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• PROCESSING WASTE 1) As mentioned in the text under the section Geology, the pile is located in a debris flow environment, however, it has been mapped by Mr. Ray Junge of the Colorado Geological Survey in 1978 as an older debris flow environment. This is evidenced by very deeply entrenched drainage across its surface and the fact that the vegetation on the fan surface is well established and appears not to have been disturbed in the recent past. The thick layers of organic material observed on the fan, ranging from as little as 6 inches to as much as 6 feet, indicates a long quiescent period in the life of the fan. The character of the materials present in the fan was observed in two test pits and, as would be expected, the materials were found to be highly stratified and somewhat variable in character ranging from relatively coarse, clean sand and gravel to silty or sandy clay. Near the toe of the fan, the materials tend to be somewhat finer with a lower overall percentage of boulder and cobble material. Higher near the apex of the fan, clays still predominate, however, a larger percentage of coarse grained m aterialis present with a much higher percentage of boulder and cobble sizes. The largest single event observed in the test pits on-site had a maximum thickness of 3 feet. This would indicate that the fans present on-site were most likely constructed by many relatively small events and were not the result of a relatively few very large events. Despite this and the evidence of relatively long inactivity on the fan, we felt it was prudent to assume that it is possible during the life of the pile for yet another debris flow event to occur, blocking the existing drainage within Sylvester Gulch. Section G.6.2 of the original permit sub mission discusses hydrologic design utilizing a 100-year flood event and • assuming blockage of Sylvester Gulch. The reconstruction of Sylvester Gulch Road in that area provides for a drainage channel sufficient to convey the 100- year flood event safely past the pile, even in the event that the natural channel now existing in Sylvester Gulch is fully blocked by debris along its full length adjacent to the pile. As discussed under the section on roads (Section 7.1 - Realignment of Sylvester Gulch Road), it is proposed that oversized material (18 inches and larger) from all stripping operations on-site will be stockpiled and placed along the east side of the Sylvester Gulch Road realignment. Therefore, the relocated Sylvester Gulch Road will provide a buffer between any flooding or debris flow events associated with Sylvester Gulch and the waste pile construction as proposed. The armoring of the east side of Sylvester Gulch Road with oversized material merely increases the level of erosion protection. 1Nith regard to the likelihood of such event occuring, as mentioned previously, past evidence all indicates that the majority of the debris flow activity in Sylvester Gulch has ceased, or at least remained inactive for a relatively long period of time. Further, the evidence indicates that the magnitude of such events is relatively sm all and that the provisions made for these events in the design should be adequate to prevent loss of processing waste into the gulch for storm events up to the 100-year event. It is not, however, possible to quantify the probability of such a debris flow event occurring within Sylvester Gulch since, unlike hydrologic events, a succession of debris flow events are not statistically independent. The movement and redistribution of loose material or debris during any given event has a definite effect on the probability assciated with subsequent events. Therefore, in our opinion, the qualitative information • available on the likelihood and the character of future debris flow events in Sylvester Gulch represents the best information available. 67