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I I I I I I I I I I I I I I I I I I I Factors which Reduce the Flood Hazard depths and backwater conditions that increase flood elevations, Oft- times 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 obstruc- tions. In addition to the vegetation obstructions, flood waters tend to erode channel banks and carry the suspended soil material as well as rocks and dislodged trees 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. Flood control improvements, including channel improvements, reservoirs and bridge improvements, as well as good flood plain management prac- tices such as flood proofing and avoiding building in flood plains, will reduce the potential hazards associated with flood prone areas. For the essentially developed flood plains delineated in this report, the enforcement of sound flood plain regulations will only help serve the needs of the study area. 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 drain- ageways. 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. The National Oceanic and Atmospheric Administration (NOAA) maintains year-round surveillance of weather and flood conditions. Although intense cloudburst activity in the general region can be forecast, such storms and resultant flooding in specific small drainage areas cannot be predicted accurately and, in any event, forecasts wculd provide very little advance warning. It is impossible to predict the degree of accumulation of debris within flood waters and the resulting effect of such accumulation. This re- port 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. Flood fighting and emergency evacuation plans are also factors which can reduce the flood hazard. No formal plans for flood fighting or emergency evacuation of people and personal property from flood plain areas are known to have been prepared for the Basin. However, during times of flooding, agencies such as the local police, the sher- iff's office" the State Highway Patrol, and road maintenance crews assist in flood fighting and evacuating people from flooded areas. The most effective means to reduce flood hazards, is to be aware of such hazards and avoid them. Implementing flood plain regulation practices will keep damageable land uses and improved property out of the flood plain and out of danger. Flood proofing will tend to reduce damage to the existing buildings and structures within the flood plain. 6