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• To determine the composite CN for each basin an area weighted value was <br />computed using information from the area table and the base CN table. This resulted in <br />the following CN's. <br />Sub-Basin Com osite CN <br />East Basin 62 <br />West Basin 66 <br />East Pond -Yard 86 <br />West Pond -Yard, Pile and <br />Adjacent Undisturbed Areas 77.6 <br />4. Hydrology <br />This section contains estimated peak discharges and volumes needed to size <br />various components of the drainage system shown in Figure 2. Pipes and channels <br />• used in routing runoff from the east and west basins around the yard are required to <br />pass the peak discharge associated with the 100yr-24hr event. Sediment pond design is <br />based on the 10yr-24hr event volume and the 25yr-24hr event peak flow for the yard <br />area. <br />The Army Corps of Engineers' HEC-1 Flood Hydrograph Computer Model was used <br />to determine the design discharges and volumes. Total storm rainfatt depths were <br />originally obtained from the National Oceanic and Atmospheric Administration (NOAA) <br />rainfall atlas and subsequently modified slightly by the Division of Mines and Geology. <br />The 10yr-24hr depth is 2.20 inches, the 25yr-24hr depth is 2.60 inches and the 100yr- <br />24hrdepth is 3.20 inches. The standard Soil Conservation Service curve number <br />method, available within the HEC-1 package, was used to determine excess rainfall. <br />Runoff routing was accomplished using the SCS unit hydrograph technique. <br />4.1 West Hillside Area <br />The west hillside drainage basin consists of the small natural area on the west side <br />and above the mine yard. Runoff from this area is routed through the west pond. The <br />drainage area lies just west of the mine shaft entrance, the scale house and the <br />daysleeper. The area is 6.27 acres with a length to the divide of 970 ft and an average <br />slope of 24.7%. The curve number is 72. <br />L <br />5 <br />January 1998 `! <br />