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Precipitation and Storm Type. The precipitation amounts for the Nucla Mine for the storm <br /> durations used in the design are listed below: <br /> 10-year, 24-hour = 1.60 inches <br /> 25-year, 24-hour = 1.80 inches <br /> 100-year, 24-hour = 2.40 inches <br /> These rainfall amounts were obtained from the NOAA Precipitation Frequency Atlas <br /> Precipitation-Frequency Atlas of the Western States Volume III - Colorado, 1973. A <br /> standard SCS Type II storm was utilized for each analysis. <br /> Unit Hydrograph and Surface Condition. The SEDIMOT II computer program provides three <br /> unit hydrograph shapes to choose from in determining the runoff characteristics of a <br /> watershed. The disturbed hydrograph (No. 1) was used for the disturbed and reclaimed <br /> areas. <br /> Time of Concentration. The time of concentration, tc, is defined as the time required for <br /> water to flow from the hydraulically most remote point in the watershed to the watershed <br /> outlet. The runoff time of concentration was calculated using the following equations <br /> (Barfield, 1985): <br /> tc = time of concentration (hrs) <br /> tL = lag time (hrs) <br /> L = hydraulic length (ft) = 7,155 ft. 0 <br /> 0 <br /> 1000 <br /> S = - 10 (where CN = 83) = 2.05 <br /> CN / <br /> Y = average land slope 4.65% V \ <br /> 0.8 0.5 tL _ L (S+l) = 71550.8 0.7 <br /> (3.05) = 0.646 hrs <br /> 1900Y0.5 1900(4.65)0.5 <br /> tc = tL _ 0.646 = 1.08 hrs <br /> 0.6 0.6 3� <br /> 7 <br />