Gustav has been and gone now. On August 31, the National Hurricane Centre gave it an 81 percent chance of remaining at a Category 3 hurricane or higher. The Saffir-Simpson scale rates hurricanes according to wind speed and storm surge on a scale of 1 to 5 (with 5 the worst). A category 3 hurricane brings with it sustained winds of between 111 and 130 mph and while it’s in the middle of the scale, it still packs a punch. The destructive winds associated with a Category 3 can cause significant structural damage and anything lacking a solid foundation is usually destroyed. For a city like New Orleans, with almost half its area below sea level, it is not the winds that cause the damage; it is the associated storm surge. Gustav was expected to bring with it a 9-12 foot storm surge. Given that the levees protecting New Orleans are around 23 feet high, there weren’t expected to be problems. At a minimum, there should have been an 11 foot margin of safety. Wishful thinking.
You just need to look at a side-elevation of the New Orleans levee system to appreciate what a precarious position the city is in. A report by the American Society of Civil Engineers in the wake of Katrina found that the city is built on an unstable mixture of soft sand, silt and clay. Worse still, the flood control structures keeping out the Mississippi are causing subsidence, not allowing the river’s natural silt deposits to replenish those that are continually being washed away. On top of this, the Mississippi is not at sea level and the average annual high water level is 14 feet. It doesn’t take much of a calculation to work out that a couple of unfortunate coincidences are going to cause a lot of trouble for New Orleans. High water plus a hurricane above Category 2 is likely to breach a levee that has already been structurally weakened as a result of the damage sustained during Katrina.
So, what happened with Gustav?
Despite the doomsday predictions of Mayor Nagin, Gustav was thankfully not the storm of the century and the city managed to escape without major damage. For once, Mother Nature smiled on New Orleans. Two days earlier, Gustav was a Category 4 monster (the same as Katrina) bearing down on Cuba. Two landfalls in Cuba seemed to knock the stuffing out of Gustav and some cooler air over the Gulf of Mexico saw the wind speed decrease from 135 mph to 115 mph as it came ashore near Grand Island, Louisiana. This further weakened Gustav to a Category 2 hurricane and about six hours after making landfall it had dissipated into tropical depression.
Despite this, the storm surge was sufficient to cause extensive flooding and went very close to breaching the New Orleans levee system. 110 deaths have been attributed to Gustav in the Caribbean and the US (compared to about 1,800 for Katrina) and the risk modelling firm Equecat Inc has estimated that it could cost the US insurance industry around $9 billion. This puts it among the ten costliest storms in US history. And this was only a Category 2 hurricane.
There is little doubt that the current levee is inadequate to protect New Orleans in the event of a major hurricane (classified as category 3 and above). A category 4 hurricane will bring a storm surge over the top of the levees and cause the widespread devastation that we saw with Katrina. Katrina cost nearly $70 billion in damage and this is likely to be a minimum cost for similar storms in the future. The only saving grace is that the advance warning systems and appropriately coordinated and managed evacuation plans should ensure that we do not see a repeat of the loss of life that we saw with Katrina.
Personally, I consider New Orleans an unsustainable location for a city. Subsiding foundations, the erosion of the natural defensive barrier of the offshore islands and a substantial chunk of coastline, a weakened levee system combined with a city that is substantially below sea level are a recipe for disaster in themselves, but combining that with a city in what is effectively hurricane alley means that it is not a question of if’, but when’.
A ringing endorsement of this view is the fact that as of July 2007, New Orleans only had around 60 percent of the population it had before Katrina. Perhaps it is time to stop fighting Mother Nature and look at a more viable location for the city.