Street sign in Louisiana reflects Katrina's devastation

The city of New Orleans is slowly sinking into the Mississippi River Delta of southeastern Louisiana. Throughout its history a network of levees, dikes, canals and pumping stations have worked to keep the water out.

After the nation's first billion-dollar hurricane ravaged the city in the mid-1960s, the U.S. Congress authorized the Army Corps of Engineers to build an extensive flood defense system.

About half that 560-kilometer project was destroyed or damaged last August by Hurricane Katrina. The repair bill could well exceed $10 billion -- triple the initial estimate.

While assessment is still being made as to why the levees failed, the race is on to

Repairing the levees

repair the critical barriers before the hurricane season begins on June 1.

The Hurricane Center at Louisiana State University issued its first advisory 45 hours before Hurricane Katrina hit landfall. Paul Kemp - a research scientist at the Center - knew the city of New Orleans was in trouble long before that. He had worked with the Army Corps of Engineers on models that predicted various disaster scenarios from storms the same strength as Katrina. They showed the levees were vulnerable to overtopping by storm surges or floodwaters. What Kemp hadn't anticipated were the engineering problems.

Research scientist Paul Kemp

"Our model had predicted where we should see water going over the levees and where we should not, and some of the failures were where we didn't expect to see water going over the levees." he says. "We looked at those areas particularly and in fact we found no evidence of overtopping and concluded pretty early on that the foundations of the levees had failed rather than being scoured by water going over the tops."

Kemp is a member of an independent team called on by the state of Louisiana to assess the levee failures. On an overcast day he walks through a largely abandoned neighborhood of two-story brick homes, some of which were still being built when flood waters surged

Abandoned neighborhood after Katrina

through the streets.

Kemp points to watermarks more than three meters high on the outside walls and to the shallow craters the water cut into in the sandy soil underneath the homes. Surveying the damage, he says the Army Corps of Engineers must understand how this happened and make moves to correct it. "Did overtopping [of the levees] occur prior to breaching or after breaching? What were the mechanisms of failure? Was it some bad connections between floodwalls and earthen levees as it was in many places? Was it a combination of oceanographic factors like waves and currents?" "[In] each of these cases -- even though we are dealing with the same storm," he says, "we are dealing with a different causal mechanism, and they are quite different from place to place around the levee system."

Colonel Lewis Setliff with reporter Roseanne Skirble

After Katrina, the Army Corps of Engineers began to study what went wrong. The Corps will issue its report in June. In coming weeks and months several independent teams - like the group from Louisiana State University - will also make assessments and recommendations. Many of these independent experts are already expressing fears that the levees won't stand up to the next big test.

Colonel Lewis Setliff with the Army Corps of Engineers doesn't share those fears. He is commander of Task Force Guardian, the short-term levee repair team. The job to bring the levee system back to pre-Katrina conditions is well over half done. Setliff employs hundreds of Army Corps staffers. He also oversees dozens of contractors.

On this windy day, he checks progress at the London Avenue Canal where crane

Crane operators haul 25-meter sections of metal sheet pile that will form a frame for new floodgates

operators haul 25-meter sections of metal sheet pile that will form a frame for new floodgates. Two major breeches to the canal floodwalls had allowed waters to swamp the city. "The structures themselves will rise out of the water about 29 feet [8.8 meters] above sea level. And the protection they will establish will be 16 feet [4.8 meters] above sea level when the gates are closed."

Independent experts have been highly critical of the Army Corps of Engineers for taking shortcuts, for using substandard materials, and for ignoring [levee] design and construction flaws. In response Colonel Setliff says, "It gets back to those who criticize us just because we are moving fast that we can't build a quality product. I would contend that is not the case," he says, noting there are many layers of quality

Tractor operator at levee site

management. "The construction companies, independent architect-engineers, independent laboratories and the Corps of Engineers are checking our quality every step of the way because this is critical. We have to do this right. There are people's lives dependent on what we are doing. We are also inviting anyone who wants to critique us to come on site and show us the science and engineering to back their claims."

Colonel Setliff says these repairs are an interim solution. He says the Corps is committed to building a "higher and better" levee system based on lessons learned from its own study and that of independent analysts.

Professor Paul Kemp from Louisiana State University says in the meantime, the City of New Orleans hangs in the balance. "Now the question is, you can build something that looks like a levee in three months, but will it actually hold the water back?"

The answer could come when hurricane season begins on June 1.