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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />5.0 SEDIMENT <br /> <br />Sediment yield in the drainage basin above the reservoir was estimated from a <br />1974 sediment yield map published by the Colorado Land Use Commission: Much <br />of the quantitati ve data on the map was based on i nformati on deri ved from <br />small dry reservoirs with drainage basins of one to five square miles. The <br />amounts of sediment refl ect the average wei ght and vol ume of sediment as <br />ordinarily deposited in these reservoirs. The information presented represents <br />an average yield over a period of time greater than 25 years. <br /> <br />The estimated sediment yield for the drainage basin above Peterson lake <br />Reservoir is shown to be very low compared tlith other drainages in Colorado, <br />and averages less than 0.1 acre-foot per square mile per year. The basin is a <br />hi gh mountai n area, wi th dense vegetative cover, and wi th resi stant rock <br />outcrops. Assuming a 100-year project life, the total expected sediment yield <br />from the watershed is 26 acre-feet. <br /> <br />For most p1 anni ng studies, the 50-year sediment accumu1 ati on is subtracted <br />from the reservoi r storage content for estimati ng project yi el ds. In thi s <br />case, since the sediment yield is extremely small, the conservation storage <br />space has not been diminished. <br /> <br />The trap effi ci ency of a reservoi r is defi ned as the rati 0 of the quanti ty of <br />deposited sediment to the total sediment inflow. Gunnar Brune has developed a <br />set of trap efficiency curves for use with normal ponded reservoirs using <br />their capacity-inflow relationships. <br /> <br />The capacity-inflow ratio for Peterson Reservoir is 0.31 for the rehabilitated <br />structure. Using the Brune curves gives a trap efficiency of 95 percent. <br />This information produces just 12 acre-feet of sediment in the reservoir over <br />50 years. Even this estimate is probably high, because the reservoir is <br />emptied or nearly emptied every year. This type of operational scheme will <br />reduce trap effi ci enci es and encourage f1 ushi ng sediment through the outl et <br />works. <br /> <br />-33- <br />