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Precipitation and Storm Type. The precipitation amounts at the Seneca <br />Mine permit area for the 10-year, 25-year, and 100-year storms were <br />obtained from NOAA Precipitation Frequency Atlas Precipitation - FreauencY <br />• Atlas of the Western States - Volume II - Colorado, 1973. A standard SCS <br />Type II storm was utilized for each analysis. <br />Unit Hvdrograph and Surface Condition. Zhe SIDCAD+ computer program <br />provides three unit hydrograph shapes to choose from in determining the <br />runoff characteristics of a watershed. The disturbed hydrograph (FAST) <br />was used for the area ahead of minim that has been stripped of topsoil. <br />(MmI[Af) was used for the spoils area and reclaimed area. <br />Time of Concentration. The time of concentration, tc, is defined as the <br />time rPmiired for water to flora frwn the hydraulically most remote point <br />in the watershed to the watershed outlet. The runoff time of <br />ooncesitration was calculated using the follaaing equation (x;rpich, 1940) <br />for the diversion ditch. <br />tC = 0.0078L0.77 (L/H)0.385 <br />where tc = minutes <br />L = maximum length of flag in feet <br />H = the difference in elevation in feet between the outlet of <br />the watershed and the hydraulically most remote point in <br />the watershed <br />The time of concentration for Pond 008 was calculated using the Barfield, <br />1981 method. This formula <br />structure. lends itself to overland flow to a pond <br />~ = L0.8(S + 1)0.7 <br />1140 y0.5 <br />where tc = lag in hours <br />L = hydraulic length of watershed in feet <br />S = maximinn retention <br />= 1000 -10 <br />CN <br />. Y = average land slope in percent <br />7-6f-9 Revised 08/21/91 <br />