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storm event which caused the water accumulation. When sediment <br />accumulation reaches 75% of the maximum storage, the sediment will be <br />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 Volume III, Exhibit 8. <br />Sediment pond J will be deepened to increase its capacity. Material from the <br />excavation will be placed in the western (narrow) end of the pond. The <br />embankment will not be modified. <br />Subdrainaae System <br />A rock subdrainage system 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 design of the rock drain will be an alternative design in <br />accordance with Rule 4.10.3(5). The drain will be about 100 feet wide near <br />where the seepage emerges below the upper diversion ditch and then taper <br />down to a two foot wide by two feet deep drain. A perforated pipe will be <br />installed in the rock 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 />TR -80 - 6 - 07/13 <br />