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Breakdown: Why “100-year flood” may not mean what you think it does

Published: Nov. 28, 2021 at 9:17 AM CST
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MEMPHIS, Tenn. (WMC) - We’ve all heard the term “100-year flood.” But do you know what it actually means?

No, it doesn’t mean it will flood every 100-years.

It means there is a 1 in 100 chance (1% probability) of a flood of that magnitude occurring any given year.

Other examples:

  • A 500-year flood is defined as a 1 in 500 chance (0.2% probability) of a flood of that magnitude occurring any given year.
  • A 25-year flood is defined as a 1 in 25 chance (4% probability) of a flood of that magnitude occurring any given year.

In order to figure out the probabilities, you divide 1 (any given year) by the flood magnitude, then move the decimal to the right two places.

  • 1/100 = 0.01 = 1%
  • 1/500 = 0.002 = 0.2%
  • 1/25 = 0.04 = 4%

Flooding is a coast-to-coast threat to some part of the United States and its territories nearly every day of the year.

According to the National Weather Service, flooding kills more people than just about any weather-related hazard.

A house and garage submerged in high flood waters in Waverly, Tennessee, after a complex of...
A house and garage submerged in high flood waters in Waverly, Tennessee, after a complex of thunderstorms dropped more than a foot of rain across parts of central Tennessee on August 21, 2021. More than 20 people died in the flash floods.(Tennessee Emergency Management Agenc)

Most deaths associated with floods occur either at night, or when people become trapped in automobiles that stall while driving in areas that are flooded.

If you know what to do before, during, and after a flood you can increase your chances of survival and better protect your property.

Learn how to better protect yourself and your family by reading these Flood Safety Tips and Resources from the National Oceanic and Atmospheric Association (NOAA).

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