New Orleans arms itself against the water
Temporary storm surge barrier and clever compartmentalization
In August 2005 Hurricane Katrina flooded around 80 percent of the city of New Orleans in a very short time. The New Orleans District of the US Army Corps of Engineers (USACE) was commissioned to provide protection for New Orleans against a hurricane with a recurrence period of one hundred years by 2011. An important part of the plan is the construction of a storm surge barrier on the east side of the city, but it will be three or four years before it becomes operational. Royal Haskoning carried out a study into the hydraulic effectiveness of two alternatives in order to reduce the effects of a hurricane in the interim period: raising the present levee and constructing a lower storm surge barrier as a precursor to the final barrier. .
A large proportion of the inhabitants of New Orleans live on three polders that lie below sea level. Several shipping channels with open links to the Gulf of Mexico run alongside and between these polders. There are now around 600 kilometres of levees and retaining walls around New Orleans. Mathijs van Ledden is advising the US Army Corps of Engineers on the technical design and the implementation of a water barrier programme. ‘The most important lesson from Katrina is that the entire flood defence system consisted of separate components. That’s one of the reasons why it went so dramatically wrong,’ explains Mathijs van Ledden. ‘So we are proposing a system-focused approach, in which strategic choices are being made on the basis of safety risks and cost considerations.’ The reinforcement programme for the levees around New Orleans will cost 15 billion dollars. Twenty percent of the flood defence programme has already been completed.
Storm surge barrier
The proposed storm surge barrier on the east of the city will not be ready until 2011. In order to increase safety until then, at USACE’s request Royal Haskoning has determined the hydraulic effectiveness of raising the levees or constructing a lower storm surge barrier ahead of the final barrier. Van Ledden continues, ‘We used results from advanced models to establish the height of the water levels. A wave model was also used to determine the wave height and wave period. In total 152 different hurricane situations were calculated. What is unique is that our methodology takes explicit account of the hydraulic uncertainties in the barrier design itself, such as the water level and the wave height.’
Connected system
Royal Haskoning’s study reveals that a temporary storm surge barrier is the most effective solution for this particular situation. The water levels in the channels would be significantly lowered, and as a result the volume of water that flows into the polders would be drastically reduced. The temporary storm surge barrier could be ready for the 2009 hurricane season. ‘But you mustn’t look at the storm surge barrier in isolation from the other protective measures,’ warns Van Ledden. ‘The storm surge barrier is not a solution for all the weak links. Strategic choices are inevitable in order to make this city safe in an affordable and effective manner. Giving all areas a hundred year protection level is a good first step. We have advised them to look beyond 2011 and to think about which areas we want to offer better protection to in the long term in order to take that into account in the current design.’
Clever compartmentalization
Royal Haskoning also looked into the idea of dividing the central part of the city into various sub-polders or compartments. Van Ledden explains, ‘This is called “from bowl to muffin pan” here. The enormous pan turns into a jam tart tin, and this limits the damage from possible flooding. Exploratory calculations indicate that the principle could be effective for the New Orleans Metro area. But in order to get a good idea of the pros and cons, we still need a good cost comparison with other alternatives such as increasing the height of the outer levee.’
Contact: Mathijs van Ledden
+31 (0)6 52 36 19 87
m.vanledden@royalhaskoning.com
