2009-01-15_REPORT - C1980007 (8)

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Fall 2008 Subsidence and Geologic Field Observations Box Canyon, Apache Rocks, and South of Divide Mining Areas would be expected, the baseline elevations for these reset monuments have changed. Both of the baseline surveys were conducted before longwall mining began and conform to previously established criteria. The subsidence grid is planned to be resurveyed at least three months, but no more than nine months, after the longwall face has moved past the end of the longitudinal survey line of the El panel. This new data will be used to quantify the amount of subsidence (vertical displacement, tilt, and strain) that has occurred and project the subsidence angle of draw. Data obtained from the monitoring of this subsidence grid will be used to calibrate the subsidence prediction model so that a more refined analysis can be made. 3.2 Stream Channel Baseline Data for Dry Fork Subsidence depressions, tilt, and strain are projected to occur in the Dry Fork drainage and its tributaries when Longwall Panels E2 through E6 are mined in the SOD mining area. Maximum vertical displacement (subsidence) projected above the center of these mined longwall panels is as much as 11.2 feet where 14 feet of coal is extracted. A maximum tilt of 6.4 percent also is expected as detailed in Appendix A (see page 5 and Table 2). It is therefore important to obtain baseline measurements of the stream channel bottoms, particularly the channel of Dry Fork, prior to mining. Survey measurements for the deepest part of the Dry Fork stream bed (thalweg), from where the flume crosses Deep Creek to Minnesota Reservoir, were completed this fall for the second consecutive year. This survey also includes the invert and outlet elevations on all flumes and corrugated metal pipe culverts along this reach. This data, as presented in Appendix C of this report, provides both baseline data sets along with a comparison of the horizontal and vertical data points from each survey. This comparison reveals changes (some larger than others) that have occurred during the time between surveys. Some of the differences can be accounted for by the increased beaver activities which prevented access to the same survey locations while other differences appear to be the result of continued erosion of the stream channel by natural means. It should also be noted that individual survey markers were not set at each survey station. As a consequence, the second survey attempted to relocate the 831-032.790 Wright Water Engineers, Inc. Page 11 December 2008