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

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West Sediment Control System and Section 26 Pond Throughout this haul -back operation the area reporting to the northern crest of the spoil pile in the Bear Draw drainage will increase until ultimately the entire pit area in that drainage and watershed areas above it, as well as the entire footprint of the now exhausted spoil pile, are reporting to the northern crest of the remaining spoil pile. The location of this northern crest will gradually move north above the valley floor in Bear Draw. Concurrently, the northern crest of this western lobe of the spoil pile will also drop in elevation. However, throughout this time period the northern crest of the remaining spoil pile will always act as a plug that prevents any runoff from directly proceeding further downstream to the Section 26 Pond. Eventually, once the final lifts are removed from Bear Draw (again, refer to the profile presented on Exh. 7-23, Figure I1), the entire watershed will be reporting to the Section 26 Pond. Also, as the temporary spoil pile is removed, the face area of the spoil pile that is directly tributary to Section Pond 26 will be gradually shrinking, thereby slightly lightening the pond's service load temporarily, until the final lift is removed (refer to profile Exh. 7-23, Figure I1). At this point, the entire western watershed will be tributary to the Section 26 Pond. To retain the full use of the Sidehill Sediment Pond for sediment control as the final spoil pile lift is removed from the Bear Draw bottoms, a diversion ditch will be employed to route the majority of the watershed runoff through the Sidehill Sediment Pond. When the temporary spoil pile surface has been lowered to just above the 7200 contour, which is at about the elevation of the Sidehill Sediment Pond, a contour ditch will be constructed across the valley bottoms to direct all runoff from the nearly complete watershed toward the Sidehill Pond before release to the Section 26 Pond via D-1 Lower Ditch. This "Side Hill Contour Ditch" appears on Map 41C (Volume 22). In the West System, use of the Side Hill Contour Ditch as described above will allow the full capacity of the Sidehill Sediment Pond to be used to pre -condition the runoff before release to the Section 26 Pond. Ultimately, when the final lift of the spoil pile is removed from the pile's footprint below this El 7200 contour ditch, thereby removing the plug above Section 26 Pond, the area below the contour ditch will report directly to Section 26 Pond. However, the majority of the watershed will still continue to report to Section 26 Pond only after having passed through the Sidehill Sediment Pond via the Side Hill Contour Ditch. Above Sta. 37+00, the D-1 Upper Ditch will be removed when the top of the remaining spoil pile has dropped below the ditch invert, which is the condition addressed in this time frame. This ditch is modeled in Part C and described in detail in Part B. It is a vegetated trapezoidal channel with an average slope of 5%, a bottom width of 12 feet, and 2H:1 V sideslopes. It formerly extended from Sta. 0+00 at the Sidehill Sediment Pond to Sta. 57+74 near the center of the south face of the temporary spoil pile (refer to Part D of this exhibit). The remaining portions of this ditch, from Sta. 37+00 northerly to Sta. 0+00 at the Sidehill Sediment Pond, will continue in service. The lower 3,700 feet of the D-1 Upper Ditch is therefore incorporated into this model (as SEDCAD Structure No's 10 and 8). Exh. 7-23I-2 Revision Date: 2/15/16 Revision No.: PR -04