Clearing Up Confusion Over '100-Year' Floods The term "100-year flood" can be confusing and misleading, scientists, local emergency officials and homeowners all agree. Experts say there's a better way to communicate about flood risk.
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When '1-In-100-Year' Floods Happen Often, What Should You Call Them?

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When '1-In-100-Year' Floods Happen Often, What Should You Call Them?

When '1-In-100-Year' Floods Happen Often, What Should You Call Them?

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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Rain is falling again on parts of the Mississippi River. That means even more flooding for communities in the Midwest that have been dealing with high water for more than a month. Even though flooding along the river is increasingly frequent and severe, many places are not prepared to handle the water. NPR's Rebecca Hersher reports one reason may be the words officials use when they talk about flood risk.

REBECCA HERSHER, BYLINE: If you live in a flood-prone area as tens of millions of Americans do, you've heard the words I'm talking about.


UNIDENTIFIED PERSON #1: We are at a thousand-year level.

UNIDENTIFIED PERSON #2: ...Called 500-year rainfall...

UNIDENTIFIED PERSON #3: One hundred years flood.

HERSHER: Is this a good phrase - hundred-year flood?

ALICE HILL: I think it's highly confusing to people. It's based on probabilities.

HERSHER: Alice Hill studies disaster resilience at the Hoover Institution, and she was a climate advisor in the Obama administration.

HILL: Many people assume that if their area has experienced the one in 100 year flood, that means that for the next 99 years, they need not worry about flooding. So...

HERSHER: And that's not the case.

HILL: It's not the case.

HERSHER: So here's what a hundred-year flood does mean. It means there's a 1% chance it will happen each year. If it happens this year, there is still a 1% chance it will happen next year.

HILL: As with the flip of a coin, if you flip heads twice in a row, that doesn't mean that you're going to get tails the next time. So you could have three very significant floods right in a row.

HERSHER: That kind of thing has happened a lot recently. For example, North Carolina got hit by two really wet hurricanes in a row. And that prompted then-Governor Roy Cooper to say this.


ROY COOPER: When you have two 500-year floods within two years of each other, it's pretty clear it's not a 500-year flood.

HERSHER: But that's not it. Both floods had a low probability of happening, but sometimes low probability things do happen. And Hill says the widespread confusion about basic flood probability is a big problem.

HILL: We are leading people to be unprepared.

HERSHER: It's not surprising that the one in 100-year language isn't helping people prepare for flooding. It was never meant to. The hundred-year flood term was adopted by Congress back in the 1970s to describe who would be required to buy flood insurance, and researchers say there are better ways to communicate flood risk. Instead of talking about how likely a given flood is to happen each year, talk about how likely that flood is to happen over many years.

For example, if there is a 1% chance of a flood happening each year, that means there is a 26% chance it will happen over the course of a 30-year mortgage. And if you live your whole life in a flood zone, you'll be more likely than not to experience a hundred-year flood. Explaining flood probability that way helps people understand their risk over time.

Robert Holmes is the national flood hazard coordinator for the U.S. Geological Survey. He says his team is trying to move away from the one in 100 year language in their public documents in part because the misunderstandings undercut the public's trust in flood science.

ROBERT HOLMES: The educated lay person or elected officials - they think well, you scientists and engineers can't get it straight because we had a hundred-year flood two years ago. Why are we having another one? You guys must have your numbers wrong, or you're doing something wrong. And that's not the case. I mean, it makes people think, well, we just don't know what we're doing.

HERSHER: Holmes says the stakes are high when it comes to flood risk. In many parts of the country, flooding is getting more frequent and severe. Climate change is part of the problem. Warmer air can hold more moisture, which falls as more extreme rain. And in many places, development is also creating more runoff, all of which puts more people in harm's way, many of whom don't know they're at risk.

HOLMES: You know, if you build in the wrong spot or you buy a house that, you know, you were unaware that you had a risk, you know, you could lose your life savings. Or worst case, you lose a member of your family or your own life.

HERSHER: Rebecca Hersher, NPR News.


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