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Chapter 3. Controlling Urban Runoff 226 

1 Appendix 2B: Procedures and Case Studies from the Section 

2 438 Guidance 

1 

3 The following information is from the Technical Guidance on Implementing the Stormwater 

4 Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and 

5 Security Act available at www.epa.gov/owow/NPS/lid/section438. 

This appendix includes procedures for calculating the 95th 6 percentile rainfall event, case studies 

of stormwater designs to retain the 95th 7 percentile rainfall event, and assumptions related to the 

8 runoff methodology calculations. 

Calculating the 95th 9 Percentile Rainfall Event 

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

determine the 95th 11 percentile rainfall event for a location. Thirty years or more of monitoring 

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

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

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

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

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

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

18 record should include the following basic information: 

19 • Location (monitoring station) 

20 • Recording time (usually the starting time of a time-step) 

21 • Total precipitation depth during the time-step 

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

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

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

25 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 26 percentile rainfall event using a 

27 spreadsheet. These steps are summarized below: 

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

29 NCDC website). 

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

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

32 multiple columns of daily precipitation records. 
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Chapter 3. Controlling Urban Runoff 227 

1 

2 

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

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

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

6 entire dataset). 

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

8 further analysis. 

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

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

11 general, does not produce runoff from most sites. 

12 

13 

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

15 spreadsheet functions. In MS Excel [Data / Sort] 

7. Calculate the 95th 16 percentile rainfall amount by applying the PERCENTILE spreadsheet 

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

18 

19 
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Chapter 3. Controlling Urban Runoff 228 

Note: The PERCENTILE function returns the nth 1 percentile of value in the entire 

precipitation data range. This function can be used to determine the 95th 2 percentile 

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

8. The 95th 4 percentile was calculated in the previous step. However, if the user would like to 

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

6 storm percentiles fall in terms of rainfall depths, the following methodology can be used. 

7 Derive a table showing percentile versus rainfall depth to draw a curve as shown below. 

8 The PERCENTILE spreadsheet function can be used for each selected percent. It is 

9 recommended to include at least 6 points between 0% and 100% (several points should 

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

11 

12 

13 

14 

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

The 95th 16 percentile storm event should correlate to the rainfall depth calculated in step 7, 

17 however the graph can be used to calculate rainfall depths at other percentiles (e.g., 50%, 90%). 
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Chapter 3. Controlling Urban Runoff 229 

Case Studies on Capturing the 95th 1 Percentile Storm 

2 Using Onsite Management Practices 

3 

4 INTRODUCTION

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

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

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

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

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

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

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

12 runoff to the maximum extent technically feasible (METF) to maintain or restore the pre13 development 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 14 percentile storm onsite. 

15 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 16 percentile storm. 

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

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

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

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

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

22 lack of general applicability such an analysis would have. 

23 Background 

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

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

95th 26 percentile rainfall event was identified as Option 1 because small, frequently-occurring 

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

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

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

30 degradation and diminished baseflow. For the purposes of this guidance, retaining all storms up 

to and including the 95th 31 percentile storm event is analogous to maintaining or restoring the pre32 development hydrology with respect to the volume, flow rate, duration and temperature of the 

33 runoff for most sites. 

Determination of the 95th 34 Percentile Rainfall Event 

The 95th 35 percentile rainfall event was determined using the long-term daily precipitation records 

36 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 37 percentile storm event can be 

determined. From a frequency analysis viewpoint, the 95th 38 percentile event is the storm event 

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

40 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 41 percentile storm event for 
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Chapter 3. Controlling Urban Runoff 230 

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

2 conditions.

3 Onsite Stormwater Management Practice Determinations 

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

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

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

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

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

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

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

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

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

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

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

goal of capturing the 95th 15 percentile rainfall event onsite. Examples of onsite stormwater 

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

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

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

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

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

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

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

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

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

25 on cost and site suitability factors. 

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

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

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

29 demonstrate the feasibility of the approach. For example, bioretention retrofits can and should 

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

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

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

33 and microbes to capture, treat and infiltrate runoff. 

34 It is assumed bioretention practices would be installed within currently landscaped 

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

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

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

38 inches of water across the surface of the bioretention cell (see Resources at end of this 

39 Appendix). The conceptual design of this storage depth would occur within the media 

40 and/or could be included as ponded storage. Further design storage beyond the 10 inches 

41 would be acceptable (and encouraged) above the media on a site-by-site basis with 

42 ponded depth generally not to exceed 12 inches. 
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Chapter 3. Controlling Urban Runoff 231 

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

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

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

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

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

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

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

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

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

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

retention capacity for the 95th 11 percentile event volume. 

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

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

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

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

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

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

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

19 or permeable pavers that are designed to infiltrate runoff. 

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

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

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

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

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

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

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

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

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

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

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

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

32 water storage depth of 2 inches. 

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

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

35 conservatively by applying the minimum infiltration rate for each soil type (see 

36 Resources at end of this Appendix). 

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

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

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

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

41 pavements is available at: 

42 http://cfpub.epa.gov/npdes/greeninfrastructure/technology.cfm#permpavements
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Chapter 3. Controlling Urban Runoff 232 

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

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

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

4 capture. The sizes of cisterns would be calculated based on site-specific rainfall, site5 specific spatial and structural conditions, use opportunities and rates, and consideration of 

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

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

8 necessary storage but should be based on the impervious area and runoff quantities which 

9 will flow to the cistern. The most efficient cost per volume storage would need to be 

10 considered on a site-by-site basis (see Resources at end of this Appendix). 

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

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

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

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

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

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

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

18 Resources at end of this Appendix). 

19 General Approach 

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

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

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

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

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

25 management practices for each site. 
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Chapter 3. Controlling Urban Runoff 233 

1 

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 goals 

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 

No

Determine location and size(s) of 

onsite or off-site control measures 

Design and implement control measure(s)

Yes

No Select control 

measure(s) to fit the site 

and confirm performance
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Chapter 3. Controlling Urban Runoff 234 

1 

2 Figure 3-10. Flow chart depicting the process for determining control measures 

using the 95th 95th 3 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 goals 

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

Yes

2. 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 

No

Determine location and size(s) of 

onsite or off-site control measures 

Design and implement control measure(s)

Yes

No Select control 

measure(s) to fit the site 

and confirm performance
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Chapter 3. Controlling Urban Runoff 235 

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

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

3 the scenario analyses that follow. 

