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West Elk Mine <br />geomorphic engineering professions based on the effects of thalweg slope changes (either increase <br />or decrease) due to mining-induced subsidence. <br />Maximum estimated change in channel width is calculated to be seven feet, and the maximum <br />change in channel depth is 0.4 feet. Changes in new regime width and depths occur in a slow <br />process that may take from three to five yeazs or more. The likely change in sediment yield is not <br />expected to be more than 5 percent. Reaches where the slope increases will undergo degradation <br />(erosion), and reaches where the channel slope decreases will experience aggradation (deposition). <br />Overall, the mining-induced subsidence impacts on existing stream channel parameters and basin <br />sediment yield are not significant. See Exhibits 55 and SSA. However, MCC commits to <br />removing any significant mine induced accumulation of sediment in Minnesota Reservoir, the plan <br />and frequency of removal being subject to any required approval by the IJSFS and MCRC. <br />As discussed in the subsidence section of this pernut document [(Section 2.05.6 (6)(e)(i)(C&D)], <br />the height of the caved zone above B Seam mining is conservatively estimated at 2.St or 30 feet. <br />The height of the fractured zone is conservatively estimated at 20t or 240 feet when t = 12 feet. <br />Summing these two figures yields a total combined thickness for the caved/fractured zone of 270 <br />feet above the B Seam. This can be contrasted with the minimum overburden thicknesses in the <br />mine plan area of 375 feet, which occurs in the South of Divide permit azea. As outlined in detail in <br />the section entitled "Potential for Hydraulic Between Mine Workings and Surface," the maximum <br />combined zones of potential fracturing is 253 feet, leaving over 100 feet of buffer. Consequently, <br />• the probability of establishing a hydraulic connection between the mine workings and surface water <br />features remains very small. <br />Ana/vsis oflmpacts to Streams <br />Subsidence in the South of Divide permit revision area has the potential to affect streams in <br />two ways. Fractures, which extend upward from the mined seam may intersect the stream or, <br />more likely, surface cracks may develop which intersect the stream. The principal problem that <br />longwall-induced subsidence creates with respect to streamflows is surface cracks that may cause <br />the loss (normally on a temporary basis) of surface flows. WWE, working closely with Messrs. <br />Rold and Dunrud, has evaluated the phenomenon of surface cracking in association with longwall <br />mining at West Elk Mine and has referred to comparable experience in the North Fork Valley and in <br />the relevant literature. It is WWE's general conclusion that, in the permit area, there will be very <br />little modification of the existing surface water flow regime as a consequence of surface cracks <br />caused by mining. <br />As discussed in Section 2.05.6 (6)(b)(i), Description of Possible Subsidence Consequences, surface <br />cracks resulting from longwall mining subsidence aze infrequent at the mine. In fact, where <br />cracks have formed above longwall panels, additional factors have usually played a role. <br />The crack above the east side of 1NW longwall panel was influenced byroom-and-pillaz mining <br />in the F Seam. <br /> <br />1.05-189 Revised June 2005 PRIO, January 2006, March 2006, Rev. May 1006 PRIG <br />