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Many unknowns rermain because the new system has yet to be tested by a major storm.

“It’s like combat: Defend your perimeter. Where are my weakest points on my perimeter? Water will find the weakest point,” said J. David Rogers, an engineer and levee expert at the Missouri University of Science and Technology in Rolla, Mo.

Mr. Rogers said Katrina exposed numerous weak points — floodgates that did not close, porous ground and sewer lines that undermined defenses.

Mr. Rogers said that while “everybody’s attention is focused on what failed the last time,” the next major storm could expose new weak points.

Despite the uncertainties, residents are feeling better about their chances.

David Warino, a 46-year-old operations manager at a shipyard in Avondale, La., said he bought a two-story house next to the site of a breach on the 17th Street Canal in New Orleans because he felt confident.

“This seemed like a safe spot,” Mr. Warino said. “We’re on high ground. The corps rebuilt the wall here.”