2024-01-22_PERMIT FILE - C1980007A (4)

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West Elk Mine basin of Minnesota Creek. The next largest percentage is Lick Creek, which contains approximately 19 percent of the South of Divide permit area. When evaluating the location of the proposed E Seam longwall panels, the emphasis on Dry Fork becomes even more pronounced. There are several distinct stream channels within the permit area. Each of these streams has been identified with an index number which provides the basis for identifying stream channel characteristics before and after mining, as presented in Exhibit 55A. WWE defined the existing stream channel characteristics for various parameters, including channel slopes, peak flow rates for a variety of frequencies of occurrence, flow velocities for each of the frequencies, sediment transport regime, stage -discharge relationships, channel profiles and channel and overbank stability. The existing channel shapes were related to the "dominant discharge," which has a two-year frequency of occurrence. Sediment transport has been defined for each stream using the annual sediment load and sediment loads associated with large floods. Many of these streams (with the exception of Raven Creek, Deep Creek and Dry Fork as previously described) within the South of Divide and Dry Fork permit areas are ephemeral, based on U.S. Geological Survey topographic maps and extrapolation of gaged streatmflow data. Flows occur oily in response to snowmelt and significant rainfalls. As discussed in Exhibits 559 55A, and 55B WWE and Mr. Pemberton and Tetratech used a variety of statistically -based methods to detennine a representative average annual yield value that would apply to these drainages. Exhibits 55, 55A and 55B concludes that for the purpose of computing pre and post -mining average annual sediment yields, an appropriate mean annual runoff for the subject basins of 475 acre-feet per square mile per year should be adopted, even though site-specific data for the basins indicate a mean amlual runoff of considerably less than this amount. From the standpoint of water rights, analyses of water yield by WWE for the Division No. 4 Colorado Water Court for the 1986 West Elk Mune water augmentation plan indicated that the typical annual water yield for tributaries to the Dry Fork of Minnesota Creek were approximately 200 acre-feet per square mile per year. This is consistent with the average annual yield of the Dry Fork basin, which is also about 200 acre-feet per square mile per year based on the available data. In short, the appropriate average annual streamflow for the channels ranges from 200 and 475 acre-feet per square mile per year, with 200 acre-feet per square mile per year being used for water rights purposes and 475 acre-feet per square anile being used for sediment yield purposes. In addition to computing average annual yields, WWE calculated floodflows for multiple return frequency events. These are presented in Exhibits 55 and 55A. This is important because it is necessary to evaluate how the stream channels will respond to large flood flows after subsidence has occurred, especially with respect to sediment transport. The projected subsidence for the stream channels was determined using output from the CISPM, Version 2.0, calibrated us -Ina site-specific subsidence data. The str?ain cinainnels S�,ere analyzed to determine the magnitude of change resulting from the change in channel slope. The changes to stream channel characteristics were analyzed using standard procedures of the sedimentation and 1.05-188 Revised June 2005 PRIO. Januay 2006, March 2006: Rev. May 2006 PRIO,Nov. 2006 TRIO Sep. 2007 PR12; Feb 2008 PR12