A New Orleans TV station has some information from the Army Corps of Engineers on its website. It looks like it will be weeks (at best) before all of New Orleans is dry again, let alone habitable. A blunt explanation is given for why the flooding is as bad as it is:

Q.2. Why did the levees fail?

A.2. What failed were actually floodwalls, not levees. This was caused by overtopping which caused scouring, or an eating away of the earthen support, which then basically undermined the wall.

These walls and levees were designed to withstand a fast moving category 3 hurricane. Katrina was a strong 4 at landfall, and conditions exceeded the design.

Q.3. Why only Category 3 protection?

A.3. That is what we were authorized to do.

Presumably, after billions are spent rebuilding New Orleans, billions more will be spent on a system of barriers that can withstand tougher conditions. In the meanwhile, let us focus on helping the hurricane victims.