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mining activities and to potentially mitigate water rights (see Tab 16 ). This structure <br />will be constructed upon approval in 1992. This structure will be maintained as a <br />sedimentation structure until one year prior [o bond release. Peabody will [hen submit a <br />permanent impoundment design evaluation of this structure for approval by Colorado Mined <br />Land Reclamation Division (CNLRD). This evaluation will include historical and <br />site-specific data related to this structure's viability as a permanent impoundment. <br />SITE DESCRIPTION <br />Land Use. Sedimentation Pond 005 has a 289.9-acre watershed that is tributary to Dry <br />Creek (see Exhibit 13-2). This watershed will be disturbed by mining activities as shown <br />on Exhibits 12-2 and 20-1 and reclaimed to a permanent impoundment as shown on Exhibit <br />20-2. The watershed will be approximately 45 percent undisturbed and 55 percent <br />disturbed. Design of the pond was conservatively based on [he maximum disturbance at any <br />one time within the proposed mining area. <br />Embankment and Excavation. The proposed embankment will be a homogeneous earth fill <br />structure. The top width of the proposed embankment will be approximately 20 feet with <br />upstream and downstream slopes of approximately 3h:1v each. The principal spillway will <br />consist of a 24-inch drop inlet CMP; the emergency spillway will be a trapezoidal channel <br />15 feet wide (see Exhibit 13-5). A portion of the pond capacity will be a result of <br />excavating topsoil and material below natural ground level. Excavated suitable material <br />will be used in embankment construction. <br />DESIGN ANALYSIS <br />General. This sedimentation pond was designed to meet or exceed the design and <br />performance requirements of LMLRD 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 and 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, 24-hour <br />storm event; <br />3. Provide adequate storage for sediment generated as deTermined from the Revised <br />Universal Soil Loss Equation as shown on Page 13 -3-8. <br />4. Provide a nonclogging dewatering device and principal spillway [hat will provide <br />adequate detention time for the 10-year, 24-hour storm event and still allow a method • <br />to dewater the pond down to the 100 percent sediment storage level in a reasonable <br />amount of time. <br />13-3-2 Revised 03/13/92 <br />