3/15/20 2

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### Chapter 3. Controlling Urban Runoff 226 

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

# 438 Guidance 2

1 

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

### Runoff Requirements for Federal Projects under Sect ion 438 of the Energy Independence and 4 

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

. 5 

### This appendix includes procedures for calculating t he 95 th 

### percentile rainfall event, case studies 6 

### of stormwater designs to retain the 95 th 

### percentile rainfall event, and assumptions related to the 7 

### runoff methodology calculations. 8 

## Calculating the 95 th 

## Percentile Rainfall Event 9 

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

### determine the 95 th 

### percentile rainfall event for a location. Thirty y ears or more of monitoring 11 

### data are desirable to conduct an unbiased statistic al analysis. The National Climatic Data Center 12 

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

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

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

### universities, water treatment plants, or other faci lities might also maintain long-term 16 

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

### record should include the following basic informati on: 18



### • Location (monitoring station) 19



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



### • Total precipitation depth during the time-step 21 

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

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

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

### quality control as prescribed by the NCDC and has b een determined to be a valid data point. 25 

### There are several data processing steps to determin e the 95 th 

### percentile rainfall event using a 26 

### spreadsheet. These steps are summarized below: 27 

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

### NCDC website). 29 

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

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

### multiple columns of daily precipitation records. 323/15/20 2

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### Chapter 3. Controlling Urban Runoff 227 1 2 

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

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

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

### entire dataset). 6 

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

### further analysis. 8 

### 6. Remove small rainfall events (typically less than 0 .1 inches), which may not contribute to 9 

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

### general, does not produce runoff from most sites. 11

12 13 

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

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


### 7. Calculate the 95 th 

### percentile rainfall amount by applying the PERCENT ILE spreadsheet 16 

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

18 193/15/20 2

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### Chapter 3. Controlling Urban Runoff 228 

### Note: The PERCENTILE function returns the n th 

### percentile of value in the entire 1 

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

### percentile 2 

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

### 8. The 95 th 

### percentile was calculated in the previous step. Ho wever, if the user would like to 4 

### see this information represented graphically and ge t a relative sense of where individual 5 

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

### Derive a table showing percentile versus rainfall d epth to draw a curve as shown below. 7 

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

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

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

11 12

13 14 

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

### The 95 th 

### percentile storm event should correlate to the rai nfall depth calculated in step 7, 16 

### however the graph can be used to calculate rainfall depths at other percentiles (e.g., 50%, 90%). 173/15/20 2

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### Chapter 3. Controlling Urban Runoff 229



### Case Studies on Capturing the 95 th 

### Percentile Storm 1 

### Using Onsite Management Practices 2 3 

### I NTRODUCTION 4 

### This section contains nine case studies that are in tended to be representative of the range of 5 

### projects that are subject to the requirements legis lated in Section 438 of the Energy 6 

### Independence and Security Act. The facility example s in the case studies were selected to 7 

### illustrate project scenarios for differing geograph ic locations, site conditions, and project sizes 8 

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

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

### redevelopment projects should be designed to infilt rate, evapotranspirate, and/or harvest and use 11 

### runoff to the maximum extent technically feasible ( METF) to maintain or restore the pre12 

### development hydrology of the site. Scenarios 1-8 ar e examples of sites where it was technically 13 

### feasible to design the stormwater management system to retain the 95 th 

### percentile storm onsite. 14 

### Scenario 9, however, was provided as an example of an METF analysis where site constraints 15 

### allowed the designers to retain only 75% of the 95 th 

### percentile storm. 16 

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

### not include site specific design features such as r unoff routing, specific site infiltration rates, th e 18 

### structural loading capacity of buildings, etc. in t erms of stormwater practice selection. 19 

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

### analysis, has not been provided in this document be cause of the complexity of factors and the 21 

### lack of general applicability such an analysis woul d have. 22 

### Background 23 

### Numerous approaches exist for determining the volum e of runoff to be treated through 24 

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

### 95 th 

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

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

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

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

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

### to and including the 95 th 

### percentile storm event is analogous to maintaining or restoring the pre31 

### development hydrology with respect to the volume, f low rate, duration and temperature of the 32 

### runoff for most sites. 33


### Determination of the 95 th 

### Percentile Rainfall Event 34 

### The 95 th 

### percentile rainfall event was determined using the long-term daily precipitation records 35 

### from the National Climate Data Center (NCDC, 2007). By analyzing the frequency and rainfall 36 

### depths from daily rainfall records over 24-hour per iods, the 95 th 

### percentile storm event can be 37 

### determined. From a frequency analysis viewpoint, th e 95 th 

### percentile event is the storm event 38 

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

### Regional climate conditions and precipitation vary across the U.S. Because of local values, it is 40 

### essential that the implementing agency or departmen t establish the 95 th 

### percentile storm event for 413/15/20 2

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### Chapter 3. Controlling Urban Runoff 230 

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

### conditions. 2 

### Onsite Stormwater Management Practice Determination s 3 

### For the purposes of the case study scenarios, the f ollowing four categories of practices were 4 

### selected as the most appropriate practices for impl ementing Section 438 requirements: 5 

### bioretention, permeable pavements and pavers, ciste rns, and green roofs. These practices were 6 

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

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

### practices were considered. Bioretention practices w ere considered first because these systems 9 

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

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

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

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

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

### goal of capturing the 95 th 

### percentile rainfall event onsite. Examples of onsi te stormwater 15 

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

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

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

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

### illustrate the range of site conditions designers m ay encounter, and how site conditions impact 20 

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

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

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

### a combination of modular paving blocks, a green roo f, and cisterns was ultimately selected based 24 

### on cost and site suitability factors. 25 

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

### category of practice. These assumptions are not nec essarily an endorsement of a particular design 27 

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

### demonstrate the feasibility of the approach. For ex ample, bioretention retrofits can and should 29 

### often be located in prior impervious locations; how ever, in all modeled scenarios bioretention 30 

### was restricted to currently landscaped areas. The a ssumptions were: 31



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

### and microbes to capture, treat and infiltrate runof f. 33 

### It is assumed bioretention practices would be insta lled within currently landscaped 34 

### pervious areas or that pervious areas would be crea ted for bioretention cells. While 35 

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

### temporary storage. Bioretention areas would be desi gned to accept up to a depth of 10 37 

### inches of water across the surface of the bioretent ion cell (see Resources at end of this 38 

### Appendix). The conceptual design of this storage de pth would occur within the media 39 

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

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

### ponded depth generally not to exceed 12 inches. 423/15/20 2

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### Chapter 3. Controlling Urban Runoff 231 

