2008-03-21_PERMIT FILE - C1980007A (5)

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West Elk Mine i MCC recognizes the natural potential for rockfalls in the reservoir area does exist. However, as discussed previously, based on annual observations in the Apache Rocks and Box Canyon mining areas during the last six years (1999-2004 inclusive), subsidence and any seismic activity caused by longwall mining is not expected to significantly affect rockfall areas with an estimated high to low rockfall potential. Only rockfall areas with an estimated very high rockfall potential were noticeably affected. However, because there are no rockfall areas in the South of Divide mining area with an estimated very high rockfall potential, longwall mining in this area will not affect the rock fall potential. In other words, mining activity should not increase the risk of rockfalls in the Minnesota Reservoir pool area. Landslides and rockfalls have occurred in reservoir areas in various locations throughout the world. Most have not had catastrophic results. If a rockfall were to occur where large boulders entered the reservoir, at a minimum displacement of water equal to the volume of the rocks would occur. If the rocks had a large mass, they entered the reservoir at relatively high velocities, and the boulders hit the water with the largest surface area facet entering the water first, a wave could be generated that would run up the face of the dam. If the wave over topped the dam, erosion of the downstream dam face could occur. Historically, however, the rockfalls that have occurred in the reservoir area do not appear to include large masses of rocks and do not appear to achieve significant velocities as they travel downslope, (generally large boulders in the Dry Fork area do not appear to have significant runout lengths). Therefore, the anticipated worst-case rockfall event would not create a significant overtopping event in the Minnesota Reservoir or displace volume of water significant enough to result in the erosion of the dam to the point of failure. Landslides are present in the reservoir area, in fact are part of both dam abutments and form portions of the shoreline. The landslides in the area tend to move in response to soil saturation conditions and do not appear to move at high velocity rates. .If a landslide did move into the reservoir basin, the subsequent displacement of water would occur gradually and should be sufficiently handled by the designed overflow structure, assuming the slide occurred when the reservoir was at full capacity. Movement of the landslides on either abutment could result in a failure of the dam. Once again, the movement would be-gradual. The first signs of failure would likely be discharges of sediment-laden waters on the-downstream dam face. The flows would gradually increase as erosion of the internal dam structure occurred. If complete catastrophic dam failure occurred as a result of the movement of the abutment landslide and internal erosion and the reservoir was at full capacity, 467 acre-feet of water would be released into the canyon below the dam. The stream channel between the dam structure and the opening of the canyon would be significantly modified, vegetation would be removed, and the Dry Canyon access road could be compromised in a few locations. Once the waters left the confines of the Dry Fork Canyon, the velocity and depth of the released water would decrease significantly but the width of the flood would increase. Road crossings and low-lying agricultural lands could be inundated. Irriaation diversions and related structures could be damaged or destroyed. Homes built within the flood plain may be damaged as well. Livestock and wildlife should be able to move to higher ground. Residents of the farms within the lowest portions of floodplain would most likely observe the steady rise in stream flow and escape to higher ground. Those displaced by the floodwaters would likely contact local emergency services that would in . turn begin the appropriate evacuation of residents living downstream along Minnesota Creek. Once the floodwaters reached the North Fork of the Gunnison, the larger channel would convey the 2.05-161 Revised June 200.1 PRIO. Rev. Alarch 2006; Alay 2006 PRIO, Nov. 2006TR707,Aprd 2007TR108; Sep. 2007 PR12; Feb. 2008 PR-12