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structures. <br />i <br />9.3.4 Ponds and Diversions <br />9.3.9.1 Sediment Pond <br />One surface runoff pond has been constructed to control sediment <br />and surface water draining from the surface facilities. This <br />sediment pond, designated Pond A, is located on the downgradient <br />side of the surface facility below the access road into the site <br />(Figure 9.2-3) This location facilitates tYie collection and <br />detention of surface runoff and sediment from the surface develop- <br />ments, including the mine portal areas. <br />Sediment Pond A also collects flow and the natural sediment load <br />from the undisturbed areas above the mine portal and surface <br />facilities. The pond is sized to allow this additional contribu- <br />tion of water and sediment from the undisturbed area. A technical <br />review was made to evaluate the feasibility of constructing a <br />single or double diversion channels above the surface facilities to <br />separate the "clean" water and sE~diment from the undisturbed area <br />(results of calculations are included in Appendix 9.3-2). Geo- <br />technically, channel erosion and erosion control structures on such <br />steep slopes (10~ to 25o grades) would be unstable and possibly <br />lead to environmental damage and maintenance problems. The best <br />solution to this problem was to allow the low volume, dispersed <br />flows and sediment to be routed through the surface facilities and <br />collected in Sediment Pond A. <br />Pond A is sized in accordance with applicable CMLRD (1982) regula- <br />tions. The Soil Conservation Service (SCS) procedure for deter- <br />mining peak flows in Colorado was used to calculate the pond volume <br />(10-year, 29-hour event) and the pond spillway capacity (25-year, <br />NCIG - Coal Ridge #1 4-38 ABC/1145/910509 • <br />