When 1-in-100 Year Floods Happen Often

Posted on: May 9th, 2019 by Leslie_Zucker

The “100-year flood” can hap­pen a lot more often than every 100 years. National Pub­lic Radio (NPR) just tried to clear up the con­fu­sion — lis­ten or read the story here. Many peo­ple assume if they’ve expe­ri­enced a 100-year flood it won’t hap­pen again for another 99 years, but this is not the case. The term “1-in-100 year flood” means there is a 1% chance a flood will hap­pen each year. If it hap­pens this year, there is still a 1% chance it will hap­pen next year. Also, every other sized flood could hap­pen in a year, they are just more or less likely.

The hundred-year flood term was adopted by FEMA to des­ig­nate a zone where flood insur­ance is required with a fed­er­ally backed mort­gage. The 100-year flood­plain is an area where flood risk is higher, but it’s not the only tool, and maybe not the best way to deter­mine your flood risk. It’s bet­ter to think in longer timeframes.

As the NPR story states, it’s bet­ter to know this fact — there is a 26% chance the 100-year flood will hap­pen over the course of a 30-year mort­gage. If you live your entire life in the flood zone, you are likely to expe­ri­ence a big flood.