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I I .~ Chapter 2 SEVEN PERCENT OF OUR LAND: THE FLOODPLAIN Flooding of land adjacent to rlvers, oceans and other bodies of water is a natural event which occurs from time to time. Problems arise only where man competes with nature for use of these floodplains. Floodplains are low areas subject to flooding from time to time. Most floodplains are adjacent to streams, lakes or oceans. The amount of land inundated by a flood depends on the flood's magnitude. Approximately seven percent of the nation I s land area--an area almost as big as Texas--is subject to flooding by the lOO-year flood. The action of water cn the land and interaction of water with vegeta- tion produce floodplains which differ appreciably from one another and from ~...r:~'" _r _ '_~fJ:",%-~_ _....,. . '.._y,.... .. . '" '.. rr' F"....~.m..o. , --,/ "-,~"'1t.,,..-'C'. ' .' ,,-?'-;c . 100 year flood level 20 year flood level 20 year flood plain 100 year flood plain FLOODS AND FLOODPLAINS The size of a flood is usually described in terms of its statistical frequency of occurrence. For example, a "20-year flood" IS one likely to be equalled or exceeded on the avera~ of only once every 20 years. However, such averages are only meaningful over decades or centuries. Several floods of that size or larger could occur in a particular year or short period ')f years. Floodplains are typically low of 'dater which are subject to to floods. For instance, the "20-year floodplain/!. areas adjacent to a rIVer, ocean, or other body flooding. They dre described in a fashion similar land flooded by the 20-year flood is called the The 100-year flood and lOO-year floodplain are particularly dgnificant. The boundary of the JOO-year flood has been widely used In federal and state programs for identifying areas where the risk of flooding is significant. For example, floodplain wning laws and other regulations controlling building in flood hazard areas are often applied to the 100-year floodplain. 5