I wonder if ultimately the whole levee can be encased in concrete? Seems to me that forms could be built and then pump it full of concrete.
But what do I know....
Actually, it is easier than that. All they need to do is create a backing behind the levee, plant steel rebars and pour concrete in the water between the backing and the levee. This is usally done with slurry (I think that is spelled) but it should work. The concrete will go to the bottom and eventually displace the water to the point where there is nothing but concrete. Then let the concrete dry. And you have a concrete steel re-enforced levee behind the original levee which should now beable hold the water back, as water is pumped out of the city.
Unless with water seeps up through the ground, which could happen in New Orleans. In which case, you have a whole new set of problems to work out.