It’s time to ditch the concept of ‘100-year floods’

By Maggie Koerth-Baker

Photos of water-covered neighborhoods and families riding floating refrigerators to safety have made clear the scale of Hurricane Harvey’s wrath. But the risks that coastal Texans faced before the storm hit — and the probability that others will be dealt a similar fate — are still a confusing mess. Surveys have shown that even people who live across the street from the bureaucratically determined risk zones known as floodplains don’t understand how those boundaries were drawn or what the risk metric that defines them really means.

That’s no surprise to experts, who say the concept of the “100-year flood” is one of the most misunderstood terms in disaster preparedness. In the wake of catastrophic flooding on the Texas coast, the media has been working hard to explain the term, turning out dozens of articles explaining that a “100-year flood” is not a flood that you should expect to happen only once every 100 years. Instead, it refers to a flood that has a 1 percent chance of happening in any given year. Over the course of a 30-year mortgage, a house in a 100-year floodplain has a 26 percent chance of being inundated at least once. 

Stories that emphasize this fact are “doing the Lord’s work,” said Wesley Highfield, professor of marine sciences at Texas A&M University at Galveston. But there are still more holy offices to perform. The concept of using a “100-year flood” as a benchmark for risk isn’t just misunderstood; it obscures fundamental statistical problems in how we assess flood risks — problems that can lead to residents and homeowners believing themselves to live in a zone of safety that isn’t there. It may be time for us to find a different way of evaluating that risk altogether.

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