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structure will be to control runoff end sediment from a watershed disturbed by proposed <br />mining activities. This structure was constructed in the summer of 1992. <br />SITE DESCRIPTION <br />Lend Use. Sedimentation Pond 005 has a 289.9-acre watershed that is tributary to Dry <br />Creek (see Exhibit 13-2). .This we[e rshed will be disturbed by ,olio ing activities es shown <br />T - <br />on Exhibits 12-2 end 20-1 and reclaimed as shown on Exhibit 20Y2: The Land condition of <br />the watershed was conservatively based on the maximum disturbance at any one time within <br />the proposed mining area, approximately 45 percent undisturbed end 55 percent disturbed. <br />Embankment end Excavation. The embankment is a homogeneous earth fill structure. The top <br />width of the embankment is approximately 15 feet with upstream slope of 3.Sh:1v entl <br />downstream slopes of approximately 2.Sh:1v. The principal spillway consists of a 24-inch <br />drop inlet CNP; the emergency spillway is a trapezoidal channel 12 feet wide (see Exhibit <br />13-SA). A portion of the pond capacity was the result of excavating topsoil end material <br />below natural ground level. Excavated suitable material was used in embankment <br />construction. <br /> <br />DESIGN ANALYSIS <br />General. This sedimentation pond was designed to meet or exceed the design and <br />performance requirements of [NERD while utilizing as much of the existing topography as <br />possible. Sedimentation Pond 005 was designed to adhere to the following specifications: <br />1. Provide storage end treatment for the runoff from the 10-year, 24-hour storm event <br />without experiencing any discharge through the emergency spillway; <br />2. Provide an emergency spillway capable of passing runoff from the 100-year, 2G-hour <br />storm event; <br />3. Provide adequate storage for sediment generated es determined from the Sedcad+ <br />computer program. <br />4. Provide a nonclogging dewatering device and principal spillway that will provide <br />adequate detention time for the 10-year, 24-hour storm event end still allow a method <br />to deweter the pond down to the 100 percent sediment storage level in a reasonable <br />amount of time. <br />i <br />13-3-2 Revised 08/11/92 <br />