A floodplain is the normally dry area, usually low land, adjacent to a stream, river, lake, watercourse, or bayou that is inundated on a periodic basis with flood waters.
The extent or size of the floodplain depends on the magnitude of the flow, as defined for a given frequency of occurrence, and the physical attributes of the bayou, river, etc., and the watershed which it drains.
Floodplains are usually referred to by a given recurrence interval with respect to the flows generated by a storm event, for example: "These homes are located in the 100-year floodplain" or "This subdivision is situated within the 500-year floodplain."
The term "100-year storm" or "100-year frequency" does not refer to a rainfall event that occurs once every 100 years. Rather, in any given year, a 1 percent chance exists of a 100-year flood event occurring.
Storm frequencies are used to refer to the average rainfall intensity for a given duration of time, the volume of rain that falls over a given period of time, or the peak flow that occurs from an event.
For example, a 100-year storm (or worse) may occur three years in a row, or maybe twice in the same year, or perhaps not at all in 100 years. The chance that a storm will occur with some given frequency is known as the "return period" or "recurrence interval." A 500-year storm event occurs, on average, once in 500 years, or has a 0.2 percent probability of occurring or being exceeded in any given year; a 25-year storm event occurs on average once in 25 years, or has a 4 percent probability of occurring or being exceeded in any given year; a 2-year storm has a 50 percent chance of occurring or being exceeded in any given year. A storm event for some specified return period such as a 2-year or 100-year storm is frequently used in order to design storm water drainage systems, and is known as the design storm for that system.
The Floodplain Management Office maintains an archive of historic Flood Insurance Rate Map (FIRM) panels that are no longer “effective” as determined by FEMA. To access the maps in pdf format, click on the following link: Historical FIRM Panels
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