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New Development <br />Required Volume (ft') =Impervious surfaces (ftZ) x 1.0 (in) x 0.9 coefficient x 1/12 (ft/in) <br />Redevelopment <br />Required Volume (ft3) =Impervious surfaces (ftZ) x 1.0 (in) x 0.45 coefficient x 1/12 (ft/in) <br />a. When using infiltration for volume reduction, the following <br />requirements must be met: <br />(I) Infiltration volumes and facility sizes shall be calculated <br />using the appropriate hydrological soil group classification <br />and design infiltration rate from Table 1. Select the design <br />infiltration rate from Table 1 based on the least permeable <br />soil horizon within the first five feet below the bottom <br />elevation of the proposed infiltration BMP. <br />(2) The applicant may complete double-ring infiltrometer test <br />measurements at the proposed bottom elevation of the <br />infiltration BMP to the requirements of ASTM D3385. The <br />measured infiltration rate shall be divided by the <br />appropriate correction factor selected fiom the Minnesota <br />Stormwater Manual. This test must be completed by a <br />licensed soil scientist or engineer. <br />Table 1. Design Infiltration Rates <br />Soil Group Soil Textures ASTM Unified Soil Rate <br /> Class S mbols <br /> Gravel, sand, sandy gravel, or silty GW, GP 1.63 in/hr <br />A gravel, loamy sand, sandy loam GM, SW, SP 0.80 in/hr <br /> SM 0.60 in/hr <br />B Loam, Silt loam <br /> ML, OL 0.30 in/hr <br />C Sandy clay loam GC, SC 0.20 in/hr <br />D Clay loam, silty clay loam, sandy CL, CH, OH, MH 0.00 in/hr <br /> cla ,silt cla , or clay <br />Source: Minnesota Stormwater Manual, November 2005. <br />(3) The infilh•ation area shall be capable of infiltrating the <br />required volume within 48 hours for surface and subsurface <br />BMPs. <br />(4) Infiltration areas shall be limited to the horizontal areas <br />subject to prolonged wetting. <br />(5) Areas of permanent pools tend to lose infiltration capacity <br />over time and shall not be accepted as an infiltration <br />practice. <br />Stormwater Ordinance 928- 7 <br />