What is a 500-Year Flood?

Courtesy PrairiePublic.org

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By Becky Ogann

Q: WHAT IS THE "500-YEAR FLOOD"?
A: The 500-year flood, like the 100-year flood is misleading. The 500-year flood is not the flood that will occur every 500 years, instead, it is the flood that has a .2% chance of occurring each year. The 500-year flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program as the standard for floodplain management and to determine the need for flood insurance.

Q: WHAT IS THE "100-YEAR FLOOD"?
A: The term "100-year flood" is misleading. It is not the flood that will occur every 100 years. Rather, it is the flood that has a one percent chance in a hundred of occurring each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. In fact, it's possible for two, 100-year floods to occur in the same week. The 100-year flood which is the standard used by most Federal and State agencies, is used by the National Flood Insurance Program as the standard for floodplain management and to determine the need for flood insurance.

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