2019-09-18_PERMIT FILE - C1981035 (9)

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. Linsley (1986) reports on a study carried out by Langbien and Schumm (1958) where average annua( sediment yield per unit area for a number of basins was related to mean annual rainfall. For a mean annual rainfall of about 18 inches (Durango), their curve results in an annual sediment production of 600 tons/mil. In another study (U.S. Department of Agriculture, 1973) cited by Linsley, selected rates of sediment production were reported for several reservoirs in the United States. From these sites the four most likely to represent conditions in southern Colorado were used to give an annua! sediment production of 1650 tons/mil. The highest value in this study was 3340 tons/mil (Morena Reservoir, California). Similar results are reported by Vanoni (1975) for additional sites around the United States. One in particular is for the North Fork of Clear Creak near Blackhawk, Colorado where the annual sediment yield was estimated to be 340 tons/mi2. Combining this information results in a potential sediment yield for the east pond drainage of 0.05 AF and for the west pond 0.1 AF annually. A stage staff Dearly marked off in 112 foot intervals shall be installed in both ponds for gaging sediment and water levels. Annual maintenance is required to assure adequate storage for the design rainfall event. 6.3 Pond Geometry The combined east and west ponds have a storage volume, below the spillway crest, of 2.78 AF. Regulations require that the sum of the embankment insfope and • outslope be no greater that 5:1 at any location. In addition a minimum topwidth of 8 ft is required. The elevation of the top of the west pond embankment is about 7501. The spillway crest is at 7500 and the bottom of the pond is at 7492.5. The volume reserved for the sediment is 0.10 AF. The elevation of the top of the east pond embankment is about 7498. The spillway crest is at 7497 and the bottom of the pond is at 7485.5. 6.4 Outlet Works and Trash Racks Outlet works for the west pond include a low level orifice and a spillway. The spillway is asharp-crested, rectangular weir. The weir notch on the spillway is 5 feet long and 0.7 feet high. As previously stated this will pass the required discharge with a maximum depth over the crest of less than 0.4 ft. One foot of freeboard is required above the high water line for each pond. The west pond low level outlet is a circular orifice sized to drain a full pond in 48 hours. The orifice.discharges water into a 9 inch diameter pipe, 37.5 feet tong which terminates in the field north of the pond. The orifice diameter is 3.1 inches. In order to r 13 January 19981