4 Collecting spatial data for a site 

5 1. Collect an aerial orthophotograph for the desired site. 

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

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

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

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

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

Determining the 95th 11 percentile, 24-hr rainfall event 

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

13 NCDC Web site or other source). 

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

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

16 multiple columns of daily precipitation records. 

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

18 further analysis. 

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

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

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

22 surface depression storage. 

6. Calculate the 95th 23 percentile rainfall volume by applying the PERCENTILE spreadsheet 

function to a range of data cells. The PERCENTILE function returns the nth 24 percentile 

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

95th 26 percentile storm event that captures all but the largest 5% of storms. In MS Excel

27 [PERCENTILE(precipitation data range,95%)]

Estimating Current Runoff and Placing onsite control measures to capture the 95th 28 percentile 

29 rainfall event 

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

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

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

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

34 Management Model (SWMM 4.x) manual (see Resources at end of this Appendix). 

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

applying the 95th 36 percentile rainfall to the existing site conditions (land use and soil 

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

38 analysis, it is assumed that the rainfall amount is distributed over a 24 hour period. Actual 
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Chapter 3. Controlling Urban Runoff 236 

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

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

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

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

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

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

7 exercise. 

5. Select onsite control practices to capture the current 95th 8 percentile runoff event; base the 

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

10 and other factors such as site use and other constraints. 

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

12 recommended that a qualified professional engineer determine or verify that stormwater 

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

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

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

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

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

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

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

20 than less frequent, longer duration precipitation events. 

21 Scenarios 

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

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

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

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

95th 26 percentile rainfall events for these locations produced rainfall depths that range from 1.00 

27 inch to 1.77 inches (Table 2). 

28 
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Chapter 3. Controlling Urban Runoff 237 

1 

2 Figure 3-11. Locations for analyzing onsite control measures Locations for Analyzing Onsite 

3 Control Measures. 

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

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

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

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

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

9 95% of the site. Aerial photos of the sites are included along with site specific rainfall runoff and 

10 soil results. 

11 Table 3-2. Summary of rainfall dataRainfall Data for the seven locationsSeven 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 

12 
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Chapter 3. Controlling Urban Runoff 238 

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

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

3 Table 3-3. Summary of landLand-use determinationsDeterminations of the study sitesStudy 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% 

4 

5 Methods for Determining Runoff Volume 

6 Direct Determination of Runoff Volume 

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

8 following equation: 

9 Runoff = Rainfall – Depression Storage – Infiltration Loss 

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

11 when calculating on a volumetric basis. 

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

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

14 Ft = fmin + (fmax – fmin) e - k t 

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

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

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

18 k = infiltration rate decay factor (/hr) and

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

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

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

22 Infiltration Loss = ∑ (f ·∆t)
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Chapter 3. Controlling Urban Runoff 239 

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

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

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

4 soil decay factor. The results of this process are further illustrated in the Resources section at the

5 end of this Appendix. 

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

7 using an area-weighted calculation as shown below: 

Runoffsite ={(Runoffroof ×Aroof)+(Runoffpavement 8 ×Apavement)+(Runoffpervious ×Apervious)}/Asite

Where Runoffsite = total runoff from the site (inches); Asite = site area (acres); Runoffroof 9 = runoff 

from rooftop (inches); Aroof = rooftop area (acres); Runoffpavement 10 = runoff from pavement area 

(inches); Apavement 11 = pavement area (acres); Runoffpervious = runoff from pervious area (inches); 

12 and Apervious = pervious area (acres). 

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

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

15 and 3. 

Runoffroof 16 = 95th Rainfall – Depression Storage 

17 = 1.23 – 0.1 = 1.13 inches 

Runoffpavement 18 = 95th Rainfall – Depression Storage 

19 = 1.23 – 0.1 = 1.13 inches 

20 Runoffpervious = 95th Rainfall – Depression Storage – Infiltration Loss 

21 = 1.23 – 0.1 – 9.73 = 0 inches (i.e., no runoff because the result is a 

22 negative number) 

Runoffsite ={(Runoffroof ×Aroof)+(Runoffpavement 23 ×Apavement)+(Runoffpervious ×Apervious)}/Asite

24 ={(1.13 ×0.10)+(1.13×0.41)+(0 ×0.19)}/0.7 = 0.82 inches 

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

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

27 runoff occurred from the pervious area. 

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

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

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

percent of the 95th 31 percentile storm event within a bioretention system another onsite 

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

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

34 of treatment cells where possible. 

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

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

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

38 loam textures with some loam, silt loam, silt, or sandy clay loam soil textures placed in this 
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Chapter 3. Controlling Urban Runoff 240 

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

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

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

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

5 aggregated, of low bulk density, or contain greater than 35 percent rock fragments (USDA6 NRCS, 2007). The application of these hydrologic soil groups was intended to give reasonable 

7 and somewhat conservative estimates of infiltration capacity. 

8 General hydrologic parameters in this analysis were assumed as follows (see Resources at the 

9 end of this Appendix for citations of assumptions):

10 • Depression storage (or initial abstraction) 

11 – Rooftop: 0.1 inches 

12 – Pavement: 0.1 inches 

13 – Pervious area: 0.2 inches 

14 • Horton Infiltration parameters 

15 – Hydrologic Soil Group B 

16 – Maximum infiltration rate: 5 in/hr 

17 – Minimum infiltration rate: 0.3 in/hr 

18 – Decay factor: 2 /hr 

19 – Hydrologic Soil Group C 

20 – Maximum infiltration rate: 3 in/hr 

21 – Minimum infiltration rate: 0.1 in/hr 

22 – Decay factor: 3.5 /hr 

23 • Design storage assumptions of control measures 

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

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

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

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

28 Other Methods for Estimating Runoff Volume 

Runoff from a site after applying the 95th 29 percentile storm can be estimated by using a number of 

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

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

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

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

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

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

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

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

38 Program – Fortran (HSPF, USDA) is a watershed and land use based lumped model that can be 
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Chapter 3. Controlling Urban Runoff 241 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

21 features. 

