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soils. <br />Assuming that the soils were co-mingled during stockpiling, the post-mining soils <br />are a result of a blended combination. In addition, the lifting action during <br />stripping and the subsequent replacement of the topsoil on a subsoil consisting of <br />overburden, results in a soil that is more permeable than an undisturbed soil. The <br />introduction of void space and a less compacted soil, together with a mixed post- <br />mining soil condition suggests that the post-mining soils are of a type B hydrologic <br />soil group. However, to be on the conservative side, HGCC assumed that the post- <br />mining topsoil has the characteristics of type B/C hydrologic soil group. <br />• The approved post-mining land uses for the mine site are lifestock grazing and <br />wildlife habitat. The mine reclamation plan was designed to create vegetation <br />conditions wnductive to livestock and wildlife utilization, and the existing <br />vegetative cover is dryland pasture or rangeland. Field vegetative sampling and <br />observations indicate excellent revegetative success, the sampling data indicate a <br />range wver condition rating of approximately 98%. Vegetative success monitoring <br />conducted by HGCC in 1993 indicate that the average percent cover for the reclaimed <br />area is 70%, with a litter cover of 28% (HGCC, 1994). <br />Figure S-3 (Wilkes and King, 1980) provides a graph for calculating a CN based on the cover type, <br />the cover condition rating, and the hydrologic soil group. Figure S-3 is included at the end of this <br />methodology and assumptions section. Using Figure S-3, assuming a pastureland vegetative <br />community, a conservative range cover condition rating of 95%, and a B/C hydrologic soil group, <br />a CN value of 65 was calculated for the runoff analysis. <br />2.2 SEDCAD Sedimentology <br />The design storm sediment yield calculations were performed with the sedimentology component <br />of the SEDCAD computer model (Schwab, 1992). <br />The sedimentation program was applied to develop a sedimentation graph utilizing the Revised <br />Universal Soil Loss Equation (RUBLE) and the following input parameters: <br />Y=95 •(Y•QP)o.s6+K•LS•CP <br />Where: Y <br />V <br />Qp <br />K <br />LS <br />CP <br />Sediment yield (tons) <br />Run off volume (acre-feet) <br />Peak discharge (cfs) <br />Soil erodibility factor <br />Representative length-slope factor <br />Control practice factor <br />c:\g7Thg8~primary4nethodhow 7/16/96 <br />