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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 />'. <br />'. <br />I <br /> <br />6 <br /> <br />FACTORS AFFECTING FLOOD HAZARD <br /> <br />It is impossible to predict the degree of accumulation of de- <br />bris within floodwaters and the resulting effect of such accumu- <br />lation. This report assumed that existing structures within the <br />flood flow area would not see blockage and reduced efficiency dur- <br />ing a major flood event. The capacity of bridge structures, how- <br />ever, was reduced to account for bridge skew. <br /> <br />Factors which Increase the Flood Hazard <br /> <br />Natural obstructions to flood flow include brush, trees and <br />other lowland vegetation which naturally grow along stream <br />banks and lowlands adjoining drainageways. During periods of <br />high flooding, these obstructions tend to slow the flood <br />velocities and as a result, cause greater depths and backwater <br />conditons that increase flood elevations. Often the natural <br />vegetation found within the flow area becomes dislodged from <br />its normal location and accumulates as debris within the flood- <br />waters to be collected at bridges and culverts and other <br />manmade obstructions. In addition to the vegetative <br />obstructions, floodwaters tend to erode channel banks <br />and carry the suspended soil material as well as rocks and <br />boulders further downstream, adding to the debris and damage <br />at the manmade obstructions. As these masses of debris accumulate, <br />the artificial barrier soon breaks loose and a wall of water <br />and debris surges further downstream to continue the hazard <br />and destruction. <br /> <br />Factors Which Reduce the Flood Hazard <br /> <br />Flood control improvements, including channel improvements, <br />reservoirs and bridge improvements, as well as good flood <br />plain management practices such as flood proofing and avoiding <br />development in flood plains, will reduce the potential hazards <br />associated with flood prone areas. For the essentially undeveloped <br />flood plains delineated in this report, the enforcement of <br />sound flood plain regulations will best serve the needs of <br />Adams County in the study area. <br /> <br />In general, both natural and manmade obstructions exist which <br />tend to restrict flood flows and cause overbank or non-channel <br />flooding. These obstructions will often result in unpredictable <br />flood flow patterns. This is particularly true of irrigation <br />canals crossing natural drainageways. The obstructions also <br />tend to become sources of debris and further obstruction when <br />structural failure results. Also, manmade flood plain <br />obstructions tend to concentrate flood flows which in turn <br />result in increased velocities and increased hazards in adjacent <br />flooded areas. <br /> <br />Existing flood control reservoirs also significantly reduce <br />flood hazards along the South Platte River in Adams County. <br />The Omaha District of the Corps of Engineers has completed <br />two flood control reservoirs and has one under construction <br />upstream of the study reach. <br /> <br />1. Cherry Creek Dam and Lake, completed in 1953, is <br />located on Cherry Creek about six miles southeast of <br />Denver. The reservoir has a flood storage capacity <br />of 79,960 acre-feet and prevented an estimated <br />$130,000,000 in flood damages in 1965 and an additional <br />$27,000,000 in damages in 1973. <br />2. Chatfield Dam and Lake, completed in 1975, is located <br />on the South Platte River just downstream from the <br />mouth of Plum Creek, about eight miles south of Denver. <br />