The rapid sinking of the Louisiana coast may have lowered New Orleans levees and contributed to their failure after Hurricane Katrina, resulting in the city's catastrophic flooding, engineers and other experts say.
Levees and storm walls may be as much as 2 feet lower than they were designed to be, both because elevation data were outdated when the levees were built and because the land has continued to sink, they say.
Experts had sounded alarms in recent years about subsidence, as the sinking is known, warning that the coast was far more vulnerable than most people realized. Federal officials and levee managers say they will begin reviewing the problem in coming weeks.
Subsidence is one of several factors experts are scrutinizing to determine why three levees failed, leaving 80% of the city underwater and hundreds dead.
Experts also are studying the Army Corps of Engineers' 1990s project that topped existing earthen levees with concrete walls to strengthen New Orleans' hurricane defenses. The approach was economical but may have left the walls weaker than intended. Three concrete walls failed after Katrina hit Aug. 29, to catastrophic result.
Engineering experts say the designs failed to use the most modern technology, had almost no redundancy to compensate for minor problems during a storm, and were further undermined by weak clay soils in the New Orleans area.
Ivor L. van Heerden, deputy director of the Louisiana State University Hurricane Center, said he was concerned that serious design defects had contributed directly to the wall failures. He has called for an intensive independent investigation.
The corps plans to conduct its own probe with outside experts.
Meanwhile, the corps has quietly been withdrawing its early conclusions about why the levees failed.
Within days of the flooding, federal engineers asserted that the flood-control system was simply never designed for such a powerful storm.
Now, with evidence suggesting Katrina's intensity fell within the range the levees should have handled, corps spokesmen are saying the organization wants to conduct a full-scale analysis of the design and construction of the levees.
The levee system was designed to withstand a Category 3 hurricane. Katrina's winds had weakened to 125 mph by the time it reached New Orleans, consistent with a Category 3 storm, according to meteorologist Chris Landsea of the National Hurricane Center in Miami.
