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Subsidence Evacuation For Exhibd 608 South o(Divide Mining Area Page 25 • The primary streams in the South of Divide mining area are Dry Fork of Minnesota Creek, Deer Creek, Poison Creek, and Lick Creek. The primary source of water to Minnesota Reservoir comes from the Deep Creek Ditch, wherein water is diverted from 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. As discussed in Section 532, no cracks were observed in the alluvium and wlluvium of Sylvester Gulch and Deep Creek during periodic field observations in the Apache Rocks and Box Canyon mining areas. The neaz-surface alluvial material consists of primazily sand, silt, clay, and soil that range in estimated thickness from 25 to 150 feet. In the two areas mentioned the drainages were located above rigid pillars and panel boundaries where the overburden depth ranges firm 800 to 1,050 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 South of Divide mining area despite the shallow overburden. In the South of Divide mining area the overburden depth to the E Seam ranges from approximately 375 feet above the western edge of longwall panel E2 to 1,300 feet above the eastern limit of longwall panels E6 and E7. However, based on observations made by the author above the Somerset Mine in the Beaz Creek area, subsidence cracks are not expected to occur in the Dry Fork alluvium where the overburden depth ranges from 375 to 800 feet. No cracks, and no change in stream flow, were observed in the Beaz Creek alluvium (estimated to be 10 to 15 feet thick) when coal was extracted by room-and-pillaz methods at depths ranging from 220 to 300 feet beneath Bear Creek (Bureau of Land Management, et al., 2002). The probable reason for the lack of cracking in alluvium is that the fine sand- to clay-sized material and overlying soil yields without cracking or bulging as it deforms as a discrete unit, or as discrete units, in the subsidence process. This same reasoning also applies to the colluvium in the area. Although subsidence cracks were locally observed in colluvium less than one foot to a few feet thick, no cracks were observed in coluuvium more than about ten feet thick. No cracks have been observed in alluvium above mined longwall panels in the Apache Rocks and Box Canyon mining areas. It is important to continue ongoing surface-water monitoring in the Dry Fork and Lick Creek drainages in order to compare the historic information derived from annual subsidence observations in the West Elk Mine area with field observations in selected areas of the South of Divide mining area- Subsidence depressions, slope changes, and strain are projected to occur in above longwall panels E2 through E8 in Dry Fork and its tributaries and in Lick Creek when the panels are mined. The maximum subsidence amount, slope change (tilt), and strain are projected to occur above solid coal barriers and mined longwall panel boundaries, such as above the west ends of longwafl panels E2 and E3, where the shallowest overburden occurs. Depressions, ranging in depth from 8.4 to 112 feet, are projected in this area. Maximum changes in slope (tih), ranging from 1.3 to 6.4 percent, are also projected for this area. The maximum horizontal tensile and compressive 831-032.690 Wright Water Engineers, Inc.