Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

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Appendix D4LID Credits Background ReportI. INTRODUCTIONMultiple environmental agencies, including the Central Valley Regional Water Quality Control Board,have recently adopted a strategy to encourage municipalities and developers to incorporate Low ImpactDevelopment (LID) into site planning and design. Low Impact Development includes a set of measuresthat reduce site imperviousness, thereby reducing storm water runoff, and/or provide filtration throughvegetation or infiltration. In theory, use of LID controls within a given site results in a reduction of theamount of storm water requiring treatment, termed Runoff Reduction. The purpose of this report is tosummarize an effort undertaken by local agencies to develop an LID, or Runoff Reduction, credit system.This system is being developed for use in the Stormwater Quality Design Manual for the Sacramento andSouth Placer Regions (Manual).The NPDES Phase I and Phase II Municipal Stormwater Permits for the County of Sacramento (and copermittees)and the City of Roseville, respectively, require that treatment of storm water occur fordevelopment projects of various types: residential and commercial being the most common. Thresholds(by size of development) for treatment vary by permit, but all participating municipalities have an interestin achieving regional consistency in the application of storm water design standards.This document and the resulting worksheets are an attempt to 1) quantify the benefit obtained through theincorporation of specific Runoff Reduction measures, and 2) provide a mechanism by which developerscan calculate the benefit for using Runoff Reduction and the resulting reduction in size of treatmentcontrols. This is achieved by assigning “credits” to the use of Runoff Reduction measures.This document focuses on the two most common types of development, residential and commercial, butthe concepts presented could be easily adapted to other types of development projects.AssumptionsThe Runoff Reduction credit system has been developed based on the following assumptions.• Pavement/asphalt and roof tops are 100% impervious.• Landscaped areas, lawns, and natural areas are pervious and any runoff generated from theseareas is assumed to be clean with no further treatment needed.• Runoff from porous pavements is partially infiltrated/reduced depending on how pervious thematerial is and on the permeability of native soils.• Runoff from impervious surfaces that flows across pervious surfaces is partially reduced with theamount of reduction dependent on the type of receiving surface.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 1
II. RESIDENTIAL CREDITS CALCULATIONBackground DataResearch was conducted in 2005 by the City of Roseville and the County of Sacramento using studies ofavailable maps of residential developments of various sizes and types within Sacramento and PlacerCounty. These maps were used to calculate the aggregate area of features of interest: streets, rooftops,sidewalks, and driveways, as well as the total area of each development. A value was calculated forrooftops, sidewalks, driveways, streets, and total impervious surface as a percentage of the total area. Thisinformation was compiled to obtain local, accurate, empirical values for the average impervious fractionof different types of residential developments with differing densities (dwelling units per acre), as well asinformation about how the different features contribute to total site imperviousness. Data collected in theresearch effort for 29 sites is summarized in Attachment A.The Sacramento/Roseville data was used to compute an average impervious fraction for each of eightcategories of residential development classified by density, which ranged from one dwelling unit per acreto 20 dwelling units per acre. Average total impervious fraction and percent imperviousness by surfacetype has been identified in Table 1.Table 1 – Average Impervious Fraction by Surface Type for Eight Categories of ResidentialDevelopmentDENSITY INDWELLING UNITSPER ACRE (DU/A) 1 2 3-4 5-6 7 8-9 10-14 15-20TOTALIMPERVIOUSFRACTION 0.17 0.25 0.35 0.40 0.50 0.55 0.60 0.70ROOFTOP 0.08 0.13 0.19 0.23 0.29 0.33 0.37 0.44SIDEWALK 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04DRIVEWAY 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04STREET 0.01 0.04 0.08 0.09 0.13 0.14 0.15 0.18Appendix D-1 in ManualThe Residential Runoff Reduction Worksheet, Form D-1, allows a designer to calculate a reducedtreatment requirement based on the incorporation of various Runoff Reduction measures into theirproject. To use Appendix D-1, the designer must obtain an accurate estimate of the project area, thenestimate how much of that area will be ‘open space and parks’ as defined in the Manual. Open space andparks does not include landscaping within individual residential lots. The open space/parks acreage issubtracted from the total acreage to find the size of the area that will require treatment, A t . Thesecalculations are completed in Step 1 of the form.The designer then proceeds through Step 2 to determine how much credit is earned for Runoff Reductiontechniques incorporated into the project. Using predetermined impervious fraction factors and Forms D-1a-d, the designer calculates the size of each area that will be affected by various Runoff Reductiontechniques, and these areas become “effectively managed” under the system. The total Effective AreaManaged (A c ) equals the amount of credit allowed for the incorporation of Runoff Reduction measures.For every technique, an efficiency multiplier is provided which reflects the fraction of runoff that is beingreduced or treated from the area being considered. For example, a porous pavement driveway reducesStormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 2
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Other Development Community Reviewe
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1 IntroductionPurposeThis Stormwate
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Chapter 4Source Control Fact Sheets
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5 Runoff Reduction Control Measures
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Technical RequirementsInfiltration
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Page 191 and 192: Glossaryexposure of construction ma
Page 193 and 194: GlossaryLow Impact Development (LID
Page 195 and 196: GlossaryRegional Stormwater Quality
Page 197 and 198: Appendix ASubmittal RequirementsThe
Page 199 and 200: Sacramento County Supplemental Appl
Page 201: Example Post Construction Stormwate
Page 204 and 205: Typically maintenance agreements an
Page 206 and 207: Example Maintenance Covenant(Sacram
Page 208 and 209: Device in accordance with the maint
Page 210 and 211: [attach]DECLARANT’S ACKNOWLEGEMEN
Page 212 and 213: Recorded at the request of:CITY OF
Page 214 and 215: 9. The obligations herein undertake
Page 216 and 217: EXHIBIT B[Map/Illustration]
Page 218 and 219: City of FolsomThe City of Folsom ow
Page 220 and 221: Appendix D-1: Residential Sites: Ru
Page 222: Form D-1d: Alternative Driveway Des
Page 225 and 226: Form D-2a: Disconnected Pavement Wo
Page 228 and 229: Appendix D-3Runoff Reduction Credit
Page 230 and 231: These tables refer to runoff reduct
Page 232 and 233: Mature TreeShape Canopy (dia.) Heig
Page 236 and 237: unoff from the driveway by 60% whil
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Page 240 and 241: Or the ASCE-WEF method (Sacramento)
Page 242 and 243: Table 3 - Multipliers Based on Rati
Page 244 and 245: Using the Effective Area Managed in
Page 246 and 247: SMRC, The Stormwater Manager’s Re
Page 248 and 249: Flow-based control measure design s
Page 250 and 251: 4. Apply the Rational Formula to ca
Page 252 and 253: 5. Enter the capture curve selected
Page 254 and 255: 1.201.000.80STORAGE (in)0.600.4024-
Page 256 and 257: 1.201.000.80STORAGE (in)0.600.4048
Page 258 and 259: Subject: VEGETATED SWALE EXAMPLEPLA
Page 260: PART II:Routing Excess Runoff throu
Page 263 and 264: 4. Design the OutletThe outlet shou
Page 265 and 266: Exhibit 1Water Quality Orifice Outl
Page 267 and 268: 3. Calculate Planter Surface Area A
Page 269 and 270: • Bottom slopes of 0.01 to 0.05%
Page 271: Appendix HDrainage Design and Storm
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