2022-07-12_PERMIT FILE - C1981019 (24)

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Exh. 7-23C-2 Revision Date: 4/28/22 Revision No.: TR-154 The selection of "L" and "S" for input into the SEDCADTM model for the near repose angle slopes of a benched spoil pile is also complex. The values of 145 feet for "L" and 31% for "S" were selected for the dumped spoil pile lifts to approximate the combined LS factor that would apply to a compound slope, in this case a concave slope with a very steep (repose angle) upper part and a very flat lower part (bench). SEDCADTM cannot handle a compound slope directly, but a method to do so is described in Chapter 5 of "Hydrology and Sedimentology for Disturbed Areas" by Barfield, Warner and Haan, 1981. The method uses the original 1965 Wischmeier and Smith USDA Handbook No. 282 formula, then adjusts each segment with a factor, and finally provides a weighted average for the compound slope. The above described method was applied to a 65 foot upper segment at repose angle (77%) followed by an 85 foot flat bench at 1%, resulting in a weighted average LS of 9.0. Several possible combinations of a single L and a single S were then examined for input into SEDCADTM to arrive at a similar LS factor. It was found that a single line with L of 145 feet with a drop of 45 feet, producing a S of 31% produces a LS of 10.1, which is slightly higher than the calculated value of 9.0 for the actual compound slope. That line also happens to be close to a simple straight line from the crest of one bench to the crest of the next one lower down the face, so it has some physical relevance to the actual spoil pile face. The L and S of 145 feet and 31%, producing a LS factor about 12% greater than the actual compound concave slope calculated were then conservatively used in the modeling of all spoil pile faces. Section 26 Pond – During Mining The location of the Section 26 Pond and its watershed is presented on Map 41B. The temporary channels that route water to the Section 26 Pond are presented on 41B. The as-built configuration for the Section 26 Pond is presented on Figure 7-23J-1, and the as-built configuration for the Sidehill Pond is presented on Figure 7-23J-2. Figure 7-23D-1 provides the breakdown of drainage areas and hydrologic conditions for all areas in the watershed reporting to these two sediment ponds during mining. The Sidehill Pond is part of the Section 26 Pond sediment control system and is utilized to reduced peak flows and provide additional sediment storage if need. All discharges from the Sidehill Ponds flow directly to the Section 26 Pond through the D-1 Ditch, and the Section 26 Pond is the final discharge location. The SEDCADTM model herein provides the results of the 10-year 24-hour design storm and demonstrates the Section 26 Pond will meet the applicable settleable solids standard under this modeled storm event. The second SEDCADTM model demonstrates that the Section 26 Pond emergency spillway elevation is capable of containing the 25-year 24-hour storm. In summary, for the mining case at the Section 26 Pond, the 10-year 24-hour storm produces 2.2 acre feet of runoff, and the peak settleable solids concentration is 0.00 ml/l. The 25-year 24-hour storm event peaks at the 6,984.1 elevation, which is below the emergency spillway elevation of 6,986.0’