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HYDROLOGIC ANALYSIS The Rational Method was used to estimate the peak runoff flows resulting from a 100- yeaz, 24-hour storm event to the sediment and retention ponds. Input requirements were runoff coefficient, storm intensity, and drainage azea. The runoff coefficient was taken as a weighted average of the impervious and pervious surfaces according to the total azea of each. Impervious surfaces were assumed to have a runoff coefficient of 0.95 and pervious surfaces, 0.40. Storm intensity was taken from anintensity-duration curve constructed fora 100-yeaz, 24-hour precipitation depth of 2.8 inches. A minimum time of concentration of 10 minutes was used and the drainage azea was measured from a site map. Using an average runoff coefficient of 0.78, a stone intensity of 3.0 in/hr, and an azea of 0.369 acres, the peak flow for the sediment pond was calculated as 0.87 cfs. Using an average runoff coefficient of 0.83, storm intensity of 3.0 in/hr, and azea of 0.499 acres, the peak flow at the stormwater retention pond was calculated as 1.24 cfs. The iwo drainage azeas above represent the surface azea upstream of the sediment pond inlet and the azea upstream of, and including, the surface azea of the retention pond, respectively. The volume of water to be contained in the retention pond as a result of the 25-yeaz, 24- hour storm event was estimated using the Soil Conservation Service (SCS) Technical Release SS, Urban Hydrology for Small Watersheds, 1986. This method requires a composite SCS curve number and total storm precipitation depth. A curve number of 91 was obtained from SCS Figure 2-3 using a percent impervious azea of 80 and a pervious azea curve number of 60. Using a total storm depth of 2.3 inches and a composite curve number of 91, SCS Figure 2-1 yielded a runoff depth of 1.45 inches. This runoff depth was multiplied by a total surface area of O.S57 acres to obtain a total runoffvolume of 2,932 cubic feet.