2008-03-21_PERMIT FILE - C1980007A (5)

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West Elk Mine . additional landslides. Damming of the ditch by landslides could result in ponding within the ditch and channel avulsions. Avulsions could also occur if the ditch develops subsidence cracks that run across the ditch. The potential for flow capture or accelerated erosion causing a significant disruption of flow in the Deep Creek Ditch is considered to be low. The potential for bedrock heaving, ponding or channel avulsion to cause a significant disruption of flow in the Deep Creek Ditch is also considered to be low. If flow disruption occurred for an extended period of time it could significantly reduce supplies of water available to irrigators, resulting in a reduction of crop/hay production and consequent economic damages. As discussed in Section 5.3.2 of Exhibit 60B, no cracks were observed in the alluvium and colluvium of Sylvester Gulch and Deep Creek during periodic field observations in the Apache Rocks and Box Canyon mining areas. The near-surface alluvial material consists of primarily sand, silt, clay, and soil that range in estimated thickness of approximately 25 to 150 feet. In the two areas mentioned the drainages were located above rigid pillars and panel boundaries where the overburden depth ranges from 800 to 1,050 feet. The alluvium and colluvium in Dry Fork and Lick Creek, which has an estimated thickness range of approximately 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 Mr. Dunrud above the Somerset Mine in the Bear 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 Bear Creek alluvium (estimated to be 10 to 15 feet thick) when coal was extracted by room-and-pillar 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 deforins 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 werc locally- observed in colluvium less than one foot to a few feet thick, no cracks were observed in colluvium 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. Surface-water monitoring in the Dry Fork and Lick Creek drainages will continue 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, slop changes, and strain are projected to occur in above longwall panels E2 through E8 i__ T'r , 1::._1K. aria its tributaries and in Lick Creels 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 longwall panels E2 and E3 where the shallowest overburden occurs. Depressions, ranging in depth from 2.05-152 Revised June 2005 PR10, Rev. Dlarch 2006; 11ap 2006 PRI Q Nov. 2006TR107,.4p7-il 2007TR108; Sep. 2007 PR12; Feb. 2008 PR-12