In this recent era of unprecedented rainstorms, the terms “100-year flood” and “500-year flood” are in the news on a regular basis. Depending on the source, I have seen Hurricane Harvey weigh in as an 800 or even 1,000-year flood. But what does this mean? Has there not been this much rain in Texas since the year 1217?

These terms are meant to give some perspective on the rarity and intensity of such weather events. Technically, a “100-year” flood has a statistical chance of 1% (1 divided by 100) of occurring in a particular place in any given year. A 500 year flood (1/500) has a 0.2% chance. These statistics, known as the annual exceedance probability (AEP), are based on historic weather patterns and a network of 7,500 stream gauges operated by the United States Geological Service (USGS). The stats do not necessarily mean that a large storm cannot occur within a few years of another one—only that over a large span of time there’s a likely average of 1 every 100 years.

Nationwide, we have had 25 “500-year” floods since 2010, including Hurricane Harvey and the 2014 floods in Pensacola. And just because we had one recently doesn’t mean we are in the clear for another 499 years. Does this mean we need to take another look at our probability rates? Probably so.

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