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<br />[<e <br />" <br /> <br />I <br />I <br /> <br />'fe <br /> <br />I; <br />lfe <br />~. <br /> <br />mately 6,600 at the upper portion of the mine site. The watershed is very <br />steep, with the slope of Ralston Creek averaging about 3 percent at the mine <br /> <br />site. <br /> <br />The watershed is predominantly forested. Mean annual precipitation <br /> <br />averages approximately 20 inches over the watershed. The majority of runoff <br /> <br />occurs from snowmelt, however peak flows generally result from summertime <br /> <br />rainstorms which can produce large peak flows in the front range area of <br /> <br />Co 1 orado. <br /> <br />Peak flow estimates for Ralston Creek were reported by the Urban Drainage <br /> <br />and Flood Control District in the "Major Drainageway Planning, Phase B Master <br /> <br />Pl an" for Ra 1 ston and Leyden Creeks. These est imates have been des i gnated <br /> <br />and approved by the Colorado Water Conservation Board. Because these peak <br /> <br />flows are the result of the most detailed study that has been done on Ralston <br /> <br />Creek, and have been approved by the agencies, they have been adopted as the <br /> <br />basis for use in this study. <br /> <br />The 100-year recurrence interval peak discharge at Ralston Reservoir was <br /> <br />reported by the Urban Drainage and Flood Control study to be 7,200 cfs. <br />This discharge represents a runoff of 150 cfs per square mile for the 48 <br /> <br />square mile watershed at the reservoir. The watershed at the mine site is <br /> <br />The runoff of 150 cfsper square mi 1 e for <br />/- , <br />/ <br />38.7 square miles results in a design discharge of 5,810 cfs for the 100- <br /> <br />approximately 38.7 square miles. <br /> <br />year event. <br /> <br /> <br />wpj -/ <br /> <br />4-2 <br />