Among the lessons driven home by Katrina 
is that a coastal city below sea level is a city at risk. Levees can only do so much; the only guarantee of protection from the ocean is to be above it.
New Orleans was not this way originally. The first settlers would not have put themselves in a place where levees and pumps were required just to throw up a cabin; the land on which the city was built was originally above sea level. But the construction of the city and the prevention of the normal deposition of silt which built the land halted the process which kept it that way, and the pumping of groundwater plus the normal consolidation of silt left the city sinking. I've read that some parts of the city are 16 feet below sea level, and subsidence runs as fast as a half-inch per year.
It would be desirable to have a city that is once again above the water. To do this, the land beneath it would have to be built up by an average of about eight feet and then continuously over time. But how to do that?
Let it flood, let it flood, let it flood
One possible method is to let floods do it as they did before. One way engineers have handled flooding is to lift buildings out of harm's way and otherwise let nature take its course. In some areas, houses have been raised on pilings to allow floodwaters to pass harmlessly beneath; the new basement area is enclosed with blow-out panels which prevent harm from the force of flowing water.
Allowing annual floods to pass over and through an "uplifted" New Orleans would be an inconvenience, to be sure. While it might be feasible to commute in boats during the wet season, clearing mud from streets afterward would be a chore perhaps more onerous than snow removal and most typical landscaping would suffer. Further, adding up to sixteen feet of material to bring the lowest parts up to sea level would take a very long time and require several intermediate reconstructions of roadways. Provision of utilities to buildings with "wet feet" would also be a challenge, and annual or more frequent interruptions in service would seriously affect the habitability of even dry buildings in the mean time. It would be better to do the job more quickly than that, or at least in just one step.
90% of everything is crap
Or so was the soil beneath habitations until fairly recently. In an ancient city, subsidence would not have been so much of a problem. Without garbage removal, much of the ephemeral goods which came to a city tended to remain there as refuse. It built up in layers, incorporating all sorts of bits of interest to archæologists, and elevated the entire city over time. As first floors became cellars, new buildings were constructed above the levels of the old and the entire city climbed with time. A sufficiently slovenly populace might keep pace with subsidence and maintain itself above sea level in a river delta even without the benefit of flood-borne silt.
We would never accept this today. On the other hand, it would not be necessary (or even desirable) to use garbage or compost. The river delivers huge quantities of silt each year to the area, which the Army Corps of Engineers spends an equally huge amount of effort to dredge out of shipping channels. Once dried somewhat, this silt forms excellent soil. Why not use it?
It appears likely that many of the lowest areas of the city are destroyed, perhaps beyond repair. Absent billions for higher levees they will be at high enough risk to perhaps be uninsurable, and will take tens of billions to reconstruct in any event. It makes little sense to spend so much money just to put so much property and so many people back in harm's way; the rebuilt areas should be engineered to remain safe for the useful lives of the structures. This requires an immediate lift and the capability for on-going elevation. So here is my proposal for a reconstructed New Orleans:
Buildings too damaged to be worth saving would be razed. Buildings to be saved would be jacked off their foundations, new foundation piers installed beneath, and elevated above what is to be the new ground level; in the lowest areas this would be at least 16 feet and could be much more than that. The attachments between the piers and the beams would not be permanent; they would be detachable at any time so that shims could be added to elevate the building further. At the new ground level and perhaps at intervals above, tie beams would be added to keep the piers in alignment and allow for the attachment of cross-bracing; this would keep the piers from shifting under horizontal loads such as hurricane winds.
The process would go something like this:
(images unavailable due to Blogger bug, will try to add later)
Regions due to be razed in their entirety could be filled to a considerable height above sea level and then allowed to consolidate before beginning new construction; this would give better support to new road surfaces, driveways and sidewalks. Roadways might be engineered like boardwalks, made of floating segments loosely connected to each other and adjusted by additions of semi-liquid material beneath. Foundation piers should be extended to well-settled material to prevent the need for frequent levelling. As subsidence continued, periodic road work would be accompanied by less-drastic lifts of buildings on their piers, addition of spacers in the foundation posts, and filling of yards and other areas to maintain their proper elevation. As the city's soil sank, it would shim itself up to keep pace.
Would New Orleans be quite the same after this? Probably not, but even pre-Katrina it was not exactly the same as it was 50 years ago, or even 20. A shift from levee-building to whole-city elevation would be a radical change in process, but it would allow further change to proceed by increments rather than catastrophic jumps. It would allow for a great deal more stability and less worry. Isn't that what everyone wants?
: This lesson was only driven home this time. It was learned long ago, written up dozens of times, and published in everything from government disaster-preparedness studies to glossy popular magazines. Unfortunately, the people who had taken the money and assumed the responsibility to do something about it decided to procrastinate. (back)