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

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