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

Or the ASCE-WEF method (Sacramento):WQV (ac-ft) = P 0 * A / 12where A = the total area of the drainage shed,P 0 = maximized detention volume using the ASCE-WEF method.Please refer to chapter 6 and Appendix E for the selection of the treatment measures and designrequirements. Then use form D-1f, Treatment – Volume Based (CASQA) for volume-based treatmentcontrols within the City of Roseville, or form D-1g, Treatment – Volume Based, for treatment controls inareas outside of the City of Roseville.Adjusted Runoff CoefficientThe Adjusted Impervious Fraction is converted to an Adjusted Runoff Coefficient, C A , using the empiricalregression equation presented in the California Stormwater BMP Handbook (CASQA, 2003).C = 0.858 I 3 – 0.78 I 2 + 0.774 I + 0.04where,C = runoff coefficientI = impervious fractionThe results of this equation for values of “I” between 0 and 1 are listed in Table D-1d, Form D-1.III.COMMERCIAL AND MULTI-FAMILY CREDITS CALCULATIONAppendix D-2 in ManualThe Commercial and Multifamily Runoff Reduction Worksheet, Appendix D-2, allows a designer tocalculate a reduced treatment requirement based on the incorporation of various Runoff Reductionmeasures into their project. To use Appendix D-2, the designer must obtain an accurate estimate of thearea, then estimate how much of that area will be ‘open space and parks’ as defined in the Manual. Thisincludes all landscaping areas and areas left in a natural state. The open space acreage is subtracted fromthe total acreage to find the size of the area that will require treatment, A T . This area (A T ) generallyincludes parking lots, rooftops, and driveways, which are assumed to be impervious. The designer thencalculates the size of each area that will be affected by various Runoff Reduction techniques, and theseareas become “effectively managed” under the system. The total Effective Area Managed (A c ) equals theamount of credit allowed for the incorporation of Runoff Reduction measures.Runoff Reduction measures for which credit can be obtained within commercial and multi-familydevelopment projects include porous pavement, disconnected roof drains, green roofs, disconnectedpavement, and interceptor trees. All Runoff Reduction measures must be designed and installed inaccordance with specifications and details provided in Fact Sheets included in the Manual. The basis forthe credit allowed for each of the Runoff Reduction measures has been detailed below.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 7