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• Drainage Area. {n some instances, the mining operations will cause the drainage area to <br />change during the life of tfie sediment structure. When the pit moves into the watershed, <br />runoff is intercepted by the pit and diverted away from the structure. In these cases, a <br />conservative procedure was used; the structure was analyzed for the largest anticipated <br />drainage area that will contribute runoff to the structure. This condition usually occurs <br />at the start of mining and again during the reclamation period. Drainage areas for <br />structures are measured on a 1" = 400' and 1" = 1000' topographic map. <br />Unit Hydrograph and Surface Condition. The SEDCAD computer program provides three unit <br />hydrograph shapes to choose from in determining the runoff characteristics of a watershed. <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. <br />The time of concentration for sedimentation structures was calculated using the following <br />equation (Barfield et al„ 1983). <br />• tc _ L .8(S + 1)0.7 <br />1140 y0.5 <br />where tc = time of concentration in hours <br />L = hydraulic length of watershed in feet <br />5 = maximum retention <br />= 1000 -10 <br />CN <br />Y = average land slope in percent <br />Fer diversions, culverts, sediment traps, and ditches, the following equation was used to <br />obtain the time of concentration, tc, to better represent the peak flows for these <br />structures (USBR, 1977). <br />tc = ( 11,9(L)a ]0.385 <br />H <br />• <br />where: L = hydraulic length of watershed (miles) <br />H = watershed elevation difference (feet). <br />13-1-9 <br />