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I I I I I I I I I I I I I I I I I I I FIGURE 2 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 down- stream to continue the hazard and destruction. In general, both natural and manmade obstructions exist which tend to restrict flood flows and to cause overbank or nonchannel flooding. These obstructions will often result in unpredictable flood flow patterns. The obstructions also tend to become sources of debris and further obstruction when structural failure results. Also, manmade floodplain obstructions tend to concentrate flood flows which in turn result in increased velocities and increased hazards in adjacent flooded areas. It is impossible to predict the degree of accumulation of debris within floodwaters and the resulting effect of such accumu- lation. This report assumed that existing structures wrthin the flood flow area would not see blockage and reduced efficiency during a major flood event. Factors Which Reduce the Flood Hazard Damage to 10th Street bridge over Tucker Gulch in Golden after flood of July 23- 26, 1965 (Denver Post photo) . Flood control improvements, including channel improvements, reservoirs and bridge improvements, as well as good floodplain management practices such as flood proofing and avoiding develop- ment in floodplains, will reduce the potential hazards associated with floodprone areas. For the essentially undeveloped floodplains delineated in this report, the enforcement of sound floodplain regulations will best serve the needs of the communities in the study area. There are 11 major reservoirs in the lower Clear Creek basin, three of which are on-stream and provide some residual flood con- trol effects downstream from each site. None of these reservoirs has a specific flood control function. Information concerning each of the on-stream structures is presented below (Reference 4). - 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 velo- cities and, as a result, cause greater depths and backwater conditions that increase flood elevations. Often 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 man- made obstructions. In addition to the vegetative obstructions, floodwaters tend to erode channel banks and carry the suspended 1. Ralston Reservoir. Ralston Reservoir was built in 1938 by the City and County of Denver. It receives water from Ralston and South Boulder Creeks and is used for municipal water supply. The outlet works -3-