One in 300 year defences seriously under-designed

FAILED FLOOD defences in New Orleans were massively underdesigned, a leading Dutch flood protection engineer said this week.

They only anticipated storm events of one in 300 years, while those in Britain and the Netherlands are built to withstand one in 1,000 year events at least.

Theo Stoutjesdijk, a senior advisor on water defences at Dutch geotechnical consultant Geodelft, said that principal dykes along Holland's coast and major rivers are designed for one in 10,000 year storm events.

'And there are debates about whether that level of protection is enough, ' he added.

The system of bunds and barriers protecting the Thames Estuary is designed to a one in 1,000 year standard.

But levees protecting New Orleans anticipated one in 200 to one in 300 year events.

This meant that the levees would inevitably be overtopped in the event of a major storm surge, such as that accompanying Hurricane Katrina.

Storm surges are bulges of water that are created by powerful storms. When they coincide with high tides they can overtop and destroy flood defences, causing devastating floods.

The Dutch flood protection standard was adopted after the 1953 storm surge that swept down the English Channel, killing hundreds in Holland, Belgium and low lying areas of England.

'Along the coast we can cope with water levels 6m above normal, and waves of 6m to 9m on top of that, ' said Stoutjesdijk.

'But New Orleans' flood event strategy appears to have been based more on evacuation than on protection.' US Army Corps of Engineers liaison officer John Hall said that New Orleans' defences had been designed to cope with 'a mild category three hurricane'. Katrina was category four.

Of the 25 hurricanes to have hit the state of Louisiana in the last century, eight have been category three, with four measuring category four to five.