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West Elk Mine <br />• SG-1 <br />Sediment pond SG-1 was constructed in 1997 to accommodate storm water runoff from the pads <br />associated with ventilation shafts #1 and #2, located in Sylvester Gulch. The pond treated <br />construction water during construction of the shags. The pond was designed to contain the 10- <br />year, 24-hour runoff volume. The pond is lined with HDPE. The principal spillway consists of a <br />channel of filter rock that filters the sediment prior to entering a slotted HDPE pipe, which then <br />becomes a solid pipe. The system is an effective means to treat the water and the dischazge pipe <br />has been designed to remain open. However, it is usually valved closed to control dischazge <br />from the pond. The emergency spillway is anopen-channel, rock-lined spillway. The pond was <br />designed to hold three years of sediment, As the pond has recently been constructed, MCC <br />cannot evaluate the frequency of cleaning that may be required. <br />RPE Ponds <br />The RPE ponds were also constructed during 1997 to accommodate storm water runoff and <br />possible leachate from the Refuse Pile Expansion (RPE). The RPE ponds are essentially one <br />pond with two cells, similar to pond MB-1. Water from the sediment ditch enters Cell A (the <br />eastern cell). Water can be treated in this cell and discharged directly to the river through piping <br />that connects the cell to the primary discharge structure in Cell B, or the water can be transferred <br />over to Cell B for further treatment if necessary. The primary discharge is through a gated pipe. <br />The emergency spillway is a pipe, but remains open. The pond has been designed to hold the <br />• runoff volume from a 100-yeaz storm event. The pond was designed to hold three yeazs of <br />sediment that could be generated during a storrn event. As the pond has recently been <br />constructed, MCC cannot evaluate the frequency of cleaning that may be required. <br />FW-1 <br />Freshwater Pond-1 (FW-1) was originally designed to provide two months storage capacity for <br />potable and domestic water uses at 250,000 tons per year production. At a projected production rate <br />of 8.2 million tons per yeaz, approximately 3.0 days of storage capacity is now provided. This pond <br />has a capacity of ten acre-feet, an embankment height of 19.5 feet, and a surface azea of 1.10 acres. <br />The pond is located on the natural bench above the valley floor at an approximate elevation of 6,290 <br />feet. <br />Normal surface runoff has been kept from entering Freshwater Pond-1 (FW-1) by providing a ditch <br />upstream of the pond to intercept and carry the Probable Maximum Precipitation (PMP) assumption <br />"B" storm flow to MB-1. This ditch keeps all nuisance flows from entering and combining with the <br />raw water stored in the fresh water pond. A spillway section has been designed to pass many <br />overland flows that may overtop this ditch and enter the pond. <br />Spillway Desi,2ns <br />Spillways have been sized using the broad crested weir equation, Q = CLFI s. A coefficient "C", <br />of 2.67 was assumed, as this is the low end of the practical range. Spillways aze constructed of <br />. riprap or gabions. The design head for each spillway has been limited to 2.50 ft. FW-1 Pond has <br />a service spillway to assure that small inadvertent flows entering the ponds aze passed without <br />2.05-33 Revised June 2005 PRIG; Rev. January 2006; Rev. March 2006; Rev. Apri12006 PRIO, May 2006 PRIO <br />