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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 - Frequency <br />Atlas of the Western States - Volume II - Colorado, 1973. A stardard SCS <br />Type II storm was utilized for each analysis. <br />Unit HvdroQraph and Surface Cordition. The SIDCAD+ computer program <br />provides three unit hydrograph shapes to choose from in detarmini m the <br />runoff characteristics of a watershed. The disturbed hydrograph (FAST) <br />was used for the area ahead of mining that has been stripped of topsoil. <br />(i+~DIUM) was used for the spoils area and reclaimed area. <br />Time of Concentration. The time of concentration, tc, is defined as the <br />time required for water to flow from the hydraulically most remote point <br />in the watershed to the watershed outlet. The runoff time of <br />concentration was calculated using the following equation (Kirpich, 1940) <br />for the diversion ditch. <br />tC = 0.0078L0.77 (L/H)0.385 <br />• where tc = minutes <br />L = maximum length of flow i.r1 feet <br />H = the difference in elevation in feet between the outlet of <br />the watershed aryl 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 lends itself to overland flow to a pond <br />structure. <br />~ = L0.8~S + 110.7 <br />1140 Y0.5 c <br />where tc = lag in hours <br />L = hydraulic length of watershed in feet <br />S = maximum retention <br />= 1000 -10 <br />CN <br />• Y = average lard slope in percent <br />7-6f-9 Revised 08/21/91 <br />