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West Elk Mine '~ geomorphic engineering professions based on the effects of thalweg slope changes (either increase or decrease) due to mining-induced subsidence. Maximum estimated change in channel width is 2.6 feet, and the maximum change in channel depth is 0.2 feet. Changes in new regime width and depths occur in a slow process that may take from three to five yeazs or more. The likely change in sediment yield is not expected to be more than 5 percent. Overall, the mining-induced subsidence impacts on existing stream channel pazameters and basin sediment yield aze not significant. See Exhibit; 55. As discussed in the subsidence section of this permit document [(Section 2.05.6 (6)(e)(i)(C&D)], the height of the caved zone above B-Seam mining is conservatively estimated at 2.St or 30 feet. The height of the fractured zone is conservatively estimated at 20t or 240 feet when t = 12 feet. Summing these two figures yields a total combined thickness for the caved/fractured zone of 270 feet above the B-Seam. This can be contrasted with the minimum overburden thicknesses in the mine plan area, as follows: Minimum overburden thickness above the east (i.e., the 14SE through 17SE longwall panels) panels where only B-Seam mining will occur, is 800 feet. j 2. Minimum overburden thickness above the Apache Rocks SW longwall panels (i.e., the 12NW through 13aNW longwall panels) where B and E-Seam (i.e., "Two Seam") mining will occur is 400 feet over the E-Seam. ~. 3. Minimum overburden thickness above the seven Box Canyon panels, where only B-Seam mining will occur, is 500 feet. These calculations demonstrate ;that fractures extending upwazd from the mined azea would not impact streamflows. Annlycic tmYnnctc t~ ,Ctranm.c -Subsidence in the current and Box Canyon permit revision azeas potentially affect streams in two ways. Fractures which extend upYvazd from the mined seam may intersect the stream or, more likely, surface cracks may develop ryhich intersect the stream. The principal problem that longwall-induced subsidence creates with respect to streamflows is surface cracks that may cause the loss (normally on a temporazy basis) of'surface flows. WWE, working closely with Messrs. Rold and Dunrud, has evaluated the phenomenon of surface cracking in association with longwall mining at the West Elk Mine and has referred to compazable experience in the North Fork Valley and in the relevant literature. It is WWE's general conclusion that, in the current permit azea, there will be very little modification of the existing surface water flow regime as a consequence of surface cracks caused by mining. As discussed in Section 2.05.6 (6)(b)(i), Description of Passible Subsidence Consequences, surface cracks resulting from longwall mining subsidence are rare at the mine. In fact, where cracks have formed above longwall panels, additional factors have always played a role. • The crack above the east side of 1NW longwall panel wash influenced by room-and-pillaz mining in the F-Seam. ~~ 2.05-/51 November 2004 PR/1