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

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The entire D-1 Lower Ditch, which conveys flows from the Sidehill Sediment Pond down to Section 26 Pond, is also incorporated into the model, as SEDCAD Structure No. 4. Ditch D-1 Lower Ditch is a rip -rapped channel with 12 foot bottom, 2H:1 V sideslopes, at an average slope of 24.2%. The above described condition has been modeled and is presented in this section. The following SEDCAD models accompany this section: Westside End of Mine Life Model, 10 Year 24 Hour Event, and Westside End of Mine Life Model, 100 Year 24 Hour Event With the Side Hill Contour Ditch and the Sidehill Pond both in service, and the upstream watershed at PMT and various stages of reclamation (various CN's), the peak pool in Section 26 Pond in a 10 year 24 hour storm does not reach the emergency spillway (peak pool Elevation 6993.52 as compared to emergency spillway Elevation 6995), and the 24-hour weighted settleable solids concentration is 0.08 ml/1 (below the 0.5 ml/1 limit). The spillway design flood for this condition (100 year, 24 hour storm) engages the emergency spillway to an elevation of Elevation 6995.93, or 0.93 feet deep in the spillway. East Sediment Control System and Section 25 Pond Similar to the progress in the West Sediment Control System, the top elevation of the temporary spoil pile will lower, and the northern crest will move north as spoil is hauled back to the pit. Also, as in the West System, the remaining pile will continue to trap runoff from the upstream pit area as the pit is backfilled with spoil hauled from the stockpile, as discussed in more detail below. Throughout this haul -back operation the area reporting to the northern crest of the spoil pile in the Little Collom Gulch drainage will increase until ultimately the entire pit area 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 Little Collom Gulch. Concurrently, the northern crest of this eastern 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 Section 25 Pond. Once the final lifts are removed from Little Collom Gulch (refer to the profile presented as Exh. 7-23, Figure I1), the entire watershed will be reporting to the Section 25 Pond. That is the condition addressed by this East Sediment Control System section. As spoil material is removed from the Little Collom Gulch, more and more surface area will become available for reclamation and for sediment control activities. Sediment ponds Section 25 Pond in Little Collom Creek, Section 36 Pond in Bear Creek, and Middle Pond in the Badger Draw drainage will remain in service through this period. However, they will be augmented with a series of six small 3 ac ft stock ponds acting as a "string of pearls". These stock ponds will located in the Little Collom Gulch at final PMT elevation but still close to the source of the Exh. 7-23I-3 Revision Date: 2/15/16 Revision No.: PR -04