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• The proposed Pit 4 backsloping plan will reduce both the size and capacity <br />of exisiir~g Sedimentation Pond 4. However, the existing pond configuration <br />provides significant excess capacity, and revegetation of the Pit 4 disturbance <br />area has significantly reduced both runoff and sediment loss. Given these <br />considerations, the modified Pond 4 configuration will still provide adequate <br />storage capacity for both runoff resulting from the 10-year, 24-hour storm <br />event, and a three year accumulation of sediment. Design adequacy is <br />documented by hydrobgic calcuiations presented in Appendix A, Drainage <br />Design Information. <br />The modified Pond 4 design utilizes the existing pond embankment and <br />principal spillway (CMP riser with 4-inch or~ce). The existing riprapped <br />trapezoidal emergency spillway will be lowered and modified to safely pass <br />the peak flows resetting from the 25-year, 24-hour storm event while <br />maintaining adequate freeboard. <br />• The proposed Pit 4 backsloping plan and required modifications to Pond 4 <br />and associated drainage structures are described and documented by the <br />following components included in this response submittal: <br />• Drainage and Sediment Control Map, (Map R-7), which illustrates the <br />backsloped configuration, regrading limits, drainage areas utilized to <br />complete dhch and pond design calculations, and mod~ed <br />configuration for Pond 4 and associated structures. <br />• Pit 4 Slope and Pond Reclamation, (Map R-2). <br />• Pond 4 Design Map, (Map R-3). <br />• Appendix A, Drainage Design Information. <br />• Appendix B, Pit 4 Backslope Area -Bond Reclamation Cost Estimate. <br /> <br />-2- <br />