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~. Sediment pond MB-5 has been designed to accommodate storm water runoff, and is located at <br />the old Bear No. 1 mine site. The pond usually is discharged only once a yeaz. The principal <br />spillway is a gated pipe, and the emergency spillway is anopen-channel, rock-lined spillway. <br />This pond was designed to store three-yeazs of sediment. The pond has never been cleaned as the <br />sediment generated is less than what was predicted in the SEDCAD model. MB-5 is sized to <br />hold the runoff from a 25-yeaz, 24-hour storm event. <br />C('.-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 shafts. The pond was designed to contain the 10- <br />yeaz, 24-hour runoff volume. The pond is lined with HDPE. Theiprincipal 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 discharge pipe <br />has been designed to remain open. However, it is usually valved closed to control discharge <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 just been constructed, MCC cannot <br />evaluate the frequency of cleaning that may be required. <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 dischazge 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 years of <br />sediment that could be generated during a storm event. As the pond has just been constructed, <br />MCC cannot evaluate the frequency of cleaning that may be requi;ed. <br />Basis for Design of 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 <br />rate of 8.2 million tons per year, approximately 3.0 days of storage capacity is now provided. <br />This pond has a capacity often acre-feet, an embankment height of 19.5 feet, and a surface azea of <br />1.10 acres. The pond is located on the natural bench above the valley floor at an approximate <br />elevation of 6,290 feet. <br />Norma! 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) <br />1.05-33 November 1004 PRIl <br />