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West Elk Mine <br />• MB-1 and MB-2R have been classed in the United States Bureau of Reclamation (USBR) Hazard <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 lazgest 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-yeaz, 24-hour storm runoff design was adopted in accordance with CDMG regulations. The <br />Revised Universal Soil Loss Equation (RUBLE) 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 dischazge in <br />compliance with effluent limitations. In 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 a of sediment. <br />A system of ditches and culverts has been designed to collect runoff from the 10-yeaz, 24-hour or <br />lazger storm event and direct it to the runoff ponds. Although not required by CDMG regulations, <br />most of the haul and access roads located within the main mine facilities area aze drained to <br />sedimentation ponds. An exception is the Sylvester Gulch hauUaccess road; however, best <br />management practices for stonnwater treatment will be utilized. Modifications to the original mine <br />pond and ditch system can be found in Exhibit 48 and Exhibit 66. Ditches and culverts associated <br />with the RPE area aze contained in Exhibit 70. Ditches and culverts aze shown on Maps 54, 54A <br />and 54B. <br />Combined ditch flows are summarized for the system as they progress toward the runoff ponds. <br />Ditch and culvert specifications aze listed on Table 44, Exhibit 66. A summary table of inflows and <br />volumes for each pond is presented in Table 42 and in Exhibit 66. <br />The hydrologic pazameters for watershed and sub-watershed basins aze summarized on Table 43 in <br />Exhibit 66. Documentafion of specific parameters such as curve number, rainfall and time of <br />concentration aze also found in Exhibit 66. This information for the RPE pond is located in Exhibit <br />70. <br />Miscellaneous Sediment Control Facilities <br />The railroad loadout facility has been treated independently of the other surface facilities. The <br />loadout is located across the river from the other surface facilities and, therefore, must have its own <br />sediment control. The sediment control is a pond designed (sized) fora 25-year, 24-hour storm <br />event and to accommodate wash-down water from the train load-out facility. Pond MB-4 was <br />relocated in 1998 from the east side of the train load-out to an azea west of the load-out between the <br />two sets of tracks. The relocated sediment pond is constructed with concrete and is preceded by an <br />oil skimmer and a sediment trap. The current design detail for this pond can be found in Exhibit 66. <br />The RPE azea has also been treated independently of the other surface facilities. The RPE area is <br />• located to the east of the lower refuse disposal azea and east of Sylvester Gulch. The sediment pond <br />has been designed to completely contain the runoff from a 100-yeaz, 24-hour storm event. The <br />2.05-17 Revised June 1005 PRIO <br />