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3-9 3.6.2 Soil Loss Analysis The Universal Soil Loss Equation (USLE) was used to evaluate potential soil losses within the watershed boundary of the FL site for the previously closed Stage 1A Partial Final Closure Plan Area. The USLE was initially intended for analysis of meadows and cropland soil loss. However, with certain engineered assumptions, it can be applied to soil cover over landfills. The USLE is: A = RKLSCP where A = average soil loss, in tons/acre R = rainfall and run-off erosivity index K = soil erodibility factor, tons/acre L = slope-length factor S = slope-steepness factor C = cover-management factor P = practice factor The soil loss analysis performed is based on a "closed landfill" condition as required by CCR Title 27, and for a post final closure construction condition to estimate sediment amounts as a basis for sizing the sedimentation basins. At closure, the potential soil loss is minimal because the landfill will have a compacted final cover, an erosion control surface of vegetation and a drainage system installed which all contribute to controlling soil erosion. The following USLE constants were utilized for the "closed landfill" condition: R = 20.41 Average annual rainfall value K = 0.28 Soil erodibility 3.20 < LS < 5.90 Dependent upon length gradient C = 0.05 Based on vegetative material P = 0.10 Practice factor Since the borrow source utilized for the closure will be the same as Stage 1A, as are the average slopes of the final grades for the Stage 1B areas, the soil loss analysis results can be considered as identical for the two partial final closure areas. As indicated for the Stage 1A soil loss analysis, the "closed landfill" average soil loss for the FL is 0.15 tons/acre/year which is below the two tons/acre/year industry standard maximum soil loss. Over the 30- year post-closure maintenance period, the average soil loss over the closure area will be approximately 0.03 inches. The 30-year soil loss represents 0.1 percent of the total final cover thickness. This analysis assumes a seventy to eighty percent ground cover of the specified vegetation will exist over the post-closure period. A copy of the landfill soil loss analysis data as prepared for Stage 1A is presented in Appendix G. 3.6.3 Erosion Control The Stage 1B Partial Final Closure design has multiple erosion control features that will reduce the potential for soil erosion due to water and wind. These features include vegetation, a slope bench system, fiber rolls, and silt fence. There is one bench proposed for construction along the westerly landfill slope, in addition to the deck access road and the perimeter berm/access road. No additional benches will be constructed on the North Slope with this partial final closure. The final slope bench system and a series of downdrains will reduce the length of travel of run-off on the slope face thus reducing the potential for erosion rills and gullies. The fiber rolls, silt fence, and Forward Landfill Stage 1B Partial Final Closure/Post-Closure Maintenance Plan-June 2011 SWT Engineering z:\projects\allied waste\forward\11-1014 partial fnl clsr pin\partial final closure plan document\tent\sec 3.doc