LAKE BORGNE, La. » The great wall of Lake Borgne is a monster. Nearly two miles long and 26 feet high, it spans a corner of the lake, 12 miles east of New Orleans. On Aug. 29, 2005, that corner funneled Hurricane Katrina’s surge into New Orleans, causing some of the city’s most violent flooding. Now the corner is being blocked.

Nearly five years after Katrina and the devastating failures of the levee system, New Orleans is well on its way to getting the protection system Congress ordered: a ring of 350 miles of linked levees, flood walls, gates and pumps that surrounds the city and should defend it against the kind of flooding that in any given year has a 1 percent chance of occurring.

The scale of the nearly $15 billion project, which is not due to be completed until the beginning of next year’s hurricane season, brings to mind an earlier age when the nation built huge works like the Brooklyn Bridge, the Hoover Dam and the Interstate highway system.

The city’s reinforced defenses are already stronger than they were before Katrina. But even after 2011, experts argue, they will still provide less protection than New Orleans needs to avoid serious flooding in massive storms.

For a region devastated by a storm and by a loss of faith in the government’s ability to safeguard it, the new system is a test of more than the prowess of the Army Corps of Engineers. Some residents say they may never fully get over the failure of the Katrina response.

"Do I trust them?" asked Beverly Crais, a Jefferson Parish resident. "No. How can I trust somebody who makes that big of an error?"

That could be part of the reason that the top of the Lake Borgne wall is crenelated like the fortifications of a castle. The indentations, engineers say, will weaken waves that splash against the top. But they will also send a clear visual message to anyone who sees them: there is safety behind this wall.

The patchwork of walls and levees that fell apart after Katrina were, in the words of the corps’ own report on the disaster, "a system in name only." But projects like the wall — vast but largely unseen, because they take the first line of defense away from the center of New Orleans — are knitted into a single barrier.

"It’s a comprehensive-system approach," said Karen Durham-Aguilera, a civilian engineer responsible for work on what is now known as the Hurricane and Storm Damage Risk Reduction System. "We’re not even in the same universe anymore."

The lessons of Katrina were learned at a tremendous cost in life and property, but they can be seen throughout the works.

Where some of the old levees were built with dredged mud and shell fillthat washed away in the storm, the new ones are toughened with clay. Many old flood walls were shaped, in cross section, like the letter I and stood on muddy soil that seemed almost eager to give way; most of the new work is sturdier, shaped like an inverted T and braced with pilings driven diagonally into the ground. The corps is strengthening some soil, by mixing cementdeep into the ground.

Victor Zillmer, the engineer in charge of the Lake Borgne barrier, stood on the roadway that runs along its top and looked at the cranes building its navigation gates. His challenge, he said, is to build "the world’s tallest surge barrier on the world’s worst soils — in the least amount of time."

Many who watch the corps declare themselves impressed.

"The system that they are building is going to keep us, I think, safe," said Timothy P. Doody, the president of the consolidated levee board created to correct the failures of fragmented local boards.

He recalled that when he left his home in St. Bernard Parish before Katrina, he patted a column and said, "See you later." He knew it was possible to lose his home — as he did — but "I couldn’t wrap my mind around it till it happened." Today, he said, "imagination is no longer necessary."

Polly Campbell recently toured the 35-foot-high flood wall that will protect her St. Bernard Parish neighborhood.

"It’s absolutely phenomenal," she said.

Early in the morning and late at night, Campbell said, she hears the pile drivers pounding — "the greatest sound in the world," she said.

Still, many experts say that the level of risk reduction will be inadequate for a city that has been devastated by storms twice in living memory, Betsy in 1965 and Katrina.

Robert G. Bea, a professor at the University of California, Berkeley, and an author of a damning report on the levee failures, said that he has seen "lots of positive changes" in the corps but said 100-year protection "is not even close to what is needed." The system needs a greater margin of safety, he said.

Corps officials argue that Congress paid for the planned level of protection and note that a study is under way to design even more. But, they say, no matter how high the walls, the city will always be at risk, and residents will need to evacuate before storms.

Another difference between the old system and the new one is resiliency, said Durham-Aguilera of the corps’ building team. If a storm were to send waves over the new walls, she said, the streets could flood and the strengthened pump stations would remove the water. She said the system was designed to stand up to the kind of storm that might come once every 500 years.

"Being overtopped and getting a little bit wet is a lot different from having a breach," Durham-Aguilera said.

The bigger problem lies beyond the walls, said Ivor van Heerden, an author of the State of Louisiana’s report on the levee failures. The nation needs to rebuild fragile wetlands, which are disappearing at a rate of about 24,000 acres a year and reduce the force of storms.

"We’re never going to succeed if we rely on concrete alone," van Heerden said.

Many residents say they just want to get on with their lives. Artie Folse lives three blocks from the section of the 17th Street canal that failed in Katrina. He rebuilt his home by hand during the year after the storm.

The flood walls have been repaired, and the canal now has a gate to close in a hurricane. With nearly five years since Katrina, he said, "honestly, I don’t usually even think about it." As for the corps, he said, "I may be naive, but hopefully they’re doing everything they can to make sure it doesn’t happen again."

He added, "If it happens to me again, I’ll be living on a mountain somewhere."

© 2010 The New York Times Company