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<br />SITE DRAINAGE NARRATIVE <br />PAGE 2 OF 2 <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 = 41,200 sf <br /> = 0.946 acres <br /> <br />Required Stormwater <br />Runoff Volume (CF) = 41,200 sf x 1.1 in x 1/12 ft/in <br /> = 3,777 cf <br /> <br />The existing low area will continue to be used for stormwater management. Some of the site <br />(rear lots, roofs and yard area) will be collected behind Block 1 and then routed to the low area <br />(which is proposed to be expanded.) The proposed road and driveways are collected and routed <br />to a pre-treatment cell at the east side of the expanded low area. The pre-treatment cell <br />overflows to the expanded low area. <br /> <br />Volume in basin = 37,741 cf (volume of 100-year storm) <br />Surface area for infiltration = 21,355 sf <br />Infiltration rate = 0.8 in/hr <br />Drawdown time = 37.0 hours <br /> <br />Rate Control <br />Because the north portion of the site is land-locked in both existing and proposed conditions, no <br />run-off leaves the low area. Thus, rate control is met. <br /> <br />To the south: <br /> Existing Proposed <br /> 2-yr 0.16 cfs 0 <br /> 10-yr 0.24 cfs 0 <br /> 100yr 0.79 cfs 0.18 cfs <br /> <br />Floodplain <br />The low area has a floodplain elevation of 864.9. Per the HydroCAD report: <br /> Existing volume below 864.9 = 35,336 cf <br /> Proposed volume below 864.9 = 37,561 cf <br /> <br />There is no net floodplain fill. <br />