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Wes( 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. FW-1 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 interstice 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, 01106, 03106, r14/r16 8 OSIrX- PRirl; r1r/r19-.?1R35r1; U4/rly- 7'R116:; 05109-AIR354; 081119- TR119