2018-09-04_REVISION - C1980007 (11)

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59 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): 1. 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, 5. Installing and periodically surveying monuments on the landslide that adjoins the south abutment of the dam, 6. Monitoring seismicity at a seismic station previously installed on Monument Dam and to be installed in selected E-seam panels, 7. Removing any significant sediment accumulation from the reservoir caused by mining, 8. Constructing a stability berm and buttress at the dam, and making other modifications to restore the dam to a 1.5 static safety factor and to enable the dam to withstand the predicted maximum seismic event from mining in the South of Divide area of 2.3 on the Richter scale and peak ground acceleration of 0.15 g, 9. Surveying monuments on the dam within two days of a seismic event that exceeds 0.15 g, and if the integrity of the dam or reservoir appears threatened, immediately notifying the Division, the Minnesota Reservoir Company, the U.S. Forest Service, the Office of the State Engineer, and other appropriate agencies,