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West Elk Mine <br />embankment height of 19.5 feet, and a surface area of 1.10 acres. The pond is located on the <br />natural bench above the valley floor at an approximate elevation of 6,290 feet. <br />Normal surface runoff has been kept from entering Freshwater Pond -1 (FW -1) by providing a <br />ditch upstream of the pond to intercept and carry the Probable Maximum Precipitation (PMP) <br />assumption "B" storm flow to the ditches that flow to sediment pond MB -5E. <br />FW-2 <br />Freshwater pond FW -2 (formerly sediment pond MB -1) is a two - celled pond that provides <br />secondary raw water storage volume for the mine's uses. Piping is in place to allow water <br />from FW -2 to be pumped to FW -1 for mine water use. Water can also be routed to the pond <br />from FW -1 with portable pumps and can be pumped back to FW -1 when needed to provide <br />raw water for the mine or for treatment for potable uses. There are four pipes in the dike that <br />separate the two cells to transfer water from the south cell to the north cell to maximize storage <br />potential. The primary spillway is located in the north cell and raw water can be directed to <br />sediment pond MB -5E for additional storage if needed. The emergency spillway is an open - <br />channel, rock -lined spillway. FW -2 is lined with a HDPE liner that was installed in early <br />1998. <br />Spillway Designs <br />Spillways have been sized using the broad crested weir equation, Q = CLF A coefficient <br />"C ", of 2.67 was assumed, as this is the low end of the practical range. Spillways are <br />constructed of riprap or gabions. The design head for each spillway has been limited to 2.50 <br />ft. FWA Pond has a service spillway to assure that small inadvertent flows entering the ponds <br />are passed without relying on the emergency spillway. The individual plan sets in Exhibit 43, <br />Exhibit 47, and Exhibit 66 includes all pertinent hydrologic and spillway data. <br />Stability Program Description <br />The stability analysis for the dams was performed using a slope stability computer program <br />(SLOPE II) developed by Geo Slope Programming, Ltd., Regina, Saskatchewan. The <br />Ordinary (Fellenius) Method and the Simplified Bishop Methods were used to compute <br />stability factors of safety for each embankment's maximum cross - section. <br />Both methodologies use the method of slices and utilize the limit equilibrium theory to solve <br />the factor of safety. The principal difference between the two methods is the treatment of <br />interslice forces. The Ordinary Method assumes that interslice forces can be ignored, while <br />the Simplified Bishop Method assumes that the interslice forces act horizontally. Merrick and <br />Company's results indicated that the Ordinary Method yielded lower factors of safety in all <br />cases. Consequently, for all subsequent trials on the given dam, only the Ordinary Method <br />was used. <br />2.05 -36 Rev. 06/05, 01/06, 03/06, 04/06 & 05/06- PRIG; 01109 - MR350; 04109- TR116 ;; 05109- MR354; 08109- TR119 <br />