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

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

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

### directly but could be applied at the point of stora ge overflow such that no overflow 4 

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

### maximize the storage and infiltration benefits of t hese systems. Designs utilizing 6 

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

### the media is permeable and the underdrain conveys t he runoff offsite through the 8 

### underdrain before it can be infiltrated. Because st andard underdrains typically discharge 9 

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

### retention capacity for the 95 th 

### percentile event volume. 11 

### The bioretention footprint for modeling purposes wa s calculated as one uniform area that 12 

### did not include side slopes. There is an expectatio n that actual bioretention cell 13 

### construction would be distributed throughout the si te with targeted locations based on 14 

### hydrology (natural flow paths) and soils with great er infiltrative capacity. Side slopes 15 

### may increase the surface excavation area required t o accommodate the footprint and 16 

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



### • Porous/permeable pavement : Transportation surfaces constructed of asphalt, c oncrete 18 

### or permeable pavers that are designed to infiltrate runoff. 19 

### Infiltration was modeled for the entire porous pave ment area with drainage pipes used 20 

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

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

### type pavers were generally applied in this design c ategory under the assumption that they 23 

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

### porous pavement applications are available that sup port heavy loads and can be designed 25 

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

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

### design depth could be achieved by specifying 10 inc hes of media depth that had 20% 28 

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

### above the bottom surface, with specified media cont aining 33% void space. This 30 

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

### water storage depth of 2 inches. 32 

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

### engineering stability. As a result, infiltration in to underlying soils was modeled 34 

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

### Resources at end of this Appendix). 36 

### Generally, porous pavement is not recommended for h igh traffic areas or loading bays 37 

### Because of this the scenarios assumed that only a p ercentage of total parking and road 38 

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

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

### pavements is available at: 41 

### http://cfpub.epa.gov/npdes/greeninfrastructure/tech nology.cfm#permpavements 423/15/20 2

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### Chapter 3. Controlling Urban Runoff 232 

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

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

### necessary to include additional storage volume to m eet the goal of onsite rainfall runoff 3 

### capture. The sizes of cisterns would be calculated based on site-specific rainfall, site4 

### specific spatial and structural conditions, use opp ortunities and rates, and consideration of 5 

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

### This total volume could be subdivided into any numb er of cisterns to provide the total 7 

### necessary storage but should be based on the imperv ious area and runoff quantities which 8 

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

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



### • Green roof : Roof designed with light weight soil media and pl anted with vegetation. 11 

### Frequently, green rooftop area is limited by struct ural capacity. In addition, other rooftop 12 

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

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

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

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

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

### Resources at end of this Appendix). 18 

### General Approach 19 

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

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

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

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

### illustrates the processes used for selecting and de termining the overall size of stormwater 24 

### management practices for each site. 253/15/20 2

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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 95 th 

### percentile 24
- hour rainfall event

Estimate the current ru noff

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)

Ye s No Select control measure(s) to fit the site and confirm performance3/15/20 2

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### Chapter 3. Controlling Urban Runoff 234 1 Figure 3-10. Flow chart depicting the process for d etermining control measures 2 using the 95 th 95 th percentile, 24-hour, annual rainfall event. 3 

### 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 95 th 

### 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)

Ye s No Select control measure(s) to fit the site and confirm performance3/15/20 2

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### Chapter 3. Controlling Urban Runoff 235 

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

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

### the scenario analyses that follow. 3 

### Collecting spatial data for a site 4 

### 1. Collect an aerial orthophotograph for the desired s ite. 5 

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

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

### 3. Categorize the digitized or planned land use/land c over based on surface hydrologic 8 

### conditions, e.g., rooftop, pavement, and pervious/l andscaped area. 9 

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

### Determining the 95 th 

### percentile, 24-hr rainfall event 11 

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

### NCDC Web site or other source). 13 

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

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

### multiple columns of daily precipitation records. 16 

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

### further analysis. 18 

### 5. Remove small rainfall events (typically less than 0 .1 inches) that may not contribute to 19 

### rainfall runoff. These small storms often produce l ittle if any appreciable runoff from 20 

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

### surface depression storage. 22 

### 6. Calculate the 95 th 

### percentile rainfall volume by applying the PERCENT ILE spreadsheet 23 

### function to a range of data cells. The PERCENTILE f unction returns the n th 

### percentile 24 

### value in the specified precipitation data range. Th is function can be used to determine the 25 

### 95 th 

### percentile storm event that captures all but the l argest 5% of storms. In MS Excel 26 [PERCENTILE(precipitation data range,95%)] 27 

### Estimating Current Runoff and Placing onsite contro l measures to capture the 95 th 

### percentile 28 

### rainfall event 29 

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

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

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

### generalized soils groups and their infiltration pro perties come from the EPA Stormwater 33 

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

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

### applying the 95 th 

### percentile rainfall to the existing site condition s (land use and soil 36 

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

### analysis, it is assumed that the rainfall amount is distributed over a 24 hour period. Actual 383/15/20 2

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### Chapter 3. Controlling Urban Runoff 236 

### rainfall event duration (and intensity) was not con sidered for determining rainfall runoff 1 

### (however, timing was considered when modeling infil tration). 2 

### 4. Determine flow paths so that management practice pl acements are in locations where 3 

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

### may require detailed topographic information or fur ther surveys this would be a task to be 5 

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

### exercise. 7 

### 5. Select onsite control practices to capture the curr ent 95 th 

### percentile runoff event; base the 8 

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

### and other factors such as site use and other constr aints. 10 

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

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

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

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

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

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

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

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

### stands to reason that more frequent, shorter durati on precipitation events are better represented 19 

### than less frequent, longer duration precipitation e vents. 20


### Scenarios 21 

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

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

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

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

### 95 th 

### percentile rainfall events for these locations pro duced rainfall depths that range from 1.00 26 

### inch to 1.77 inches (Table 2). 27
283/15/20 2

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### Chapter 3. Controlling Urban Runoff 237 1 Figure 3-11. Locations for analyzing onsite control measures Locations for Analyzing Onsite 2 Control Measure s . 3 

