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What Are 100 and 500 Year Floods?

According to the FEMA FAQ website, a 100-year flood is defined as:

The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation 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. The 100-year flood, which is the standard used by most Federal and state agencies, is used by the NFIP as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30 year mortgage.

Similar to a 100-year flood, a 500-year flood has a 0.2-percent chance of being equaled or exceeded each year (FEMA, 2009). Information on specific flood zones is available through the FEMA Flood Zones website.

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