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Evacuation remains essential to New Orleans flood plans

Engineers this week warned that New Orleans’ residents will still have to flee the city in the event of another Hurricane Katrina-style event, despite a massive $15bn (£9.7bn) investment in flood defences.

Defences are being built to reduce the risk of storm damage from a 1 in 100 year event even though the city was struck by the 1 in 400 year storm five years ago this week.

This system is now well advanced. However, the Hurricane and Storm Damage Risk Reduction System (HSDRRS) will only reduce the risk of damage from, rather than fully protect against, a 1 in 100 year storm.

The floods that followed Katrina killed close to 2,000 people.

“I can only speculate but if you had an exact replication of Katrina tomorrow the levees wouldn’t wash away as they did and less water would get in [to the city]. But it’s likely people would die if they didn’t leave,” said Arcadis Corps Projects vice president, program manager Dan Hitchings.

Consultant Arcadis is working on the key design contracts for flood and storm structures within the HSDRRS.

Louisiana has improved evacuation plans via contra-flows and widening routes out of the city, to ensure full protection of human life.

A 1 in 100 year standard is acceptable to insurance firms and the federal government. Despite the threat from hurricanes to£65bn worth of infrastructure in Louisiana, it is considered too costly to create more robust protection.

“I don’t know that there is anyone reviewing the [1 in 100 year] standard,” said Hitchings, adding that the Coastal Protection and Restoration Authority of Louisiana had recently submitted a technical evaluation of the risks of such storms to Congress, but that Congress has expressed no interest in receiving recommendations for reviewing standards.

“The annual budget for the US Army Corps of Engineersis £2.9bn. You think the rest of the country is going to throw £32bn – 10 years of the Corps’ budget – just at Louisiana?”

Dan Hitchings, Arcadis

The HSDRRS includes work on 218km of levees and flood walls, 73 pumping stations, three canal closure structures and four gated outlets.

“It costs tens of billions of dollars just to do the minor things,” said Hitchings.

“The annual budget for the US Army Corps of Engineers [responsible for building up the country’s flood walls and levees] is $4.5bn [£2.9bn]. You think the rest of the country is going to throw $50bn [£32bn] – 10 years of the Corps’ budget – just at Louisiana?”

The Corps had been carrying out work on a smaller version of the scheme when Katrina hit. But work had not finished, and where it had, levees still suffered breaches in the aftermath.

“Prior to Katrina, flood protection in New Orleans was considered much more than a 1 in 100 year [system]. That’s why this [the HSDRRS] changed from a flood protection to a risk reduction system,” said Hitchings.

“It’s substantially stronger than it was five years ago and in most places the levees and walls are higher than they were before. But it is very difficult to project the future.”

“Built-in redundancy”

Others said the new system had built in redundancy that meant it would be more robust than was suggested by the design standard used.

“It’s robust for a 1 in 100 year system,” said US Army Corps resident engineer Vic Zilmer of the 3km long Lake Borgne surge barrier, which is one of the most important structures in the system. “[The barrier] is close to the equivalent of a 1 in 10,000 year Dutch design.”

Arcadis global director of water resources Pieter Dircke said the experiences following Katrina had acted as a “wake-up call” for flood risk assessment in the Netherlands, despite having robust and extensive flood defences.

Redundancy within the HSDRRS – boosted by a need to speed up construction and therefore over-engineer structures to ensure their structural integrity – has created a 1 in 500 year resiliency level.

This means that the system could be overtopped in another Katrina-size storm, but it would not be breached.

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