### The government facilities in the 8 case study locat ions were selected because they represent 4 

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

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

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

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

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

### soil results. 10 Table 3-2. Summary of rainfall data Rainfall Data for the seven locations Seven Locations

. 11 NCDC Daily Precipitation Data Rainfall Depth (inches) No Location Period of record Coverage Annual average 95 th 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% 3 5.8 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% 4 8.9 1.77 8 Norfolk, VA 1/1/1957 - 12/31/2006 (50 yrs) 99% 45 .4 1.68 123/15/20 2

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### Chapter 3. Controlling Urban Runoff 238 

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

### rooftop, pavement, and pervious area, as shown in T able 3. 2 Table 3-3. Summary of land Land -use determinations Determinations of the study sites Study Sites

. 3 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 

### Methods for Determining Runoff Volume 5 

### Direct Determination of Runoff Volume 6 

### Runoff from each land cover was estimated using a s implified volumetric approach based on the 7 

### following equation: 8 

### Runoff = Rainfall – Depression Storage – Infiltrat ion Loss 9 

### Again, this methodology does not consider routing o f runoff; therefore slope is not considered 10 

### when calculating on a volumetric basis. 11 

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

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

### Ft = f min 

### + ( f max 

### – f min 

### ) e - k t 14 

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

### f

min 

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

### f

max 

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

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


### t = time (hr) measured from time runoff first discharge d into infiltration area 19 

### Infiltration loss for the 24-hr rainfall duration w as estimated by the following equation with 20 

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

### Infiltration Loss = ∑ ( f ·∆ t ) 223/15/20 2

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### Chapter 3. Controlling Urban Runoff 239 

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

### equation, infiltration loss was integrated over a 2 4-hour period using a half hour time step while 2 

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

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

### end of this Appendix. 5 

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

### using an area-weighted calculation as shown below: 7 

### Runoff site 

### = {( Runoff

roof 

### ×A roof 

### )+( Runoff

pavement 

### ×A pavement 

### )+( Runoff pervious 

### ×A pervious 

### )}/

A site 8 

### Where Runoff site 

### = total runoff from the site (inches); A site 

### = site area (acres); Runoff roof 

### = runoff 9 

### from rooftop (inches); A roof 

### = rooftop area (acres); Runoff pavement 

### = runoff from pavement area 10 

### (inches); A pavement 

### = pavement area (acres); Runoff pervious 

### = runoff from pervious area (inches); 11 

### and A pervious 

### = pervious area (acres). 12 

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

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

### and 3. 15 

### Runoff roof 

### = 95th Rainfall – Depression Storage 16 

### = 1.23 – 0.1 = 1.13 inches 17 

### Runoff pavement 

### = 95th Rainfall – Depression Storage 18 

### = 1.23 – 0.1 = 1.13 inches 19 

### Runoff pervious 

### = 95th Rainfall – Depression Storage – Infiltrati on Loss 20 

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

### negative number) 22 

### Runoff site 

### = {( Runoff

roof 

### ×A roof 

### )+( Runoff

pavement 

### ×A pavement 

### )+( Runoff pervious 

### ×A pervious 

### )}/

A site 23 

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

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

### above. Because the volume removed from surface runo ff through infiltration was substantial, no 26 

### runoff occurred from the pervious area. 27 

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

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

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

### percent of the 95 th 

### percentile storm event within a bioretention syste m another onsite 31 

### management practice was selected to manage the runo ff that could provide the necessary 32 

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

### of treatment cells where possible. 34 

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

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

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

### loam textures with some loam, silt loam, silt, or s andy clay loam soil textures placed in this 383/15/20 2

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### Chapter 3. Controlling Urban Runoff 240 

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

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

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

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

### aggregated, of low bulk density, or contain greaterthan 35 percent rock fragments (USDA5 

### NRCS, 2007). The application of these hydrologic so il groups was intended to give reasonable 6 

### and somewhat conservative estimates of infiltration capacity. 7 

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

### end of this Appendix for citations of assumptions): 9



### • Depression storage (or initial abstraction) 10



### – Rooftop: 0.1 inches 11



### – Pavement: 0.1 inches 12



### – Pervious area: 0.2 inches 13



### • Horton Infiltration parameters 14



### – Hydrologic Soil Group B 15



### – Maximum infiltration rate: 5 in/hr 16



### – Minimum infiltration rate: 0.3 in/hr 17



### – Decay factor: 2 /hr 18



### – Hydrologic Soil Group C 19



### – Maximum infiltration rate: 3 in/hr 20



### – Minimum infiltration rate: 0.1 in/hr 21



### – Decay factor: 3.5 /hr 22



### • Design storage assumptions of control measures 23



### – Bioretention: up to 10 inches (but variable based o n balancing necessary storage 24 

### volume, media depth for plant survivorship, and sur face area limitations) 25



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



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

### Other Methods for Estimating Runoff Volume 28 

### Runoff from a site after applying the 95 th 

### percentile storm can be estimated by using a numbe r of 29 

### empirical, statistical, or mathematical methods. Se veral methods were considered in this 30 

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

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

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

### hydrograph. The EPA Stormwater Management Model (SW MM) can be used to simulate 34 

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

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

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

### Program – Fortran (HSPF, USDA) is a watershed and l and use based lumped model that can be 383/15/20 2

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### Chapter 3. Controlling Urban Runoff 241 

### used to compute the movement of water and pollutant s when evaluating the effects of land use 1 

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

### regionally calibrated modeling parameters are incor porated into HSPF. QUALHYMO is a 3 

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

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

### storage tracking, baseflow and deep volumetric loss es, and other variables. 6 

### Many of the existing tools for analyzing distribute d systems use some part or all of the principles 7 

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

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

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

### Calculator (Center for Neighborhood Technologies) e stimates the benefit of onsite GI/LID 11 

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

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

### control measure sizing on neighborhood or site leve l spatial scales. The WWHM (Western 14 

### Washington Hydrology Model) is a regionally calibra ted HSPF model intended for use in sizing 15 

### stormwater detention and water quality facilities t o meet the Washington State Department of 16 

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

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

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

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

### features. 21
223/15/20 2

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### Chapter 3. Controlling Urban Runoff 242 Table 3-4. Potential methods for analyzing control measures Potential Methods for Analyzing 1 Control Measure s . 2

