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REFUSE PILE EXPANSION location constraints, the sedimentation pond was designed to rely on storage capacity to control peak runoff events. The pond is designed with a factor of safety of 2.0 to contain runoff and sediment load from a 25-yr, 24-hr storm event without discharge, plus contain the annual sediment load for each phase. The outlet structures for the sediment pond were analyzed to derive a peak flow discharge capacity. The following sections describe the design of the sediment control ditches, the clear water ditches, and the sedimentation pond for the RPE. maximum runoff possible during each construction phase was used for [he analysis. Runoff from the undisturbed upland terrain was assumed to remain constant at the level estimated to occur during Phase One, although progressive construction bf the RPE will reduce undisturbed surface area. A summary of estimated total runoff for each phase is presented in Table 11 and was used for sediment pond sizing and runoff control ditches. Positive drainage will be maintained on the pile during construction and all drainage control features will be installed prior to preparation of the area for the RPE construction. Surface runoff from the refuse pile will be collected in the designed perimeter sediment ditches and conveyed to the RPE sediment pond. The drainage from the undisturbed upgradien[ area toward [he south will be diverted westward toward Sylvester Gulch, and eastward toward the unnamed shallow gulch which lies between Sylvester Gulch and Box Canyon. At no time will runoff be allowed to collect at the head of the pile (Note that the pile will be rough and some minor ponding/puddling may temporarily occur within shallow undulations during construction, but these areas will be minimized). The location of the ditches for each phase is shown on the maps in Appendix B. The sediment ditches for the final configuration are shown in Figure B-18 in Appendix B. 6.2 Runoff Estimation Peak and cumulative runoff from undisturbed terrain, exposed refuse, and reclaimed refuse pile side slopes for each phase of construction were estimated using the U.S. Department of Agriculture Soil Conservation Service (SCS) runoff curve number (CN) method, using specific procedures defined in the Procedures for Determinine Peak Flows in Colorado TR-~5 supplement. Parameters for the estimates are presented in Table 9. 6.3 Sediment Load Estimates Sediment carried by precipitation runoff from disturbed areas will be routed with sediment control ditches into a two-chambered settling pond. Sediment quantities expected from each phase of construction were estimated using the Universal Soil Loss Equation (LISLE): A = (R)(K)(LS)(CP), where A =Sediment Loss (tons/acre) R =Rainfall Factor K =Soil Erodability Factor LS =Length Slope Factor CP =Erosion Control Practice Factor The factors for the equation were obtained from A~olied Hvdroloey and Sedimentoloev for Disturbed Areas, by B.J. Barfield, Oklahoma Technical Press, 1981. Sediment quantities for each phase of the RPE are summarized in Table 12. To limit erosional effects from surface runoff, numerous 1-foot high berms called contour furrows will be placed at regular intervals across the RPE finished face. The berms will be constructed using a bulldozer blade during soil replacement and regrading operations and will be formed with reclamation soil. Berms will be staegered to prevent overland runoff from Flowing more than 50 vertical feet without encountering a contour furrow. Refer to Figure L-3 for a typical erosion control contour furrow detail. Estimation methods are presented in Appendix L; 6.4 Sediment Control Ditch Design runoff results are presented in Table 10. The surface area for each phase is divided into: I) Sediment control ditches will be constructed along •,? exposed refuse, and ?) reclaimed areas. The [he perimeter of the RPE during each phase as shown in Appendix B. The ditches were designed to Harding Lawson Associates 12