22 
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Chapter 3. Controlling Urban Runoff 242 

1 Table 3-4. Potential methods for analyzing control measuresPotential Methods for Analyzing 

2 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). 

3 

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

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

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

7 Table 3-5. 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 
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Chapter 3. Controlling Urban Runoff 243 

1 

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

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

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

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

6 the overall area-weighted runoff coefficient can be determined. 

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

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

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

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

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

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

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

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

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

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

17 step was applied for the SWMM model runs. 

18 Runoff Methodology Results 

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

Determination approach to capture the volume of runoff generated in a 95th 20 percentile rainfall 

event at each location. Total acreage, impervious area, the 95th 21 percentile rainfall event, the 

current expected runoff for the 95th 22 percentile rainfall event, and the future runoff with 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

42 The 10-year design storms were selected from the NRCS TR-55 Manual (USDA, 1986) for both 
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Chapter 3. Controlling Urban Runoff 244 

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

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

3 represents a common design standard used by state and local governments in order to manage 

4 peak rates of runoff and prevent flooding. 

5 Cost Estimates for Selected Scenarios 

6 Scenarios #2 and 7 include cost estimates comparing the capital costs for a design to comply 

with Section 438 (retention of the 95th 7 percentile rainfall event) and capital costs for a traditional 

8 stormwater management design (e.g., typical curb and gutter, off-site pond for stormwater 

9 management). These costs are based on average unit costs to construct both traditional and 

10 GI/LID controls. 
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Chapter 3. Controlling Urban Runoff 245 

1 Scenario #1 - Charleston, West VirginiaWV

2 A 0.7-acre site with 73% impervious area was selected from Charleston, West Virginia (Figure 

12). If the 95th 3 percentile rainfall event (1.23 inches) occurred on the existing site (i.e., with no 

4 control measures), 0.82 inches of runoff using the Direct Determination method would be 

generated and require management. The runoff from the 95th 5 percentile rainfall event could be 

6 retained by the installation of bioretention systems totaling 0.03 acres if hydrologic soil group B 

7 is present, or 0.06 acres if hydrologic soil group C (Table 6) is the predominant soil type on the 

8 site. Assuming that bioretention practices are placed in areas that are currently pervious or 

9 landscaped, a total of 0.2 acres of pervious area would be available for the placement of 

10 bioretention systems. The effective design storage depth within the designated bioretention area 

11 was 8 inches. 

12 

13 Figure 3-12. Actual Site and Onsite Control Measures (Charleston, WV) 
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Chapter 3. Controlling Urban Runoff 246 

1 Table 3-6. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 0.10 0.12 Off-site storage necessary to control the 10-yr 

event of 3.9 inches (acre-ft) Without onsite controls 0.16 0.17 

3 Note: The two hydrologic methods used (direct determination and SWMM) estimated similar 

4 bioretention sizes. 
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Chapter 3. Controlling Urban Runoff 247 

1 Scenario #2 - Denver, ColoradoCO

2 A 4.5-acre site with 55% impervious area was selected from Denver, Colorado (Figure 13). If the 

95th 3 percentile rainfall event (1.07 inches) occurred on the existing site (i.e., with no control 

4 measures), 0.53 inches of runoff from the site would be generated and require management. The 

runoff from the 95th 5 percentile rainfall event could be retained by the installation of bioretention 

6 systems totaling 0.16 acres if the hydrologic soil group B is present or 0.3 acres if hydrologic soil 

7 group C (Table 7) is the predominant soil type on the site. Assuming that bioretention practices 

8 are only placed in areas that are currently pervious or landscaped, a total of 2 acres of pervious 

9 area is available for the placement of bioretention systems. The design storage depth of media 

10 within the designated bioretention area was 6 inches. 

11 

12 Figure 3-13. Actual siteSite and onsite control measuresOnsite Control Measures (Denver, CO) 
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Chapter 3. Controlling Urban Runoff 248 

1 Table 3-7. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 0.35 0.52 Off-site storage necessary to control the 

10-yr event of 3.2 inches (acre-ft) Without onsite controls 0.64 0.64 

3 

4 Cost estimates were also developed for this scenario (Table 8) to compare the costs of installing 

onsite control measures to retain the 95th 5 percentile rainfall event versus the costs to install 

6 traditional stormwater management controls (e.g., curbs and gutters combined with off-site 

7 retention such as extended detention wet ponds). In a GI/LID scenario, the bioretention cell 

8 would occupy a specified area. This same area in a traditional design would be covered in turf 

9 since the pond would typically be offsite and not occupy the area planted in turf. Table 8 

10 includes this cost under the traditional column. Note: typical land development practices involve 

11 mass clearing and grading so little or no pre-existing vegetation is typically retained. It is also 

12 assumed that the use of GI/LID practices would require less underground infrastructure because 

13 the traditional design typically routes stormwater underground to an off-site pond via pipes or 

14 culverts while GI/LID practices are designed to manage runoff onsite and as close to its source as 

15 possible. They are also dispersed across the site and routing occurs through surface drainage via 

16 bioswales and overland flow. As a result GI/LID practices do not require as much or any hard or 

17 grey infrastructure. The cost estimates were developed for Hydrologic Soil Group B. 
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Chapter 3. Controlling Urban Runoff 249 

1 Table 3-8. Estimated costsCosts 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% 

2 
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Chapter 3. Controlling Urban Runoff 250 

1 Scenario #3 - Cincinnati, OhioOH

2 A 19-acre site with 51% impervious area was selected in Cincinnati, Ohio (Figure 14). If the 

95th 3 percentile rainfall event (1.45 inches) occurred on the existing site (i.e., no control measures 

4 were in place), 0.68 inches of runoff from the site would be generated and require management. 