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 Yes b Yes Yes No No Neighborhood Yes Yes Yes Yes Possible Possible Spatial scale Regional Yes Yes c Yes No Yes Yes

Peak Discharge Yes Yes Yes No Yes Yes
Runoff Volume Yes Yes Yes Yes Yes Yes Hydrograph Yes a 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 

### From the viewpoint of modeling both lot-level and n eighborhood scale projects, the Rational 4 

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

### scenario analyses. Strength and weakness of these m ethods are presented below: 6 Table 3-5. Comparison of approaches for determining runoff volume

. 7 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 skills3/15/20 2

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### Chapter 3. Controlling Urban Runoff 243 1 

### Each method requires specific parameters for estima ting runoff from a site. Runoff coefficients 2 

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

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

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

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

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

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

### CN was determined on the basis of the hydrologic so il group and the status of grass cover 9 

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

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

### estimated by using an area-weighted calculation (US DA-SCS, 1986). 12 

### In SWMM modeling, infiltration was modeled using Ho rton’s equation. The same infiltration 13 

### parameters and depression storage values used in th e direct determination method of runoff 14 

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

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

### step was applied for the SWMM model runs. 17 

### Runoff Methodology Results 18 

### Stormwater management practice sizes (and depth) we re determined using the Direct 19 

### Determination approach to capture the volume of run off generated in a 95 th 

### percentile rainfall 20 

### event at each location. Total acreage, impervious a rea, the 95 th 

### percentile rainfall event, the 21 

### current expected runoff for the 95 th 

### percentile rainfall event, and the future runoff w ith 22 

### stormwater management controls were reported for ea ch site. Results were summarized for the 23 

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

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

### practices were placed only on undeveloped or landsc aped areas without regard for true flow 26 

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

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

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

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

### areas that are most suitable for infiltration. In o ther cases, areas that are currently impervious 31 

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

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

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

### To compare other approaches of runoff estimation, a lternate methodologies were also employed 35 

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

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

### Although flood control is not the focus of this gui dance, most localities have flood control 38 

### requirements that will need to be considered in des igning control measures to comply with 39 

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

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

### The 10-year design storms were selected from the NR CS TR-55 Manual (USDA, 1986) for both 423/15/20 2

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### Chapter 3. Controlling Urban Runoff 244 

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

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

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

### peak rates of runoff and prevent flooding. 4


## Cost Estimates for Selected Scenarios 5 

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

### with Section 438 (retention of the 95 th 

### percentile rainfall event) and capital costs for a traditional 7 

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

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

### GI/LID controls. 103/15/20 2

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### Chapter 3. Controlling Urban Runoff 245 

## Scenario #1 - Charleston, West Virginia WV 1 

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

### 12). If the 95 th 

### percentile rainfall event (1.23 inches) occurred o n the existing site (i.e., with no 3 

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

### generated and require management. The runoff from t he 95 th 

### percentile rainfall event could be 5 

### retained by the installation of bioretention system s totaling 0.03 acres if hydrologic soil group B 6 

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

### site. Assuming that bioretention practices are plac ed in areas that are currently pervious or 8 

### landscaped, a total of 0.2 acres of pervious area w ould be available for the placement of 9 

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

### was 8 inches. 11

12 Figure 3-12. Actual Site and Onsite Control Measure s (Charleston, WV) 133/15/20 2

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### Chapter 3. Controlling Urban Runoff 246 Table 3-6. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 1 #1 (Charleston, WV) 2 Total Area (acres) 0.7 Estimated Imperviousness (%) 73% 95 th Percentile Rainfall Event (inches) 1.23 Expected Runoff for the 95 th Percentile Rainfall Event (inches) 0.82 Stormwater Management Area Required Hydrologic Soil Group B C Bioretention estimated by Direct Determination meth od (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 

### Note: The two hydrologic methods used (direct deter mination and SWMM) estimated similar 3 

### bioretention sizes. 43/15/20 2

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### Chapter 3. Controlling Urban Runoff 247 

## Scenario #2 - Denver, Colorado CO 1 

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

### 95 th 

### percentile rainfall event (1.07 inches) occurred o n the existing site (i.e., with no control 3 

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

### runoff from the 95 th 

### percentile rainfall event could be retained by the installation of bioretention 5 

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

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

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

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

### within the designated bioretention area was 6 inche s. 10

11 Figure 3-13. Actual site Site and onsite control measures Onsite Control Measure s (Denver, CO) 123/15/20 2

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### Chapter 3. Controlling Urban Runoff 248 Table 3-7. Estimated sizes Sizes of onsite control measures Onsite Control Measure s for Scenario 1 #2 (Denver, CO) 2 Total Area (acres) 4.5 Estimated Imperviousness (%) 55% 95 th Percentile Rainfall Event (inches) 1.07 Expected Runoff for the 95 th 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 

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

### onsite control measures to retain the 95 th 

### percentile rainfall event versus the costs to inst all 5 

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

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

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

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

### includes this cost under the traditional column. No te: typical land development practices involve 10 

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

### assumed that the use of GI/LID practices would requ ire less underground infrastructure because 12 

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

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

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

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

### grey infrastructure. The cost estimates were develo ped for Hydrologic Soil Group B. 173/15/20 2

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### Chapter 3. Controlling Urban Runoff 249 Table 3-8. Estimated costs Costs for Scenario #2 (Denver, CO) 1 Sizes of Onsite Control Practices Controls for 95 th Percentile Event Traditional Stormwater Controls Rainfall depth (in) 1.07 Bioretention (acres) 0.1 Paver blocks (acres) 0 Green roof (acres) 0

WQV (ac-ft) - 0.18 Off-site Pond 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

WQV (ac-ft) $14,833 Off-site Pond 10-Yr Fld Cntr (ac-ft) $10,073 $9,527

Pipe $8.990 $16,982 Infrastructure Inlet $9.920 $14,880 Land Area (assumes $300K/acre) $14,500 $31,500 Sum $75,978 $91,909 % difference from Traditional -17.3% 23/15/20 2

