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The practical maximum flood boundary of a flood hazard is defined, for purposes of the National Flood Insurance Program (NFIP), as the area expected to be inundated by a one in 500-year flood. Floodplains are areas expected to be inundated by a one in lOO-year flood. A IOO-year flood is also known as the base flood. This does not mean that the floodplain will be under water only once every lOO-years. It indicates that there is a I % chance of the floodplain being inundated in any given year. SeverallOO-year floods can strike in back to back years. Notes ..... Flood Hazard Area ~l River Channel Flood Hazard Boundary (500 Year Flood) I I I I I !H I I I ~ I I ~_ Floodplain ~ (100 Year Flood) Water enters a river system from a variety of sources. When large water runoffs enter a water network too fast or in excessive quantities, flooding develops. Heavy rains on water saturated or impermeable land in urban areas are major sources of runoff to . river systems. Snow melt, particularly when the melt is accelerat- ed by rainfall, is another major source of water runoff. ----------Water enteflng the flver system has two noticea e e ects on the river. The increased flow in the river raises the level of water in the channel. As the river rises, the velocity of the-water flow will also increase. The combination of increasing water levels and higher velocities can greatly multiply the hydrodynamic loads placed on water and flood control structures. Flood control structures must be well maintained to survive these greatly increased dynamic loads. A series of benchmarks chart the increasing water levels. The first benchmark reached by the waterway is bank full stage, which is also known as the channel capacity. This is the maximum amount of water that II channel can carry without overflowing its' banks at the average ground elevation. However, there may be some minor flooding of low lying areas before bank full stage. Page 1 - 4