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Katrina anniversary: Will New Orleans levees hold next time?

Hurricane predictive models change every time a new one strikes—so the numbers being used as guidelines for construction keep changing.

New Orleans' levees are better than they were 10 years ago, but maybe not good enough to fend off another Katrina.

The reinforced levee stands in the Lower Ninth Ward, on July 22, 2015, in New Orleans.
Melanie Stetson Freeman | The Christian Science Monitor | Getty Images
The reinforced levee stands in the Lower Ninth Ward, on July 22, 2015, in New Orleans.

The city's redesigned levees and floodwalls should be far more resilient against storms than the ones that broke open and let Hurricane Katrina flood nearly the entire city a decade ago. But civil engineers warn that much more needs to be done to keep New Orleans safe, and they caution that people should not assume the levees will protect them from a big storm.

Hurricane Katrina killed more than 1,800 people and caused an estimated $75 billion in damage after it made landfall 10 years ago this week. During the storm, several of the levees shielding New Orleans broke, flooding nearly 80 percent of the city in up to 15 feet of water.

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Soon after, Congress authorized the U.S. Army Corps of Engineers to spend $14.6 billion to improve the levees and other structures designed to protect the city from extreme storms.

At the time, Congress told the Corps to design the system to withstand a storm that has a 1 percent chance of happening every year—the so-called 100-year storm. That standard would allow people in New Orleans to qualify for the U.S. government's National Flood Insurance Program, which requires that policyholders live behind adequate flood protection structures.

But experts point out that the 100-year standard is not a safety benchmark at all, but a statistical guideline used most commonly by insurers to estimate the costs of insuring property against flooding damage.

"If you looked at the NOAA data in 1965, Katrina would have looked like a 1,000-year event." -Ed Link, director, Interagency Performance Evaluation Task Force

"It is a number which is based on protecting real estate, not protecting lives, and that is an incredibly important distinction," said Greg Baecher, a professor of engineering at the University of Maryland and a member of the Interagency Performance Evaluation Task Force, a group started by the Army Corps of Engineers in 2005 to investigate the levee systems built before and after the hurricane.

The 100-year threshold is also a statistical guess based on data on past storms and assessments of whether they'll occur in the future. That means the models change every time a new hurricane strikes. The numbers being used as guidelines for construction are changing as time passes.

The standard also does not mean—can't possibly mean—that a 100-year storm will occur only once per century. It means that such a storm has a 1 percent chance of happening in any given year. So for example, it's technically possible for several 100-year floods to occur in just a few years, although it's highly unlikely.

Needless to say, there's disagreement among researchers as to whether Katrina was a 100-year storm, or something else.

"If you looked at the NOAA data in 1965, Katrina would have looked like a 1,000-year event," said Ed Link, director of the Interagency Performance Evaluation Task Force (IPET).

"Fast forward to 1979: NOAA redid the analysis based on a lot more data—and remember it wasn't until the 1960s that we had weather satellites," Link said. "Katrina in 1979 would have been more like a 1-in-300 event."

When the National Oceanic and Atmospheric Administration redid the analysis again in 2005, Katrina was more like a 100-year event, Link said.

But given that the 100-year standard comes with so much uncertainty, what are the alternatives? One alternative measure is the Saffir-Simpson scale, commonly used by meteorologists to rate hurricanes. Storms receive ratings between 1 and 5, in order of severity.

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But Saffir-Simpson ranking is based on wind speed, not the height of the storm surge, which is what made Katrina so deadly. Katrina was rated only a Category 3 when it made landfall, but the size of the storm meant the surge was significantly higher than it normally would have been for similarly rated events.

In IPET's analysis, Hurricane Katrina was more like a 400-year event, considering its capacity to create a surge, Link told CNBC.

"Katrina created the highest level of storm surge and waves that North America has ever seen," he said. "Hurricane Camille, for example, was a Category 5 hurricane, and yet it did not create the kind of surge Katrina created."

These statistical models will continue to evolve in the future, and may undermine confidence civil engineers have in the levees designed to meet standards that have been revised.

Big improvements have been made

Having said that, the new system is a considerable improvement over what was in place when Katrina struck.

For example, the levees since are in the process of being armored to help prevent the erosion that had caused many of them to fail 10 years ago. The IPET report noted that at least half of the direct damage from Katrina resulted from those breaches. The new levees are much stronger than the ones that failed during the storm, said Bob Jacobsen, a Baton Rouge, Louisiana-based engineer who has worked with local levee boards—the state-created agencies responsible for overseeing the day-to-day monitoring and maintenance of the system.

Jacobsen told CNBC that the new levees were built with better techniques for compacting the clay that makes up the levees, and some of the structures are being armored to prevent them from eroding, especially if water spills over the top of them.

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They are also higher, though by varying degrees. Some will likely need to be raised further—due to factors both natural and man-made, New Orleans is sinking, and much of the city is already below sea level. The levees and other structures will require frequent monitoring and occasional strengthening.

If all elements in the new design perform "as advertised," Jacobsen said, the structures should be able to withstand water levels from a 500-year event without collapsing or letting excessive amounts of water spill over the wall.

But he also noted that trying to prepare for disasters relies on "scientific guesstimates," and that more improvements are needed.

Ultimately, he said, the levees are designed only to protect property and reduce the potential damage to the city.

"An evacuation is still the only way to secure people's lives," Jacobsen said. "If there's a big storm coming, you get out."