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

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Flow-based control measure design standards apply to control measures whose primary mode ofpollutant removal depends on the rate of flow of runoff through the facility or device. Examplesof control measures in this category include swales, sand filters, diversion structures for off-linecontrol measures, and many proprietary products. Typically, flow-based design criteria calls forthe capture and infiltration or treatment of the flow runoff produced by rain events of a specifiedmagnitude. For the local area, the intensity of such a storm event is 0.20 inches/hour for theCities of Folsom and Roseville and 0.18 inches/hour for other cities in Sacramento County andunincorporated Sacramento County.This method satisfies the provisions of the Sacramento Areawide and City of Roseville NPDESMunicipal Stormwater Permits, which requires that flow-based measures be designed for at leastthe maximum (peak) flow rate of runoff produced by the 85 th percentile hourly precipitationintensity multiplied by a factor of two, referred to here as the flow-based 85 th percentile method.(CDM, 2003). This criterion is the same as the one prescribed by the 2003 California BMPHandbook. From Appendix D of that handbook, the 85 th percentile hourly precipitation intensityfor the Sacramento gage is approximately 0.09 inches/hour. Multiplying by two, the requiredintensity is at least 0.18 inches/hour. The factor of two specified for this method by the municipalstormwater permits appears to be provided as a factor of safety: therefore, caution should beexercised when applying additional factors of safety during the design process so that overdesign can be avoided. (CASQA, 2003).The flow-based BMP design criteria should be used in conjunction with the Rational Formula, asimplified, easy to apply formula that predicts flow rates based on rainfall intensity and drainagearea characteristics. The Rational Formula is as follows:whereWQF = flow in ft 3 /si = rain intensity in inches/hrA = drainage area in acresC = rational runoff coefficientWQF (cfs) = C i AThe Rational Formula is widely used for hydrologic calculations, but it does have a number oflimitations. For stormwater treatment control measure design, a key limitation is the ability of theRational Formula to predict runoff from undeveloped areas where runoff coefficients are highlyvariable with storm intensity and antecedent moisture conditions. This limitation is accentuatedwhen predicting runoff from frequent, small storms used in stormwater quality treatment designbecause many of the runoff coefficients in common use were developed for predicting runoff fordrainage design where larger, infrequent storms are of interest. Table 5-3 in the California BMPHandbook (May 2003) provides some general guidelines on use of the Rational Equation. Insummary, the Rational Formula, when used with commonly tabulated runoff coefficients inundeveloped drainage areas, will likely result in predictions higher than will be experiencedunder actual field conditions. However, given the simplicity of the equation, its use remainspractical and it is therefore the preferred method recommended by the local permitting agencies.The following steps describe the approach for application of the flow-based design criteria. Forsimplicity, the worksheets presented in Appendix D (see Step 3 of Appendix D-1 and D-2)already incorporate these steps.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 2
1. Identify and delineate the drainage shed that drains to the proposed control measure. Thisincludes all areas that will contribute runoff to the proposed control measure, includingpervious areas, impervious areas, and off-site areas, whether or not they are directly orindirectly connected to the control measure.2. Select design rainfall intensity for the project area:0.20 inches/hour – projects in the Cities of Folsom and Roseville0.18 inches/hour – projects located in other cities in Sacramento County andunincorporated Sacramento County3. Calculate the composite runoff coefficient “C” for the drainage shed identified inStep 1using table E-1. For contributing areas with multiple coefficients, use the weightedcoefficient for the contributing area.Table E-1: Runoff coefficients for the Rational Formula:Type of Drainage AreaBusiness:Downtown areas0.95Neighborhood areas0.70Residential:Single-family areas0.50Multi-units, detached0.60Multi-units, attached0.75Apartment dwelling areas0.70Industrial:Light areas0.80Heavy areas0.90Parks, cemeteries 0.25Playgrounds 0.40Railroad yard areas 0.40Unimproved area 0.30Lawns:Sandy soil, flat, 2%Sandy soil, average, 2 – 7%Sandy soil, steep, 7%Heavy soil, flat, 2%Heavy soil, average 2 – 7%Heavy soil, steep, 7%Streets:AsphalticConcreteBrickRunoff Coefficient, C0.100.150.200.170.220.350.950.950.85Drives and Walks 0.85Roofs 0.95Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix E Page 3
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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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IntroductionIn July 2003, the permi
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IntroductionFor redevelopment proje
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Introduction• The Appendices prov
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2 An Integrated Approach toEffectiv
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Integrated ApproachIt is also helpf
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Integrated Approachfactors will be
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Chapter 3Figure 3-1. Process for Ad
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Chapter 3runoff reduction first (ev
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Chapter 3As the name implies (and a
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Chapter 3Step 9: Establish Long Ter
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Chapter 4Source Control Fact Sheets
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Source Control Measure Fact SheetSt
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5 Runoff Reduction Control Measures
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Runoff Reduction Control MeasuresDe
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Runoff Reduction Control Measures
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Chapter 6For projects using any of
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Runoff Treatment Control Measure Fa
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Technical RequirementsInfiltration
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Technical RequirementsSand FilterSa
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Technical RequirementsVegetated Swa
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Glossaryexposure of construction ma
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GlossaryLow Impact Development (LID
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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 234 and 235: Appendix D4LID Credits Background R
Page 236 and 237: unoff from the driveway by 60% whil
Page 238 and 239: The designer must determine how man
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 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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