Definition Of Floods

Streams are linear water features that flow under the impetus of gravity. The amqunt of water contained in a stream is usually regulated by contributions of groundwater and surface runoff to the stream channel (Zaslavsky and Sinai, 1981; Knighton, 1998). Much of the time water in a stream flows within the confines of its channel. When inputs of water increase sufficiently, stream discharge leaves the stream channel and covers all or parts of the adjacent floodplain. Since the floodplain surface is usually a virtually flat surface and near the elevation of the stream channel, water can easily spread over the floodplain once water exceeds the elevation of the stream's banks. Most floods develop over a period of days or months as discharge increases gradually (Hirschboeck, 1987, 1988). Flash floods by contrast occur suddenly with little warning and are of short duration. Semiarid and arid areas are likely to experience flash floods (Reid and Frostick, 1987; Hassan, 1990). Flooding is not always associated directly with stream channels. Flooding occurs any time when water covers a surface that is normally not under water. Flooding can occur in coastal areas, low lying areas with poor drainage, or locations with inadequate urban drainage systems.

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