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HYDROLOGIC-ANALYSIS <br />The Rational Method was used to estimate the peak runoff flows resulting from a 100- <br />year, 24-hour storm event to the sediment and retention ponds. Input requirements were <br />runoff coefficient, storm intensity, and drainage area. The runoff coefficient was taken as <br />a weighted average of the impervious and pervious surfaces according to the total area of <br />each. Impervious surfaces were assumed to have a runoff coefficient of 0.95 and <br />pervious surfaces, 0.40. Storm intensity was taken from an intensity-duration curve <br />constructed for a 100-year, 24-hour precipitation depth of 2.8 inches. A minimum time <br />of concentration of 10 minutes was used and the drainage area was measured from a site <br />map. <br />Using an average runoff coefficient of 0.78, a storm intensity of 3.0 in/hr, and an area of <br />0.369 acres, the peak flow for the sediment pond was calculated as 0.87 cfs. Using an <br />average runoff coefficient of 0.83, storm intensity of 3.0 in/hr, and area of 0.499 acres, <br />the peak flow at the stormwater retention pond was calculated as 1.24 cfs. The two <br />drainage areas above represent the surface area upstream of the sediment pond inlet and <br />the area upstream of, and including, the surface area of the retention pond, respectively. <br />The volume of water to be contained in the retention pond as a result of the 25-year, 24- <br />hour storm event was estimated using the Soil Conservation Service (SCS) Technical <br />Release 55, Urban Hydrology for Small Watersheds, 1986. This method requires a <br />composite SCS curve number and total storm precipitation depth. A curve number of 91 <br />was obtained from SCS Figure 2-3 using a percent impervious area of 80 and a pervious <br />area curve number of 60. <br />Using a total storm depth of 2.3 inches and a composite curve number of 91, SCS Figure <br />2-1 yielded a runoff depth of 1.45 inches. This runoff depth was multiplied by a total <br />surface area of 0.557 acres to obtain a total runoff volume of 2,932 cubic feet. <br />4 <br /> <br />4* 4W-• <br />