Immediately after Hurricane Katrina pummeled New Orleans last August, President George W. Bush and the U.S. Department of Homeland Security declared that no one could have predicted such devastation. Yet scientists, engineers and Louisiana state politicians had warned for years that a Category 4 or 5 storm crossing the Gulf of Mexico from a certain direction would drown the region. In 1998 computer models at Louisiana State University simulated such a terrible inundation. That same year the state proposed a $14-billion plan to restore the delta's natural wetlands--which, by absorbing water, can help protect inland areas from sea surges. But Congress turned it down.

What is more, engineering firms, as well as the U.S. Army Corps of Engineers, which is largely responsible for flood protection, had proposed constructing higher earthen levees as well as huge gates that could have prevented storm surges from pouring into inner-city canals and bursting their concrete flood walls. Indeed, documents show that various gates had been recommended as far back as 1968 and in each decade since.

Furthermore, because the storm surge entered New Orleans from Lake Pontchartrain to the north and navigation channels to the east, "those plans would not have stopped Katrina," observes Hassan S. Mashriqui, an assistant research professor at L.S.U.'s Hurricane Center who has enhanced the university's 1998 surge models. He says the models show that gates across certain channels into the city would also have been needed to divert the surge.

Those gates would have done nothing for the rest of southeastern Louisiana, however. Scientific American therefore asked a wide range of experts to present solutions for the region. Three strategies emerged: a tight ring around the New Orleans metropolitan area alone; a comprehensive, 440-mile levee system that would snake from the Mississippi border halfway to Texas but lie only partway to the shoreline, leaving the coast for lost; and an outer shield around the region's perimeter, such as the one in the Netherlands, which would spare every locale. The ring and comprehensive plans would inevitably leave some people "outside the wall." All three plans include gates of some kind that are not now in place.

Although each approach has its proponents, the parties agree on one thing: critics who say it is foolish to rebuild in such a vulnerable place are missing the big picture. In addition to being a cultural center, "the Gulf Coast is the economic engine that drives the country," Bahr declares. "We can't possibly abandon it." The delta produces one fifth of the country's oil, one quarter of its natural gas, and one third of its seafood. Trillions of dollars of goods and crops flow through the ports there. These activities require extensive infrastructure and tens of thousands of employees who cannot live in temporary trailers or in homes two hours away.

A New Path
Before any plan is implemented, designers should understand fully why the existing levees failed, so weaknesses can be avoided in the future. Four teams are investigating the levee and canal wall collapses in New Orleans--from the American Society of Civil Engineers, the Corps, the state of Louisiana and the National Science Foundation.

In October, Secretary of Defense Donald Rumsfeld announced that the National Academy of Sciences would review all the reports to ultimately determine why flood walls crumbled [see Working Knowledge, on page 92]. Rums­feld said the academy would finish by June 2006. Yet hurricane season officially begins June 1, and Richard Wagonaar, a U.S. Army colonel and commander of the Corps' New Orleans district, says his goal is to restore all federal levees to their pre-Katrina level of protection (able to withstand Category 3 hurricanes) by that date. President Bush had asked Congress to appropriate $1.6 billion to repair levee damage from Katrina and in December requested another $1.5 billion to improve city levees at Category 3 levels.

Wagonaar and others see restoration as only the first step, however. They agree that the city's protection must be raised to Category 5 levels. Congress would have to appropriate much more money for this work, whether it went to the Corps or a different contractor.

When it comes to the comprehensive plans, people become more contentious. Despite local differences, the plans outlined by Shaw Coastal (part of the Shaw Group), Mashriqui and Wagonaar follow a somewhat similar route. To the south and west, the course would link and raise existing, piecemeal levees now maintained by independent levee districts and would protect most major towns. The planners disagree, though, on who should do the work as well as on how long construction would take and what it would cost. Wagonaar estimates the Corps' plan "would take us five to 10 years to build" and that the cost "would be $25 billion to $35 billion--but remember, this is still in the concept stage."

That time frame seems unrealistic to O'Neil P. Malbrough, president of Shaw Coastal, an engineering firm that has built smaller levees and floodgates elsewhere in the country. "The Corps began building levees around New Orleans and Lake Pontchartrain in 1965, and there are still segments that are incomplete," Malbrough says. "That's inexcusable. If private enterprise were doing this, it would cost half the money and take half the time. The Corps is still building the same wall it was building then."

Wagonaar says he is not familiar with Malbrough or with Shaw's plan and notes that companies are unlikely to be involved unless Congress opens bids for the work. The Corps has had a de facto monopoly on federally funded flood protection for a century.

