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

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4. Apply the Rational Formula to calculate the water quality design flow (WQF):WQF = C i ASizing Volume-Based Treatment Control MeasuresVolume-based design standards apply to control measures whose primary mode of pollutantremoval depends on the volumetric capacity of the facility. Examples of control measures in thiscategory include water quality detention basins, constructed wetlands, stormwater planters, andinfiltration basins/trenches. Volume-based design criteria calls for the capture and infiltration ortreatment of a certain percentage of the runoff from the project site, usually in the range of the75th to 85th percentile average annual runoff volume.The agencies in Sacramento County require use of the Urban Runoff Quality Managementmethod (also for sizing volume-based control measures, while the City of Roseville requires useof the CASQA method, as described below. These methods are two of the three alternativedesign approaches allowed by the NPDES municipal stormwater permits.Sacramento County Volume-Based Design MethodFor projects in Sacramento County, volume-based control measures shall be designed to captureand treat the maximized stormwater quality capture volume for the area, based on historicalrainfall records, determined using the formula and volume capture coefficients set forth in UrbanRunoff Quality Management (WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87,(1998), pages 175-178). The Urban Runoff Quality Management approach (also known asWEF/ASCE approach) is based on the translation of rainfall to runoff using two regressionequations. The first regression equation relates rainfall to runoff. The rainfall to runoff regressionequation was developed using 2 years of data from more than 60 urban watersheds nationwide.The second regression equation relates mean annual runoff-producing rainfall depths to the“Maximized Water Quality Capture Volume” which corresponds to the “knee of the cumulativeprobability curve”. This second regression was based on analysis of long-term rainfall data fromseven rain gages representing climatic zones across the country. The Maximized Water QualityCapture Volume corresponds to approximately the 85th percentile runoff event, and ranges from82 to 88%.The two regression equations that form the Urban Runoff Quality Management approach are asfollows:C = 0.858 I 3 – 0.78 I 2 + 0.774 I + 0.04P 0 = (a • C) • P 6WhereC = runoff coefficient;I = watershed imperviousness ratio which is equal to the percent total imperviousnessdivided by 100;P 0 = Maximized Detention Volume, in watershed inches;a = regression constant, a= 1.312 for 12 hrs, a=1.582 for 24 hrs, and a=1.963 for 48-hourdraw down time.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 4
P 6 = mean annual runoff-producing rainfall depths, in watershed inches.The following steps describe the use of the approach. For simplicity, the worksheets presented inAppendix D (see Step 3 of Appendix D-1 and D-2) already incorporate these steps.1. Identify the drainage shed (A in acres) that drains to the proposed control measure. Thisincludes all areas that will contribute runoff to the proposed facility, including perviousareas, impervious areas (such as roofs, roads, parking lots, etc), and off-site areas,whether or not they are directly or indirectly connected to the control measure.2. Determine the “Maximized Detention Volume” (P 0 ) in inches for the drainage shed.Please refer to the attached figures (figure E-1 though figure E-4).3. Calculate the required water quality volume of the control measure by multiplying thedrainage shed area from Step 1 by the “Maximized Detention Volume” from Step 2.WQV (ac-ft) = P 0 • A / 12City of Roseville Volume-Based Design MethodFor projects in the City of Roseville, volume-based control measures shall be designed to captureand treat stormwater runoff equal to eighty (80) percent of the volume of annual runoff,determined in accordance with the methodology set forth in the California BMP Handbook,using local rainfall data. Also referred to as the “CASQA approach”, the approach is simple toapply, and relies largely on commonly available information about a project.The following steps describe the use of the sizing curves contained in the California BMPHandbook. For simplicity, the worksheets presented in Appendix D (see Step 3 of Appendix D-1and D-2) already incorporate these steps.1. Identify the drainage shed that drains to the proposed control measure. This includes allareas that will contribute runoff to the proposed facility, including pervious areas,impervious areas, and off-site areas, whether or not they are directly or indirectlyconnected to the control measure.2. Calculate the composite runoff coefficient “C” for the area identified in Step 1.3. Select a capture curve representative of the site and the desired drain down time usingAppendix D of the California BMP Handbook (not to be confused with Appendix D inthis Sacramento/Placer Design Manual). Curves are presented for 24-hour and 48-hourdraw down times. The 48-hour curve should be used in most areas of California. Use ofthe 24-hour curve should be limited to drainage areas with coarse soils that readily settleand to watersheds where warming may be detrimental to downstream fisheries. Drawdown times in excess of 48 hours should be used with caution, as vector breeding can bea problem after water has stood in excess of 72 hours.4. Determine the applicable requirement for capture of runoff (Capture, % of Runoff).Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 5
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Table of contentsOutdoor Work Areas
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Table of contentsSF-1.SF-2.SF-3.SF-
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Other Development Community Reviewe
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1 IntroductionPurposeThis Stormwate
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Chapter 3runoff reduction first (ev
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Chapter 3Step 9: Establish Long Ter
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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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GlossaryRegional Stormwater Quality
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Appendix ASubmittal RequirementsThe
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Page 201: Example Post Construction Stormwate
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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
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Page 232 and 233: Mature TreeShape Canopy (dia.) Heig
Page 234 and 235: Appendix D4LID Credits Background R
Page 236 and 237: unoff from the driveway by 60% whil
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Page 242 and 243: Table 3 - Multipliers Based on Rati
Page 244 and 245: Using the Effective Area Managed in
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Page 248 and 249: Flow-based control measure design s
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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