The Army Corps of Engineers has released this statement:
Q. Why did the levees fail?
A. 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.
The vertical walls are supported on the city side by 45-degree embankments of earth. Not concrete, but earth. In a few places, water poured over the top of the wall and eroded the earth embankment below. Without the bank support, the wall collapsed.
The walls didn't burst under extra pressure from the storm surge, but from the diminished structural support on the city side. So the importance of support
from those banks of earth seems critical. Now here's the point I haven't read anybody raising yet:
All these supporting banks of earth on the city side will have been totally submerged and saturated for weeks before pumping begins. They will be mud. Even assuming no erosion, they will be mud. And mud yields to pressure. It oozes away. Will walls supported by mud withstand the pressure of the lake?
I'm not an engineer, but given what we've seen, the possibility of widespread wall collapse seems kinda nontrivial, at least for the earth-supported sections.
Has anyone found any sort of engineering discussion forum where this sort of stuff is being batted around?