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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 />longwall mining. A quantitative evaluation of this subsidence and its pre- and post- mining <br />effects on West Fork Terror Creek are discussed in the remainder of this section. <br />5.1 Subsidence Evaluation of the Terror Creek Mining Area <br />In 2012, C. Richard Dunrud, P.E. performed a subsidence evaluation of the Terror Creek Mining <br />Area as a means to quantitatively assess the expected limits and magnitude of the subsidence <br />associated with the Proposed Project. This evaluation is summarized in a report entitled <br />"Subsidence Evaluation of the Western Mining District and Spruce Stomp Lease Area" and is <br />included with this report as Appendix 4. The following discussion summarizes the detailed <br />analysis presented in Appendix 4. <br />From this evaluation, Mr. Dunrud found that the average draw angle (angle of influence) <br />associated with the current upper B -seam mining is 22° from vertical as measured from the edge <br />of the longwall panel (see Figure 4 in Appendix 4) and the maximum observed vertical <br />displacement (subsidence) is 7.5 feet with an overburden thickness of about 860 feet. This same <br />draw angle is expected for lower B -seam mining in the Potential Project area given to proximity <br />and similar geology. With lower B -seam overburden thicknesses in excess of 950 feet in the <br />Potential Project area, the projected maximum subsidence value is about 5.7 feet at a distance <br />approximately 300 feet south of the West Fork Terror Creek channel. Along West Fork Terror <br />Creek, the maximum subsidence is expected to be about 5.1 feet. Mr. Dunrud further concludes <br />that there will be no reduction in surface flow in West Fork Terror Creek resulting from the <br />subsidence in the Potential Project area. This conclusion is based on experience with similar, <br />nearby longwall coal mines and the amount and makeup of the colluvium material underlying <br />West Fork Terror Creek in the Potential Project area. <br />5.2 Characterization of Surface Impacts <br />Based on the magnitude of subsidence predicted by Mr. Dunrud, there will be some <br />morphological changes to West Fork Terror Creek drainage. This will include (1) changes in the <br />channel profile and slope angles, (2) development of potential surface fractures, and (3) potential <br />influences on existing geologic hazards. Each of these areas is discussed below. <br />121 - 014.000 Wright Water Engineers, Inc. Page 17 <br />January 2013 <br />