2013-07-29_REVISION - C1996083 (3)

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Water Control Measures Rule 4.09.1(13) requires that an appropriate underdrain be installed if seeps, springs or waterways, natural or manmade, are in the coal waste disposal footprint area. A June 2013 inspection of the site found that seepage from the Fire Mountain Canal had surfaced in the upper diversion ditch. Wet areas were also noted below the upper diversion ditch. The seepage did not cause any perceptible flow. The design of a rock subdrainage system is detailed below. The run -off and collection ditches are shown on Map 15. Run -off from the benches will be directed to the east and west to collection ditches. The east and west collection ditches D -J8 and D -J9 are; 1) lined with riprap where necessary to ensure they are stable; 2) constructed off the fill and; 3) designed to safely pass the run -off from the 100 -year, 24 -hour precipitation event. See Volume III, Exhibit 8 for detailed design information of the run -off and collection ditches. Sedimentation Pond Sediment Pond J will be modified so it will contain the additional run -off from the coal mine refuse disposal area. The sediment pond and the general drainage plan for the waste disposal site were prepared under the direction of J. E. Stover, Colorado Professional Engineer, No. 19230. See Map 22 -J. The total drainage area associated with the new coal mine waste disposal area is approximately 25 acres. The area was divided into separate watersheds draining into run -off and collection ditches designed to handle the run -off from the 100 -year, 24 -hour precipitation event. The drainage areas are shown on Map 20 in Volume II. The modified sediment pond was sized to contain the run -off from the 10 -year 24 -hour precipitation event (1.8 inches). This storm event will generate approximately 2.6 acre feet of runoff. Sediment accumulation was estimated with the use of the Universal Soil Loss Equation. The pond has been designed to contain approximately 2.6 acre feet of water and 0.2 acre feet of sediment. The sediment pond will be dewatered if it contains water above the maximum sediment level. This dewatering event should occur within 48 hours after the TR -80 - 5 - 07/13