The runoff from the 95th 5 percentile rainfall event could be retained by the installation of 

6 bioretention systems totaling 0.8 acres if the hydrologic soil group B is present or 1.3 acres if 

7 hydrologic soil group C (Table 9) is the predominant soil type on the site. Assuming that 

8 bioretention practices are only placed in areas that are currently pervious or landscaped, a total of 

9 9.4 acres of pervious area is available for the placement of bioretention systems. The design 

10 storage depth of media within the designated bioretention area was 8 inches. 

11 

12 Figure 3-14. Actual site and onsite control measuresActual Site and Onsite Control Measures

13 (Cincinnati, OH) 
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Chapter 3. Controlling Urban Runoff 251 

1 Table 3-9. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 2.42 3.24 Off-site storage necessary to control the 10-yr 

event of 4.2 inches (acre-ft) Without onsite controls 3.29 3.73 
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Chapter 3. Controlling Urban Runoff 252 

1 Scenario #4 - Portland, OregonOR

2 A 27-acre site with 95% impervious area was selected in Portland, Oregon (Figure 15). If the 

95th 3 percentile rainfall event (1.0 inches) occurred on the existing site (i.e., no control measures), 

4 0.86 inches of runoff would be generated and require management. This site has the greatest 

5 imperviousness among the 7 sites. 

6 Given these site conditions, there is not enough pervious area to manage the entire runoff volume 

discharged by the 95th 7 percentile rainfall event with bioretention. As a result, other practices 

8 were evaluated and selected. The practices integrated into the design included a green roof, 

9 cisterns, and porous pavement. Based on the technical considerations of constructing and 

10 maintaining control measures at the site, it was assumed that approximately 30% of the available 

11 pervious area could be converted into bioretention cells; 20% of total rooftop area could be 

12 converted into green roofs; 40% of paved area could be converted into paver blocks; and 50,000 

13 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 14 percentile rainfall event would be 

15 retained (Table 10). 

16 

17 Figure 3-15. Actual Site and Onsite Control Measures (Portland, OR) 
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Chapter 3. Controlling Urban Runoff 253 

1 Table 3-10. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 5.37 5.62 Off-site storage necessary to control the 10-yr 

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. 

3 

4 A total of 1.3 acres of the site is pervious area or landscaped of which, 0.4 acres (30% of the 

5 pervious area) could be converted to bioretention cells that have a storage depth of 10 inches. Of 

6 the 8.8 acres of current rooftop area, 1.7 acres (20% of the rooftop area) could be retrofitted into 

7 green roof areas. Of the 16.9 acres of paved area, 1.4 acres (8% of the paved area) for hydrologic 

8 soil group B, or 3.5 acres (20% of the paved area) for hydrologic soil group C, of paver block 

9 systems could be implemented. One or more cisterns (as indicated in Figure 15) could be used to 

10 capture up to 50,000 gallons of runoff from rooftop areas. Note: The high percentage of 

11 imperviousness of the site (95%) requires that all infiltration designs be based on resident soil 

12 type and design volumes, or with adequate sub-bases or amended soils. 
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Chapter 3. Controlling Urban Runoff 254 

1 Scenario #5 – Near Phoenix, ArizonaAZ

2 A 2-acre site with 47% impervious area was selected near Phoenix, Arizona (Figure 16). If the 

95th 3 percentile rainfall event (1.0 inches) occurred on the existing site (i.e., with no control 

4 measures), 0.42 inches of runoff would be generated and require management. The runoff from 

the 95th 5 percentile rainfall event could be retained by installing bioretention systems totaling 0.06 

6 acres if the hydrologic soil group B is present or 0.1 acres if hydrologic soil group C (Table 11) 

7 is the predominant soil type on the site. Assuming that bioretention practices are only placed in 

8 areas that are currently pervious or landscaped, a total of 1.1 acres of pervious area is available 

9 for the placement of these practices. The design storage depth of media within the designated 

10 bioretention area was 6 inches. Note: If the design storage depth were increased to 10 inches, the 

11 off-site storage necessary for the 10-year event could be reduced to 0.03 acre-ft for type B soils 

12 and 0.08 acre-ft for type C soils. 

13 

14 Figure 3-16. Actual siteSite and onsite control measuresOnsite Control Measures (Phoenix, AZ) 
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Chapter 3. Controlling Urban Runoff 255 

1 Table 3-11. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 0.05 0.12 Off-site storage necessary to control 

the 10-yr event of 2.4 inches (acre-ft) Without onsite controls 0.18 0.18 

3 
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Chapter 3. Controlling Urban Runoff 256 

1 Scenario #6 - Boston, MassachusettsMA

2 A 3.5-acre site with 69% impervious area was selected in Boston, Massachusetts (Figure 17). If 

the 95th 3 percentile rainfall event (1.52 inches) occurred on the existing site (i.e., with no control 

4 measures), 0.98 inches of runoff would be generated and require management. Given these site 

5 characteristics, there is adequate area to place appropriately sized bioretention cells to capture the

95th 6 percentile storm event. However, for the purposes of this analysis, unspecified conditions 

7 preclude the use of bioretention. As a result, a paver block system was selected as the best onsite 

8 control measure and the system was designed such that the necessary design parameters could be 

9 achieved by storing some of the volume in the paver media and by infiltrating the remainder of 

the volume. The runoff from the 95th 10 percentile rainfall event could be retained by installing a 

11 paver block area totaling 0.4 and 0.8 acres assuming soil types B and C, respectively (Table 12). 

12 For the purposes of this case study, a total of 1.5 acres of parking lot was made available to 

13 accommodate the paver block system. The area retrofitted with paver blocks would primarily be 

14 dedicated for use as parking stalls. 