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### Chapter 3. Controlling Urban Runoff 250 

## Scenario #3 - Cincinnati, Ohio OH 1 

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

### 95 th 

### percentile rainfall event (1.45 inches) occurred o n the existing site (i.e., no control measures 3 

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

### The runoff from the 95 th 

### percentile rainfall event could be retained by the installation of 5 

### bioretention systems totaling 0.8 acres if the hydr ologic soil group B is present or 1.3 acres if 6 

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

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

### 9.4 acres of pervious area is available for the pla cement of bioretention systems. The design 9 

### storage depth of media within the designated bioret ention area was 8 inches. 10

11 Figure 3-14. Actual site and onsite control measures Actual Site and Onsite Control Measure s 12 (Cincinnati, OH) 133/15/20 2

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### Chapter 3. Controlling Urban Runoff 251 Table 3-9. Estimated sizes Sizes of onsite control measures Onsite Control Measure s for Scenario 1 #3 (Cincinnati, OH) 2 Total Area (acres) 19 Estimated Imperviousness (%) 51% 95 th Percentile Rainfall Event (inches) 1.45 Expected Runoff for the 95 th 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 3.733/15/20 2

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### Chapter 3. Controlling Urban Runoff 252 

## Scenario #4 - Portland, Oregon OR 1 

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

### 95 th 

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

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

### imperviousness among the 7 sites. 5 

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

### discharged by the 95 th 

### percentile rainfall event with bioretention. As a result, other practices 7 

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

### cisterns, and porous pavement. Based on the technic al considerations of constructing and 9 

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

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

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

### gallons of total volume could be captured in cister ns for use on this urbanized site. Using this 13 

### system of four different practices, all runoff for the 95 th 

### percentile rainfall event would be 14 

### retained (Table 10). 15

16 Figure 3-15. Actual Site and Onsite Control Measure s (Portland, OR) 173/15/20 2

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### Chapter 3. Controlling Urban Runoff 253 Table 3-10. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 1 #4 (Portland, OR) 2 Total Area (acres) 27 Estimated Imperviousness (%) 95% 95 th percentile Rainfall Event (inches) 1.00 Expected Runoff for the 95 th 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 (acr es) 0.4 Green Roof estimated by Direct Determination (acres ) 1.7 Cistern volume estimated by Direct Determination (g allons) 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 bec ause the other control options were maximized based on the site-specific design assumptions. 3 

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

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

### the 8.8 acres of current rooftop area, 1.7 acres (2 0% of the rooftop area) could be retrofitted into 6 

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

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

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

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

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

### type and design volumes, or with adequate sub-bases or amended soils. 123/15/20 2

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### Chapter 3. Controlling Urban Runoff 254 

## Scenario #5 – Near Phoenix, Arizona AZ 1 

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

### 95 th 

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

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

### the 95 th 

### percentile rainfall event could be retained by ins talling bioretention systems totaling 0.06 5 

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

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

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

### for the placement of these practices. The design st orage depth of media within the designated 9 

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

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

### and 0.08 acre-ft for type C soils. 12

13 Figure 3-16. Actual site Site and onsite control measures Onsite Control Measure s (Phoenix, AZ) 143/15/20 2

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### Chapter 3. Controlling Urban Runoff 255 Table 3-11. Estimated sizes Sizes of onsite control measures Onsite Control Measure s for Scenario 1 #5 (Phoenix, AZ) 2 Total Area (acres) 2 Estimated Imperviousness (%) 47% 95 th Percentile Rainfall Event (inches) 1.00 Expected Runoff for the 95 th 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 33/15/20 2

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### Chapter 3. Controlling Urban Runoff 256 

## Scenario #6 - Boston, Massachusetts MA 1 

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

### the 95 th 

### percentile rainfall event (1.52 inches) occurred o n the existing site (i.e., with no control 3 

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

### characteristics, there is adequate area to place ap propriately sized bioretention cells to capture the 5 

### 95 th 

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

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

### control measure and the system was designed such th at the necessary design parameters could be 8 

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

### the volume. The runoff from the 95 th 

### percentile rainfall event could be retained by ins talling a 10 

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

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

### accommodate the paver block system. The area retrof itted with paver blocks would primarily be 13 

### dedicated for use as parking stalls. 14

15 Figure 3-17. Actual site Site and onsite control measures Onsite Control Measure s (Boston, MA) 163/15/20 2

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### Chapter 3. Controlling Urban Runoff 257 Table 3-12. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 1 #6 (Boston, MA) 2 Total Area (acres) 3.5 Estimated Imperviousness (%) 69% 95 th Percentile Rainfall Event (inches) 1.52 Expected Runoff for the 95 th 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 0.963/15/20 2

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### Chapter 3. Controlling Urban Runoff 258 

## Scenario #7 - Atlanta, Georgia GA 1 

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

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

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

### the 95 th 

### percentile rainfall event could not be adequately retained solely with bioretention 5 

### systems. Based on the technical considerations of c onstructing and maintaining control measures 6 

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

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

### the stormwater management techniques used on the si te includes both bioretention and paver 9 

### blocks as presented in Table 13, then all runoff fo r the 95 th 

### percentile rainfall event would be 10 

### controlled. 11

12 Figure 3-18. Actual site Site and onsite control measures Onsite Control Measure s (Atlanta, GA) 133/15/20 2

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### Chapter 3. Controlling Urban Runoff 259 Table 3-13. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 1 #7 (Atlanta, GA) 2 Total Area (acres) 21 Estimated Imperviousness (%) 70% 95 th Percentile Rainfall Event (inches) 1.77 Expected Runoff for the 95 th 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 Determinat ion (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 assu mptions. **Because TR-55 estimated smaller runoff in this sc enario, bioretention can retain all of the 95 th percentile runoff if the site has soil group B. 

