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RWMWD Watershed District Volume Control Worksheet <br />Permit Name <br />Permit Number <br />Permit Contact & Phone Number <br />►Complete the steps below to determine the volume required to be reduced and calculate the proposed amount of <br />volume to be retained. A site plan /map showing impervious surfaces shall also be submitted for review along with <br />other submittal requirements as stated in the District rules. <br />► Instructions: Fill in the yellow boxes with the requested information. Numbers in the black - outlined boxes will be <br />calculated for you (if filling out electronically). Orange boxes are key totals. <br />STEP 1: Determine Impervious Surface Areas <br />(a) Total Site Area: <br />(b) Total Impervious Area: <br />3.52 1 <br />2.4 <br />acres <br />acres x 43,560 sq. ft. /acre = <br />01104044IN <br />square feet <br />STEP 2: Determine Required Volume Reduction Runoff Volume from Impervious Surfaces <br />I 104,544 Ix 1.0 x 0.9 x 0.083333 = <br />Impervious Runoff Inches per <br />Area (sq, ft.) Inches <br />from (b). Coefficient Foot <br />Required Volume Reduction <br />(Cu. Ft.) <br />(c) <br />STEP 3: Determine Infiltration Rate, Drawdown Time and Volume Reduction for Proposed Onsite BMPs. <br />►For each BMP, select the appropriate hydrological soil group classification and design infiltration rate from Table A. <br />Select the design infiltration rate from Table A based on the least permeable soil horizon within the first five feet below <br />the bottom elevation of the proposed BMP. Enter the infiltration rate in 'Table B. <br />►The applicant may complete double -ring infiltrometer test measurements at the proposed bottom elevation of the <br />BMP to the requirements of ASTM D3385. The measured infiltration rate shall be divided by the appropriate correction <br />factor selected from the Minnesota Stormwater Manual. The test must be completed by a licensed soil scientist or <br />engineer. <br />Table 4 Design Infiltration Rates <br />Soil Group <br />Rate <br />Soil Textures <br />ASTM Unified Soil Class <br />Symbols <br />A <br />1.63 in /hr <br />Gravel, sand, sandy gravel, silty gravel, loamy sand, sandy <br />GW, GP <br />0.80 in /hr <br />X0.60 <br />loam <br />GM, SW, SP <br />13 <br />in/hr <br />SM <br />0.30 in/hr <br />Loam, silt loam <br />ML, OL <br />C <br />0.20 in/hr <br />Sandy clay loam <br />GC, SC <br />D <br />0.00 in /hr <br />Clay loam, silty clay loam, sandy clay, silty clay, or clay <br />CL, CH, OH, MH <br />Source: Minnesota Stormwater Manual, November 2005. <br />37 <br />