2018-09-04_PERMIT FILE - C1980007A (44)

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West Elk Mine Shallow Colluvial Slides Shallow colluvial slides are quite thin, usually only 2 to 10 feet thick. They involve slippage of colluvium on steep bedrock-colluvium contacts. They are triggered by extensive, high intensity rainfall, rapid snowmelt or seepage of groundwater into the bedrock-colluvium interface. Depending upon the saturation of the colluvium and the steepness of the slopes, they can slowly ooze down the slope or become fast moving debris avalanches. They aze usually fairly small being only a few feet to a few tens of feet wide, with failed material moving a few tens of feet to as much as 200 to 300 feet downslope. The shallow slides usually occur as a single event but can repeat if the slide leaves colluvium in an unstable position. Sometimes, the slides can occur several to an azea, almost in a cluster. Many examples of these features can be seen as fresh scars on steep slopes in the area. Old slides that have revegetated are difficult to see from aerial photos or even on the ground. Predicting the location of future individual slides is extremely difficult because of the widespread, but irregulaz, occurrence of colluvium on the steep slopes. If structures were present, any shallow colluvium slides upslope of the structures could represent a hazard. They would not be a hazazd or detriment to the underground mining or the current land uses of the permit area. Debris Avalanches Debris avalanches have occurred in narrow, steep channels on steep slopes along the cliff faces above Minnesota Creek. Recent ones show dramatically as bare, lineaz streaks on aerial photographs or at a distance when viewed in the field. If and when vegetation becomes established in the channels, they are difficult to see. This vegetation, however, tends to trap the material and inhibit the debris avalanching. Debris avalanches occur when colluvium in a steep channel becomes saturated and mobilized by intense thunderstorms or very rapid snowmelt. The loose saturated material slide rapidly to the base of the slope. In most cases, stormwater in the ravines at the base of the slope lacks sufficient volume or force to entrain the material and it simply clogs the channel. In lazger drainages, during a 100-yeaz or greater storm, the material from lazger debris avalanches might be mobilized into a debris flow. In most of the north-facing channels, thick vegetation will entrap the debris and inhibit debris flow formation. These features could be a maintenance nuisance to roads crossing them. Only one public road, USFS 711, exists within the azea of mining influence in the South of Divide permit revision azea. USFS 711 has limited public use, with recreational hunting and drill access by MCC being the historic primary uses. Debris Flows The most significant debris flow noted in review of the aerial photographs or field reconnaissance was the one previously described in the channel of the Dry Fork immediately below the Minnesota Reservoir embankment. In the past, probably 50 to 100+ yeazs ago, debris flows occurred in the channel of Sylvester Gulch and its eastern tributary, apparently originating in Section 22. It would be expected that debris flows could form in the lazger drainages, such as Sylvester Gulch or Box Canyon, as a result of 100-yeaz or greater storms, These natural]y occurring debris flows aze not anticipated to affect MCC's operations in the azea. 1.04-42 Revised November 1004 PRIO