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Page last updated at 18:58 GMT, Sunday, 31 August 2008 19:58 UK

How good are New Orleans defences?

By Joe Boyle
BBC News

A huge storm surge caused by Hurricane Katrina in August 2005 deluged New Orleans, overwhelming the city's flood defences. As another powerful storm approaches, could the same thing happen?

Rebuilt floodwall (top) compared with floodwall after Katrina
The city's floodwalls were swiftly repaired after Katrina

Flood protection in New Orleans has been a hot topic for more than four decades - since Hurricane Betsy devastated the city in 1965.

After Betsy, a new system was proposed - but many of the most ambitious measures were shelved and a more modest programme of repairing levees and building concrete floodwalls was undertaken.

Katrina laid bare the inadequacies of these measures.

A storm surge pushed up the canals from Lake Pontchartrain and Lake Borgne - the sheer force of the water buckling and breaching the floodwalls and smashing through levees.

In other areas, the surge came over the top of the walls and crashed down on to the soil, wearing it away in a process known as scouring, weakening the walls and allowing them to be breached.

Heavy rain added to the problems and about 80% of the city was flooded.

100-year storm

The city's mayor, Ray Nagin, explained that the breaching of levees and floodwalls was such a concern because New Orleans was shaped like a bowl, with the centre of the city being the area furthest below sea-level.

So if water comes over the top of the defences, or a storm dumps huge amounts of rainwater, the city centre is vulnerable to severe flooding.

We're still aware that a large enough storm surge will inundate the system
Major Tim Kurgan
Army Corps of Engineers

The Army Corps of Engineers, charged with rebuilding the city's defences, say they managed to get New Orleans back to pre-Katrina levels within a year of the disaster.

"But we didn't just get back to pre-Katrina levels and stop. We kept on ploughing on," says Major Tim Kurgan, the Corps' spokesman.

He says levees and floodwalls were repaired - many of them raised and strengthened with steel rods plunging more than 100ft (30m) into the ground.

Learning the lessons of Katrina, floodwalls were protected from scouring, and crucially, floodgates were installed at several points in the city's canal system.

Should the gates be closed, new water-pumping stations - described by Mayor Nagin as the "best in the world" - would begin pumping rainwater out of the canals.

By 2011, the Corps aims to have enough protection in place to cope with a so-called 100-year storm - a severe weather event with a 1% likelihood of occurring each year.

'Funnel effect'

Yet doubts persist about the city's defences, and the Corps' work has been under constant scrutiny.

As Hurricane Gustav approached, homeland security chief Michael Chertoff said the city was better prepared than it had been before Katrina.

But he added that there was a "real possibility" of water coming over the top of the flood defences if Gustav hit with the force of a Category Four storm.

"Rain is a big factor here. Even if there is no overtopping, you may get some flooding," he said.

Mayor Nagin expressed particular concern over parts of the city where flood-defence work was ongoing.

He said Lake Borgne on the east side tended to create a "funnel effect" and force water into the Industrial Canal - which caused huge flooding during Katrina.

And he said levees in the West Bank area had not been armoured or raised to the same level as those on the East Bank.

He added that construction was taking place in the Harvey Canal, but defences there still had several "gaps".

The mayor warned that these issues could lead to severe flooding.

Despite there being more to do, Maj Kurgan said he was confident in the work the Corps had done.

"The defences are better than they ever were. But we're still aware that a large enough storm surge will inundate the system."

New Orleans map




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