Two 500-year Floods in 15 Years? USGS Explains

When the U.S. Geologic Survey (USGS) used the term "500-year flood" to describe the recent devastating flooding in the Midwest, many people in the region who recalled the Great Midwest Flood of 1993 – also designated as a 500-year flood – were understandably confused. Shouldn't such a flood happen no more often than once every 500 years?

It’s All About the Odds: The USGS explains that the term "500-year flood" actually refers to the likelihood – the odds – of such an extraordinary flood happening during any given year in a particular location. In other words, a 500-year flood has a .02 percent -- or 1-in-500 -- chance of happening in any given year in the same place. Similarly, a 100-year flood has a 1 percent – or 1-in-100 – chance of happening in the same place. Thus, while not likely, it is entirely possible for 100- or even 500-year floods to happen in the same place in consecutive years.

"The term '500-year flood' can be a little misleading," said Robert Holmes, USGS National Flood Specialist in a recorded interview. "We hydrologists realize the term has instant public recognition and we use it to point to the extraordinary nature of such floods."

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