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West Elk Mine <br />• Facilities Area. See Exhibit 46 for the original design specifications and for the current designs for <br />MB-4, Unit Train Loadout Sedimentation Pond. Pond MB-4 was relocated in 1998 from the east <br />side of the train load-out to an azea west of the load-out between two sets of train tracks. The <br />relocated sediment pond is constructed of concrete and is preceded by an oil skimmer and a <br />sediment trap. Design criteria for Ponds MB-5 aze contained in Exhibit 47, and an as-built survey is <br />contained in Exhibit 66. Ponds MB-2 and MB-6 no longer exist and were replaced by new pond <br />MB-2R. Design criteria for the Refuse Pile Expansion (RPE) pond, designated the "RPE" pond, aze <br />included in Exhibit 70. <br />HydrolopicDesiQn of Runoff Ponds <br />Seven storm drainage sediment control structures are used as settling ponds for storm water runoff <br />from the mine site. These ponds have been designed according to CDMG regulations (see Exhibits <br />43, 47, 66, and 70). <br />These ponds have been constructed to contain an estimated three yeazs of sediment storage volume <br />and a runoff storage volume resulting from the 10-year 24-hour rainfall event. Sediment will be <br />excavated, allowing for additional storage, when sediment has reached the maximum storage <br />capacity. Sediment volumes are estimated by a quarterly visual evaluation, The smaller ponds MB- <br />3, MB-4, and MB-5, usually dry during a portion of the year and sediment levels, relative to the <br />primary dischazge structure elevations, can be visually determined. The lazger ponds usually <br />contain water and visual evaluations are less accurate. These ponds, MB-1, MB-2R, and SG-1 will <br />be surveyed at least every three years to detemrine the actual sediment levels. <br />The CDMG requirements for the emergency spillway sizing are 25-year, 24-hour storm flows, or <br />100-yeaz, 24-hour storm flows, depending on the size of the structure. The spillways for MB-3, <br />MB-4 and MB-5 were sized based on the 25-yeaz, 24-hour storm, while MB-1, MB-2R, and SG-1 <br />were sized for the 100-year, 24-hour storm event in accordance with the requirements of the State <br />Engineer. Drainage basins for the spillway sizing are shown on Map 54, Sediment and Drainage <br />Control Facilities. The RPE pond has been designed to completely contain the runoff from a 100- <br />year, 24-hour storm event. Designs are included in Exhibit 70. <br />MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (iJSBR) Hazazd <br />Assumption "B" category. Accordingly, the probable maximum precipitation inflow design flood <br />has been reduced to the "B" assumption. This assumption is justified in that the largest storm <br />drainage/sediment control pond has a storage volume of less than 15 acre-feet. Furthermore, the <br />potential for loss of life is minimal as immediate downstream households are limited in number and <br />could be evacuated quickly. In addition, the impact on the North Fork of the Gunnison River is <br />minimal since the energy from such a small volume of water would be quickly dissipated. <br />The 10-year, 24-hour storm runoff design was adopted in accordance with CDMG regulations. The <br />Revised Universal Soi] Loss Equation (RUSLE) was used to predict the sediment yield from three <br />years of runoff. MCC will maintain the ponds in compliance with the designs to effect discharge in <br />compliance with effluent limitations. 1n some cases, MCC has installed concrete sediment traps <br />above the pond inlets to facilitate sediment removal. These traps typically have inside dimensions <br />of 24 feet in length, 10 feet in width, and 5 feet in depth, and can contain 600 ft s of sediment. <br />2.05-19 Revised June 1005 PR/0; Rev. January 1006; Rev. March 2006; Rev. Apri(1006 PRIO, May 2006 PRIO <br />