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2-31 <br />• 2.4.4 Hydrologic ]mpact Evaluation <br />Hydrologic analyses for the coal refuse disposal site were <br />performed using the methods described in the Colorado Water <br />Conservation Board Technical Manual No. 1 (McCain, 1976) and USDA <br />Soil Conservation Service publication - Peak Flows in Colorado. <br />Technical Manual No. 1 was prepared by the Water Conservation <br />Board in cooperation with the U.S. Geological Survey and is <br />regarded as one of the most applicable methods for determining <br />peak flows in ungauged streams at this time. <br />The watershed in which the refuse pile is to be located comprises <br />approximately 115 acres in the southern plateau region of <br />• Colorado. The watershed ranges from approximately elevation 5,180 <br />feet at the confluence with East Salt Creek to 6,240 feet at the <br />highest point. Peak discharges in this region are computed by a <br />regression equation described in the manual. Discharges were <br />computed for the upper twenty percent of the watershed, for the <br />upper seventy five percent of the watershed and for the entire <br />watershed. Discharges are shown below as a function of distance <br />from the highest elevation. The capture of refuse pile runoff in <br />sediment ponds will result in lesser discharge from this <br />watershed. See Section 2.3.5 and Appendix As. <br />DESIGN DISCHARGES FOR COAL REFUSE AREA WATERSHED <br />Distance from Top Drainage Discharge in CFS for Several <br />of Watershed" Area Recurrence Intervals <br />(Feet) (Acres) 2-Yr. 10-Yr. 25-Yr. 50-Yr. 100-Yr. <br />1,600 24 2.9 5.8 7.4 8.6 9.9 <br />3,400 86 7.3 14.4 18.4 21.5 24.9 <br />4,900 115 9.0 17.7 22.6 26.4 30.5 <br />• See Figure 4.3-ls for locations. <br />2!16/89 <br />