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Summary of Base CN's <br />Land Use - Hydrologic Average CN <br />Ground Cover Group <br />40% <br />50% <br />60% <br />Native D 76 73 70 <br />Native B 57 52 48 <br />Mine Yard D or B 86 <br /> <br />To determine the composite CN for each basin an areal weighted value was computed using <br />information from the area table and the base CN table. This resulted in the following CN's. <br />Sub-Basin Composite CN <br />East Basin 62 <br />West Basin 66 <br />Fast Pond -Yazd 86 <br />West Pond -Yard and <br />Adjacent Undisturbed Areas 77 <br />4. Hydrology <br />i <br />This section contains estimated peak discharges and volumes needed to size various <br />components of the drainage system shown in Figure 2. Pipes and channels used in routing runoff <br />from the East and West Basins around the yard are required to pass the peak discharge <br />associated with the 100yr-24hr event. Sediment pond design is based on the l0yr-24hr event <br />volume and the 25yr-24hr event peak flow for the yard area. <br />The Army Corps of Engineers' HEC-1 Flood Hydrograph Computer Model was used to <br />determine the design discharges and volumes. Total storm rainfall depths were originally <br />obtained from the National Oceanic and Atmospheric Administration (NOAA) rainfall alas and <br />subsequently modified slightly by the Division of Mines and Geology. The l0yr-24hr depth is <br />2.20 inches, the 25yr-24hr depth is 2.60 inches and the 100yr-24hr depth is 3.20 inches. The <br />standard Soil Conservation Service curve number method, available within the HEC-1 package, <br />was used to determine excess rainfall. Runoff routing was accomplished using the SCS unit <br />hydrograph technique. <br />4 <br />June 1995 <br />