09-24-08 Council Agenda - Laserfiche WebLink

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1Vew Development Required Volmne (ft') =Impervious surfaces (ft2) x 1.0 (in) x 09 coefficient x 1/12 (ft/in) Redevelopment Required Volume (ft') =Impervious surfaces (fP) x 1.0 (in) x 0.45 coefficient x 1/12 (ft/in) a. When using infiltration for volume reduction, the following requirements must be met: (1) Infiltration volumes and facility sizes shall be calculated using the appropriate hydrological soil group classification and design infiltration rate fiom Table 1. Select the design infiltration rate from Table 1 based on the least permeable soil horizon within the first five feet below the bottom elevation of the proposed infiltration BMP. (2) The applicant may complete double-ring infilhometer test measurements at the proposed bottom elevation of the infiltration BMP to the requirernents of ASTM D3385. The measured infiltration rate shall be divided by the appropriate correction factor selected from the Minnesota Stormwater Manual. This test must be completed by a licensed soil scientist or engineer. Table i. Desi n Infiltration Rates _ Soil Group Soil Textures ASTM Unified Soil Class S mbols Rate Gravel, sand, sandy gravel, or silty GW, GP 1.63 in/hr A gravel, loamy sand, sandy loam GM, SW, SP _ 0.80 in/hr SM 0.60 in/hr B Loam, Silt loam ML, OL 0.30 inlhr C Sandy clay loam GC, SC 0.20 in/hr ___ D Clay loam, silty clay loam, sandy clay, silt cla , or cla _ CL, CH, OH, MH _ 0.00 inlhr Source: Minnesota Stormwater Manual, November 2005. (3) The infiltration area shall be capable of infiltrating the required volume within 48 hours for surface and subsurface BMPs. (4) Infiltration areas shall be limited to the horizontal areas subject to prolonged wetting. (5) Areas of permanent pools tend to lose infiltration capacity over time and shall not be accepted as an infiltration practice. Stormwater Ordinance 925- 7 9