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100-yeaz 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-yeaz 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 aze above the flood plain elevation and should experience minimal damage. Cross Section The cross section of a typical collection and diversion channel are shown in Exhibit 47 and the minimum ditch designs aze shown in Exhibit 66. Exhibit 66 discusses the hydrologic design of ponds, channels, and culverts in the azea 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, aze 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 aze ui Exhibit 43. Acreages of drainage azeas relating to certain diversion and collection channels aze shown in Exhibit 66. Locations of culverts are shown on Map 54, Map 54A, and Map 54B. Details and the culvert specifications are found in Exhibit 66. Details and culvert specifications for the ditches and culverts at the RPE azea aze found in Exhibit 70 and Exhibit 66. MCC may install "internal" culverts (i.e., located within the mine site drainage systems) to provide access. If added, the culverts would be sized equivalent to or greater than the next downstream ditch or culvert. Prior to the disturbance of the area for surface construction at the West Elk Mine, four sedimentation ponds and associated collection structures were constructed to effectively retain runoff from the azea to be disturbed. The other ponds were developed prior to the use of the old Beaz Mine area, construction of the Lower Refuse Pile, formerly referred to as the U.S. Steel laydown area, construction of the Sylvester Gulch Facilities azea, and construction of the RPE azea. Prior to construction, topsoil was salvaged from all construction azeas, stockpiled, and/or applied to 2.05-3 8 Revised Jan. 7997 TR79