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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 />The subdrainage discussion below pertains to both subdrainage systems and <br />are not individually analyzed as the seeps are nearly the same in size and flow. <br />See location of both subdrainage systems on Map 21-1. <br />Two rock subdrainage systems consisting of durable non -acid-forming and non- <br />toxic -forming rock will be constructed to intercept seepage from the Fire <br />Mountain Canal to assure the foundation of the coal mine waste pile does not <br />become saturated. The drain will be about 100 feet wide near where the <br />seepage emerges below the upper diversion ditch and then taper down to a two <br />Volume XI <br />TR -109 - 6 - 09/16 <br />