What Is A 500-Year Flood?

Three 100-year floods in five years? How can that happen? What is a 500-year flood?

“100-year” and “500-year” is really misleading and not the best way to communicate the risk.

It doesn’t mean it’ll happen “once in 100 years” or once in 500 years”.

For a 100-year flood it means there’s a 1% chance it could happen EVERY year.

For a 500-year flood it means there’s a 0.2% chance it could happen EVERY year.

While the statistics aren’t high… they’re based on a recurrence interval. The current flood is a 500-year flood event. The Corps of Engineers has a flood map which shows potential inundation areas for 1% and 0.2% areas. This map is slightly different from the FEMA Flood Risk Map and communicates more of the percentage chance of experience inundation and different levels.

US Army Corps of Engineers Inundation Maps (Click Inundation Forecast for 0.2% to see the 500-year Inundation Zones)

-Garrett

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