BioEd Online

The flood waters in New Orleans are, at last, subsiding. The US Army, given the dirty task of plugging the levees and pumping out the water, has announced that the city should be dry by 18 October. But experts are still left with the question of how to stop such a disaster from happening again.

It is no secret that the levees were never designed to meet the once-in-a-lifetime force of a storm like Katrina. To repair the damage and allow draining to get under way, troops have been busy dropping sandbags and concrete blocks into the levees' yawning gaps, most notably the fatal 100-metre rift in the walls of the 17th Street canal.

More than 70 pumps are currently churning 300 cubic metres per second out of New Orleans' sodden streets. And Dan Hitchings, a spokesman for the US Army Corps of Engineers, which is leading the effort, says that with no further heavy rains the drainage will be complete well ahead of initial estimates.

But as thoughts turn from short-term relief to long-term rebuilding, engineers and city officials are considering how to replace the patched-up levees with something more permanent.

Dyke works

One country with a wealth of experience in this area is the Netherlands. In 1953, a flood that killed some 1,800 people hastened the start of a huge flood-protection project called the Delta Works. This guards the low-lying country with four 40-metre-high dykes (up to 10 metres of which are above mean sea level) that withstand a constant pressure of water, and an 11.5-metre storm-tide barrier for use in severe storms.

New Orleans, which faces a lake rather than the battering tides of the North Sea, had dykes of 4 metres, and no storm-tide barrier at all.

What New Orleans needs to do first is rebuild levees over the mishmash piles of rubble currently being constructed, says Jos Dijkman, a flood-management specialist with the Dutch research institute Delft Hydraulics. "This has been a rough and dirty job to plug the hole," he says. "It's not the ideal cross-section of a brand-new levee."

A good levee should consist of a ridge of sand covered with good-quality clay that will resist water and not crack if the structure subsides with time, Dijkman explains. On top of that, an outer covering of grass or rocky blocks is laid to protect the levee from water erosion.

The levee's dry side should have a gentle slope, Dijkman adds, to minimize damage should the waters slop over its top. If flood waters brim over the levee, a steep slope causes the water to cascade more quickly down the other side, increasing its power to erode. New Orleans's levees, some of which were vertical walls, are thought to have toppled in exactly this way.

Obviously there is a trade-off between space and protection: levees with shallower slopes take up more room. Dutch dykes maintain a 33% slope on their dry sides, so that any overflowing flood water forms a gentle trickle rather than a rushing torrent. The Japanese go even further: flood barriers there are constructed with a 1% gradient, and houses are simply built on this well-drained slope.

Perhaps building this type of sprawling flood barrier, which effectively raises the ground level of a strip of the city, would be a good move for New Orleans, Dijkman suggests.

Storm specials

Another point to consider is whether New Orleans will want a special storm barrier.

The city came close to building one in the late 1960s. After Hurricane Betsy struck the city in 1965, Congress approved some $85 million (half a billion dollars in today's terms) to improve defences. But the project, having been signed into law by President Lyndon Johnson, was abandoned after a 1977 legal challenge on the grounds of adverse environmental impacts.

The project would have seen the construction of two huge gates, the larger some 240 metres long, capable of closing off Lake Pontchartrain from the Gulf of Mexico.

It would surely have stopped the 3.65-metre surge in the lake's levels that had such disastrous results, says Joseph Towers, a former member of the army's engineering corps. "If we had built the barriers, New Orleans would not be flooded."