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percent chance of being equaled or exceeded in any year. Perhaps more significantly, it has about a 25 percent chance of occurrence during a 30-year mortgage period. Hazards of Large Floods The hazards to life and extent of damage caused by any flood depend on the topography of the area flooded, depth and duration of flooding, velocity of flow, rate of rise, and develop- ments on the flood plains. velocities greater than 3 feet per second combined with depths of 3 feet or more are generally considered hazardous to life as well as to property. Water flowing in excess of 4 feet per second is capable of transporting sediment and causing severe erosion of streambanks and fill around bridge abutments. Where velocities drop below 2 feet per second, debris and silt deposits can build up, ex- tending the flood damages and creating adverse health conditions. The saturation effects of long-duration flooding can weaken bridge abutments, levees, and other embankment works that may fail as the floodwaters recede. Property damage from great floods in developed areas can be overwhelming. The entire community suffers when streets, bridges, sewers, and other public utilities are destroyed or otherwise made inoperative. The delineation of what will occur during a flood cannot be totally measured in dollars. The possible loss of life, health, and general well-being of a community are items that cannot be measured and must be considered. A great flood can unleash ill- ness and deadly epidemic disease. Damaged or destroyed sanitary sewage facilities can pollute the floodwaters and create a health hazard. Newspapers sometimes report a "lighter side of the news" in picturing children at play in the mud and standing water left by a receding flood. Unfortunately, however, the contamination threat to domestic water supplies cannot be illustrated with similar ease. 13