15 

16 Figure 3-17. Actual siteSite and onsite control measuresOnsite Control Measures (Boston, MA) 
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Chapter 3. Controlling Urban Runoff 257 

1 Table 3-12. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 0.59 0.71 Off-site storage necessary to control 

10-yr event of 4.5 inches (acre-ft) Without onsite controls 0.89 0.96 
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Chapter 3. Controlling Urban Runoff 258 

1 Scenario #7 - Atlanta, GeorgiaGA

A 21-acre site with 70% impervious area was selected in Atlanta, Georgia (Figure 18). If the 95th 2 

3 percentile rainfall event (1.77 inches) occurred on the existing site (i.e., with no control 

4 measures), 1.17 inches of runoff would be generated and require management. The runoff from 

the 95th 5 percentile rainfall event could not be adequately retained solely with bioretention 

6 systems. Based on the technical considerations of constructing and maintaining control measures 

7 at the site, it was assumed that up to 15% of the pervious area could be converted into 

8 bioretention cells and up to 40% of paved area could be converted into a paver block system. If 

9 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 10 percentile rainfall event would be 

11 controlled. 

12 

13 Figure 3-18. Actual siteSite and onsite control measuresOnsite Control Measures (Atlanta, GA) 
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Chapter 3. Controlling Urban Runoff 259 

1 Table 3-13. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

With onsite controls 5.85 6.62 Off-site storage necessary to control 

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. 

3 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 4 

5 percentile rainfall event. Assuming that bioretention practices are only placed in areas that are 

6 currently pervious or landscaped, a total of 6.2 acres of pervious area is available for the 

7 placement of bioretention systems. The design storage depth of media within the designated 

8 bioretention area was 10 inches. Permeable pavement systems could be used to treat the 

9 remaining volume on the 10.8 acres of existing paved area. 

10 In applying the TR-55 model, the overall curve numbers for the site were 87 and 91 for Group B 

11 and C soils, respectively. TR-55 was used to estimate 0.73 inches of runoff for soil group B and 

12 0.97 inches for soil group C, which are smaller numbers than the 1.17 inches of runoff estimated 

13 by the Direct Determination method. As a result, the sizes of the onsite control measures 

14 designed using the TR-55 model were smaller than those designed using the Direct 

15 Determination method. Note: It is recommended that caution be exercised when using TR-55 to 

16 model storms less than 0.5 inches per event. See application of TR-55 in Table 5. 

17 Cost estimates were also developed for this scenario (Table 14) to compare the costs to install 

onsite control measures to retain the 95th 18 percentile rainfall event, and costs to install traditional 

19 stormwater management controls (e.g., primarily curb and gutter with off-site retention). The 

20 cost estimates were developed for Hydrologic Soil Group B. 
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Chapter 3. Controlling Urban Runoff 260 

1 Table 3-14. Estimated costsCosts 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% 

2 
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Chapter 3. Controlling Urban Runoff 261 

1 Scenario #8 - Cincinnati, OhioOH

2 A 19-acre site with 51% impervious area was selected in Cincinnati, Ohio (Figure 19). If the 

95th 3 percentile rainfall event (1.45 inches) occurred on the existing site (i.e., with no control 

4 measures), 0.68 inches of runoff would be generated and require management. The runoff from 

the 95th 5 percentile rainfall event could be retained by the installation of bioretention systems 

6 totaling 0.8 acres if the hydrologic soil group B is present or 1.3 acres if hydrologic soil group C 

7 (Table 9) is the predominant soil type on the site. Assuming that bioretention practices are only 

8 placed in areas that are currently pervious or landscaped, a total of 9.4 acres of pervious area is 

9 available for the placement of bioretention systems. The design storage depth of media within 

10 the designated bioretention area was 8 inches. 

11 Scenario #8 represents an alternative to the Cincinnati, scenario in #3 (Figure 14). In this case, 

12 hydrologic soil group D was selected to represent the soil characteristics present for the entire 

13 site. Alternatively, simulations could have been run under the assumption that the use of 

14 infiltration practices were precluded by contaminated soils or high ground water tables. Under 

15 these site conditions, bioretention options are severely limited and cannot be used to adequately 

capture the entire 95th 16 percentile storm event. As a result, options such as cisterns and green 

roofs were considered. In the absence of management practices, the 95th 17 percentile rainfall event 

18 discharges 1.45 inches of stormwater and 0.53 inches of this runoff is captured by onsite 

19 depression storage. The difference, 0.92 inches of runoff, would then require capture and 

20 management. Based on the technical considerations of constructing and maintaining controls at 

21 the site, it was assumed that up to 20% of pervious area can be converted into bioretention areas; 

22 up to 30% of paved area can be converted into porous pavement; and up to 30% of the rooftop 

23 area can be converted into green roofs. Cisterns can be added to the system if additional storage 

24 volume is required. It should be noted that green roofs were selected lowest in the hierarchy of 

25 practices evaluated because of cost and potential structural issues associated with design and 

26 placement on existing buildings. By using the four onsite control options as presented in Table 

15, all runoff for the 95th 27 percentile rainfall event would be retained. From a management 

28 perspective, it was assumed that the design storage depth within the designated bioretention area 

29 was 6 inches because of the low infiltration rates adopted for this scenario. 

30 
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Chapter 3. Controlling Urban Runoff 262 

1 

2 Figure 3-19. Actual siteSite and onsite control measuresOnsite Control Measures (Cincinnati, OH) 

3 Table 3-15. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

4 #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 

5 

6 This site contains a total of 9.4 acres of pervious area, 8.0 acres of paved area, and 1.6 acres of 

7 rooftop area. If 1.9 acres (20%) of the pervious area were converted to bioretention cells; 2.4 

8 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 9 percentile 

10 storm would be captured on site. By also adding one or more cisterns (as indicated in Figure 19), 
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Chapter 3. Controlling Urban Runoff 263 

1 an additional 13,000 gallons could be captured, thus illustrating that 100% of the rainfall from 

the 95th 2 percentile event can be managed onsite with GI/LID practices.
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Chapter 3. Controlling Urban Runoff 264 

1 Scenario #9 – Norfolk, VirginiaVA

2 A 1.6 acre site with 91% impervious area was selected from Norfolk, Virginia. Table 16 contains 

3 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 4 percentile 

storm onsite (1.27 inches). The remaining portion of the 95th 5 percentile rainfall event (0.41 

6 inches would be discharged off of the site. 

