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ENGBR <br />SUBJECT: Bear Creek Basin, South Platte River and Tributaries, Colorado, <br />Wyoming, and Nebraska <br />through September. The largest and most destructive flood occurred in <br />September 1938. The most recent flood occurred in July 1965. The <br />typical flood of the basin is the flash flood occurring suddenly from <br />intense rainfall. The steep terrain produces rapid concentration of <br />runoff and high velocity flow. High velocities are the principal cause <br />of the hazard to life and large property damage. Because of the changed <br />economic characteristics of the basin and regional storm characteristics, <br />the flood history of the basin provides only a limited indication of the <br />flood damage potential under existing and prospective future economic <br />conditions. These changed conditions result from metropolitan growth <br />at Denver and the fundamental circumstances associated with metro- <br />politan growth which require optimum use of the stream valley. Under <br />existing economic conditions, the average annual flood damages are <br />estimated at $930,000. At the levels of economic development <br />expected by the year 2000, the estimated average annual damage <br />potential is $3,873,000. The majority of these damages would be <br />urban. <br />6. Improvements desired.- -Local interests have expressed a <br />virtually unanimous desire for flood protection by a dam and reservoir <br />at the site of the proposed project, known locally as the Mount <br />Carbon site. Recreation interests urge corollary recreation develop- <br />ment to meet the needs of water -based recreation in the region. <br />7. Plan of improvement. - -The District Engineer finds that a <br />dam and reservoir on Bear Creek about 8 miles above the confluence <br />of Bear Creek and the South Platte River at Denver, would provide <br />flood protection for metropolitan Denver and the rural reaches of the <br />South Platte River below Denver. The multiple- purpose project would <br />consist of an earthfill dam 143 feet high, with a crest length of about <br />4,000 feet. A supplementary earthfill dike would be constructed to <br />a height of 33 feet and a crest length of 2 , 100 feet to the south of the <br />main embankment in order to develop the storage capacity required. <br />An earthen and rock -cut emergency spillway would be provided to <br />insure the safety of the structure. The reservoir would provide storage <br />of 28,400 acre -feet, of which 2,400 acre -feet would be for sediment <br />reserve and recreation and 26,000 acre -feet for flood control. An <br />ungated service outlet would be provided for flood control releases, <br />and a gated low -flow outlet for release of streamflow requirements. <br />3 <br />