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• Len tg h Slope Factor "LS" <br />• <br />• <br />The effects of topography on soil erosion are determined by the dimensionless "L" and "S" <br />factors, which account for both rill and interrill erosion. These factors have been <br />considerably revised since the initial Universal Soil Loss Equation. The new RUSLE <br />computer database for the combined "LS" factor is based on the soil's ratio of rill to <br />interrill erosion. For most western rangeland soils, the equations for low rill to interrill <br />ratio are used. Table 1 (from Predicting Soil Erosion by Water: A Guide to Conservation <br />Planning With the Revised Universal Soil Loss Equation - US Department of Agriculture <br />1997) shows a chart based on these equations. The average slope of the release areas in <br />both pre-mine and post-mine is determined with an algorithm in SurvCAD, using a 3D grid <br />of the topography in both cases. The slope length will be 1000 feet, as this is the limit of <br />the "LS" table. <br /> Calculated Avg. Land LS Factor <br /> Slope <br />Pre-Mine Dryland Pasture 6.94 1.7 <br />Post-Mine Dryland 5.06 1.1 <br />Pasture <br />Pre-Mine Irrigated 5.35 1.2 <br />Pasture <br />Post-Mine Irrigated 4.90 1.1 <br />Pasture <br />From Table 2, it is seen that the pre-mine "LS" factor for Irrigated Pasture is 1.2 based on <br />a length of 1000 feet and a grade of 5.35%. The "LS" factor for pre-mine Dryland Pasture <br />is 1.7 based on a length of 1000 feet and a grade of 6.9%. Post-mine "LS" factors are 1.1 <br />for both irrigated pasture and dryland pasture areas based on 4.9% and 5% average slopes <br />respectively. These average slopes are based on the final Phase 2 release areas for 2008 for <br />dryland pasture and irrigated pasture. <br />16 <br />