Published online by Cambridge University Press: 10 January 2011
Introduction
The word flood has a common usage, implying a situation where water in a river spills over its banks. It can be defined as a hydrologic condition where the river discharge exceeds the storage capacity of the channel and the excess water overflows and inundates part of the valley bottom. The height of the overflow and the extent of inundation depend on the size of the flood, which in turn is related to its frequency. In any given location, bigger floods occur rarely and smaller floods are common. Probability of a flood of a given size is frequently expressed in terms of its recurrence interval, i.e., the time period within which the flood of a given size is expected to occur once. Flooding is a normal and expected phenomenon for a river, but as riverbanks are commonly populated, larger floods carry a hazardous component.
All rivers flood, although the magnitude and frequency of floods vary among them. Rivers flowing through certain environments tend to be more flood prone, e.g., those draining hilly basins located across common paths of tropical cyclones. A flood discharge is capable of enlarging the channel, transporting large quantities of sediment, and physically transforming part of the valley flat. Flood pulses also control valley bottom ecology, ranging from rivers with large floodplains, as documented for the Amazon (Junk, 1997), to steep rock-cut canyons, as for the Colorado (Webb et al., 1999).
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