### For the example site in Atlanta, GA, areas of 1.8 a cres for hydrologic soil group B, and 4.1 acres 3 

### for hydrologic soil group C, would be required to m anage the runoff discharged from a 95 th 4 

### percentile rainfall event. Assuming that bioretenti on practices are only placed in areas that are 5 

### currently pervious or landscaped, a total of 6.2 ac res of pervious area is available for the 6 

### placement of bioretention systems. The design stora ge depth of media within the designated 7 

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

### remaining volume on the 10.8 acres of existing pave d area. 9 

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

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

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

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

### designed using the TR-55 model were smaller than th ose designed using the Direct 14 

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

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

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

### onsite control measures to retain the 95 th 

### percentile rainfall event, and costs to install tr aditional 18 

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

### cost estimates were developed for Hydrologic Soil G roup B. 203/15/20 2

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### Chapter 3. Controlling Urban Runoff 260 Table 3-14. Estimated costs Costs for Scenario #7 (Atlanta, GA) 1 Sizes of Onsite Control Practices Controls for 95 th Percentile Event Traditional Stormwater Controls Rainfall depth (in) 1.77 Bioretention (acres) 0.94 Paver blocks (acres) 0.86

WQV (ac-ft) - 1.75 Off-site Pond 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

WQV (ac-ft) $0 $72,888 Off-site Pond 10-Yr Fld Cntr (ac-ft) $39,648 $0

Pipe $54,827 $191,095 Infrastructure Inlet $52,080 $79,360 Land Area (assumes $300K/acre) $84,000 $175,000 Sum $700,356 $637,368 % difference from Traditional 9.9% 23/15/20 2

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### Chapter 3. Controlling Urban Runoff 261 

## Scenario #8 - Cincinnati, Ohio OH 1 

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

### 95 th 

### percentile rainfall event (1.45 inches) occurred o n the existing site (i.e., with no control 3 

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

### the 95 th 

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

### totaling 0.8 acres if the hydrologic soil group B i s present or 1.3 acres if hydrologic soil group C 6 

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

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

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

### the designated bioretention area was 8 inches. 10 

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

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

### site. Alternatively, simulations could have been ru n under the assumption that the use of 13 

### infiltration practices were precluded by contaminat ed soils or high ground water tables. Under 14 

### these site conditions, bioretention options are sev erely limited and cannot be used to adequately 15 

### capture the entire 95 th 

### percentile storm event. As a result, options such as cisterns and green 16 

### roofs were considered. In the absence of management practices, the 95 th 

### percentile rainfall event 17 

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

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

### management. Based on the technical considerations o f constructing and maintaining controls at 20 

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

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

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

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

### practices evaluated because of cost and potential s tructural issues associated with design and 25 

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

### 15, all runoff for the 95 th 

### percentile rainfall event would be retained. From a management 27 

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

### was 6 inches because of the low infiltration rates adopted for this scenario. 29
303/15/20 2

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### Chapter 3. Controlling Urban Runoff 262 1 Figure 3-19. Actual site Site and onsite control measures Onsite Control Measure s (Cincinnati, OH) 2

Table 3-15. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 3 #8 (Cincinnati, OH) 4 Total Area (acres) 19 Estimated Imperviousness (%) 51% 95 th Percentile Rainfall Event (inches) 1.45 Expected Runoff for the 95 th Percentile Rainfall Event (inches) 0.92 Stormwater Management Applied Hydrologic Soil Group D Bioretention estimated by Direct Determination (acr es) 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 

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

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

### acres (30%) of parking lot converted to paver block s; and 0.5 acres (30%) of rooftop area were 8 

### retrofitted to green roof areas for this site, then 97% of stormwater runoff from the 95 th 

### percentile 9 

### storm would be captured on site. By also adding one or more cisterns (as indicated in Figure 19), 103/15/20 2

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### Chapter 3. Controlling Urban Runoff 263 

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

### the 95 th 

### percentile event can be managed onsite with GI/LID practices. 23/15/20 2

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### Chapter 3. Controlling Urban Runoff 264 

## Scenario #9 – Norfolk, Virginia VA 1 

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

### the land use categories for the site. Figure 20 dep icts the site and associated facilities. Site 3 

### specific factors based on an METF analysis allow ma nagement of 75 % of the 95 th 

### percentile 4 

### storm onsite (1.27 inches). The remaining portion o f the 95 th 

### percentile rainfall event (0.41 5 

### inches would be discharged off of the site. 6

Table 3-16. Land use determination after redevelopment La nd Use Determin ation After 7 Redevelopment 8 

### 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 Figure 3-20. Proposed redevelopment scenario Proposed Redevelopmen t Scenario 113/15/20 2

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### Chapter 3. Controlling Urban Runoff 265 1

2 Figure 3-21. Location of facility Facility (Norfolk, VA) 3


### Site conditions and intended uses limited the numbe r of practices that were technically feasible 4 

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

### project includes the construction of an airplane ha nger which lacks the structural strength to 6 Redevelopment Site

Redevelopment Site3/15/20 2

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### Chapter 3. Controlling Urban Runoff 266 

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

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

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

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

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

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

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

### use 5% of the 95 th 

### percentile event. Because of the relatively large volume of water that would 8 

### need to be collected and used, cisterns were not co nsidered a feasible option to manage a 9 

### significant volume of runoff at the site. 10 

### However, site conditions did allow for the use of b oth permeable pavement and bioretention 11 

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

### for bioretention. Based on Department of Defense fa cility requirements, ten percent of the 13 

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

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

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

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

### undeveloped land was assumed to be available for bi oretention because of conflicts with site 18 

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

### practices directly adjacent to the hanger. 20

21 Figure 3-22. Actual site Site and onsite control measures Onsite Control Measures (Norfolk, VA) 223/15/20 2

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### Chapter 3. Controlling Urban Runoff 267 Table 3-17. Estimated sizes Sizes of onsite control measures Onsite Control Measures for Scenario 1 #9 (Norfolk, VA) 2 Total Area (acres) 1.6 Estimated Imperviousness (%) 91% 95 th Percentile Rainfall Event (inches) 1.68 Expected Runoff for the 95 th 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 meth od (acres) 0.08 3 

