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08 <br /> Existing stormwater runoff into proposed detention structure is as <br /> follows: <br /> Existing Impervious Surface Area = 0.25 Acres <br /> Runoff Coeff. = 0.95 <br /> Intensity of Rain = 7.5 in./hr. <br /> Q = 1.8 cis <br /> Existing Grass and Woodland = 5.15 Acres <br /> Runoff Coeff. = 0.5 <br /> Intensity of Rain = 7.5 in./hr. <br /> Q = 19.5 cfs <br /> Total stormwater runoff into proposed detention structure after <br /> development is as follows: <br /> Additional. Impervious Surface = 0.42 Acres <br /> Total Impeirvious Surface = 0.67 Acres <br /> Runoff Coeff. = 0.95 <br /> Intensity of Rein = 7.5 in./hr. <br /> Q = 4.8 cfs <br /> Remaining Grass and Woodland = 4.73 Acres <br /> Runoff Coef. = 0.5 <br /> Intensity of Rain = 7.5 in./hr. <br /> (i = 17.9 cfs <br /> The total quantity of stormwater runoff flowing into the proposed <br /> detention structure before development is 21.3 cfs. After <br /> development the stormwater runoff into this area is 22.7 cfs. The <br /> difference in runoff ( Q) is 1.4 cfs. The storage for the <br /> detention structure is computed by using the formula: <br /> Storage = ( 0) 2 (Tc) (60 sec/min) <br /> ( 0) = 1.4 cfs <br /> Tc = 3.7 minutes <br /> Storage = 621.6 cubic feet <br /> A small stilling basin will be used as the stormwater detention <br /> structure (see Exhibit 1 and 2) . This basin will have a 22' <br /> controlled discharge of the stormwater. The stilling basin, <br /> located as it is in an area which has almost no grade held several <br /> times the minimum storage (621.6 cubic feet) . The stilling basin <br /> will slow down and detain runoff from this site. This will allow <br /> the pollutants to settle. The stormwater passing through rock dam <br /> section (see Exhibit 2) will provide further cleansing of <br /> pollutants by filtration. <br /> 2 <br />