2016-08-08_PERMIT FILE - C1981019A (115)

Home Browse Search

EXHIBIT 7, ITEM 23, PART I Sediment Control System Analysis at End of Mining Near the end of active mining, and concurrent with the completion of all coal removal, the temporary spoil pile will be is removed over a period of about four years and hauled back to the pit. For planning purposes at present, this is anticipated to commence in approximately Year 2030. For the existing downstream sediment control structures, this time period is likely to be the worst case hydrologic condition. It produces more runoff volume than the storm modeling presented in Part C primarily due to the larger contributory drainage area. For example, the East side model presented in Part C has a drainage area of 707 acres and produced about 12.3 acre feet of runoff into the Section 25 Pond, the East Side model in this Part I analysis contains about 1,541 acres of drainage and produces more runoff. A series of six additional 3 acre-foot stock ponds are proposed under this Part I to minimize flows to the Section 25 Pond. Progression of this haul -back will obviously be top down, but at all times through this period the remaining spoil pile will continue to intercept runoff from the upstream watershed until the last lift at the northern extreme of the spoil pile is removed and hauled back. Watershed boundaries for the hydrologic evaluation are presented on Map 41C, "Watershed Boundaries for Hydrologic Modeling, Collom Area - Near End of Mining" (Volume 22). Exhibit 7-23 Figure I1 "'Profile Along Reconstructed Little Collom Gulch and Bear Draw" presents a profile along the channels showing the surface with the spoil pile still in place and the surface after the pile is removed Also presented is a similar profile along Bear Draw just to the west. The profiles also show the final lift in the northern toe of the spoil pile footprint in both the Little Collom Creek and Bear Draw drainages. This will be useful for discussion purposes in this Part I. Reclamation progress is assumed to closely follow pit backfilling progression shown on Map 29C (Volume 22). Referral to both of these maps will aid in understanding this complex end -of -mining hydrologic analysis. In all, the west side SEDCAD model includes 16 separate structures and 10 subwatersheds, while the east side model has 39 separate structures and 50 subwatersheds, each with individual slope, curve numbers, time of concentration parameters, and sedimentology input data. As the temporary spoil pile lifts are removed, the final post -mining topography will gradually re- appear, starting in the pit as areas are backfilled up to final grade, and extending northerly through the footprint of the former temporary spoil pile as increasing portions of the pile's former footprint are excavated down to final grade, which in this case is the original topography within the footprint of the pile. This will occur in both the Bear Draw and the Little Collom Gulch drainages. The following discussion separately addresses conditions in the Bear Draw drainage, referred to as the "West Sediment Control System" and in the Little Collom Gulch/Badger Draw drainage, referred to as the "East Sediment Control System." The final structure in the West System is Section 26 Pond, while the final structure in the East System is Section 25 Pond. Exh. 7-23I-1 Revision Date: 2/15/16 Revision No.: PR -04