MARY LOUISE KELLY, HOST:
Elsewhere on the program, we talked to a vegetable farmer and an outdoor goods store owner in Vermont about that state's devastating floods and all the cleanup facing them now. Some officials in Vermont are calling this week's deluge a thousand-year flood. Climate change is making events like this more common. As NPR's Rebecca Hersher reports, the words officials use when they talk about flood risk can be misleading.
KELLY: If you live in a flood prone area, as tens of millions of Americans do, you've heard the words I'm talking about.
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UNIDENTIFIED PERSON #1: We are at a thousand-year level.
UNIDENTIFIED PERSON #2: Also called 500-year rainfall...
UNIDENTIFIED PERSON #3: It is a 100 years' flood.
REBECCA HERSHER, BYLINE: 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.
HILL: Many people assume that, if their area has experienced the 1-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: That'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's still a 1% chance it will happen next year.
HILL: As with a 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 is happening more and more frequently. For example, North Carolina got hit with two really wet hurricanes in a row, in 2018 and 2019. And that prompted then-Governor Roy Cooper to say this.
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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 1-in-100-year language isn't helping people prepare for flooding. It was never meant to. The 100-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's a 1% chance of a flood happening each year, that means there's 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're more likely than not to experience a hundred-year flood. Explaining flood probability that way helps people understand their risk over time.
And 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.
Rebecca Hersher, NPR News.
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