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ATTY/ORD.0010/CC ORD AMENDING MUNI CODE CH.18, 27A, 30 – EXHIBIT B REV: 09-17-24 VR Page 3 of 4 1.Volume Hydraulic Design Basis: Stormwater treatment measures that detain stormwater for a certain period and treat primarily through sedimentation and infiltration, whose primary mode of action depends on volume capacity, such as detention/retention units, green roofs, or infiltration structures, shall be designed to treat stormwater runoff equal to: a. The maximized stormwater quality capture volume for the area, based on historical rainfall records, determined using the formula and volume capture coefficients set forth in Urban Runoff Quality Management, WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87 (1998), pages 175—178 (e.g., approximately the eighty-fifth (85th) percentile twenty-four (24) hour storm runoff event); or b. The volume of annual runoff required to achieve eighty percent (80%) of more capture, determined in accordance with the methodology set forth in the California Stormwater Best Management Practices Handbook for New Development and Redevelopment (2003), using local rainfall data. 2. Flow Hydraulic Design Basis: Stormwater treatment measures that treat pollutants from a moving stream of water through filtration, infiltration, sedimentation and/or biological processes, whose primary mode of action depends on flow capacity, such as bio-retention areas, flow-through planters, tree well filters, or media filters, shall be sized to treat: a. Ten percent (10%) of the fifty (50) year peak flow rate; b. The flow of runoff produced by a rain event equal to at least two (2) times the eighty-fifth (85th) percentile hourly rainfall intensity for the applicable area, based on historical records of hourly rainfall depths; or c. The flow of runoff resulting from a rain event equal to at least 0.2 inches per hour intensity (also known as the "4 percent method"). 3. Combination Flow and Volume Design Basis: For projects on sites where infiltration should be avoided, or that are planned to maximize density at a redevelopment or infill site, a combination flow and volume design basis may be used for bio -retention areas and flow-through planters. These treatment measures shall be sized to treat at least eighty (80) percent of the total runoff over the life of the project, using local rainfall data and the methodology set forth in the C.3 Stormwater Technical Guidance, written by the San Mateo Countywide Water Pollution Prevention Program. C. Stormwater treatment measures proposed as part of a project's permanent stormwater pollution prevention measures shall be designed in accordance with the