7 Table 3-16. Land use determination after redevelopmentLand Use Determination After 

8 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%

9 

10 

11 Figure 3-20. Proposed redevelopment scenarioProposed Redevelopment Scenario
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Chapter 3. Controlling Urban Runoff 265 

1 

2 

3 Figure 3-21. Location of facilityFacility (Norfolk, VA) 

4 Site conditions and intended uses limited the number of practices that were technically feasible 

5 to use onsite to manage runoff. For example, the use of a green roof was not feasible because the 

6 project includes the construction of an airplane hanger which lacks the structural strength to 

Redevelopment Site 

Redevelopment Site 
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Chapter 3. Controlling Urban Runoff 266 

1 support a green roof. Cisterns were also not included in the set of suitable practices based on the 

2 analysis, which considered the number of people and amount of daily water use at the site, i.e., 

3 40 people x 3.5 toilet flushes per day would use only 280 gallons of runoff per day or 2,000 

4 gallons per week. Stormwater use for HVAC make-up would also be negligible based on the 

5 typical cooling system design. To put things in perspective, if the hanger rooftop covers the 

6 entire building footprint, 41,000 gallons of runoff would be generated from a 1.68 inch rainfall. 

7 Assuming a drawdown of 2,000 gallons per week based on toilet flushing, the users would only 

use 5% of the 95th 8 percentile event. Because of the relatively large volume of water that would 

9 need to be collected and used, cisterns were not considered a feasible option to manage a 

10 significant volume of runoff at the site. 

11 However, site conditions did allow for the use of both permeable pavement and bioretention 

12 practices. Approximately 0.15 acres (6,500 sf) of the proposed site is undeveloped and available 

13 for bioretention. Based on Department of Defense facility requirements, ten percent of the 

14 parking area is designed with landscaping, usually around the perimeter and in landscaped 

15 islands. If this ten percent were designed as bioretention cells, then 0.035 acres of bioretention 

16 would be achieved. If bioretention cells were also placed in about 30% of the undeveloped area 

17 of the project, then an additional 0.045 acres of bioretention could be implemented. Note: not all 

18 undeveloped land was assumed to be available for bioretention because of conflicts with site 

19 utilities, security and anti-terrorism requirements and slopes that limited the use of infiltration 

20 practices directly adjacent to the hanger. 

21 

22 Figure 3-22. Actual siteSite and onsite control measuresOnsite Control Measures (Norfolk, VA) 
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Chapter 3. Controlling Urban Runoff 267 

1 Table 3-17. Estimated sizesSizes of onsite control measuresOnsite Control Measures for Scenario 

2 #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 

3 

4 The bioretention cells were designed with an effective storage depth of 10 inches, which 

5 included a depth from media surface to outlet of 10 inches. In this case study, state regulations 

6 precluded the project from taking credit for the storage potential provided by the void space 

7 within the bioretention cell media. Similarly, approximately 0.55 acres of the proposed site is 

8 impervious due to parking lots, streets, and sidewalks. Due to manufacturer’s recommendations 

9 that permeable pavement materials not be used in applications subject to heavy loads and 

10 potential pollutant exposure the access roads and parking lot access isles were assumed to be 

11 constructed from conventional impervious concrete or asphalt. Thus 60% of the parking area 

12 (primarily parking stalls and sidewalks), which is about 38% of the entire paved area, is assumed 

13 to be suitable for paver blocks. A high water table at the site limited the modeled net storage 

14 depth under paver blocks placed in the parking areas and sidewalks to four inches. This storage 

15 was calculated using the assumption that the pavement sub-base of 12 inches would have a 

16 minimum void space of approximately 30%. 

17 Comparison of the Runoff Estimation Methods 

As illustrated in each of the case studies above, runoff of the 95th 18 percentile storm was estimated 

19 in order to size onsite control measures. These estimates were produced by applying four 

20 different methods: the Direct Determination method, the Rational Method, the NRCS TR-55, and 

21 the EPA SWMM. The results comparing each of these methods for scenarios 1-7 are presented in 

22 Table 3-18. 

23 Table 3-18. 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 
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Chapter 3. Controlling Urban Runoff 268 

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

1 

2 As shown in the above table, the estimated runoff results from direct determination, the Rational 

3 Method, and SWMM are relatively similar. Runoff volumes using TR-55 are lower than the 

4 other estimates. SWMM modeling results using NRCS 24-hour rainfall distributions were nearly 

5 identical to the results based on uniform distribution. 

6 Table 3-19. 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 

7 

8 Conclusions 

9 Although sites varied in terms of climate and soil conditions, in most of the scenarios selected, 

the 95th 10 percentile storm event could be managed onsite with GI/LID systems. There are other 

11 infiltration, evapotranspiration and capture and use stormwater management options available 

12 than those used in these analyses. These options provide site managers additional flexibility to 

13 choose appropriate systems and practices to manage site runoff. 
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Chapter 3. Controlling Urban Runoff 269 

1 References 

2 Booth, Derek Direct Testimony. 2008. Pollution Control Hearings Board for the State of 

3 Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County 

4 Public Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish 

5 County; Clark County; and Pacificorp and Puget Sound Energy, Appellants, vs. 

6 Department of Ecology, Respondent, and King County; City of Seattle; Port of Tacoma, 

7 and Washington State Department of Transportation, Intervenors, August 2008. 

8 California Department of Transportation (Caltrans). 2003. Caltrans Storm Water Quality 

9 Handbooks. California Department of Transportation. 

10 Casey Trees. 2007. The Case for Trees – Relief from Summer Heat, 

11 www.caseytrees.org/resources/casefortrees.html. 

12 Galli, John. 1991. Thermal Impacts Associated with Urbanization and Stormwater Best 

13 Management Practices in Maryland, Anacostia Restoration Team for the Maryland 

14 Department of the Environment, Washington, DC. 

15 Grant, Gary, Luke Engleback and Barry Nicholson. 2003. Green Roofs: Their Existing Status 

16 and Potential for Conserving Biodiversity in Urban Areas, Report Number 498, English 

17 Nature Research Reports. 

18 Hathaway, J., and W.F. Hunt. 2007. Stormwater BMP Costs. North Carolina Department of 

19 Environment and Natural Resources. 

20 www.bae.ncsu.edu/stormwater/PublicationFiles/DSWC.BMPcosts.2007.pdf. 

