Design flood

Projects dealing with design of flood control structures, or with evaluation of flood risk or potential damage in floodplains, are based on design floods. Design floods represent values of flood levels, flows or volumes used for sizing the structures or for delineation of flood zones.

The design flood is determined according to some criterion, which reflects a desired degree of protection against floods or an acceptable level of risk from flooding. Such a criterion is usually probabilistic. Typically, a design flood is the flood of a specified return period, such as 25-year or 100-year flood. Other probabilistic criteria are also possible, e.g. specified probability of the design flood to be exceeded in structure's lifetime.

Non-probabilistic criteria for design floods are sometimes used. For example, the largest observed flood is used as the design flood for flood control structures in some parts of Canada. Large dams and major bridges are usually designed on the basis of the probable maximum flood, which is a hypothetical flood event that represents most severe, but reasonably possible, combination of meteorological and hydrologic conditions in the region.

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Hydrology of Floods






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