Overall, has the system of levees along the Mississippi made communities along the river safer from flooding or have they exacerbated the problem?
This depends on where you look. Certainly, in a grandiose view of the entire MR&T system, it is much safer than it was in 1927 in terms of the flood threat posed to those areas that were then under cultivation, like the cotton fields surrounding Greenville, Mississippi. But, high-value areas like Memphis and places where engineering infrastructure crosses the rivers—bridges for example—we have constrained the channels with considerable severity, for which we later pay an exacting price. One of the biggest challenges in flood routing is shifting flow regimes. The Mississippi River has been narrowed down to just a 3,000-foot [900-meter] width where at passes downtown Saint Louis, when the natural low-flow channel, prior to Corps of Engineers involvement, was about 5,000 feet [1,500 meters] wide. When you squeeze the channels adjacent to densely populated urban centers, the result is usually back-eddy under-scour problems in the flow transitions upstream and downstream of such constructions. And, you heighten the flood depths in the very areas you are seeking most to protect. You have to "store" the excess floodwater somewhere. The big debate is always about where the somewhere else is going to be.
Are earthen levees still being built, or have they largely been replaced by newer technology? What are some of the alternatives—for example soil reinforcement?
Earthen levees are still being built because they are cheap, not because they are good. The average levee in the United States is 10,000 times more likely to fail than the average dam. That's not a matter of expert opinion. It's simply the actuarial figure. We can make levees more survivable by engineering them—hardly any levees are "engineered," per se—which means examining their respective foundation conditions and designing the dikes accordingly, instead of employing a one-size-fits-all mentality, which had been the norm until Katrina. There are all sorts of engineered measures that could be undertaken to make levees more resilient and survivable, if the will exists to do so.
That said, it is very tough to construct robust levees in the Mississippi–Atchafalaya delta because of grievous and pervasive ground settlement and low soil strength. When you stack up three feet [one meter] of earth to begin a levee, it settles about two and a half feet, due to exceedingly low bearing capacity. In those kinds of conditions, we need to develop methods and techniques that are more cost-effective than simply piling up earth like they did during the Civil War. This includes using clay as a binder. Clay wasn't used in New Orleans's levees prior to Katrina, so they didn't have any soil cohesion within the levees [making them more susceptible to erosion once they were overtopped].
What progress has been made in building levees more as energy-dissipation systems as opposed to simply being used as barriers?
I have been one of the advocates of energy dissipation systems because that is the type of system that has been developed by coastal engineers to design resilient breakwater systems. It could easily be applied to protective structures for hurricanes and tsunamis, which are much easier engineering problems than 100-year floods because of their very short duration. You only have to survive six to eight hours of overtopping, even in the largest hurricane.
What are "green" levees?
This is a term I've coined to describe a new type of protective barrier system, which would employ woody vegetation as a critical defensive component, along with a series of low-weight reinforced soil berms, which would be crafted to enhance recreation opportunities—hiking, biking and boating paths, for example—and be capable of surviving overtopping by the largest hurricanes.
View a slide show of the Army Corps of Engineers opening up the Bonnet Carre Spillway and blowing up a portion of the Birds Point Floodway.