The furious storm: one wild hurricane could drown a major American City. Can scientist prevent the disaster in time?

Science World, Oct 18, 2002 by Larry O'Hanlon

Here's a tip from the experts: If you're in New Orleans when the "Big One" hits, have a lifeboat handy. Some scientist warn that the right hurricane--a tropical cyclone with at least 74-mile-per-hour winds--could strike the Gulf Coast in a way that would hurl millions of gallons of water to turn the city known as the Big Easy into the Big Soup Bowl (see map, next page).

A major flood could submerge much of central New Orleans beneath 20 feet of water, leaving many of the metropolitan area's 1.3 million residents clinging to rooftops--a prospect that has engineers and city planners scrambling for defensive strategies. "It's the luck of the draw," says hurricane expect Hugh Willoughby at the National Oceanic and Atmospheric Administration (NDAA). He thinks it's a matter of when--not if--the Big One will pound New Orleans During some annual hurricane season between June and November.

Why is New Orleans so vulnerable? Try these three main reasons:

* Sandwiched between Lake Pontchartrain and the Mississippi River, most of the city lies below sea level. A flood that gushes over shielding levees (earthen walls built in the late 1800s to protect against river overflow) would submerge New Orleans underwater.

* Marshes, fresh and saltwater swamps of mud and diverse plant life, divide New Orleans from the Gulf of Mexico. They once acted as barriers from storm surges--high water accompanying storms. Now marshes are quickly eroding, or wearing away. This is partly because levees block and reroute the Mississippi's periodic flooding cycles, which spread mud and sediment (rock particles) that shore up marshes. In some places, the gulf has receded 32 to 48 kilometers (20 to 30 miles) closer to New Orleans.

* The number and intensity of Atlantic Ocean hurricanes tend to increase in cycles every few decades, experts say. "We've just entered a more active phase," says Willoughby (see "How Hurricanes Form," p. 24).

New Orleans hasn't always faced such danger. When first built in 1718, the city sat on higher land beside the Mississippi. But it was erected on soft river mud--a mix of silt (loose rock particles) and clay minerals--deposited over millions of years by flowing water at the delta, or mouth where the Mississippi meets the gulf. Trouble is, the soft ground beneath central New Orleans has sunk nine feet in nearly 300 years. (Most New Orleans skyscrapers are supported by deep piles, so they don't rely on the soft ground for support)

PERFECT STORM

Not just any hurricane could engulf New Orleans, Willoughby explains; otherwise, the city would have drowned long ago. New Orleans' nightmare will be a "perfect" storm--one that strikes in just the right way.

Every year, an average of five or six hurricanes that form in the Atlantic Ocean churn toward Central and North America--often with Florida and the Caribbean islands dead in their paths. But changes in wind direction and Earth's air currents cause most hurricanes to sweep around and roll up the U.S. East Coast, weakening as they move over colder seawater. About once a year, however, a hurricane stomps right over Florida, where warm water in the Gulf of Mexico can reenergize it as a monster storm, thrusting it westward.

The perfect storm could either strike New Orleans east of the city, with gale-force winds blowing south, shoveling water from Lake Pontchartrain over the lake levees; or the storm could strike west of the city, causing winds to heave Gulf of Mexico seawater up the Mississippi River and crash over its levees.

Joseph Suhayda, former director of the Louisiana Water Resources Research Institute at Louisiana State University, uses computer models to study potential hurricane hits. His surprising finding: A severe but not catastrophic Category 3 storm (see next page) would be enough to swamp New Orleans if it slowed down and hovered east of the city. "A slow storm has more time to build up the wind effect over the lake," says Suhayda. Waves can add four to five feet to surging lake waters, he adds.

RIVER WALLS

Engineers and city planners are racing to soften a hurricane's blow to New Orleans. In addition to hashing out elaborate evacuation plans, one strategy calls for slowing the loss of marshlands by building control gates. These would let the Mississippi overflow once again, spreading sediment-rich water to rebuild marshes. Another idea: shoring up barrier islands in the gulf. But no plan would remove the hurricane threat immediately, and public officials say costs for all schemes are prohibitive.

With early storm detection, most of New Orleans can be safely evacuated, Suhayda says. For those who can't get out, Suhayda offers his own controversial scheme: Engineers would construct a 20-foot-high east/west wall along the north edge of the French Quarter, which would seal off a downtown section. The existing Mississippi River levees would surround the "haven" on three sides and are high enough now for Category 5 hurricane protection. The sealed off "bowl" could provide safety for several hundred thousand people.