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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 />A rock subdrainage system 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 />foot wide by two feet deep drain. A perforated pipe will be installed in the rock <br />drain. See Map 21 -1 for details. <br />The majority of the rock drain will have 4 square feet of 3/4" screened or washed <br />rock. The upper section of the rock drain will have up to 200 square feet of <br />screened or washed rock. The drain will have a slope of 2% to 8 %. Darcy's Law <br />can be used to calculate the flow though the drain. <br />Darcy's Law is Q = kA dh /dl <br />where: <br />Q = rate of water flow calculated by formula (0.16 to 64.0 cubic feet per minute) <br />k = hydraulic conductivity of clean gravel (2.0 to 200 ft/minute)' <br />A = column cross sectional area (4 SF) <br />dh /dl = hydraulic gradient, the change in head over the length of interest (0.02 to 0.08). <br />1 Principles of Geotechnical Engineering, Braja M. DAS, 1985, page 84. <br />Volume XI <br />TR -84 - 6 - 12/13 <br />