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0 1. Sediment Loss from Phase 2 Areas <br />The land requested for this bond release is irrigated pasture, which was originally irrigated <br />before mine disturbance. Since a comparison must be made of the sediment loss from the <br />reclaimed area to that of the original mine community, the best manner to accomplish this <br />is to use the vegetation statistics from the reclaimed areas and the same areas prior to <br />mining in 1987. <br />The Universal Soil Loss Equation was developed in 1965 by Wischmeier and Smith using <br />earlier equations by Musgrave in 1947. The equation was modified in 1993 to account for <br />steeper slope areas, change in erosion over time among other improvements. The revised <br />equation (RUSLE) does account for rill and inter-rill erosion. Its complete form is listed <br />below: <br />A= R x K x (L x S) x C x P <br />where: <br />A = annual soil loss in tons/acre <br />R = rainfall factor (from standard chart of area) <br />K = soil erodibility factor (function of site specific soil) <br />LS = length slope factor (from contour map & chart) <br />C = management factor dependent upon vegetation & mulch <br />P = erosion control practice factor (constant) <br />Rainfall Factor "R" <br />The rainfall factor is the product of rainfall energy times the maximum 30-minute intensity <br />for a given rainstorm. It is considered as the erosive power of the rain for that particular <br />area. A chart of "R" values for Colorado developed by the Transportation Research Board <br />in 1980 shows that the "R" value for the entire Nucla area is 26. This can be seen in <br />Appendix C. <br />Since the reclaimed land slope is similar to the pre-mine slopes, the same rainfall factor <br />will be used for both pre mine and reclaimed conditions. <br />For a discussion of this adjustment to the rainfall factor, see Design Hydrology and <br />Sedimentology of Small Catchments by Han, Barfield and Hayes. <br />0 SL-12 DRAFT 20AUG10.wPd 16