When civil engineers talk about the threat of a “100-year flood,” a common assumption is that such disasters will be separated by a century.

But a 100-year storm is merely shorthand for an event that has a 1 in 100 chance of occurring in any given year. Such a storm could hit two years in a row, or not at all for two centuries.

Trusting to long intervals between 100-year floods can contribute to bad decisions, from homeowners feeling secure enough to drop flood insurance to members of Congress refusing to finance higher levels of protection.

Some state and local officials are seeking clearer verbiage for discussing floods, hurricanes and the levels of protection the government provides.

“We must settle on a new way of explaining risk, a new vocabulary that lay people can understand,” said John Barry, who has written extensively on Mississippi River flooding.

“The phrase ‘100-year flood’ doesn’t communicate to the public or to policymakers the real risk of flooding. They think it means a flood that occurs once every 100 years, when in fact, there’s something like a 60 percent chance of experiencing it in your lifetime.”

The Federal Emergency Management Agency began moving some years ago to redefine the “100-year storm” as “a statistical event that has a 1 percent chance of occurring each year.” In doing so, FEMA acknowledged that the lexicon of engineers and statisticians wasn’t informing the public at large.

“The term ‘100-year flood’ is misleading,” FEMA says on its Web site. “It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1 percent chance of being equaled or exceeded each year.”

Spring deluges that caused widespread flooding in the New Orleans area in 1978 and 1995 were both considered 100-year storms.

But Barry and other levee authorities say most people still don’t understand their risk of flooding each year.

“Mark Twain said the difference between the right word and the wrong word is the difference between lightning and lightning bug,” Barry said. “Residents need to understand … members of Congress need to understand that the risk of a ‘1,000-year flood’ occurring in the average life span of an individual is well over 5 percent.”

FEMA has tried to clarify the probability of a 100-year flood by saying that it has about a 26 percent chance of occurring during the life of a 30-year home mortgage. That’s about triple the risk of a fire during that period.

Barry said the key players in any potential change – such as FEMA, the Army Corps of Engineers and the National Weather Service – have not agreed on a new metric.

“I don’t have a solution, but someone has to take the lead in actually making a change,” Barry said. “The entire flood community is behind this, but someone has to make it happen.”

FEMA and the Corps of Engineers (though not the weather service) similarly have abandoned the Saffir-Simpson category scale to rate hurricanes. The scale historically is based on wind speed but also includes a generalized description of hurricanes’ other damaging effects, including the heights of storm surge.

Along the Gulf Coast, a Category 1 or 2 storm was one that didn’t worry most people because their accompanying surge was believed to be a threat only outside levees. A Category 3, depending on forward speed, was known to create surge high enough to top levees and trigger evacuations. Category 4 and 5 storms were understood to have catastrophic potential.

That characterization died for most federal agencies after Katrina. With Category 3 winds but a Category 5 storm surge on the eastern edge of New Orleans, it killed more than 1,500 people and caused billions of dollars’ worth of damage.

Research after the storm found that Katrina’s surge height was governed by the radius of its hurricane-force winds, which did not necessarily follow the Saffir-Simpson categories.

For example, Hurricane Camille in 1969 was a Category 5 storm based on its wind speed and raised a 23-foot surge when it went ashore in Mississippi. But its hurricane-force winds extended only 10 miles from its eye.

Katrina, however, created a 26-foot surge along a much wider swath of the Mississippi Coast – with at least one report of water 32 feet high – while having only Category 3 wind speed. That’s because its hurricane-force winds extended 75 miles from its center, and it had been at Category 5 strength only 12 to 18 hours before landfall.

The National Hurricane Center is studying ways of updating or replacing the Saffir-Simpson scale. In the aftermath of Katrina, it has played down the surge portion of the scale. Instead, the center has begun running maps predicting surge heights for specific storms along specific sections of coastlines 24 hours in advance of a hurricane’s predicted landfall.

RB END GRISSETT/SCHLEIFSTEIN

(Sheila Grissett and Mark Schleifstein are staff writers for The Times-Picayune of New Orleans and can be contacted at sgrissett(at)timespicayune.com or mschleifsteintimespicayune.com)

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