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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 />However, the terminal points of landslide /debris flow deposits are located on the north and <br />south edge of the West Fork Terror Creek channel where the flow gradient begins to increase <br />eastward ( Dunrud, 1989). These deposits are visible on a detailed satellite or aerial image of <br />the area. The northern, more extensive deposit shows no sign of recurring movement. The <br />lower part of the southern slide, however, shows signs of renewed movement since it was <br />initially deposited. These landslides /debris flows consist of unconsolidated basalt boulders <br />and Wasatch clays that were remobilized during periods of very high precipitation, <br />precipitation that was probably much higher than any recorded during the approximately <br />hundred -year history of coal mining in the North Fork Valley. There is no indication of any <br />recent movement. <br />Subsidence related to longwall mining has the potential to trigger landslides, debris flows and <br />rockfalls in areas currently prone to wasting events. However, these events have occurred <br />naturally in this area for a long time without adversely impacting the continued existence of <br />wildlife and vegetation. <br />5.3 Reasonableness Checking— Comparison to Nearby Mines <br />Mr. Dunrud has over 17 years' experience monitoring subsidence above more than 25 longwall <br />panels at the nearby West Elk Mine. During this time, Mr. Dunrud reports that cracks were not <br />observed in the stream and impacts to stream flow were not identified. The overall magnitude <br />and scope of subsidence impacts modeled for the Potential Project were compared to those <br />observed for the West Elk Mine and were found to be comparable. <br />6.0 POTENTIAL IMPACTS TO WEST FORK TERROR CREEK'S ASSOCIATED <br />AQUATIC LIFE <br />Projections for the impacts to aquatic life in West Fork Terror Creek resulting from the Potential <br />Project's subsidence were developed using the existing habitat and biologic conditions, the <br />sediment transport analysis (Appendix 2), and Mr. Dunrud's subsidence characterization <br />(Appendix 4). The potential impacts are summarized in Table 7. <br />121 - 014.000 Wright Water Engineers, Inc. Page 21 <br />January 2013 <br />