Fifteen years ago, after the Midwest was swamped with what was pronounced a "100-year" or even a "500-year" flood, some folks figured they would never again see such a disaster in their lifetime. Some even dropped their flood insurance.
Now, with the region struck by a supposedly once-in-a-lifetime flood for the second time since 1993, some scientists and disaster officials say the use of terms like "100-year flood" should be re-evaluated because they are often misunderstood and can give the public a false sense of security.
"We, the United States Geological Survey, almost need to quit using the term `100-year flood,'" said hydrologist Gary Wilson with the USGS Missouri Water Science Center in Rolla, Mo. "It could happen twice a year, if you're unlucky." Or 200 years could go by without a 100-year flood, he said.
Villanova University professor Robert Traver, who specializes in storm water management, was more succinct: "Whoever invented that term should be shot."
Several government scientists say they have tried to move away from using the terms, yet they also say they routinely fall back on the labels as shorthand for measuring a flood's severity.
Costly consequences of terminology
The terms have practical consequences; they are used for such things as classifying a levee's protection level and setting insurance requirements for people who live in flood-prone areas.
Many people seem to believe that a 100-year flood should happen once every 100 years, or that a 500-year flood should happen every 500 years. But that's not how it works.
A 100-year flood is defined as a flood so big that it has a 1 percent chance of happening in any given year. A 500-year flood is one with a 0.2 percent chance of happening in a given year — a 1-in-500 chance.
Scientists say it is not unusual to hear from people who want to know if they have lived through a "100-year" event and want to cancel their flood insurance, believing one recent big flood lowers the risk of another. But that's not the case.
While the rules of probability say that the odds are 50-50 that a coin will come up heads, it is entirely possible to flip a quarter and come up with heads four or five times in a row.
Mike Russell, an alderman in the Missouri town of Clarksville where a huge sandbagging effort has protected the community's historic downtown, suggested terms like "100- or 500-year flood" don't make sense.
"I was in my 20s then," he said of the Great Flood of '93. "Here it is 2008 and I'm 40. I didn't think 500 years had already gone by."
Some critics argue that it's the government's flood forecasts that are faulty.
FEMA maps may underestimate risk
Carolyn Kousky, a researcher who has studied natural disaster policy, wrote in an opinion piece in Monday's St. Louis Post-Dispatch that maps produced by the Federal Emergency Management Agency's National Flood Insurance Program may underestimate flood risks, though she noted that work to modernize those maps is under way.
She said more paved areas leave less ground to absorb rainwater, so more runs into creeks and rivers. The construction of levees and the building of new river channels are also contributing to the risks, she said. And she said it's possible climate change will increase flooding in the Midwest.
Government agencies say their flood estimates are sound. The government uses information about a river's elevation and flow rate, along with historical records, to determine the chances of a flood at the 100- or 500-year level.
"We're using the best data we have," said Army Corps spokesman Alan Dooley in St. Louis. He said the agency has improved the way it gathers data over time, and continues to explore ways to improve it. "There's so many variables. Anyone who doesn't want more information and better information isn't doing their job."
Residents gambled on flood insurance
Michael Moran, a 52-year-old pipe insulator in the flood-stricken Missouri community of Winfield, canceled his flood insurance after the Flood of 1993. In recent days, his house swamped again. He said he was misled by the 100-year and 500-year terminology.
"I gambled on it. I thought I had time," he said. "Who knew in 15 years we'd see it again?"
The trouble is, terms that might improve public understanding don't exactly roll off the tongue.
For a 100-year flood, "we should be talking about a 1 percent annual chance flood," said Rich Leonard, flood plain management chief for the Federal Emergency Management Agency office in Kansas City, Mo.
Paul and Evelyn Dixon, who live in a 500-year floodplain in Old Monroe, Mo., along the Mississippi River, were not fooled by the terminology. In 1993, floodwaters filled the basement of their ranch-style home and crept a few inches upstairs. This year, their home was threatened again.
The 60-something couple said they have always carried flood insurance.
"I wouldn't live here without it," Paul Dixon said.