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Sedimentation Pond <br />Sediment Pond J was modified so it will contain the additional run-off from the <br />coal mine refuse disposal area. The sediment pond and the general drainage <br />plan for the waste disposal site were prepared by Tammerin Kae Stover -Bishop, <br />Colorado Professional Engineer, No. 43402. See Map 22-J. <br />The total drainage area associated with the new coal mine waste disposal area <br />is approximately 25 acres. The area was divided into separate watersheds <br />draining into run-off and collection ditches designed to handle the run-off from <br />the 100 -year, 24-hour precipitation event. The drainage areas are shown on <br />Map 20 in Volume II. <br />The modified sediment pond was sized to contain the run-off from the 10 -year <br />24-hour precipitation event (1.8 inches). This storm event will generate <br />approximately 2.6 acre feet of runoff. Sediment accumulation was estimated <br />with the use of the Universal Soil Loss Equation. The pond has been designed <br />to contain approximately 3.93 acre feet of water and 0.58 acre feet of sediment. <br />The sediment pond will be dewatered if it contains water above the maximum <br />sediment level. This dewatering event should occur within 48 hours after the <br />storm event which caused the water accumulation. When sediment <br />accumulation reaches 75% of the maximum sediment storage elevation, the <br />sediment will be removed and placed in the waste pile. <br />The mine waste pile and associated sedimentation structures are designed for <br />compliance with the effluent limitations of Rule 4.05.2. Drainage calculations are <br />presented in Appendix B. <br />Sediment pond J has been designed to increase its capacity. Material from the <br />excavation will be placed in the foundation of the access road along the southern <br />border of the pond. The embankment will not be modified. <br />Subdrainage System <br />There will be two subdrainage systems located at gob pile #3, see location of the <br />West subdrainage system on Map 21-1, and the East subdrainage system on <br />Figure -1 (Operational). The subdrainage systems will be very similar in design, <br />the main difference being the amount of rock in the trench. The subdrains will be <br />referred to as the "West" and "East" subdrainage systems. <br />Volume XI <br />TR -114 - 6 - 10/17 <br />