2021-02-05_PERMIT FILE - C1980007A (5)

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Subsidence Evaluation for the Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 35 Sunset Trail mining areas are undermined. The clay-rich material that lines these ponds is expected to provide a seal against subsidence effects. Numerous observations have been made of the stock watering ponds and U.S.Forest Service water resources over both B-seam and E-seam longwall mining. Based on years of field studies, none of these resources has been noticeably affected when longwall mining occurred beneath them. These observations are documented in the annual Subsidence and Geologic Field Observations reports. 11.3 Streams and Ditches The primary streams in the Southern Panels mining area are Dry Fork of Minnesota Creek, Deep Creek, Poison Creek, and Lick Creek. A primary source of water to the Dry Fork and Minnesota Reservoir comes from the Deep Creek Ditch, wherein the trans-basin water is conveyed through the upper drainage of Deep Creek and transmitted to Dry Fork. The Deep Creek Ditch was constructed in debris flows or colluvium and alluvium derived from the debris flow, as described in Section 11.2,this debris flow material is not expected to be impacted by longwall mining. There is an area of the Deep Creek Ditch that has a hard rock bottom. This area is limited to the lower gradient areas above the first landslide in the Dry Fork mining area (Refer to Map 1). The Deep Creek Ditch is not anticipated to be affected by longwall mining based on the eastern extent of E- seam longwall panel E5 and the projected extent of longwall panel E6. Longwall mining of the E-seam panels E1 to E5 has occurred under Dry Fork where the overburden depth above the longwall panels drops as low as about 375 feet. No adverse impacts were observed during or subsequent to the longwall panel mining. The proposed B-seam panels B26 to B29 will have 200 to 300 feet of additional overburden, and do not extend as far west as the E-seam panels where the overburden under the Dry Fork channel is at a minimum. In the Apache Rocks West mining area, Horse Gulch enters Minnesota Reservoir from the north and has already had longwall mining occur beneath it in B-seam longwall panels B 12 to B 13-A with no apparent cracking in the alluvial material. Although E-seam mining had less overburden(down to about 350 feet),the experience of mining beneath Dry Fork with similar overburden indicated that there will not be adverse impacts to the Horse Gulch channel even though the alluvial thickness is less than Dry Fork. As discussed in Section 5.3.2,no cracks were observed in the alluvium and colluvium of Sylvester Gulch, Deep Creek, and Dry Fork during periodic field observations. The near-surface alluvial material consists of primarily sand, silt, clay, and soil that ranges in estimated thickness from 25 to 75 feet. The alluvium and colluvium in Dry Fork and Lick Creek, which also has an estimated thickness range of 25 to 75 feet,contains more clay than does the Deep Creek alluvium. Therefore, it is even less likely that cracks will occur in colluvium and alluvium in the stream valleys of the Southern Panels mining area despite the shallow overburden. The Sunset Trail mining area includes South Prong and some of its tributaries. The channels all have steep gradients and are not anticipated to be adversely affected by longwall mining of Panels SS1 to SS4. Exhibit 55B addresses potential impacts to South Prong in more detail. 831-032.912 Wright Water Engineers, Inc. December 2020