### The bioretention cells were designed with an effect ive storage depth of 10 inches, which 4 

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

### precluded the project from taking credit for the st orage potential provided by the void space 6 

### within the bioretention cell media. Similarly, appr oximately 0.55 acres of the proposed site is 7 

### impervious due to parking lots, streets, and sidewa lks. Due to manufacturer’s recommendations 8 

### that permeable pavement materials not be used in ap plications subject to heavy loads and 9 

### potential pollutant exposure the access roads and p arking lot access isles were assumed to be 10 

### constructed from conventional impervious concrete o r asphalt. Thus 60% of the parking area 11 

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

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

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

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

### minimum void space of approximately 30%. 16 

## Comparison of the Runoff Estimation Methods 17 

### As illustrated in each of the case studies above, r unoff of the 95 th 

### percentile storm was estimated 18 

### in order to size onsite control measures. These est imates were produced by applying four 19 

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

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

### Table 3-18. 22 Table 3-18. Comparison of the estimated runoff (uni t: inches) 23

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 0.173/15/20 2

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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 

### As shown in the above table, the estimated runoff r esults from direct determination, the Rational 2 

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

### other estimates. SWMM modeling results using NRCS 2 4-hour rainfall distributions were nearly 4 

### identical to the results based on uniform distribut ion. 5 Table 3-19. Applicability of the methods for analyz ing onsite control measures 6 Purpose Direct Determination Rational Method TR-55* SWMM Planning Tool Applicable Applicable Applicable Appl icable 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 sma ll storms
7


## Conclusions 8 

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

### the 95 th 

### percentile storm event could be managed onsite wit h GI/LID systems. There are other 10 

### infiltration, evapotranspiration and capture and us e stormwater management options available 11 

### than those used in these analyses. These options pr ovide site managers additional flexibility to 12 

### choose appropriate systems and practices to manage site runoff. 133/15/20 2

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### Chapter 3. Controlling Urban Runoff 269 

## References 1 

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

### Washington , Puget Soundkeeper Alliance and People for Puget S ound; Pierce County 3 

### Public Works and Utilities Department; City of Taco ma; The Port of Seattle; Snohomish 4 

### County; Clark County; and Pacificorp and Puget Soun d Energy, Appellants, vs. 5 

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

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

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

### Handbooks . California Department of Transportation. 9 

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

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

. 11 

### Galli, John. 1991. Thermal Impacts Associated with Urbanization and St ormwater Best 12 

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

### Department of the Environment, Washington, DC. 14 

### Grant, Gary, Luke Engleback and Barry Nicholson. 20 03. Green Roofs: Their Existing Status 15 

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

### Nature Research Reports. 17 

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

### Environment and Natural Resources. 19 

### www.bae.ncsu.edu/stormwater/PublicationFiles/DSWC.B MPcosts.2007.pdf

. 20 

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

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

### www.cwp.org/postconstruction

. 23 

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

### of Washington , Puget Soundkeeper Alliance and People for Puget S ound; Pierce County 25 

### Public Works and Utilities Department; City of Taco ma; The Port of Seattle; Snohomish 26 

### County; Clark County; and Pacificorp and Puget Soun d Energy, Appellants, vs. 27 

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

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

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

### Washington , Puget Soundkeeper Alliance and People for Puget S ound; Pierce County 31 

### Public Works and Utilities Department; City of Taco ma; The Port of Seattle; Snohomish 32 

### County; Clark County; and Pacificorp and Puget Soun d Energy, Appellants, vs. 33 

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

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

### National Climatic Data Center (NCDC). 2007. NCDC pr ecipitation data, CD-ROM, National 36 

### Climatic Data Center. 373/15/20 2

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### Chapter 3. Controlling Urban Runoff 270 

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

### National Academies Press, Washington, DC. 2 

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

### Management: Technical and Institutional Issues – 2n d Edition , North American Lake 4 

### Management Society, Madison, WI. 5 

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

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

### Engineers, New York, NY. 8 

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

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

### Protection, Ellicott City, MD. 11 

### U.S. Department of Agriculture, Natural Resources C onservation Service. 2007. National 12 

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

. 13 

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

. 14 

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

### Watersheds . Technical Release No. 55. Second Edition. Washing ton, D.C. 16 

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

### www.epa.gov/greeninfrastructure

. 18 

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

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

### Sciences Division. 21 

### Wisconsin DNR. 2008. Impact of Redevelopment on TSS Loads, Runoff Manage ment , available at 22 

### http://www.dnr.state.wi.us/runoff/pdf/rules/nr151/I mpact_of_RedevTSSLoads_021308.pdf

. 233/15/20 2

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### Chapter 3. Controlling Urban Runoff 271 

## Resources: Runoff Methodology Parameter Assumptions 1 

### Runoff from each land cover was estimated by the fo llowing equation: 2 

### Runoff = Rainfall – Depression Storage – Infiltrat ion Loss (1) 3 

### Depression Storage 4 Reference depression storage (inches) 5 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 

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

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

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

### Francis. 10 

### 3. Walesh, S. G. (1989). Urban Surface Water Management . John Wiley & Sons, Inc. 11 12 

### Based on the above reference data, depression stora ge (or initial abstraction, the rainfall required 13 

### for the initiation of runoff) to the direct determi nation method was assumed as follows: 14



### • Rooftop: 0.1 inches 15



### • Pavement: 0.1 inches 16



### • Pervious area: 0.2 inches 17


### Infiltration 18 

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

### Horton’s equation: 20 

### Ft = f min 

### + ( f max 

### – f min 

### ) e - k t (2) 21
22 

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

### f

min 

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

### f

max 

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

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


### t = time (hr) measured from time runoff first discharge d into infiltration area 27 283/15/20 2

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### Chapter 3. Controlling Urban Runoff 272 Reference infiltration parameters 1 Maximum infiltration rate (in.hr), f max 2 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 

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

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

### Athens, GA. 5 6 Minimum infiltration rate (in/hr), f min 7 Hydrologic Soil Group Infiltration (in/hr) A 0.45 - 0.30 B 0.30 - 0.15 C 0.15 - 0.05 D 0.05 - 0 

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

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

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

### Athens, GA. 11 12 Decay coefficient, k 13 Soils k (sec -1 ) k (hr -1 )

0.00056 2 0.00083 3 0.00115 4 Sandy

Clay 0.00139 5 

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

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

### Athens, GA. 16 17 

### Based on the above reference data, infiltration par ameters to the direct determination method 18 

### were assumed as follows: 19



### • Hydrologic Soil Group B 20



### – Maximum infiltration rate: 5 in/hr 213/15/20 2

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### Chapter 3. Controlling Urban Runoff 273 

