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West Elk Mine sedimentation ponds MB-l, the original MB-2, and MB-3, and the freshwater pond, FW-1. Approval was received on February 3, 1981, and is included in Exhibit 45. Exhibit 66 contains the new designs for ponds MB-2R and MB-3. The Exhibit 43 and Exhibit 44 designs are no longer valid for these ponds. Exhibit 66 contains an as-built drawing that reflects the addition of a dike in the pond to create a north and south cell. However, the drawings included in Exhibit 43 do accurately reflect other structures associated with MB-1 (i.e., fire primary and emergency spillways). Exhibit 66 also contains the design information for SG-l located at the SylvesterGulch Facilities Area. See Exhibit 46 for the original design specifications and for the current designs for MB-4, Unit Train Loadout Sedimentation Pond. Pond MB-4 was relocated in 1998 from the east side of the wain load-out to an azea west of the load-out between two sets of train tracks. The relocated sediment pond is constructed of concrete and is preceded by an oil skimmer and a sediment trap. Design criteria for Ponds MB-5 are contained in Exhibit 47, and an as-built survey is contained in Exhibit 66. Ponds MB-2 and MB-6 no longer exist and were replaced by new pond MB-2R. Design criteria for the Refuse Pile Expansion (RPE) pond, designated the "RPE" pond, are included in Exhibit 70. Hydrologic Design of Runoff Ponds Seven storm drainage sediment control structures are used as settling ponds for storm water runoff from the mine site. These ponds have been designed according to CMLRB regulations (see Exhibits 43, 47, 66, and 70). These ponds have been constructed to contain an estimated three years of sediment storage volume . and a runoff storage volume resulting from the 10-yeaz 24-hour rainfall event. Sediment will be excavated, allowing for additional storage, when sediment has reached the maximum storage capacity. Sediment volumes are estimated by a quarterly visual evaluation. The smaller ponds MB-3, MB-4, and MB-5, usually dry during a portion of the year and sediment levels, relative to the primary dischazge structure elevations, can be visually determined. The lazger ponds usually contain water and visual evaluations aze less accurate. These ponds, MB-1, MB-2R, and SG-] will be surveyed at least every three years to determine the actual sediment levels. The CMLRB requirements for the emergency spillway sizing are 25-yeaz, 24-hour storm flows, or 100-year, 24-hour storm flows, depending on the size of the structure. The spillways for MB-3, MB-4 and MB-5 were sized based on the 25-year, 24-hour storm, while MB-1, MB-2R, and SG-1 were sized for the 100-year, 24-hour storm event in accordance with the requirements of the State Engineer. Drainage basins for the spillway sizing are shown on Map 54, Sediment and Drainage Control Facilities. The RPE pond has been designed to completely contain the runoff from a 100- year, 24-hour stonrr event. Designs are included in Exhibit 70. MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (USBR) Hazard Assumption "B" category. Accordingly, the probable maximum precipitation inflow design flood has been reduced to the "B" assumption. This assumption is justified in that the lazgest storm drainage(sedimentcontrol pond has a storage volume of less than 15 (acre-feet). Furthermore, the potential for loss of life is minimal as immediate downstream households aze limited in number and could be evacuated quickly. In addition, the impact on the North Fork of the Gunnison River is minimal since the energy from such a small volume of water would be quickly dissipated. 2.05-29 RevisedJan 1996 RV03: Revised A'ov. 1996 PROi;ReviseJJan. 1997 TRi9: RevsedJul. 1997 TR8?: Revised Jun. 1999 TR90