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Dames & Moore <br /> <br />Flatiron Companies <br />May 17, 1982 <br />Page 3 <br />Topographic slopes on the dump will be flatter than the pre-existing <br />gradients of the natural terrain in the dump area. <br />Watershed and Drainage Modification <br />The existing drainage course in the area of the planned East Dump will be <br />eliminated by the dump. Runoff formerly following this drainage course <br />will be diverted mainly to the drainage depression which lies immediately <br />southwest of the dump. A natural drainage course already exists in this area <br />which will be extended by stripping and recontouring of the small hilt that <br />presently forms the head of the drainage. The catchment which collects runoff <br />tributary to Mann Draw will therefore be shifted about 800 feet to the south- <br />west from its present position but will not be significantly changed in areal <br />• dimension. <br />Infiltration Capacity <br />The area to be covered by the East Dump is underlain principally by <br />Pierre Shale. The dump material will contain a significant proportion of <br />sand and silt derived from the excavated Coalmont Formation and will be <br />compacted to a density less than its original in-place density. The East <br />Dump, therefore, will be more permeable than the underlying Pierre Shale and <br />should promote a higher rate of infiltration from rainfall sad snowmelt. <br />HYDROLOGIC IMPACTS <br />Surface Water <br />Runoff from the East Dump is expected to be less than the pte-dump runoff <br />naturally occurring in this area. Runoff in the recontoured aide-draw <br />southwest of the East Dump may be slightly increased. The net change in <br />amount of runoff to Mann Draw from the disturbed area on the Bourg property is <br />expected to be negligible. <br />• <br />