DROWNING CITY: Battered by the August 29 arrival of Hurricane Katrina, levees protecting New Orleans crumbled, such as this one along the Inner Harbor Navigational Canal. Tattered wetlands around the city offered little protection against stormy seas. Image: VINCENT LAFORET AP Photo/Pool
In the aftermath of Hurricane Katrina's devastation, the nation has vowed to rebuild New Orleans and Gulf Coast communities while improving protection against raging storms. But before engineers redesign a single levee, they must consider a fundamental question: Can the Mississippi Delta be restored as a lush, hardy buffer that can absorb surges and rising seas? Or is it too far gone, necessitating a 300-mile wall to hold back the Gulf? Researchers have known for at least five decades that wetlands help to stop storm surges from crashing inland. But for a century, the U.S. Army Corps of Engineers leveed the Mississippi River to its mouth to stop annual floods. That spared New Orleans but starved the wetlands south and east of the city of the sediment, nutrients and freshwater they need to thrive. The levees also cut off sediment flow that builds barrier islands ringing the delta.
In 1998 scientists and engineers proposed a $14-billion master plan, which Congress never funded. Called Coast 2050, it detailed strategies to revive the delta and control flooding [see "Drowning New Orleans," by Mark Fischetti; Scientific American, October 2001]. Seven years and a bad hurricane later, the wetlands and barrier islands are so much more tattered that traditional restoration techniques may no longer suffice. "I raised this issue at a Corps meeting several years ago already," notes S. Jeffress Williams of the U.S. Geological Survey, who has studied the coast for 20 years. "It was not well received. But considering the rate of wetland loss, land subsidence, sea-level rise, and increasing frequency and severity of storms, the question should be on the table."
Coast 2050-style measures carry some significant complications. For one, massive superlevees such as those being raised in Osaka, Japan, would be needed and would overrun city streets and private land. The alternative would be to connect the barrier islands and outer marshes with large levees, dams and floodgates, creating a continuous rim around the delta. After a 1953 North Sea storm sent a surge into the Netherlands that killed 2,000 people, the Dutch government agreed to that very kind of network, which today safeguards 400 miles of coast. Joop Weijers, a senior engineer at the Dutch Ministry of Transport, Public Works and Water Management, which oversees the installation, known as Delta Works, says that "building the whole system right now would cost $15 billion to $16 billion."
A front wall against the Gulf could condemn to death the wetlands behind it, by changing the tidal mixing patterns of saltwater and freshwater. Weijers admits that in the first decades of Delta Works marshland was lost. To prevent that, the Dutch engineers changed strategy; instead of installing solid dams, they erected a long series of sluices whose huge doors remain open year-round, allowing the sea in. The doors close only when storms approach. Other retrofits are creating more natural water flows. As a result, the loss of wetlands has stopped.
Insight can also be gained from storm surge work going on in Venice, Italy, says Rafael Bras, a civil and environmental engineer at the Massachusetts Institute of Technology. The venerable city will rely on mobile floodgates in its lagoon that lie flat on the seabed under normal conditions and rise only during extremely high tides. Further lessons, he notes, are coming from current efforts to restore natural freshwater flows throughout the Florida Everglades and from the successful restoration of 15,000 acres of wetlands in south San Francisco Bay.
At the same time, "we need a new vision to revive southeastern Louisiana," says Denise Reed, a marsh specialist at the University of New Orleans. Even if a Mississippi version of Delta Works defends wetlands, certain Coast 2050-style projects will still be needed to negate damage done. The region produces one third of the country's seafood and provides wintering for 70 percent of the nation's migratory waterfowl, yet at least 25 square miles are still disappearing every year. And Katrina may have accelerated the destruction. Although investigations are just beginning, Reed expects that Katrina's surge pushed saltwater much farther into the marsh than normal. "That salt has likely baked into the soil," Reed says, "and won't readily wash out," which could kill wide areas of marsh grass from the roots up.