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West Elk Mine <br />All of these streams (with the exception of Deep Creek) within the South of Divide permit area aze <br />ephemeral, based on U.S. Geological Survey topographic maps and extrapolation of gaged <br />streamflow data. Flows occur only in response to snowmelt and significant rainfalls. As discussed <br />in Exhibits 55 and SSA, WWE and Mr. Pemberton used a variety of statistically-based methods to <br />determine a representative average annual yield value that would apply to these drainages. Exhibits <br />55 and SSA concludes that for the purpose of computing pre and post-mining average annual <br />sediment yields, an appropriate mean annual runoff for the subject basins of 475 acre-feet per <br />square mile per year should be adopted, even though site-specific data for the basins indicate a mean <br />annual runoff of considerably less than this amount. <br />From the standpoint of water rights, analyses of water yield by WWE for the Division No. 4 <br />Colorado Water Court for the 1986 West Elk Mine water augmentation plan indicated that the <br />typical annual water yield for tributaries to the Dry Fork of Minnesota Creek were approximately <br />200 acre-feet per squaze mile per year. This is consistent with the average annual yield of the Dry <br />Fork basin, which is also about 200 acre-feet per squaze mile per yeaz based on the available data. In <br />short, the appropriate average annual streamflow for the channels ranges from 200 and 475 acre-feet <br />per squaze mile per year, with 200 acre-feet per square mile per year being used for water rights <br />purposes and 475 acre-feet per square mile being used for sediment yield purposes. <br />In addition to computing average annual yields, W WE and Mr. Pemberton calculated floodflows for <br />multiple return frequency events. These are presented in Exhibits 55 and SSA. This is important <br />because it is necessary to evaluate how the stream channels will respond to lazge flood flows after <br />subsidence has occurred, especially with respect to sediment transport. <br />The projected subsidence for the stream channels was determined using output from the CISPM, <br />Version 2.0, calibrated using site-specific subsidence data. The stream channels were analyzed to <br />determine the magnitude of change resulting from the change in channel slope. The changes to <br />stream channel characteristics were analyzed using standazd procedures of the sedimentation and <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 years 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. <br />As discussed in the subsidence section of this pemut 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 400 feet. <br />1.05-158 Revised November 2004 PRIG <br />