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When '100-year floods' happen often, what should you call them?

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Hurricane Florence floods South Carolina in 2018
Hurricane Florence flooded Nichols, S.C., in September 2018. It was the second catastrophic flood in the region in less than two years.
Associated Press

Just when we thought it might finally be over, the shoulder season between winter and spring has asserted itself again. 

A snowstorm forecast to bring a possible foot of snow to areas around Lake Superior is barreling toward northeastern Minnesota Wednesday. And farther south Iowa, along the Mississippi, communities are bracing for more rainfall — and more flooding.

While Minnesota is now taking stock of this year's flood season, Gov. Tim Walz plans to request federal disaster assistance to help pay for the damage caused by flooded rivers.

State emergency management director Joe Kelly said initial estimates put the total damages at more than $30 million, four times the threshold for federal aid.

"Given the amount of overland flooding that we saw across the state, and then of course the wind storm that knocked down all the power poles," he said, "I'm not surprised by that number at all, and I do think the odds are pretty good that it's going to hold or even go a little higher."

FEMA officials are visiting 50 counties and four tribal communities across the state to assess the damage.

— MPR News staff

It's been a long time: St. Paul hits flooding longevity record

Mississippi River flooding in St. Paul this spring swamped the record for the longest period of flooding ever measured in the city.

The St. Paul Pioneer Press reports that the river was above flood stage for 42 straight days, easily surpassing the 2001 record of 33 days.

The National Weather Service says the river fell below flood stage last weekend. The city is continuing to assess and clean up riverside streets that were submerged before reopening them. All of the city's flood plain parks, including boat launches, remain closed.

— The Associated Press  |  St. Paul

When '100-year floods' happen often, what should you call them?

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, then-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."

— Rebecca Hersher | NPR

Along the Mississippi: Crests aside, more rains mean more worries

Davenport, Iowa
An aerial photo of Davenport, Iowa, shows the Figge Art Museum and the SkyBridge surrounded by Mississippi River floodwaters Friday. Iowa Gov. Kim Reynolds visited Davenport Friday. Several blocks of downtown Davenport were flooded this week when a flood barrier succumbed to the onslaught of water.
Kevin E. Schmidt | Quad City Times via AP

Rainfall in the coming days could raise water levels in the Mississippi River higher than expected and lead to more flooding, the National Weather Service at St. Louis said Saturday.

At some points, including Davenport, Iowa, the past week's levels were the highest ever. Davenport saw a large part of its riverfront and downtown flooded when a section of a temporary flood barrier broke after it had held back the swollen river for 38 days.

The river began dropping Friday at Davenport after eclipsing a record set in 1993. Officials said it could be days before the water is once again confined within the river's banks.

The good news is that no significant rain is expected in the region over the weekend. The bad news is that rain and thunderstorms will come roaring back in starting Monday night, said meteorologist Mark Fuchs at St. Louis.

Starting Monday night, up to a couple of inches of rain could fall on Kansas, Missouri and Iowa and soon reach Illinois, Fuchs said.

"Tuesday evening through Thursday evening, we could be seeing quite a bit of rain -- several inches," he said. "It will have an impact."

The Mississippi crested a few feet shy of 1993 levels at several other places in Iowa, Missouri and Illinois. Crests further to the south in towns like Cape Girardeau, Missouri, aren't expected until the middle of next week.

If rain amounts stay on the low end of the prediction models, it will cause the swollen river to linger at current elevated levels. At worst, the service said, additional rain will push river levels back up, leading to more flooding.

A flood warning continues for areas on either side of the river from Minnesota all the way to Louisiana, where the river empties into the Gulf of Mexico.

— The Associated Press  |  Omaha, Neb.