 0.02 0.15 0.05 0.01 

Sum: Infiltration loss during 24 hours c 9.743 3.430 0.769 a

 Calculated infiltration rate at each time by Equation (2) 

b

 Calculated infiltration volume from the previous time to the current time by Equation (3) c

 Integrated infiltration volume for 24 hours with a half hour ∆t by Equation (4) 

Based on the above calculation, 24-hr infiltration losses for pervious areas and bioretention areas 

were modeled as follows: 

 Soil Group B: 9.743 inches 

 Soil Group C: 4.430 inches 

 Soil Group D: 0.769 inches 

Infiltrations of underlying soils at paver blocks were modeled conservatively by applying the 

minimum infiltration rate for each soil type (Infiltration loss = fmin · 24) because the soils under 

the paver blocks may require or be subjected to some compaction for engineering stability. The 

estimated infiltration losses for each soil are presented below: 

 Soil Group B: (0.3 in/hr) · (24 hrs) = 7.2 inches 

 Soil Group C: (0.1 in/hr) · (24 hrs) = 2.4 inches 

 Soil Group D: (0.02 in/hr) · (24 hrs) = 0.48 inches 

Design Storage of Management Practices 

Bioretention 

Reference Ponding 

(inches)1

Mulch 

(inches) 

Soil 

media (ft) 

Soil Media 

Porosity Underdrain 

1 up to 12 2 - 4 

(optional) 1 - 1.5 about 40% bioretention systems utilize infiltration rather than an underdrain 

2 6 - 12 2 - 3 2.5 - 4 about 40% recommended, especially if initial 

testing infiltration rate < 0.52 in/hr 

3 6 - 12 2 - 4 

4 2 - 3 1.5 - 4 if necessary 

5 up to 6 1.5 - 2 30 - 40% Optional 

6 6 - 18 as 

needed 2 - 4 if necessary 

1. State of New Jersey. (2004). New Jersey Stormwater Best Management Practices Manual

www.nj.gov/dep/stormwater/tier_A/pdf/NJ_SWBMP_9.1 print.pdf. 

2. Maryland Department of the Environment (MDE), (2000). 2000 Maryland Stormwater Design Manual, 

Volumes I & II, prepared by the Center for Watershed Protection and the Maryland Department of the 

Environment, Water Management Administration, Baltimore, MD. 

www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp. 

 

1

 Ponding is a measure of retention capacity 

60 
Section 438 Technical Guidance December 2009 

61 

3. Clar, M. L. and R. Green, (1993). Design Manual for Use of Bioretention in Storm Water Management, 

prepared for the Department of Environmental Resources, Watershed Protection Branch, Prince 

George's County, MD, prepared by Engineering Technologies Associates, Inc. Ellicott City, MD, and 

Biohabitats, Inc., Towson, MD. 

4. U.S. Environmental Protection Agency. (1999). Storm Water Technology Fact Sheet: Bioretention. 

EPA 832-F-99-012. Office of Water. US Environmental Protection Agency. Washington, D.C. 

www.epa.gov/owm/mtb/biortn.pdf. 

5. Prince George's County. Bioretention Design Specifications and Criteria. Prince George's County, 

Maryland. 

www.co.pg.md.us/Government/AgencyIndex/DER/ESG/Bioretention/pdf/bioretention_design_manual

.pdf. 

6. City of Indianapolis. (2008). Indianapolis Stormwater Design Manual. 

www.sustainindy.org/assets/uploads/4_05_Bioretention.pdf. 

Paver Blocks 

Reference Media (inches) Void Space 

1 12 or more 40% 

2 9 or more 40% 

3 12 - 36 40% 

1. Univ. of California at Davis. (2008). Low Impact Development Techniques: Pervious Pavement. 

http://extension.ucdavis.edu/unit/center_for_water_and_land_use/pervious_pavement.asp. 

2. AMEC Earth and Environmental, Center for Watershed Protection, Debo and Associates, Jordan Jones 

and Goulding, and Atlanta Regional Commission. (2001). Georgia Stormwater Management Manual 

Volume 2: Technical Handbook www.georgiastormwater.com/. 

3. Subsurface Infiltration Bed. www.tredyffrin.org/pdf/publicworks/CH2 - BMP4 Infiltration Bed.pdf. 

Green Roofs 

Reference Media (inches) 

1 3 - 4 

2 1 - 6 

3 2 - 6 

1. Charlie Miller. (2008). Extensive Green Roofs. Whole Building Design Guide (WBDG). 

www.wbdg.org/resources/greenroofs.php. 

2. Great Lakes WATER Institute. Green Roof Project: Green Roof Installation. 

www.glwi.uwm.edu/research/genomics/ecoli/greenroof/roofinstall.php. 

3. Paladino & Company. (2004). Green Roof Feasibility Review. King County Office Project. 

http://your.kingcounty.gov/solidwaste/greenbuilding/documents/KCGreenRoofStudy_Final.pdf. 

Based on the above reference data, design storages to the direct determination method were 

assumed as follows: 

 Bioretention: up to 10 inches (depending on practice used, site conditions, etc.) 

 Green roof: 1 inch (2.5 inches deep media with 40% void space) 

 Porous pavement: 4 inches (10 inches deep media with 40% void space) 

Factors that influence total storage available include, ponding depth, available media void space, 

and supplemental storage if the system is designed with gravel or open pipes underneath the 

media.