The flood levee system designed to protect New Orleans, US, has failed, allowing water into the city and adding to the damage already caused by Hurricane Katrina.

Water has poured in from Lake Pontchartrain through a 61m hole in a levee on the city’s north side, and vessels along the nearby Mississippi river reported a 30m breach in another on the south side of town. Downtown New Orleans is in a basin several metres below sea level and has depended for years on its system of pumps and levees to keep it dry.

Local reports suggest that the Louisiana city’s flood defences were not designed to deal with a category 5 storm like Hurricane Katrina, which had top winds of 160mph.

The natural disaster has laid waste to a 241km stretch of New Orleans, with Mayor Ray Nagin estimating that 80% of the city was submerged, with waters as deep as 6m.

Mayor Nagin has ordered the evacuation of the city, while the Pentagon sent six Navy ships and eight maritime rescue teams to the Gulf coast to help the rescue operation. Over 1M people could be left homeless.

Experts have warned about the vulenrability of New Orleans for years, especially since the state of Lousiana has lost over 4ha of coastal wetlands in the past 70 years. This great plain of swamps and bayous south of the city acts as a buffer to partly absorb waters that a hurricane pushes ashore.

There has also been criticism of the city’s system of surrounding levees, designed to keep flood waters out but with the twin effect of containing the vast stagnent pond that covers New Orleans now the city be has been breached. How to remove this unwanted body of contaminated water will be of paramount importance to authorities.

The economic cost of the Hurricane Katrina may be as high as US$26B, risk analysts estimate.