How does a 100-year flood occur more than once a century?

People measure floods by the probability of their occurrence. However, how floods are labeled can be misleading.

After a big flood event you may hear things like, “That was a 100-year flood.” This doesn’t mean floods of that severity only occur every 100 years. The same principle applies to 50-year floods, 25-year floods, and even 500-year floods.

Each of these floods are actually calculations from years and years of meteorological and flood data. The 100-year flood is actually just another way of saying a flood that severe has a one percent chance to occur any given year.

The major flood reference points and their probabilities of happening any given year look like this:

This means a significant flooding event such as a 100-year flood could occur in back-to-back years or even within the same year, if conditions are right.

A few good examples can be found along the Ohio River. The earliest known flood in this river was 1773, and according to the few records, it surpassed the 500-year flood mark.

Less than 200 years later, in 1937, another flood not only surpassed the 500-year flood mark, but waters also rose above the 1773 flood.

Now is the time to be thinking about flooding and whether or not you’re at risk. Significant winter snowfall and a wet spring is putting some areas in the Midwest and the northern Plains at a moderate risk for flooding this year, according to the National Weather Service.

Follow Storm Shield Meteorologist Jason Meyers via the Storm Shield app on twitter, @StormShieldApp and Facebook. Download the Storm Shield Weather Radio App for your iPhone or Android device and get severe weather alerts wherever you are.

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