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

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unoff from the driveway by 60% while a Hollywood driveway reduces runoff by 75%, based on eachdriveway’s ability to infiltrate stormwater. After an efficiency multiplier is determined, a use multiplierrepresenting the percentage of units in the development that are using the technique is determined. So, if50% of the units in the development use the Runoff Reduction measure, then 50% of possible reduction isachieved. Finally, a reduction factor is selected, which represents the fraction of land that the surface ofinterest represents. For example, in calculating the amount of credits allowed for an alternative driveway,the reduction factor reflects the area of driveway surface as a fraction of the total area requiring treatment(A t ); in all residential developments, driveways comprise 4% of the total area. These fractions are allmultiplied together to get a fraction of total site area effectively managed. This is multiplied by the totalacreage of A T to find the area of impervious surface that is effectively managed (A c ) by using the RunoffReduction technique.So the effective area managed is equal to:efficiency multiplier * use multiplier * reduction factor * total acreageRunoff Reduction measures for which credit can be obtained within residential development projectsinclude disconnected roof drains, disconnected pavement, interceptor trees, and alternative driveways.All Runoff Reduction measures must be designed and installed in accordance with specifications anddetails provided in Fact Sheets included in the Manual. The basis for the credit allowed for each of theRunoff Reduction measures has been detailed below.Disconnected Roof Drains (DRDs)Disconnected roof drains (DRDs) can achieve the functional equivalent of reducing a large amount ofimperviousness by directing rooftop runoff to a pervious surface, dispersal trench, or dry well, as allowedby the local permitting agency. Use of DRDs as a Runoff Reduction measure has been recommended inguidance manuals such as Start at the Source (1999) and The Practice of Low Impact Development(2003.) If the roof drainage is connected to a dispersal trench or a dry well that have been designedaccording to specifications provided in the fact sheet, then 100% of the impervious surface attributed torooftop is considered treated for that unit, thus the efficiency multiplier is 1.00. Design standards for thedispersal trench and dry well are adapted from High Point Community Site Drainage Technical Standards(2004) and Virginia Stormwater Management Program Handbook (1999).If the runoff, via the roof drain, is directed across the surface of the landscaping, then the amount ofrunoff that will be treated is dependent on the amount of vegetation the water will flow through beforeentering the storm drain system. The design specifications for Sacramento County require a 20-footsetback between the house and the sidewalk. Most new developments obtain variances to reduce theirsetback distances to 15 feet or 12.5 feet. The efficiency multiplier for disconnected roof drains draining tolandscaping is then found using manning’s equation to solve for hydraulic residence time. The manning’sequation used is the equation described in Filter Strip Worksheet 2005 Surface Water Design ManualSizing Method published in the King County Surface Water Design Manual published by King CountyWater and Land Resources Division.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 3
QL = tW⎛1.49W⎜⎝ Q ns ⎞⎟⎠0.6Where,L = filter strip length (feet)t = hydraulic residence time (seconds)Q = design flow (cfs)W = filter strip width (feet)n = Manning’s roughness coefficients = longitudinal slope along pathWQF = C iAWhere,WQF = design flow (cfs)C = Rational Runoff coefficienti = rainfall intensity (in/hr)A = Area (acres)The equation is solved for hydraulic residence time, using various setback lengths and thefollowing assumptions.Residential Disconnected Roof Drain Assumptions:W = 5 feet (recommended by local hydrologist as typical average)s = 0.01n = 0.35 (residential is likely to have short grasses)C = 0.9 (rational C) (corresponds to rooftop’s 100% imperviousness)Rooftop area = 2,500 sq feet (average roof area for 5-7 DU/A)The hydraulic residence time is then computed as a percentage of the value for full treatment, identified asbeing 7 minutes by many stormwater manuals. The results are summarized in Table 2, and multipliers arerounded to the nearest 0.05 in the form.Table 2 – DRD Efficiency Multipliers Based on Length of Front Yard SetbackLength of Setback Residence Time Percent of 7 minutes Efficiency multiplier25 feet 8.6 min 123% 120 feet 6.9 min 98% 0.9815 feet 5.2 min 74% 0.7410 feet 3.4 min 48% 0.485 feet 1.7 min 24% 0.24Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 4
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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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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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5 Runoff Reduction Control Measures
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Runoff Reduction Control MeasuresDe
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Runoff Reduction Fact SheetDisconne
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Chapter 6For projects using any of
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Technical RequirementsInfiltration
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Technical RequirementsVegetated Swa
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Page 185 and 186: Runoff Treatment Control Measure Fa
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 234 and 235: Appendix D4LID Credits Background R
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 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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