21 Hirschman, David and John Kosco. 2008. Managing Stormwater in Your Community: A Guide 

22 for Building an Effective Post-Construction Program, Center for Watershed Protection, 

23 www.cwp.org/postconstruction. 

24 Holz, Thomas Written Direct Testimony. 2008. Pollution Control Hearings Board for the State 

25 of Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County 

26 Public Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish 

27 County; Clark County; and Pacificorp and Puget Sound Energy, Appellants, vs. 

28 Department of Ecology, Respondent, and King County; City of Seattle; Port of Tacoma, 

29 and Washington State Department of Transportation, Intervenors, August 2008. 

30 Horner, Richard Direct Testimony. 2008. Pollution Control Hearings Board for the State of 

31 Washington, Puget Soundkeeper Alliance and People for Puget Sound; Pierce County 

32 Public Works and Utilities Department; City of Tacoma; The Port of Seattle; Snohomish 

33 County; Clark County; and Pacificorp and Puget Sound Energy, Appellants, vs. 

34 Department of Ecology, Respondent, and King County; City of Seattle; Port of Tacoma, 

35 and Washington State Department of Transportation, Intervenors, August 2008. 

36 National Climatic Data Center (NCDC). 2007. NCDC precipitation data, CD-ROM, National 

37 Climatic Data Center. 
3/15/202-22-10 Chesapeake Bay Executive Order Section 502 Guidance — Do Not Cite or Quote — 

Chapter 3. Controlling Urban Runoff 270 

1 National Research Council. 2008. Urban Stormwater Management in the United States, The 

2 National Academies Press, Washington, DC. 

3 Shaver, E., R. Horner, J. Skupien, C. May, and G. Ridley. 2007. Fundamentals of Urban Runoff 

4 Management: Technical and Institutional Issues – 2nd Edition, North American Lake 

5 Management Society, Madison, WI. 

6 Schueler, T. and M. Helfrich. 1988. “Design of Wet Extended Detention Pond Systems,” Design 

7 of Urban Runoff Controls, L. Roesner and B. Urbonas eds., American Society of Civil 

8 Engineers, New York, NY. 

9 Schueler, T and H. Holland. 2000. The Practice of Watershed Protection: Techniques for 

10 Protecting our Nation’s Streams, Lakes, Rivers, and Estuaries. Center for Watershed 

11 Protection, Ellicott City, MD. 

12 U.S. Department of Agriculture, Natural Resources Conservation Service. 2007. National 

13 Engineering Handbook, title 210–VI. Part 630, chapter 7. Washington, DC. 

14 http://directives.sc.egov.usda.gov. 

15 U.S. Department of Agriculture, Soil Conservation Service. 1986. Urban Hydrology for Small 

16 Watersheds. Technical Release No. 55. Second Edition. Washington, D.C. 

17 U.S. Environmental Protection Agency, Managing Wet Weather with Green Infrastructure, 

18 www.epa.gov/greeninfrastructure. 

19 Vingarzan and Taylor. 2003. Trend Analysis of Ground Level Ozone in the Greater Vancouver/ 

20 Fraser Valley Area of British Columbia, Environment Canada – Aquatic and Atmospheric 

21 Sciences Division. 

22 Wisconsin DNR. 2008. Impact of Redevelopment on TSS Loads, Runoff Management, available at 

23 http://www.dnr.state.wi.us/runoff/pdf/rules/nr151/Impact_of_RedevTSSLoads_021308.pdf.
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Chapter 3. Controlling Urban Runoff 271 

1 Resources: Runoff Methodology Parameter Assumptions

2 Runoff from each land cover was estimated by the following equation: 

3 Runoff = Rainfall – Depression Storage – Infiltration Loss (1) 

4 Depression Storage 

5 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 

6 

7 1. ASCE, (1992). Design & Construction of Urban Stormwater Management Systems. New York, NY. 

8 2. Marsaleck, J., Jimenez-Cisreros, B., Karamouz, M., Malmquist, P-R., Goldenfum, J., and Chocat, B. 

9 (2007). Urban Water Cycle Processes and Interactions. Urban Water Series, UNESCO-IHP, Tyler & 

10 Francis. 

11 3. Walesh, S. G. (1989).Urban Surface Water Management. John Wiley & Sons, Inc. 

12 

13 Based on the above reference data, depression storage (or initial abstraction, the rainfall required 

14 for the initiation of runoff) to the direct determination method was assumed as follows: 

15 • Rooftop: 0.1 inches 

16 • Pavement: 0.1 inches 

17 • Pervious area: 0.2 inches 

18 Infiltration 

19 Infiltration loss occurs only in pervious areas. In this analysis, infiltration was estimated by 

20 Horton’s equation: 

21 Ft = fmin + (fmax – fmin) e - k t (2)

22 

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

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

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

26 k = infiltration rate decay factor (/hr), and

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

28 
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Chapter 3. Controlling Urban Runoff 272 

1 Reference infiltration parameters 

2 Maximum infiltration rate (in.hr), fmax

Partially dried out with Dry soils with Infiltration 

(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 

3 Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

4 Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

5 Athens, GA.

6 

7 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 

8 A: well drained sandy; D: poorly drained clay 

9 Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

10 Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

11 Athens, GA.

12 

13 Decay coefficient, k 

Soils k (sec-1) k (hr-1) 

0.00056 2 

0.00083 3 

0.00115 4 

Sandy 

Clay 0.00139 5 

14 Reference: Huber, W. C. and Dickinson, R. (1988). Storm Water Management Model User’s Manual, 

15 Version 4. EPA/600/3-88/001a (NTIS PB88-236641/AS), U.S. Environmental Protection Agency, 

16 Athens, GA.

17 

18 Based on the above reference data, infiltration parameters to the direct determination method 

19 were assumed as follows: 

20 • Hydrologic Soil Group B 

21 – Maximum infiltration rate: 5 in/hr 
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Chapter 3. Controlling Urban Runoff 273 

