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risk of overall instability (geotechnical study results, Appendix 2.05.4-1, con- • ducted by Fox Consultants). The post-mine drainage channels and channel cross sections configuration approximate the pre-mine geomorphic characteristics. Map 17 illustrates the designed post-mine topographic surface, and Maps 18A and B illustrate approxi- mate grading profiles. Using this base, a grading plan was developed. Cut and fill calculations were performed by computer techniques to determine cut and fill requirements for pit restoration. The volume of overburden material moved to return the area to approximate original contour was approximately 600,000 cubic yards plus 50,000 yards of topsoil. During the development of the post-mine topographic design, consideration was also given to preserving the essential hydrologic functions (minimizing off- • site erosion and sedimentation impacts) of the reclaimed area by incorporating appropriate sediment control structures. The reclaimed mine area was approved to support future ranching activities and to establish drainage characteristics similar to pre-mine conditions. Sun Coal Company recognized potential erosion problems could be induced if the backfilled area was graded to form long, unin- terrupted slopes. Therefore, the basin shape was slightly modified from the pre-mine configuration in order to geotechnically and hydrogeologically stabi- lize the backfilled area. Slight drainage modifications were incorporated into the plan [compare Map 9 (Pre-Mine Water Control) to Map 10 (Post-Mine) - Area A] to provide topographic diversity while stabilizing the backfilled reclaimed mine area. The slight topographic modification appears to have little or no effect on the existing hydrologic balance. During the bonding period, the backfilled area will be monitored to deter- • mine and, if necessary, mitigate erosion and sedimentation impacts. In addi- - 317 - (Rev. 5/86)