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

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jo A summary of the stream channel hydraulic characteristics for each of the 12 drainage basins is provided in Table 3. The slope in the lower reach of the channels is shown in addition to the average channel slope since only the lower reach will receive the full basin runoff. The dominant discharge is the flow for the stream that tends to shape the stream channel and help establish the width and depth of the defined channel. The dominant discharge flow in cubic feet per second for each basin is the peak flow expected to occur there once every 2 years. Erosion and sediment yield of a stream basin is normally related to multiple factors, including rainfall, ground cover, land use, topography, upland erosion, runoff, soil types, geology, sediment, and channel hydraulic characteristics. All of these factors were considered. The channel characteristics shown in Table 3 for the dominant discharge are for the stream near the mouth or at a point just downstream of each segment of stream channel. The width and depth values at the dominant discharge in Table 3 are approximate values based upon computed physical basin regime characteristics and identification of characteristics from photographs taken on selected stream channels in the area. The adopted mean annual runoff for the subject basins is used for sediment and channel stability purposes only. It is not proposed for use in water rights studies or for site-specific water budgets. IMPACTS OF SUBSIDENCE ON STREAM CHANNELS The projected subsidence under each of first 12 stream channels in the South of Divide mining area has been determined using the output from the Comprehensive and Integrated Subsidence Prediction Model (CISPM), Version 2.0 (Department of Mining Engineering, College of Mineral & Energy Resources, West Virginia University), as calibrated using site-specific subsidence data and as reviewed by Mr. Richard Dunrud, recognized subsidence expert. Exhibit 60E presents a detailed discussion of the subsidence evaluation. In utilizing the CISPM program, each stream channel reach was defined as to location by "X"and "Y" coordinates. Channel slope changes along with the magnitude of the subsidence were determined for relatively short stream lengths of approximately 100 feet or less. The results of the computer modeling for all 12 of the potentially affected stream channels are shown in Table 4. The slope change, either positive or negative, was either added to or subtracted from the existing channel thalweg slope as determined from computations based upon USGS topographic maps with a contour interval of 40 feet. Table 4 shows that the most significant changes will occur in the tributaries overlying the southern portion of the mined permit area where E Seam mining thickness is projected to be at a maximum (14 feet). For these channels, the maximum changes in slope are an increase of 2.5 percent and a decrease of 2.5 percent. Since the pre-mining average slope of these channels ranged from 8.5 percent to 19.5 percent, the slope changes due to subsidence do not create any reaches with negative or flat slopes. Slope impacts magnitudes are greatest in the lower reaches of Basins 29 and 32, where the pre-mining slopes are approximately 4.8 percent and 4.0 percent, respectively. The pre- and post-mining profiles for Basins 29, 32 and 36, shown in Figures 15, 16, and 18 respectively, demonstrate that the overall channel slope is not significantly impacted by subsidence. Basins 5, 6, 26 and 31 generally overly mining panels with thicknesses ranging from 9 to 12 feet. The subsidence from mining in these areas will result in changes in the slopes of the stream channels ranging from a maximum increase of 1.9 percent to a maximum decrease of 1.7 percent. These slope changes will be insignificant on the stream channels where existing --- - - - - -aver-age-slopes--range from-5:3--percent-to- 8-A -per-cent-:- - -- --- - Tetra Tech - 0907161P