Residents in the hurricane danger zones hope that whoever gets the job incorporates new understanding of how Katrina and Rita ravaged New Orleans, and some worry that the Corps may not be fully responsive to external scientific information. For example, Mashriqui at L.S.U. has determined that a wide breach in the Industrial Canal, which flooded the eastern section of the city after Katrina, was caused by what is called a funnel effect. Computer simulations, and physical evidence Mashriqui obtained in October as a member of the state inspection team, show that Katrina pushed water from the east up a wide navigation channel called MRGO and simultaneously up an adjacent channel, the Intracoastal Waterway. The two wave fronts met where the inlets join and narrow into the Industrial Canal. This geometry amplified the height of the water by 20 to 40 percent, Mashriqui says. That increase raised the water pressure so high that the canal wall burst.

Scientists had predicted the funnel effect, and Mashriqui says a preparedness exercise run in 2004 by the Federal Emergency Management Agency based on a simulated hurricane called Pam also involved a funnel effect in that place. "The wind pushes a huge volume of water into this narrow strait, and the water just piles up," he says.

MRGO must be closed and gates must be put across the Intracoastal Waterway to protect the city, Mashriqui says. Numerous other experts speaking to the national press have concurred, noting that it supports very little ship traffic and costs a lot to maintain. But the Corps built MRGO, Mashriqui notes, "and for them to agree, they would be admitting a mistake was made in the first place."

Mashriqui is not the only one worrying about the need to avoid funnel effects. This very phenomenon was a chief reason for building the Thames Barrier in 1982, to prevent storm surges from piling up along the Thames Estuary close to London. "As the channel narrows by 50 percent, the water piles up by 100 percent," says Martin Earlam, chief engineer for the barrier at the country's Environment Agency, which operates it. And S. Jeffress Williams, a coastal scientist with the U.S. Geological Survey who worked for 20 years in Louisiana and has studied deltas worldwide, says funnel effects "have been underappreciated."

Coastal administrator Len Bahr says an outer shield is a wild idea, but given the delta's alarming deterioration, "we may need some wild new ideas." Williams of the USGS concurs: "Considering the rates of sea-level rise, land subsidence, and the increasing frequency and severity of storms, it's a legitimate option."

Take a Scientist to Lunch
Panicked by the devastation in 1953, the Dutch quickly set out to build solid dams across several wide estuaries to the North Sea. Those berms, however, cut off the interchange of saltwater and fresh­water and altered the environment. In recent decades, Delta Works has changed direction, emphasizing man-made barriers that close only when surges are imminent. "It took us a long time to learn that we could work with nature, not just defy it," Weijers says. He advises U.S. planners to "really think it through."

Wagonaar at the Corps, Malbrough at Shaw Coastal, and Mashriqui at L.S.U. agree that certain aspects of the original Coast 2050 and LCA plans that would revive the marshes must be incorporated into any plan adopted. Diversions--large doorways inserted into the levee along the Mississippi River's banks--would supply freshwater and sediment to the wetlands. The Corps' plan includes a significant number of gates to "let water flow in and out to support the LCA objectives," Wagonaar says.

Although the Netherlands and Britain probably have much advice to offer, Malbrough says, "we don't need the Dutch to tell us how to build a levee system." The Corps apparently felt that way at one time, too; its officials made similar statements to Scientific American during research for a 2001 article about restoring the delta [see "More to Explore" below]. But collaboration may improve. In 2004 the Corps and Delta Works leaders signed a memo of understanding to exchange information, and in October several Dutch engineers helped the Corps analyze New Orleans's flood-wall failures.

The need to coordinate physical protection across levee districts and to incorporate coastal restoration means a clear leader is needed. The experts interviewed for this article acknowledged that political infighting in Louisiana killed smaller proposals in the past. Malbrough hopes this message may be sinking in; he was hired by various levee boards together to present the state and federal government with one coordinated plan. Both he, Mashriqui and Bahr think a federally run consortium should oversee the work. They oppose putting the Corps in charge, seeing it as too slow, too politicized by Congress, and too unwilling to entertain novel technical approaches. "I'm inclined to change horses," Mashriqui says.

He and others also strongly recommend that scientist input be sought much more extensively. "I don't care who wants to control the work," Mashriqui asserts. "But whoever it is has to listen to the data. If our models show that the funnel effect will kill people, then there is no excuse not to close the funnel."

Bahr agrees that engineers "have not so far engaged research scientists nearly enough. And we need anthropologists and social scientists to help figure out the people issues. Some parishes [counties] were virtually wiped out by Katrina. Where does it make sense, socially, to rebuild?" Williams, the USGS veteran who also worked at the Corps for 13 years, says scientists continually "put data out there, but it has no effect on its own. Politicians and planners have to take it from there."

Wagonaar, a longtime Corps member who took over the New Orleans district last July, says that in the future the Corps will better integrate outside experts: "We are generally a lot more open than we were five or six years ago, especially with regard to environmental issues. But we can't study this situation forever either. Someone will have to make a decision."