Big Easy back to normal?
Engineer Mead Allison says it can happen, with help.
After Hurricane Katrina devastated New Orleans, coastal geologist Mead Allison was forced to relocate and stay with family. While much of his city was still under water, Allison spoke optimistically with Earth & Sky’s Jorge Salazar about rebuilding the Lousiana coast. Allison is associate professor in the Department of Earth and Environmental Sciences at Tulane University in New Orleans. (Updated April 2006)
Salazar: Can New Orleans get back to normal?
Allison: From an engineering standpoint, there’s no reason that at the end of this century, places like New Orleans couldn’t be thriving.
Of course, that’s up to the will of the American taxpayers, whether they consider that cultural resource important enough to spend the money to maintain those levee systems. I hope that people will realize that the reconstruction of New Orleans is not just an engineering of levees issue, but one that involves actual coastal restoration and a restoration of the adjacent wetlands in order to reach a successful conclusion.
There’s no reason why we can’t maintain the levee system around New Orleans indefinitely.
The Dutch have been living with similar problems in the Rhine–Meuse delta for centuries. They have areas that are behind levee systems that are far lower in elevation than anything in the New Orleans area. I think that something like one third of the Netherlands is actually below sea level and protected from the adjacent ocean by levee systems.
Salazar: What will New Orleans look like in the future?
Allison: There’s a consensus that if we could get back to where we were in the early part of the 20th century, in terms of wetlands area, we’d be doing very well for the health of the system. We would have much greater protection against major storms for places like New Orleans. That’s a pretty optimistic viewpoint. It takes a tremendous amount of sediment to build that much wetland.
There’s a more conservative group that says, can we get and keep it at the point it is now? That?s not necessarily saying to halt all wetland loss, but to build as much wetland as we’re going to lose in the future. That’s probably a lot more realistic impression.
Now, there’ve been some fairly alarmist things in the media saying, “Nothing’s going to be there in a century anyway. Why rebuild New Orleans?? I think those are extreme views.
The evidence seems to suggest that the wetlands loss rates have decreased substantially since the peak in the 1950s–1970s. That seems to be timed to the major period of oil and gas withdrawal.
Salazar: How can the post–Katrina Louisiana coast be rebuilt?
Allison: There needs to be a restoration of wetlands. And that means actually building new wetlands. But how do we rebuild wetlands in an area that is slowly sinking?
The answer is, we’ve got to tap into the sediments that are being carried within the Mississippi river. That can be done through what?s called diversion, where you cut a hole in a levee and allow the river to flow into an adjacent upward water region.
Or it can be done through a long distance pipeline. That?s where you stick a pipe into a river and suck sediments into a pipeline. The sediments are then carried for miles from the river channel. They?re then placed at the site where you actually need them to build new wetlands.
Salazar: What about the levees?
Allison: Even if we put the wetlands back to the way they were early in the 20th century, we still need protection from major storm surges from hurricanes. The levee system has to be rebuilt, at the very least, back to what it was pre–Katrina, and hopefully to an even stronger level to survive a storm as large or even larger than Katrina.
Salazar: How do hurricanes affect the wetlands of the Mississippi delta?
Allison: A hurricane like Katrina can do a tremendous amount of damage, mainly due to the wave attack. When a storm surge with large waves covers a marsh, that?s a lot of energy to cut away at the marsh surface.
There’s a fairly general agreement that if the marsh system were much more extensive, such as it was early in the 20th century before we had this tremendous spike in wetland loss, we would have had a lot more protection for New Orleans. That?s because the marshes and nearby barrier islands have a very strong effect on dampening the storm surge and lessening the threat to places immediately inland, like New Orleans.
Salazar: Have humans somehow speeded up New Orleans? subsidence, or sinking?
Allison: There’s been a lot of discussion of late about to what extant the human effect of oil and gas withdrawal has had on increasing the local subsidence rates. A large area of the wetlands of south Louisiana were a major focus of the oil and gas industry. Most of these areas are tapped out, and the oil industry is moving further offshore into the Gulf of Mexico into deep water, in search of oil and gas.
But particularly in the 1950s to the 1970s, there was a maximum in the rate of oil and gas extraction from the delta region of south Louisiana. And there also appears to have been a spike in the areas of wetlands that were lost. Wetlands loss rates in the 1950s and 1970s are higher than they are at present by a factor of two or three, which seems to suggest a linkage to the oil and gas industry.
But it shouldn’t be forgotten that the natural system in the Mississippi was one where this subsidence was balanced by the input of new sediments from the Mississippi river and its tributaries, like the Atchafalya. Even though you have a centimeter or two of subsidence on an annual basis, if you’re putting in new sediments and replacing them from material that’s coming out of the river, you can balance that. And wetlands can be retained, even in these high subsidence regions.
The reason that subsidence has been allowed to take over is that the river now is levied all the way down its course to its entrance point with the Gulf of Mexico. Normal river flooding that would have been the process by which these sediments get out into the adjacent wetlands has been halted. So, sediments are now being dumped directly into the Gulf of Mexico and aren’t being used to balance this subsidence issue.
