2024-08-29_PERMIT FILE - C1980007 (2)

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West Elk Mine 2.05-36 Rev. 06/05, 01/06, 03/06, 04&05/06- PR10; 01/09- MR350; 04/09- TR116; 05/09- MR354; 08/09- TR119; 5/20- MR443; 10/20- MR450; 10/21- MR456; 6/24-TR156 Freshwater Pond-1 (FW-1) was originally designed to store and provide two months storage capacity for potable and domestic water uses at the initial planned coal production rate of 250,000 tons per year. At the projected coal production rate of 8.2 million tons per year, approximately 3.0 days of water storage capacity is provided. This pond has a capacity of ten acre-feet, an embankment height of 19.5 feet, and a surface area of 1.10 acres. The pond is located on the natural bench above the valley floor at an approximate elevation of 6,290 feet. Normal surface runoff has been kept from entering Freshwater Pond-1 (FW-1) by providing a ditch upstream of the pond to intercept and carry the Probable Maximum Precipitation (PMP) assumption "B" storm flow to the ditches that flow to sediment pond MB-5E. FW-2 Freshwater pond FW-2 (formerly sediment pond MB-1) is a two-celled pond that provides secondary raw water storage volume for the mine’s uses. Piping is in place to allow water from FW-2 to be pumped to FW-1 for mine water use. Water can also be routed to the pond from FW-1 with portable pumps and can be pumped back to FW-1 when needed to provide raw water for the mine or for treatment for potable uses. There are four pipes in the dike that separate the two cells to transfer water from the south cell to the north cell to maximize storage potential. The primary spillway is located in the north cell and raw water can be directed to sediment pond MB-5E for additional storage if needed. The emergency spillway is an open-channel, rock-lined spillway. FW-2 is lined with a HDPE liner that was installed in early 1998. Spillway Designs Spillways have been sized using the broad crested weir equation, Q = CLF1.5. A coefficient "C", of 2.67 was assumed, as this is the low end of the practical range. Spillways are constructed of riprap or gabions. The design head for each spillway has been limited to 2.50 ft. FW-1 Pond has a service spillway to assure that small inadvertent flows entering the ponds are passed without relying on the emergency spillway. The individual plan sets in Exhibit 43, Exhibit 47, and Exhibit 66 includes all pertinent hydrologic and spillway data. Stability Program Description The stability analysis for the dams was performed using a slope stability computer program (SLOPE II) developed by Geo Slope Programming, Ltd., Regina, Saskatchewan. The Ordinary (Fellenius) Method and the Simplified Bishop Methods were used to compute stability factors of safety for each embankment's maximum cross-section. Both methodologies use the method of slices and utilize the limit equilibrium theory to solve the factor of safety. The principal difference between the two methods is the treatment of interslice forces. The Ordinary Method assumes that interslice forces can be ignored, while the Simplified Bishop Method assumes that the interslice forces act horizontally. Merrick and Company's results indicated that the Ordinary Method yielded lower factors of safety in all cases. Consequently, for all subsequent trials on the given dam, only the Ordinary Method was used.