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<br />SITE DRAINAGE NARRATIVE <br />PAGE 2 OF 4 <br />work has determined that this low area has an existing piped outlet. The remainder of the site <br />drains south/east to Edgerton (which continues to drain south.) <br /> <br /> Approximate site area = 3.12 acres <br /> <br /> Existing impervious area = 0.074 acres <br /> <br />Proposed Conditions <br />The proposed impervious surface will include the construction of a public road, preparation of <br />single-family lots and stormwater management improvements. <br /> <br /> Approximate disturbed area = 2.9+/- acres <br /> <br />Run-off Volume Control <br />Stormwater runoff shall be retained onsite in the amount equivalent to 1.1 inches of runoff over <br />the new and reconstructed impervious surfaces of the development. <br /> <br /> Proposed new impervious area = 40,229 sf <br /> <br />Required Stormwater <br />Runoff Volume (CF) = 40,229 sf x 1.1 in x 1/12 ft/in <br /> = 3,688 cf <br /> <br />Because the BMP area has clay soils, alternate compliance sequencing allows iron-enhanced <br />sand filter (IESF) as an acceptable method of treatment. However, only 80% credit is allowed. <br /> <br />Required Stormwater <br /> Runoff Volume (CF) = 3,688 cf ÷ 0.80 <br /> = 4,610 cf <br /> <br />Some of the site (rear lots, roofs and yard area) will be collected behind Block 1 and then routed <br />to a revised low area. The proposed road and driveways are collected and routed to an IESF <br />prior to discharge into the revised low area. Pre-treatment for the IESF is provided via a sumped <br />manhole and then some overland flow prior to reaching the IESF. <br /> <br />Water quality volume = 5,046 cf (volume below outlet) <br />Surface area for filtration = 2,200 sf (area of filtration media) <br />Filtration rate = 0.8 in/hr <br />Drawdown time = 34.4 hours <br /> <br /> Per Haugo’s borings, it appears that the water table will be well below the bottom of the IESF. <br /> <br />