2016-08-08_PERMIT FILE - C1981019A (7)

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EXHIBIT 7, ITEM 23, PART E SEDIMENT CONTROL FOR TOPSOIL STOCKPILES Map 28C shows the location of the 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 elsewhere in Exhibit 7-23 does not address runoff from any topsoil stockpiles. A separate SEDCAD analysis is provided herein to evaluate the sediment control needed for each of the topsoil stockpiles. Several different methods of sediment control (silt fences, rock check dams, 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. In addition to a small impoundment sized to contain runoff from the topsoil stockpile, each stockpile will also have a ring ditch/diversion berm established around the toe of the stockpile to prevent offsite runoff from the pile. This ditch/berm system 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 outside slope of the ditch will be continued up above original grade to form a low berm 2 feet high, pushed up from the base of the ditch, with side slopes at 2H:1 V on the inside and the outside. The inside and outside slopes of the ditch/berm system will be revegetated with the temporary seed mix used on the topsoil stockpile. In each case the ditchiberm system will divert all runoff from areas above the topsoil pile away from the ditch and impoundment, while directing all runoff from the topsoil pile into the below grade sediment pond described in more detail below. In locations where there is no upstream area tributary to the ditch/berm system, such as below the topsoil pile, the above grade berm portion of the system will not be constructed. Attached is a SEDCAD run for Topsoil Stockpile 25A which calculates the potential runoff from the 10 year -24 event using a curve number of 83 (topsoiled and seeded). This model indicates that 1.52 acre-feet of runoff will occur. A similar SEDCAD analysis was run for both of the stockponds, although only the model for Topsoil Stockpile 25A 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 I IA 8.4 acres 0.39 acre-feet Topsoil Stockpile 25A 32.6 acres 1.53 acre-feet One or more small impoundments will be used for each of the stockpiles. Each impoundment will be sized to contain 110 percent of the volume shown above. Example of the ring ditch and small incised impoundment is shown on Exh. 7-23, Figure E1. Once vegetation is established on the topsoil pile, the runoff volume would be substantially reduced. Exh. 7-23E-1 Revision Date: 3/08/15 Revision No.: PR -04