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purpose of the structure will be to control runoff and sediment from a <br />watershed disturbed by mining activities. This structure was constructed in <br />the summer of 1992. <br />SITE DESCRIPTION <br />Land Use: Sedimentation Pond 005 has a 289.9 acre watershed that is tributary <br />to Dry Creek (see Exhibit 13-2). This watershed will be disturbed by mining <br />activities as shown on Exhibit 12-2, and reclaimed as shown on Exhibit 20-2, <br />Postmining Topography and Drainage. The land condition of the watershed used <br />for the pond design was conservatively based on the maximum disturbance at any <br />one time within the mining area - approximately 45 percent undisturbed and 55 <br />percent disturbed. <br />Embankment and Excavation: The embankment is a homogeneous earth fill <br />structure. The top width of the embankment is approximately 15 feet with <br />upstream slope of 3.5H:1V and downstream slopes of approximately 2.5H:1V. The <br />principal spillway consists of a 29-inch drop inlet corrugated metal pipe <br />(CMP) riser; the emergency spillway is a trapezoidal channel 12 feet wide (see <br />Exhibit 13-5A, Pond 005 As-Built). A portion of the pond capacity was the <br />result of excavating topsoil and material below the natural ground level. <br />Excavated suitable material was used in embankment construction. <br />DESIGN ANALYSIS <br />General: The pond was designed to meet or exceed the design and performance <br />requirements of the Regulations of the Colorado Mined Land Reclamation Board <br />for Coal Mining and adhere to the following specifications: <br />1. Provide storage and treatment for the runoff to the pond from a 10-year, <br />24-hour storm event to assure compliance with receiving water quality <br />standards in accordance with Rule 4.05.6(3)(a). <br />2. Provide storage for the runoff from a 10-year, 24-hour storm event without <br />experiencing any discharge through the emergency spillway in accordance <br />with Rule 4.05.9(2)(b). <br />3. Provide adequate storage for sediment generated as determined from the <br />SEDCAD modeling demonstration. <br />4. Provide a nonclogging dewatering device and principal spillway that will <br />provide adequate detention time for the 10-year, 24-hour storm event and <br />still allow a method to dewater the pond down to the 100 percent sediment <br />storage level in a reasonable amount of time. <br />TR-59 13-3C-2 Revised 05/07 <br />