Experts: Term ‘100-year’ flood misleads public

updated 7:42 p.m. ET June 30, 2008

ST. LOUIS - 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.

Big mistake.

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.

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