2009-01-14_PERMIT FILE - C1980007A

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West Elk Mine Impacts of Overtopping Hydrologic Structures All diversion structures are designed for use throughout the life of the West Elk Mine. See Exhibit 44, Exhibit 47, and Exhibit 66 for details of the design of these structures, including stability analyses, where appropriate. The ditches have been designed and constructed to follow existing drainage routes. Overtopping the collection and diversion ditches would create localized flooding of the immediate area; however, damage would be minimal as flows would most likely enter the next downstream ditch or culvert and flow to the sediment pond. The future access road to the upper waste rock disposal area would be closed to traffic during such times. Since most of the ditches are lined with shotcrete or other such energy dissipaters, problems would be minimal. Ponds MB-1 and MB-2R have been shown to safely pass through the spillways the runoff from a 100-year precipitation event into the North Fork of the Gunnison River. Overtopping impoundment structures would create erosion along the embankments and contribute to localized flooding. In the "greater than a 100-year flood precipitation event" the south bank of the North Fork would experience more damage from the river itself than from overtopping the ponds. Furthermore, the facilities on the south bank are above the floodplain elevation and should experience minimal damage. Collection and Diversion Channels t"'rnee .Cnrtinn The cross-section of a typical collection and diversion channel is shown in Exhibit 47 and the minimum ditch designs are shown in Exhibit 66. Exhibit 66 discusses the hydrologic design of ponds, channels, and culverts in the area of the surface facilities, including the Sylvester Gulch Facilities Area. The locations of the culverts used for collection and outfall may be found on the Plot Plans showing Hydrologic Structures in Exhibit 43 and drawings in pertinent Appendices. The hydrologic design of the RPE pond is located in Exhibit 70 and Exhibit 66. Design Dimensions The hydrologic criteria of the diversion and collection channels, are located in Exhibit 66. Exhibit 46, Exhibit 47, Exhibit 49, Exhibit 66, and Exhibit 70 contains all the construction specifications for Ponds MB-1 through MB-5, the SG-1 pond, and the RPE pond (note that MB-2 has been redesigned and is included in Exhibit 66 as MB-2R, and pond MB-6 no longer exists). The designs for the trash racks that may be installed on some culverts, 30 inches or less minimum cross-sectional dimension are in Exhibit 43. Acreages of drainage areas relating to certain diversion and collection channels are shown in Exhibit 66. t"'~ilvnrte 2.05-39 Revised November 2004 PR10; Rev. March 2006; Rev. Apri12006 PR10; Rev. May 2006 PRIG; Rev. January 2009 MR-350