...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and storm walls may be as much as 2 feet lower than they were designed to be, both because elevation data were outdated when the levees were built and because the land has continued to sink, [some experts] say.
...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 [including Ivor L. van Heerden, deputy director of the Louisiana State University Hurricane Center] 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...
...In other areas, including eastern New Orleans, overflow destroyed many miles of levees.
...[Louisiana State University engineering professor Roy K. Dokka] had warned in the last two years of an approaching disaster. "The most recent data shows that all of the previous subsidence work was faulty," he said.
On average, coastal Louisiana has been sinking half an inch annually, he said. But New Orleans East, which saw devastating flooding when levees overflowed, is sinking faster. Many levees are at least 6 inches lower than they were designed to be, and those in New Orleans East and badly damaged Plaquemines Parish are perhaps 2 feet lower.
...Geological and human forces are causing land elevations to drop, said Virginia R. Burkett of the U.S. Geological Survey's National Wetlands Research Center in Louisiana.
Over thousands of years, coastal subsidence was offset by Mississippi River sediment deposits, Burkett said. But dams and other water projects on tributaries have halved river sediment since 1950, and levees on the main river channel now carry the remaining sediment deep into the gulf rather than spreading it along the coast.
Offshore oil and gas extraction have accelerated subsidence in some areas, Burkett said. And swamp drainage, particularly under New Orleans, has caused organic soil matter to decay and compact, adding even more to the subsidence.
As levees have sunk, melting glaciers have lifted ocean levels globally, she added.
The problem is expected to worsen in the coming century...
...In 2001, the National Geodetic Survey warned Congress that elevation surveys for Louisiana were "obsolete, inaccurate and unable to ensure safety."
...Two years ago, the agency began recalibrating Louisiana elevation data using the Global Positioning System, Deputy Director David Zilkoski said. But Katrina struck before the project could make a difference.
In the year before Katrina, Zilkoski said, he went from town to town to warn local officials that elevation data in the region were inaccurate, including those that showed evacuation routes.
...Meanwhile, LSU's Van Heerden expressed concern that the 1990s work to place concrete walls atop levees might have contained fundamental design flaws.
The new walls were tied into existing steel sheeting inside the earthen levees. In some cases, the tops of the steel and the footings of the new concrete walls overlapped by just 2 feet. Steel reinforcement bars were welded or looped into the sheeting to tie the structures together, he said.
Van Heerden said he believed the walls on the 17th Street and London Avenue canals failed because they could not resist the water pressure, not because of overflow or foundational erosion.
He said the storm walls snapped where the steel sheeting met the concrete walls. He said it was generally bad practice to use old sheeting as a foundation.
Katrina would also have weakened the levees' clay soils, a further potential cause of failure.
Geotechnical engineering professor Dobroslav Znidarcic of the University of Colorado at Boulder said he was surprised concrete walls were used for the levees.
Few retaining walls are made of reinforced concrete anymore, he said. Instead, "mechanical stabilized earth" can provide greater strength and resistance to total failure. The system, developed in the late 1980s, sandwiches fabric grids between layers of soil.
"I am concerned their design is prone to catastrophic failure if anything goes wrong," he said. "It does not have a sufficient level of redundancy."