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A-4 WHY UPGRADE EXISTING PLANS AND REGULATIONS? Existing community maps and regulations for flood hazard areas often provide in- adequate protection for high risk areas. Most local regulations, adopted to implement state and federal floodplain management guidelines, are designed to address normal flood haz- ards, Here the depth of inundation is the primary factor causing damage and waters are relatively free of sediment and debris. Flooding is temporary and the configuration of the flood channel is relatively stable. Areas subject to normal flood hazards include low ve- locity flow areas along major rivers and streams and relatively flat coastal areas inun- dated by the storm surge where waves and erosion are not major factors. State and national criteria based upon such a concept of the "norm" have proven satisfactory for 70 to 80 percent of the country. However, the failure to consider other flood damage factors has resulted in serious deficiencies in management approaches for the high risk areas, including the following: Maps usually understate hazards in areas with velocity, debris or other high risk problems, For example, flood maps for alluvial fan areas designating them as shallow flooding areas incorrectly imply low risk. Usually the risks there are quite serious due to high velocities, debris and erosion, Regulatory criteria designed for the "norm" also underestimate the hazard from high velocity flow, erosion, debris loading and duration of inundation. This can result in damage or destruction of structures built in compliance with regulatory criteria. THE HIGH RISK FACTORS Depth of inundation is the basic cause of damage in most floods and is also an im- portant factor in high risk areas. Factors that cause additional damages in high risk areas include: High velocity. The damage potential of flood waters increases dramatically, some- times exponentially, with velocity. Velocity is determined by slope, waves and several other factors. Unless the potential for high velocities is considered in building design, flood proofed structures often collapse from the pressures and stresses applied by fast moving water. Water moving at speeds of ten (or more) feet per second can undermine pil- ings and slab foundations. Water velocity is a major cause of damage in four areas: n Areas subject to coastal wave action (velocity zones), coastal inlets and overwash areas;