2010-12-17_REVISION - C1981019 (128)

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• EXHIBIT 7, ITEM 23, PART D SEDIMENT CONTROL FOR TOPSOIL STOCKPILE RING DITCHES Map 28C shows the location of the 5 topsoil stockpiles to be used with the Collom Mine Plan. Each stockpile has been designed with 3H:1 V side slopes. The SEDCAD modeling presented in Exhibit 7 -23 Part A assumes that the drainage from each pile is self contained. A separate SEDCAD analysis is provided herein to evaluate the sediment control needed for each of the five topsoil stockpiles. Several different methods of sediment control (silt fences, rock check darns, etc.) were evaluated, but the relatively large size and area of the stockpiles suggests that a small impoundment to completely contain the runoff is the most practical approach. The following analysis is provided to identify the size of this impoundment. Each topsoil stockpile will have a ring ditch established around the toe of the stockpile to prevent offsite runoff. This ring ditch will be a triangular ditch cut into the original ground immediately adjacent to the toe of the completed topsoil stockpile. As such, it is constructed of non - topsoil materials and will vary in depth from 2 to 3 feet. The ring ditch will in each case direct runoff from the Exhibit 7 -23D„ Figure 1 shows the ring ditch and sediment control measures associated with • Topsoil Stockpile 35A. This is the stockpile with the largest footprint. When completed, the stockpile will potentially cover about 37.8 acres. Attached is a SEDCAD run for Topsoil Stockpile 35A which calculates the potential runoff from the 10 year -24 event using a curve number of 83 (topsoiled and seeded). This model indicates that 1.77 acre -feet of runoff will occur. A similar SEDCAD analysis was run for each of the five stockponds, although only the model for Topsoil Stockpile 35A is presented herein. The only difference between the models is the contributory drainage area (footprint) of each stockpile. The following results were obtained: Stockpile Area Runoff Volume Topsoil Stockpile 24A 17.7 acres 0.83 acre -feet Topsoil Stockpile 25A 13.9 acres 0.65 acre -feet Topsoil Stockpile 25B 24.7 acres 1.16 acre -feet Topsoil. Stockpile 35A 37.8 acres 1.77 acre -feet Topsoil Stockpile 36A 16.8 acres 0.79 acre -feet • A separate, small impoundment will be used for each of the five stockpiles. Each impoundment will be sized to contain 110 percent of the volume shown above. The small impoundment design presented in Volume 1, Figure 2.05 -6 (under TR -87) will be used as a design basis, except that each structure will be entirely below grade with no embankment. Once vegetation is established on the topsoil pile, the runoff volume would be substantially reduced. Exh. 7 -23D -1 Revision Date: 12/15/10 Revision No.: PR -03