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I I I I I I I I I I I I I I I I I I I 5 These floods have been generated by intense rainfall over the tributary basins. Even though no recorded information is avail- able for the tributary streams under study, these streams were surely running out of their normal banks during the periods of flooding on Cherry Creek and the South Platte River. The U.S.G.S. reported the peak flow on Piney Creek in June, 1965, to be ap- proximately 14,100 cfs. This is over 1.5 times the estimated 100-year flood magnitude for this stream. to erode channel banks and carry the suspended soil material as well as rocks and boulders further downstream, adding to the debris and damage at the manmade obstructions. As these masses of debris accumulate, the artificial barrier soon breaks loose and a wall of water and debris surges further downstream to continue the hazard and destruction. Figures 1 and 2 show the devastation on the South Platte River in 1965 which resulted from unwise encroachment into the flood plain. Figures 3 and 4 show flooding in Arapahoe County in 1965. As evidenced, the tributary streams do flood but the high water has damaged little as the flood plain has not been misused. In general, both natural and manmade obstructions exist which tend to restrict flood flows and cause overbank or non-channel flooding. These obstructions will ofttimes result in unpredictable flood flow patterns. This is particularly true of irrigation canals crossing natural drainageways. The obstructions also tend to become sources of debris and further obstruction when structural failure results. Also, manmade flood plain obstructions tend to concentrate flood flows which in turn result in increased velocities and increased hazards in adjacent flooded areas. - FACTORS AFFECTING FLOOD HAZARD - Factors which Increase the Flood Hazard Natural obstructions to flood flow include brush, trees, and other lowland vegetation which naturally grow along stream banks and lowlands adjoining drainageways. During periods of high flooding, these obstructions tend to slow the flood velocities and as a result, cause greater depths and backwater conditions that in- crease flood elevations. Ofttimes the natural vegetation found within the flow area becomes dislodged from its normal location and accumulates as debris within the flood waters to be collected at bridges and culverts and other manmade obstructions. In addition to the vegetative obstructions, flood waters tend It is impossible to predict the degree of accumulation of debris within flood waters and the resulting effect of such accumulation. This report assumed that existing structures within the flood flow area would see variable clogging and reduced efficiency during a major flood event. The capacity for bridge structures and box culverts was reduced by 25% to account for debris accumulation at the inlet. Culvert structures greater than 36 inches in diameter were analyzed for blockage of 75%. Culvert structures smaller than 36 inches in diameter were considered to be totally blocked and ineffective.