1 – Minimum infiltration rate: 0.3 in/hr 

2 – Decay factor: 2 /hr 

3 • Hydrologic Soil Group C 

4 – Maximum infiltration rate: 3 in/hr 

5 – Minimum infiltration rate: 0.1 in/hr 

6 – Decay factor: 3.5 /hr 

7 • Hydrologic Soil Group D 

8 – Maximum infiltration rate: 1 in/hr 

9 – Minimum infiltration rate: 0.02 in/hr 

10 – Decay factor: 5 /hr 

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

12 assumptions of a half hour ∆t: 

Infiltration Loss at the nth time-step = (f ·∆t) = {( )2/) } 1f f t 13 n− + n⋅∆ (3)

14 Integrated Infiltration Loss for 24 hours = ∑ (f ·∆t) (4)

15 Integrating infiltration loss during 24 hours with a half hour ∆t 

Infiltration rate (in/hr) aInfiltration 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 
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Chapter 3. Controlling Urban Runoff 274 

Infiltration rate (in/hr) aInfiltration volume (inches) b

timestep t (hr) 

Soil B Soil C Soil D Soil B Soil C Soil D 

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 

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 c9.743 3.430 0.769 

a

1 Calculated infiltration rate at each time by Equation (2) 

b

2 Calculated infiltration volume from the previous time to the current time by Equation (3) 

c

3 Integrated infiltration volume for 24 hours with a half hour ∆t by Equation (4) 

4 
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Chapter 3. Controlling Urban Runoff 275 

1 Based on the above calculation, 24-hr infiltration losses for pervious areas and bioretention areas 

2 were modeled as follows: 

3 • Soil Group B: 9.743 inches 

4 • Soil Group C: 4.430 inches 

5 • Soil Group D: 0.769 inches 

6 Infiltrations of underlying soils at paver blocks were modeled conservatively by applying the 

7 minimum infiltration rate for each soil type (Infiltration loss = fmin · 24) because the soils under 

8 the paver blocks may require or be subjected to some compaction for engineering stability. The 

9 estimated infiltration losses for each soil are presented below: 

10 • Soil Group B: (0.3 in/hr) · (24 hrs) = 7.2 inches 

11 • Soil Group C: (0.1 in/hr) · (24 hrs) = 2.4 inches 

12 • Soil Group D: (0.02 in/hr) · (24 hrs) = 0.48 inches 

13 Design Storage of Management Practices 

14 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 

15 

16 1. State of New Jersey. (2004). New Jersey Stormwater Best Management Practices Manual

17 www.nj.gov/dep/stormwater/tier_A/pdf/NJ_SWBMP_9.1 print.pdf. 

18 2. Maryland Department of the Environment (MDE), (2000). 2000 Maryland Stormwater Design Manual, 

19 Volumes I & II, prepared by the Center for Watershed Protection and the Maryland Department of the 

20 Environment, Water Management Administration, Baltimore, MD. 

21 www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp. 

22 3. Clar, M. L. and R. Green, (1993). Design Manual for Use of Bioretention in Storm Water Management, 

23 prepared for the Department of Environmental Resources, Watershed Protection Branch, Prince 

24 George's County, MD, prepared by Engineering Technologies Associates, Inc. Ellicott City, MD, and 

25 Biohabitats, Inc., Towson, MD. 

 

1 Ponding is a measure of retention capacity 
3/15/202-22-10 Chesapeake Bay Executive Order Section 502 Guidance — Do Not Cite or Quote — 

Chapter 3. Controlling Urban Runoff 276 

1 4. U.S. Environmental Protection Agency. (1999). Storm Water Technology Fact Sheet: Bioretention. 

2 EPA 832-F-99-012. Office of Water. US Environmental Protection Agency. Washington, D.C. 

3 www.epa.gov/owm/mtb/biortn.pdf. 

4 5. Prince George's County. Bioretention Design Specifications and Criteria. Prince George's County, 

5 Maryland. 

6 www.co.pg.md.us/Government/AgencyIndex/DER/ESG/Bioretention/pdf/bioretention_design_manual

7 .pdf. 

8 6. City of Indianapolis. (2008). Indianapolis Stormwater Design Manual. 

9 www.sustainindy.org/assets/uploads/4_05_Bioretention.pdf. 

10 

11 Paver Blocks 

Reference Media (inches) Void Space 

1 12 or more 40% 

2 9 or more 40% 

3 12 - 36 40% 

12 

13 1. Univ. of California at Davis. (2008). Low Impact Development Techniques: Pervious Pavement. 

14 http://extension.ucdavis.edu/unit/center_for_water_and_land_use/pervious_pavement.asp. 

15 2. AMEC Earth and Environmental, Center for Watershed Protection, Debo and Associates, Jordan Jones 

16 and Goulding, and Atlanta Regional Commission. (2001). Georgia Stormwater Management Manual 

17 Volume 2: Technical Handbook www.georgiastormwater.com/. 

18 3. Subsurface Infiltration Bed. www.tredyffrin.org/pdf/publicworks/CH2 - BMP4 Infiltration Bed.pdf. 

19 

20 Green Roofs 

Reference Media (inches) 

1 3 - 4 

2 1 - 6 

3 2 - 6 

21 

22 1. Charlie Miller. (2008). Extensive Green Roofs. Whole Building Design Guide (WBDG). 

23 www.wbdg.org/resources/greenroofs.php. 

24 2. Great Lakes WATER Institute. Green Roof Project: Green Roof Installation. 

25 www.glwi.uwm.edu/research/genomics/ecoli/greenroof/roofinstall.php. 

26 3. Paladino & Company. (2004). Green Roof Feasibility Review. King County Office Project. 

27 http://your.kingcounty.gov/solidwaste/greenbuilding/documents/KCGreenRoofStudy_Final.pdf. 

28 Based on the above reference data, design storages to the direct determination method were 

29 assumed as follows: 

30 • Bioretention: up to 10 inches (depending on practice used, site conditions, etc.) 
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Chapter 3. Controlling Urban Runoff 277 

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

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

3 Factors that influence total storage available include, ponding depth, available media void space, 

4 and supplemental storage if the system is designed with gravel or open pipes underneath the 

5 media.