Nearly nine years after Hurricane Katrina, most of metropolitan New Orleans boasts America's most elaborate system for knocking down storm surge.
Yet, some experts believe the $14.5 billion "Hurricane Risk Reduction System," as the Corps calls it, leaves the city vulnerable in the future.
The system includes: the 2-mile-long Lake Borgne Surge Barrier in Eastern New Orleans and St. Bernard Parish; The Seabrook Navigation Canal Closure, a giant gate at the mouth of the Industrial Canal adjacent to the Seabrook Bridge; and the West Closure Complex, twin gates adjoining the world's largest drainage pumping station south of Harvey on the west bank.
In all, 133 miles of gates, pumps, walls and levees provide protection against the 100-year storm, or a tropical system with a 1-percent chance of happening in any given year.
"There is a lot of work still to be done," said Col. Richard Hansen, Commander of the U.S. Army Corps of Engineers New Orleans District.
On the New Orleans lakefront, Hansen said work is about 25 percent complete on three permanent pumping stations designed to eliminate one of the weakest links pre-Katrina, its outfall canals that drain water into Lake Pontchartrain.
In a sense, the old system left the front doors to the city wide open.
With no closure in place, lake waters invaded the Orleans, London and 17th Street Canals and blew to bits the poorly-constructed flood walls.
Already, temporary pumps close off the lake, providing an additional layer of defense.
"The modeling effort that we followed really has led to a much more robust system than Congress would have envisioned and Corps would have envisioned prior to Katrina," Hansen said.
In fact, Corps computer modeling shows the new levees would significantly reduce flooding from a 500-year storm, one even larger than Katrina.
Computer-generated maps from the Corps depict pockets of the city 8 or more feet under water in from a suite of five or six possible storm scenarios.
However, other areas would have less than 2 feet of water or no water at all, according to the model runs.
One of the final steps in completing the system will involve armoring earthen levees with a geo-textile fabric, then topping them with grass.
"That'll make the system much more resilient to a 500-year level storm and protect it from potential overtopping," Hansen said.
To design the system, engineers ran computer models of 152 possible storms.
Today, in Eastern New Orleans, 25-foot-high levees dwarf the 15-foot speed bumps that were in place on Aug. 29, 2005.
As impressive as the new structures may be, some experts believe they are inadequate to defend a major American city.
"It's a minimum daily requirement," said H.J. Bosworth Jr., a civil engineer and lead researcher for the group levees.org.
Bosworth points out most American cities, including others on the Mississippi River, enjoy much higher levels of protection.
"They all have 500-year protection and don't even think that's enough," Bosworth said.
In reality, Bosworth says not everyone within the new walls enjoys the same level of security.
Because of the design features and redundancy, those living in the interior of the system may enjoy the greatest benefit.
Bosworth notes studies showing the main basin of New Orleans - from Lake Pontchartrain to the Mississippi River and the 17th Street Canal to the Industrial Canal - is less vulnerable than St. Bernard, New Orleans or even areas of Kenner and East Jefferson.
"It has a lot of defenses," Bosworth said. "It's insulated from things that could happen in New Orleans East, in St. Bernard and the Lower 9th Ward.
The mathematics of 100-year protection can be sobering. Consider that, with 100-year protection, a baby born today and living to age 80 would have a better than 50-50 chance of experiencing a major storm exceeding the parameters of the current protection system.
In the wake of Hurricane Betsy in 1965, Congress authorized New Orleans' hurricane defenses, but it gave the metro area a "system" in name only.
"We have something that's more honest and reliable," said Mark Davis, Director of the Tulane Institute on Water Law and Policy. "But it's not enough."
Davis has serious long-term concerns about the risk-reduction system.
"We've given ourselves a second chance," Davis said. "Not many people get that."
Davis said the key question is, "whether we stand behind 1-in-100-year levees and say, 'mission accomplished,' or if we say, 'we made it into the lifeboat. Now, let's get to shore.'"
He argues maintaining the system at the 100-year level of protection will present a challenge for flood authorities and taxpayers.
"It's not going to be a federal responsibility," Davis said. "The real burden is just about to land on us."
Already, three sections of the new levees in New Orleans East required to the Corps to race to finish repairs this spring. Despite extraordinary steps during construction, one spot near the giant surge barrier sank 3 to 6 inches below the design specifications.
Contractors for the Corps had used deep-mixing of soil, drilling down and pumping cement in order to build the levees higher than otherwise would have been possible.
"We are committed to not turning over a levee if it is below design grade," Col. Hansen said.
Both Hansen and Bosworth note the rate of subsidence affecting the levees should slow in the future.
As land subsides, engineers explain the ground compacts and effectively grows stronger.
Tulane's Mark Davis worries maintenance costs could suck up dollars that would otherwise build coastal projects, restoring or simply protecting areas that once provided natural defenses.
"That's why we have to be in the business of explaining why you need wetlands, why you need certain levees and why you need to live smart behind them," Davis said.
Bosworth believes some of the remaining earthen levees could be upgraded and flood walls could be raised near Kenner, steps that would provide more protection for a fraction of the money the government has already invested.
"Be careful not to go down the path of believing the levee is your final defense," Davis cautions people living within the fancy, new flood walls. "It would be a tragedy if we went back to that kind of thinking through what we're willing to invest in."