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each soil and vegetation type was calculated using pre- and postmining 1:400 scale maps. <br />• Curve numbers of 80.1 and 74.2 were calculated for pre- and postmining conditions, <br />respectively. <br />Time of concentration was determined for the watershed under both conditions using the <br />relationship: <br />1140 (Y)~S <br />where: <br />tc = time of concentration (hrs) <br />L =hydraulic length (ft) <br />S = (1,000/curve number) -10 <br />and <br />Y =average Watershed slope (percent) <br /> The average watershed slope was calculated by measuring at least ten unbroken slopes on <br /> both the pre- and postmining watershed maps (see Exhibits 17-1 and 17-2). The hydraulic <br />• length of the watershed was determined by measuring the longest flow path from the most <br /> distant watershed point to the outlet. Time of concentrations were calculated t o be .731 <br />and .965 hours for both pre- and postmining conditions, respectively. <br />Sedimentology input parameters include particle size distributions, sediment routing <br />parameters (average specific gravity, coefficient for distributing sediment load and bulk <br />specific gravity), soil erodibility (K), slope (S), slope length (L), cropping or cover <br />(C) and conservation practice (P) factors. For both conditions, all of the Sedimentology <br />input parameters with the exception of the K, L, and 5 factors were selected from SEDIMOT <br />II batch input files used for designing the sediment ponds located in both mining areas <br />(see Tab 13, Facilities). <br />An average K factor was determined in both the pre- and postmining watersheds by figuring <br />the areal distribution of the same soil and vegetation types previously discussed. <br />K-factors have been assigned to the soil types mapped for baseline studies (see Tab 9, <br />Soil Resources Survey). K-factors of .312 and .308 were respectively assigned to the <br />• postmining sail (and vegetation) types of irrigated pasture (loam to sandy loam) and <br />rangeland (loam to sandy loam). The postmining K-factors were based on the premining <br />17-85 Revised 04/11/88 <br />