2008-03-21_PERMIT FILE - C1980007A (7)

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INTRODUCTION Wright Water Engineers, Inc. (WWE) and Ernest Pemberton, P.E.', determined of extent to which projected subsidence would impact stream channel stability and sediment transport at Mountain Coal Company's (MCC) West Elk Mine for the South of Divide (SOD) mining area in November 2004 (Wright Water Engineers, 2004) Subsequent to that report Mountain Coal Company obtained the mining lease to the adjacent Dry Fork mining area. This report is an update of the previous report and has been prepared by Tetra Tech. Most of the report is the same as Exhibit 55A. The projected mining into the Dry Fork Mining area will occur by increasing the lengths of panels E2, E3, E4, E5, and E6 by between 500 feet (for E2) to 3000 feet (for E4). This will extend the longwall mining under the upper areas of Dry Fork, a tributary to Minnesota Creek, and the upper areas of Deep Creek. Panels E1, E7, E8, and E9 are still totally within the boundaries of the South of Divide Mining Area. The effects on the lower reaches of the affected streams are not changed by mining in the Dry Fork mining area. Figure 1 shows the drainage basins of streams in the vicinity of the mining areas. Each basin has an identification number that will be used throughout this discussion of stream channel parameters and subsequent changes. As shown in Figure 1, Basins 4 through 6 and 26 through 35 are located in the Dry Fork of Minnesota Creek (Dry Fork) drainage; Basin 36, Lick Creek, is a direct tributary to Minnesota Creek. Minnesota Creek is a tributary to the North Fork of the Gunnison River (North Fork). Basin 37 is Deep Creek, a tributary to Raven Creek of the Gunnison River (North Fork). Table 1 identifies which streams may be impacted by the proposed mining. Due to the fact that impacts to tributaries would be maximized in the lower segments of the streams, this study focuses on the lower reaches of the tributary watersheds. As shown in Table 1, the mine area will underlie portions of 14 separate minor tributaries of the Dry Fork and Lick Creek in the study area as well as Deep Creek. A detailed discussion of the mine plan and projected subsidence for these areas is given in Exhibit 60E. The remaining two basin channels were determined to be outside the influence of mining activities. The stream channels for Basins 4 and 34 were modeled to verify that they were outside the influence of mining. The first portion of this hydraulic and hydrologic evaluation establishes pre-mining, or baseline, conditions. The second portion of this evaluation describes the potential and likely impacts of mining operations on the surface drainage system and channel characteristics. EXISTING STREAM CHANNEL CHARACTERISTICS The existing stream channel characteristics have been defined 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 over-bank stability. The existing channel shapes were related to the dominant discharge (2-year frequency of occurrence). Sediment transport has been defined for each stream using the annual sediment load; however, sediment transport is a long-term value and represents the full range of flows including the 10-year and 100-year frequency peak discharges. For each of the existing stream channels, many hydrologic and stream channel parameters were defined and evaluated, including: 1 Adjunct Scientist with WWE, Former Head of Sedimentation and River Hydraulics Section, Hydrology Department, Chief Engineer and Assistant Commissioner's Office, U.S. Bureau of Reclamation. Tetra Tech - 0907161P