2009-01-23_REVISION - C1980007 (3)

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which have a relatively small subsidence potential. The angle of draw of • longwall mining activity does not intersect landslide bodies in the area. MCC monitors monuments it has installed on a landslide mass in the mine's surface facilities area. Monument Dam and Minnesota Reservoir - MCC predicts the dam or reservoir will not be subsided by mining because the nearest mining will be 800 ft. away in panel E9. The angle of contact between the reservoir is 69 degrees, significantly more than the 20-degree predicted maximum angle of draw in the South of Divide area. MCC has committed to monitoring the angle of draw of panel E9 in order to verify the prediction. MCC predicts that ground vibration (seismicity) created by longwall mining could affect the dam, the reservoir, and the landslide abutting the dam because the static safety factor of the dam has been found to be less than 1.0, as shown in an analysis conducted by MCC's consultant and contained in Exhibit 72 of the permit application. The suspected cause of the relatively low factor of safety is a landslide that underlies the dam's south abutment. Possible effects from mining-induced seismicity are: rockfall into the reservoir; overtopping of the dam by water in the reservoir; catastrophic failure of the dam; flooding, sediment deposition, and erosion downstream from the dam; flood damage to roads, houses and other structures downstream from the dam. MCC has summarized in the permit application the predicted effects of a catastrophic failure of the Monument Dam determined in a Dam Breach Analysis, dated • February 8, 1984. Predicted out-of-channel floodwater depths are in the range 0.3 to 3.5 ft. in the lower 6 miles of Minnesota Creek. Predicted floodwater velocity is in the range 13 feet per second to 42 feet per second. As many as four houses could suffer shallow flooding, although loss of life would not be expected. Depending on proximity to the creek, houses could suffer structural collapse as a result of foundation erosion. Extensive erosion and resource damage would occur along Minnesota Creek and a sediment bar would probably be deposited in the North Fork of the Gunnison River. In order to mitigate any potential impacts to the dam from mining-induced seismicity, MCC has committed to the following measures (which are set forth in more detail in Section 2.05.6(6) of the permit application): Surveying the structures downstream from the dam that could be impacted by dam failure six months prior to longwall mining in the South of Divide area, and incorporating the survey into the permit application as a revision. 2. Conducting annual aerial photo surveys of landslides located north and south of the reservoir, 3. Installing and periodically surveying monuments on the dam and the . north, south, and east edges of the reservoir, 4. Monitoring water levels in piezometers installed in the dam, 48