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Evaluation of Potential Subsidence Impacts of Longwall Mining in the Spruce Stomp Lease Area <br />to Aquatic Life and Water Supply <br />5.2.1 Pre- and Post- Project Channel Profile <br />The existing West Fork Terror Creek channel profile within the Spruce Stomp Lease Area was <br />investigated using 2 -foot LiDAR elevation information collected at the direction of Bowie. The <br />"Post- Project" channel profile prediction was developed by applying Mr. Dunrud's subsidence <br />model to the existing condition (see Appendix A for model description). The Post - Project <br />profile study was limited to subsidence model stations that were established at locations along <br />the channel where the expected impacts of subsidence are greatest, i.e., where the channel <br />transitions from an area with longwall panel mining to an area with barrier pillars or no mining <br />(see Figures 14a — 14g). These are the locations where the slope of the channel are most likely <br />affected. <br />Based on the review of current conditions, the West Fork Terror Creek channel currently has an <br />approximate average slope of about 6 percent within the Potential Project area (see Figure 13). <br />Within the Spruce Stomp Lease Area and at a resolution of 100 -foot long reaches, West Fork <br />Terror Creek has maximum slopes of 10.6 percent and minimum slopes of 3.0 percent (see <br />Figures 13). Along profiles A -F (Figures 14a -14g), the existing channel has a maximum slope of <br />7.3 percent and a minimum slope of 3.0 percent. <br />The Post - Project channel profile indicates that the studied portions of West Fork Terror Creek <br />would have a new maximum slope of 7.0 percent and a minimum slope of 3.5 percent between <br />the established stations (see Figures 14a — 14g). <br />5.2.2 Characterization of Pre- and Post- Project Area Slopes <br />Existing terrain slopes in the Spruce Stomp Lease Area were characterized using USGS 10 meter <br />(approximately 30 foot) contour information. In the vicinity of West Fork Terror Creek this <br />characterization also used 2 -foot LiDAR elevation information. Based on this analysis, terrain <br />that is draining to West Fork Terror Creek in the Spruce Stomp Lease Area often have <br />approximate slopes in excess of 20 percent. Based on observations made during site visits, <br />isolated side slopes can be even steeper, approaching 1 H:1 V by visual observation. <br />121 - 014.000 Wright Water Engineers, Inc. Page 18 <br />January 2013 <br />