What is a “100-year flood” and how is it different from a “1-percent-annual-chance flood” or a “base flood?
The term “100-year flood” is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood that has a 1-percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time or even within the same month. Because this term is misleading, FEMA has also defined it as the “1-percent-annual-chance flood.” The “1-percent-annual-chance flood” is the term that is now used by most Federal and State agencies and by the National Flood Insurance Program.
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