Page "Floodplain" Paragraph 21

In the United States the National Flood Insurance Program regulates development in mapped floodplains based on the 100-year flood ( 1 % annual chance of a flood of this magnitude ).

The Flood Insurance Rate Maps, typically depict both the 100-year floodplain and the 500-year floodplains.

Where a detailed study of a waterway has been done, the 100-year floodplain will also include the floodway, the critical portion of the floodplain which includes the stream channel and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters.

When a floodway is shown on the Flood Insurance Rate Maps, the portion of the 100-year floodplain outside of the floodway is known as the flood fringe.

Another commonly encountered term is the Special Flood Hazard Area, which is any area subject to inundation by the 100-year flood.

A problem is that any alteration of the watershed upstream of the point in question can potentially affect the ability of the watershed to handle water, and thus potentially affects the levels of the periodic floods.

A large shopping center and parking lot, for example, may raise the levels of the 5-year, 100-year, and other floods, but the maps are rarely adjusted, and are frequently rendered obsolete by subsequent development.