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W � � ! � <br /> (2) Significant Redevelopment Projects that result in an increase of, or <br /> replacement of, fifty percent (50%) or less of the impervious surFace <br /> of a previously existing development shall include Permanent <br /> Stormwater Pollution Prevention Measures sufficient to reduce <br /> water quality impacts of stormwater runoff from the increased or <br /> replaced portion of the site for the life of the project. <br /> (c) Stormwater Treatment Measure proposed as part of a project's <br /> Permanent Stormwater Pollution Prevention Measures shall be designed <br /> in accordance with the following hydraulic sizing criteria to treat <br /> stormwater runoff. <br /> (1) Volume Hydraulic Design Basis. Stormwater Treatment Measures <br /> whose primary mode of action depends on volume capacity, such <br /> as detention/retention units or infiltration structures, shall be <br /> designed to treat stormwater runoff equat to: <br /> i. The maximized storm water quality capture volume for the area, <br /> based on historical rainfall records, determined using the <br /> formula and volume capture coefficients set forth in Urban <br /> Runoff Quality Management, WEF Manual of Practice No. <br /> 23/ASCE Manual of Practice No. 87 (1998), pages 175-178 <br /> (e.g., approximately the 85 percentile 24-hour storm runoff <br /> event); or <br /> ii. The volume of annual runoff required to achieve eighty percent <br /> (80%) of more capture, determined in accordance with the <br /> methodology set forth in the Califomia Stormwater Best <br /> Management Practices Handbook for New Development and <br /> Redevelopment (2003), using local rainfall data. <br /> (2) Flow Hydraulic Design Basis. Stormwater Treatment Measures <br /> whose primary mode of action depends on flow capacity, such as <br /> swales, sand filters, or wetlands, shall be sized to treat: <br /> i. Ten percent (10%) of the fifty (50) year peak flow rate; or <br /> ii. The flow of runoff produced by a rain event equal to at least two <br /> times the 85�' percentile hourly rainfall intensity for the <br /> applicable area, based on historical records of hourly rainfall <br /> depths; or <br /> iii. The flow of runoff resutting from a rain event equal to at least <br /> 0.2 inches per hour intensity. <br /> Atty/Ord/Ord.216 <br /> 063004 4 <br />