The Mississippi River is rising again as torrential rain falls across much of the Midwest. It's the latest in a series of storms that have flooded major cities and small communities along the length of the Mississippi and Missouri rivers on and off for more than a month.
In some places, homes and businesses in what's known as the 100-year flood plain have been hit by multiple floods in a matter of weeks. One St. Louis suburb has now suffered three major floods since 2015, at least two of which were approximately 1-in-100-year events.
When these sorts of floods happen back to back, many residents might start to wonder: Why are they even called 100-year floods?
"The educated layperson or elected officials, they think, 'Well, you scientists and engineers can't get it straight because we had a 100-year flood two years ago! Why are we having another one? You guys must have your numbers wrong.' It makes people think we don't know what we're doing," says Robert Holmes, the national flood hazard coordinator at the U.S. Geological Survey.
"I think the use of this 1-in-100-year and 1-in-500-year is confusing to people," says Alice Hill, a senior researcher at the Hoover Institution and former official with the National Security Council in the Obama administration. "Many people assume that if their area has experienced the 1-in-100-year flood, that means for the next 99 years they need not worry about flooding."
That's because the probability is hard to understand.
After Hurricane Florence hit North Carolina last year, Gov. Roy Cooper told reporters, "When you have two 500-year floods within two years of each other, it's pretty clear it's not a 500-year flood."
During record-breaking flooding in South Carolina in 2015, then-Gov. Nikki Haley attempted to explain the storm's magnitude, saying, "We are at a 1,000-year level of rain in parts of the low country. What does that mean? We haven't seen this level of rain in the low country in 1,000 years. That's how big this is."
Neither governor was correct. While it's unlikely that two large storms that cause flooding will happen in close succession, it's not impossible.
A 1-in-100-year storm has a 1% chance of happening every year.
"As with the flip of a coin, if you flip heads twice in a row, that doesn't mean you'll flip tails the next time," Hill says. "So you could have three very significant floods right in a row."
And, studies say, there is a better way to communicate that reality, by telling people what their risk of flooding is over time rather than each year.
For example, if there is a 1% chance that a home will flood each year, that means there's a 26% chance it will flood over the course of a 30-year mortgage.
Put another way, if you lived your entire life — say, a happy 85 years — in a flood-prone area, you'd be more likely than not to experience a 1-in-100-year flood.
Hill says transitioning to that kind of language around flood risk is extremely important as floods become more frequent and severe in much of the U.S.
"Perhaps in the past this wouldn't have mattered so much, but with development and climate change — warming temperatures and more evaporation of water that falls very quickly — we need to let people know how they can better protect themselves against flooding," she says.
Hydrologists at the U.S. Geological Survey say they are making a big effort to communicate risk more effectively, in part by transitioning away from the 1-in-100 and 1-in-500 language in public documents and instead referencing the annual probabilities.
"It's a very complex process to try to give people a proper idea of the risk of living in a particular location," says Holmes of the USGS. The climate is changing, as is the physical environment. Both development and global warming add uncertainty to flood-risk calculations, which Holmes says is frustrating for local officials who want clear information about future flood probability so they can make decisions accordingly.
Holmes says a lot is on the line when it comes to communicating flood risk.
"If you build in the wrong spot, or you buy a house that you were unaware that you had a risk, you could lose your life savings," Holmes says. "Worst case, you could lose a member of your family or your own life. So there is a lot riding on getting the answer right."
ARI SHAPIRO, HOST:
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.
(SOUNDBITE OF MONTAGE)
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.
(SOUNDBITE OF ARCHIVED RECORDING)
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.
(SOUNDBITE OF FAYE WEBSTER SONG "SHE WON'T GO AWAY") Transcript provided by NPR, Copyright NPR.