1991-05-16_REVISION - C1981035 (2)

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most likely value for this site. This technique will give at least an order of magnitude estimate of the true sediment yield to be expected. The design sedimentation volume for the ponds will be equal to the estimated annual yield. Sediment levels in the ponds will be monitored and adjustments to the sediment storage will be made after 1- year if necessary. Linsley (1986) reports on a study carried out by Langbien and Schumm (1958) where average annual 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 productions of 600 tons/miZ. For the East Pond, (drainage area = 43.5 acres) this result in a sediment storage requirement of about 0.02 AF. 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 annual sediment production of 1650 tons/mil. This translates to a sediment storage volume of 0. 05 AF for the East Pond. If the highest value in this study is used, 3340 tons/miZ (Morena Reservoir, California) , the annual volume is 0. 10 AF. 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/miZ. Using this rate the sediment volume for the East Pond would be 0. 01 AF. Combining this information results in a potential sediment yield for the East Pond drainage which ranges from 0. 01 AF to 0. 1 AF. Since the drainage area and basin characteristics for the West Pond are similar to those of the East Pond the same sediment storage-volume can be used for both. The high end estimate, 0. 1 AF, will be used in sizing both ponds. A stage staff clearly marked off in 1/2 -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. Pond Geometry Existing geometry of the East Pond was measured by National King Coal in a topographic survey in March 1991. The existing volume was computed to be 1. 36 AF, short of the required volume, 2.2 AF, by 0.84 AF. Expansion of the pond is accomplished by increasing both the length and width of the pond. In addition the depth of the pond is increased throughout. The base elevation of the design pond will be at elevation 86.5 and the top of the embankment will be at elevation 99 (maximum depth of 12.5 ft) . The design volume (2 .2 AF) must exist between the bottom and the crest of the emergency spillway, elevation 97.7. A schematic of the pond and the outlet works is shown in figure 1. 4