New Orleans Levee SystemEvolving DangerDespite rising hurricane risks, the Army Corps of Engineers hasn't revised its levee designs for the New Orleans area, and some areas may be more vulnerable to floods than the Corps maintains.By John McQuaid and Mark SchleifsteinStaff writersThe New Orleans area's last line of defense against hurricane flooding is a 475-mile-long system of levees, locks, sea walls and floodgates averaging about 16 feet high. The Army Corps of Engineers says the system will protect the city and suburbs from a Category 3 hurricane that pushes in enough seawater to raise Lake Pontchartrain 11.5 feet above sea level -- high over the head of anyone standing on the other side of a levee.That margin of error is critical because a storm that pushes the lake any higher can force water over the top of the levees and inundate the city. The water could quickly rise 20 feet or higher. People would drown, possibly in great numbers.The corps doesn't know what that safety margin is anymore.Generally speaking, the corps says the powerful, slow-moving storms capable of overwhelming the system are rare and the levees are safe. But corps engineers say their own safety estimates are out of date, and an independent analysis done for The Times-Picayune suggests some levees may provide less protection than the corps maintains.The corps' original levee specifications are based on calculations made in the early 1960s using the low-tech tools of the day -- manual calculators, pencils and slide rules -- and may never have been exactly right, corps officials say. Even if they were, corps officials and outside scientists say levees may provide less protection today than they were designed for because subsidence and coastal erosion have altered the landscape on which they were built.Experts dispute corps' estimatesAccording to the rough statistical analysis done by engineering consultant Lee Butler, the risk of levee overtopping in some areas -- St. Bernard Parish, eastern New Orleans and the Lower 9th Ward -- may actually be close to double what the corps once thought it was. The corps disputes Butler's numbers but has no current alternative figures.The agency is undertaking a new study to reassess the level of protection and another to determine whether the levees need to be raised still higher.Measuring the risks of disaster is a technical feat that few understand. But such exercises are critical to the future of New Orleans. If the new corps study confirms that protection is less than previously thought, the answers could have major effects on issues such as flood insurance rates, future levee expansions, emergency planning, evacuation and long-term business decisions.Thanks to its low, flat profile and its location on the Gulf of Mexico, south Louisiana is more at risk from a major natural disaster than most other places in the country. The risk of a catastrophic levee-topping flood in New Orleans is roughly comparable to the risk of a major earthquake in Los Angeles. Because of coastal erosion and subsidence, that risk is growing.But judging that risk and how to protect against it can be difficult. Recent experience tends to confirm the idea that catastrophic hurricane floods are rare. Even if a powerful hurricane comes close to New Orleans, only certain storm tracks could flood all or part of the city and suburbs. Twelve storms rated Category 3 and above have hit the Louisiana coast in the past 100 years, but only four produced major flooding in the New Orleans area. The levee system was built largely in response to those storms, to prevent or reduce flooding in similar events