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I I I I I I I I I I I I I I I I I I I 5.0 SEDIMENT Sediment yield in the drainage basin above the reservoir was estimated from a 1974 sediment yield map published by the Colorado Land Use COl1l1lission. ~Iuch of the quantitative data on the map was based on information derived from small dry reservoirs with drainage basins of one to five square miles. The amounts of sediment reflect the average weight and volume of sediment as ordinarily deposited in these reservoirs. The information presented represents an average yield over a period of time greater than 25 years. The estimated sediment yield for the drainage basin above Elkhead Lake is low to moderate compared with other drainages in Colorado, and averages about 0.25 acre-foot per square mile per year. The basin characteristics vary from mainly forest lands and grasslands with good vegetative cover in the upper basin to mostly rangeland in the lower basin. Assuming a 100-year project life, the total expected sediment yield from the watershed is 5100 acre-feet. The trap efficiency of a reservoir is defined as the ratio of the quantity of deposited sediment to the total sediment inflow. Gunnar Brune has developed a set of trap efficiency curves for use with normal ponded reservoi rs usi n!l their capacity-inflow relationships. The capacity-infloYl ratio for Elkhead Dam is about 0.2 for the rehabilitated or slightly enlarged structure. Using the Brune curves gives a trap efficiency of 92 percent. This information produces about 2350 acre-feet of sediment accumulation expected in the reservoir over 50 years. For mo st pl anni ng studi es, the 50-year sediment accumul ati on is subtracted from the reservoir storage content for estimating project yields. In this case, since the sediment accumulation can be contained in the inactive recreation pool, the conservation storage space has not been diminished. - 30 -