### – Minimum infiltration rate: 0.3 in/hr 1



### – Decay factor: 2 /hr 2



### • Hydrologic Soil Group C 3



### – Maximum infiltration rate: 3 in/hr 4



### – Minimum infiltration rate: 0.1 in/hr 5



### – Decay factor: 3.5 /hr 6



### • Hydrologic Soil Group D 7



### – Maximum infiltration rate: 1 in/hr 8



### – Minimum infiltration rate: 0.02 in/hr 9



### – Decay factor: 5 /hr 10


### Infiltration loss for the 24-hr rainfall duration w as estimated by the following equations with 11 

### assumptions of a half hour ∆ t : 12 

### Infiltration Loss at the n th 

### time-step = ( f ·∆ t ) = }

)2/) {( 1 

### t f f n n 

### ∆

⋅ +

− 

### (3) 13 

### Integrated Infiltration Loss for 24 hours = ∑ ( f ·∆ t ) (4) 14 Integrating infiltration loss during 24 hours with a half hour ∆ t 15 Infiltration rate (in/hr) a Infiltration volume (inches) b

timestep t (hr) Soil B Soil C Soil D Soil B Soil C Soil D 0 0 5 3 1 0 0 0 1 0.5 2.03 0.60 0.100 1.757 0.901 0.275 2 1 0.94 0.19 0.027 0.741 0.198 0.032 3 1.5 0.53 0.12 0.021 0.368 0.076 0.012 4 2 0.39 0.10 0.02 0.230 0.054 0.01 5 2.5 0.33 0.1 0.02 0.179 0.05 0.01 6 3 0.31 0.1 0.02 0.161 0.05 0.01 7 3.5 0.30 0.1 0.02 0.154 0.05 0.01 8 4 0.3 0.1 0.02 0.15 0.05 0.01 9 4.5 0.3 0.1 0.02 0.15 0.05 0.01 10 5 0.3 0.1 0.02 0.15 0.05 0.01 11 5.5 0.3 0.1 0.02 0.15 0.05 0.01 12 6 0.3 0.1 0.02 0.15 0.05 0.01 13 6.5 0.3 0.1 0.02 0.15 0.05 0.01 14 7 0.3 0.1 0.02 0.15 0.05 0.01 15 7.5 0.3 0.1 0.02 0.15 0.05 0.01 16 8 0.3 0.1 0.02 0.15 0.05 0.01 17 8.5 0.3 0.1 0.02 0.15 0.05 0.01 18 9 0.3 0.1 0.02 0.15 0.05 0.013/15/20 2

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### Chapter 3. Controlling Urban Runoff 274 Infiltration rate (in/hr) a Infiltration volume (inches) btimestep 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 c 9.743 3.430 0.769

a 

### Calculated infiltration rate at each time by Equat ion (2) 1
b 

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

c 

### Integrated infiltration volume for 24 hours with a half hour ∆ t

by Equation (4) 3 43/15/20 2

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### Chapter 3. Controlling Urban Runoff 275 

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

### were modeled as follows: 2



### • Soil Group B: 9.743 inches 3



### • Soil Group C: 4.430 inches 4



### • Soil Group D: 0.769 inches 5 

### Infiltrations of underlying soils at paver blocks w ere modeled conservatively by applying the 6 

### minimum infiltration rate for each soil type (Infil tration loss = f min 

### · 24) because the soils under 7 

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

### estimated infiltration losses for each soil are pre sented below: 9



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



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



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

### Design Storage of Management Practices 13 Bioretention 14
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 

### 1. State of New Jersey. (2004). New Jersey Stormwater Best Management Practices Man ual 16 

### www.nj.gov/dep/stormwater/tier_A/pdf/NJ_SWBMP_9.1 p rint.pdf

. 17 

### 2. Maryland Department of the Environment (MDE), (2 000). 2000 Maryland Stormwater Design Manual, 18 

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

### Environment, Water Management Administration, Balti more, MD. 20 

### www.mde.state.md.us/Programs/WaterPrograms/Sediment andStormwater/stormwater_design/index.asp

. 21 

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

### prepared for the Department of Environmental Resour ces, Watershed Protection Branch, Prince 23 

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

### Biohabitats, Inc., Towson, MD. 25

1 Ponding is a measure of retention capacity3/15/20 2

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- 10 Chesapeake Bay Executive Order Section 502 Guidance — Do Not Cite or Quote —



### Chapter 3. Controlling Urban Runoff 276 4. U.S. Environmental Protection Agency. (1999). Storm Water Technology Fact Sheet: Bioretention

. 1 

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

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

. 3 

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

### Maryland. 5 

### www.co.pg.md.us/Government/AgencyIndex/DER/ESG/Bior etention/pdf/bioretention_design_manual 6 

### .pdf

. 7 

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

. 8 

### www.sustainindy.org/assets/uploads/4_05_Bioretentio n.pdf

. 9 10 Paver Blocks 11 Reference Media (inches) Void Space

1 12 or more 40% 2 9 or more 40% 3 12 - 36 40% 12 

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

### http://extension.ucdavis.edu/unit/center_for_water_ and_land_use/pervious_pavement.asp

. 14 

### 2. AMEC Earth and Environmental, Center for Watersh ed Protection, Debo and Associates, Jordan Jones 15 

### and Goulding, and Atlanta Regional Commission. (200 1). Georgia Stormwater Management Manual 16 

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

. 17 

### 3. Subsurface Infiltration Bed. www.tredyffrin.org/pdf/publicworks/CH2 - BMP4 Infil tration Bed.pdf

. 18 19 Green Roofs 20 Reference Media (inches)

1 3 - 4 2 1 - 6 3 2 - 6 21 

### 1. Charlie Miller. (2008). Extensive Green Roofs. W hole Building Design Guide (WBDG). 22 

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

. 23 

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

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

. 25 

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

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

. 27 

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

### assumed as follows: 29



### • Bioretention: up to 10 inches (depending on practic e used, site conditions, etc.) 303/15/20 2

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### Chapter 3. Controlling Urban Runoff 277 

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



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

### Factors that influence total storage available incl ude, ponding depth, available media void space, 3 

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

### media. 5