2019-07-09_PERMIT FILE - C1981022A (2)

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Section—2.05. 1 Final backfilling and grading will be completed to create a stable final topography appropriate to the postmining land use. The final topographic configuration of the Elk Creek Facilities area is shown by the Facilities Postmining Topography Map,(Maps 2.05-M6)and the Post mining Cross-Sections,(Map 2.05- M7). The backfilling and regrading plan for this area is based on the following objectives and constraints: • Backfilling will be limited to those areas where fill is necessary to stabilize exposed highwall areas, provide sufficient cover over demolition debris,or achieve effective drainage. Fill material will be placed and compacted by equipment traffic to approximately 80 percent of maximum dry density. • Final grading plans are designed to establish slope gradients consistent with upgradient natural slopes but not exceeding 2H:1 V,unless features of the natural topography dictate otherwise. Final grading will be conducted, to the extent operationally feasible, along the contour to minimize subsequent erosion and instability. Final grading will eliminate any large depressions that could impound water. • Highwall areas will be backfilled and graded to blend with adjacent undisturbed slopes and reclaimed areas. The Somerset Mine B and C Seam portals were sealed and backfilled in 1986 and associated face-up areas have already been backfilled and graded. Final grading activities for the Elk Creek Facilities area will not redisturb these regraded areas. • Because the source of the material used to construct the Elk Creek Mine portal bench was a natural landslide above the portal area,all of the bench material cannot be utilized in establishment of the design postmining topography for the Elk Creek Mine facilities area and it cannot be replaced in its original location in a stable configuration. For this reason,approximately 72,000 cubic yards of bench material will be hauled down-valley from the Elk Creek Mine facilities area and placed in the Elk Creek surface facilities area as supplemental fill. Inclusion of this supplemental fill is reflected in the design postmining topography and cross-sections. Portions of existing bench areas may be retained, if compatible with the postmining topography and reclamation objectives, to control runoff and direct surface flows to reestablished drainages. Where portions of benches are retained,bench crests will be rounded and the resulting cut material will be pushed downslope to reduce overall outslope gradients. Bench outslopes will generally be less than 50 feet in length,so contour furrows or other runoff control measures will not be necessary. The combination of short slope lengths and limited drainage areas will result in stable slopes when revegetated. The Elk Creek culvert will be removed, except for two short sections to be retained or replaced for the Sanborn Creek and Elk Creek Roads. The culvert will be excavated, removed, and hauled off-site for disposal. Following culvert removal, the corresponding segments of the Elk Creek channel will be reconstructed. Channel reconstruction will involve grading to establish the design postmining channel configuration as illustrated by the Elk Creek Profiles,(Map 2.05-M7),and the Postmining Topography Map and Cross-Sections,(Maps 2.05-M6 and 2.05-M7),placement of riprap materials,as required; replacement of available soil material on the upper channel slopes;and revegetation of both channel and sideslope areas. The reconstructed Elk Creek channel has been designed to safely pass the peak flows resulting from a 100- year,24-hour stor►n event. The designed channel has 2H:1 V sideslopes,a 10-to 12-foot bottom width,and overall gradients consistent with both the original channel and the existing culvert. Channel design calculations for the reconstructed Elk Creek channel are included in Exhibit 2.05-E3, Drainage and Sediment Control Plan. Where the existing culvert does not sit on the original channel bottom, suitably sized riprap material will be placed and bedded in the reconstructed channel to minimize channel erosion. Riprap placement will extend approximately 3 to 4 feet up the channel MR-111 2.05-57 Revised July 2019