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~ "I ~,). ..'~... ':l' , ,) '''''';' 1,;1.",. . 35,400 and 98,000 cfs are anticipated for the 2 rivers. Four bridges in the area will obstruct flood flows. Reservoirs on both rivers reduce flood waters, and levees in the area provide some protection. 47. Army [U.5.] Engineer District, 1978, Flood plain information: Big Thompson River, Weld County, Colorado: Omaha, Nebr., Prepared for Weld County Larimer-Weld Regional Council of Governments and Colorado Water Conservation Board, 33 p. The study area is the Big Thompson River basin in Weld County, Colorado. This basin drains an area of about 830 square miles in north central Colorado. The most common form of development in the flood plain within the study area is agriculture, but overall development is relatively sparse. Flood data were gathered from 2 U.S. Geological Survey gaging stations, Corps of Engineer's flood records, topographic maps, and local newspaper. Intense rainfall of cloudburst magnitude occurs in the area. Floods generally occur from May through July, but peak discharges have been recorded from March through September. No effective flood control structures have been constructed. Olympus .Dam, located in the upper part of the river basin, has negligible effect in reducing flood damages. The worst recorded flood near the Drake gaging station occurred in July 1976, discharging 31,200 cubic feet per second (CFS). The Intermediate Regional Flood at the Larimer-Weld County line would discharge 10,000 CFS, while Standard Project Flood (SPF) at the same location would discharge 18,500 CFS. Downstream from Little Thompson River, the SPF would discharge 20,000 CFS. This report was prepared as an aid to local officials in planning the use and regulation of the flood plain. 48. Army [U.5.] Engineer District, 1971, Special flood hazard report: to revise flood plain information, Metropolitan Region, Denver, Colorado; Volume II: Sand, Toll Gate and Lower Cherry Creeks, South Platte River Basin: Omaha, Nebr., 21 p. Cherry Creek channel is alluvial, flat-bottomed, and follows a meandering course, sloping downstream from Cherry Creek dam at 25 ftlmi, with some channel improvements between the dam and Havana Street. Sand Creek has been improved by channel realignment while portions of Toll Gate Creek improved during interstate highway construction. Cherry Creek study reach is highly urbanized and is crossed by 38 bridges which can obstruct flood flow. Sand Creek and Toll Gate Creek are less urbanized, crossed by 17 and 5 bridges respectively. A zone of frequent cloudbursts over highlands at 6,000 to 7,000 feet covers major portions of these basins. Cloudburst storms cause floods from March through August. The Cherry Creek reservoir impounded a flood which had a peak inflow of 58,000 cfs in June, 1965, the greatest known are flood, saving an estimated $130 million in flood damages downstream. At Toll Gate Creek, flow was estimated to be 17,000 cfs, and at Sand Creek it was 18,900 cfs. Their flooding caused extensive damages, destroying nearly every bridge crossing. At the mouths of Cherry, Sand, and Toll Gate Creeks, discharges of an intermediate regional flood would be 10,900 cfs, 49,500 cfs, and 21,900 cfs, respectively, and of a standard project flood, 21, 200 cfs, 91,200 cfs, and 31,700 cfs. BIBLIOGRAPHY 17