2008-04-15_REVISION - M1980244 (351)

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• • ?J The temporary and permanent surface-water diversion systems for the VLF and the OSAs have been designed to safely pass the flows computed to arise as the result of a 100-year, 24-hour precipitation event (3.5"). Detention structures have also been designed so that the most downgradient structure retains the flows from two 10-year, 24-hour storm events with spillways capable of passing the runoff from a 100 year, 24-hour storm event. The storm water management designs for the SGOSA Extension and ECOSA are provided in Appendix 10 in Volume VII of this application. The storm water management designs for the VLF are provided in Appendix 8 in Volumes V and VI of this application. The designs are summarized below. 9.2 North Cresson and SGOSA Diversion and Sediment Control The North Cresson and SGOSA diversions and sediment control structures were analyzed to determine their capacity to also contain flows from the additional mining in the North Cresson and SGOSA Extension. Runoff from the SGOSA will be diverted to the Bateman Creek, Grassy Valley, and Poverty Gulch drainages during and after operations. The SGOSA diversions are trapezoidal structures of varying depths and side slopes. Where velocities exceed five feet per second, outside of bedrock areas, the channels will be lined with riprap. Under the current approved drainage system, drainage from the SGOSA and North Cresson Mine area would flow to EMP007, EMP008, and EMP009. EMP008 and EMP009, as currently approved but not been constructed, are located in steep terrain and would be difficult to construct to the approved size. The MLE drainage system proposes to construct an EMP008a and EMP008b to replace EMP008. These ponds would be located at higher elevations than the previously approved EMP008. The existing EMP007 was determined to have insufficient capacity to retain the design requirement of twice the 10-year, 24-hour storm event when evaluated in conjunction with the proposed MLE and the size of EMP007 has been increased by 50% to allow sufficient retention. EMP009 will be constructed as planned, but will be smaller than currently approved. During mining in the Globe Hill portion of the North Cresson Mine area, EMP009 will be eliminated and flows will be directed into the mine area. During mining, other changes may also occur to the sediment control system such as changes in diversion locations or temporarily directing storm water flows into mine areas, however upon closure only EMP007 and EMP008b will have contributing watersheds. EMP008a and EMP009 will remain as small wildlife watering ponds. Cripple Creek & Victor Gold Mining Company Cresson Project Mine Life Extension 9-2