(Corrects university's name in 23rd paragraph in story published Sept. 3.)

Sept. 3 (Bloomberg) -- New Orleans survived Hurricane Gustav without flooding, thanks in part to improvements in its levees after Hurricane Katrina. It might not be so lucky with a stronger storm.

A $15 billion upgrade of the system after Katrina struck three years ago is only 25 percent complete, and Gustav made landfall southwest of the city as a Category 2 storm, the second-lowest on the hurricane-wind scale, with 110 mph (177 kph) winds.

The city is still vulnerable to a hurricane of the potency of Katrina, which slammed into Louisiana as a Category 3 with winds of 120 mph, engineers and weather scientists said. There have been 10 deaths in Louisiana from Gustav, compared with 1,800 in the region from Katrina.

``We were very, very lucky,'' said Sandy Rosenthal, 51, founder and executive director of levees.org, a New Orleans- based educational group, who fled her home in the city for Gulfport, Mississippi. ``If the storm had hit directly, the levees would not have held. New Orleans' vulnerability is still there. It is better than before Katrina but not good enough.''

Katrina's winds and storm surge pushed a wall of water 20 feet (6 meters) high from the Gulf toward New Orleans, breaking through levees and flooding 80 percent of the city over the course of four days after the storm hit on Aug. 29, 2005.

Improvements Helped

No punctures of the levees were reported during Gustav, whose storm surge the National Hurricane Center estimated at about 14 feet. The only incident of concern was the levee along the Inner Harbor Navigation Canal in the Lower Ninth Ward, where water sloshed over the top.

Joel Dupre, president of Southern Recycling, said improvements to flood walls around his business on the bank of the Canal prevented a major inundation.

``If those walls were the same as the last one, the same thing would have happened,'' said Dupre, whose company is housed in a structure on stilts.

The Army Corps of Engineers is strengthening 220 miles of levees around New Orleans under a program to be completed in 2011.

August Martin, a hurricane-protection engineer with the Corps in New Orleans, said the work includes replacing concrete footings on some levees with concrete barriers designed to prevent water from undermining the dry side of levees when water overflows.

Levees Rebuilt

This was done at the Inner Harbor Canal levee, which was breached during Katrina. He said engineers also installed pumps to keep canal levels below the levee tops.

``We built three outflow stations, at the 17th Street, Orleans and London Avenue canals,'' Martin said.

The Corps also rebuilt some levees along the Mississippi River in New Orleans with packed clay instead of material pumped up from the river bottom, he said.

For the Canal that Dupre's business borders, Gustav still may have been a close call.

``It came pretty close to going over,'' said Howard Botts, a professor of geography at the University of Wisconsin at Whitewater who has researched storm surges.

Gustav struck the Louisiana coast about 30 miles farther from New Orleans than Katrina, easing pressure on the city's flood barriers, he said.

``Several miles makes a vast difference,'' said Botts, also executive vice president of First American Proxix Solutions, a researcher for the insurance industry in Madison, Wisconsin.

Reduced Intensity

``You could have the same wind speed and pressure,'' said Tom Jeffrey, the chief hazard scientist at Proxix. ``If it shifted to the east, you could have had more significant storm surge in New Orleans.''

Gustav's reduced intensity also made it a lesser test for the levees, said Jim Rouiller, a meteorologist for Planalytics Inc., a private forecaster in Wayne, Pennsylvania.

``It went over Cuba and spent less time in the warm waters of the Gulf,'' he said.

Gustav made landfall in Cuba as a Category 4 hurricane with winds as strong as 150 mph.

``Gustav was not a major hurricane,'' said Stephen P. Leatherman, director of the International Hurricane Research Center and Laboratory for Coastal Research at Florida International University in Miami. ``The levees that broke last time,'' including some off Lake Pontchartrain, ``were not truly tested this time.''

Geology a Challenge

J. David Rogers, a professor of geology and engineering at the Missouri University of Science and Technology in Rolla, Missouri, said the local geology makes it difficult to build a system that provides 100 percent protection for New Orleans, which sits below sea level.

``You have some of the weakest soils in the U.S.,'' said Rogers, who was among scientists who assessed the effectiveness of the levee system after Katrina. ``Some of it you'd sink to your hips. We're building on that. That takes a lot of care.''

The Corps of Engineers aims for a levee system that can withstand a storm with a strength level that occurs once every 100 years, said Wade Habsher, a spokesman in Baton Rouge, Louisiana. Katrina was a 400-year storm.

Botts at the University of Wisconsin and Rouiller, the private forecaster, pointed out that storm activity has been above normal. Colorado State University said yesterday that tropical activity in the Atlantic for September would be ``well above average.''

``We've been dealing with a lot of intense hurricanes,'' Rouiller said. ``One of those can destroy New Orleans.''

To contact the reporter on this story: Jerry Hart in Miami at jhart@bloomberg.net.

Last Updated: September 4, 2008 11:12 EDT