Not to be controversial but I do have a picture from the 17th St breech, showing the overtopping scar as it was filled in during the creation of the new wall.
Without seeing that specific photo, I have talked with those engineers on the scene.  In the undermine scenario, the concrete plates would fall underneath the waves as the dirt disappeared underneath.  The intact concrete plates next to the breach showed debris lines a foot from the top.  Since all plates were within an inch of each other in height before Katrina, if it did not overtop next to the breach AND there is clear evidence of a sand boil in the yard of a neighbor, then this supports the undermining explanation.

Note, more than one breach on the 17th Street Canal (love the Quality control at the Corps of Engineers !), so there may be more than one mechanism for failure.

This is a typical overtopping scour (from the Industrial Canal). The width is about 3-4 ft and you can see the sheet piles exposed under the concrete wall.  The sheet piles are then moved forward and tilt.

This was from down at Port Sulfur. Note that the water flow has gouged out down to bedrock in front of the sheet piles.
The concrete slabs on top are then thrown forward

This is from the center of the 17th St failure. You can see the slabs fell forward and not back,

We have many photos that show similarities across the floodwall failures around NOLA and the delta, but these I can use, since they are mine, the rest will only be available when the report  is released.

I can likley get you additional photos taken on onsite Wednesday after Katrina fro 17th Street & Orleans failures.

eMail with directions (friend of mine with Public Works took them) Alan_Drake@Juno.com

I didn't know Louisiana had any "bedrock".  Most of my projects, we only hit stiff clay and oyster shells sometimes if we were lucky.   I worked a lot up at Taft and then down in Homa, Lake Charles, Lafayette, Morgan City and a lot we did only on piles set deep into muck with a build up of shell on the surface.  I thought most all of its the kind of gumbo you can't eat, clay gumbo; kinda' too thick to drink, but too thin to plow.
I may have got that wrong - I wasn't about to go down and look, though it did look like a rock slab it could have been a hard layer of clay, and it was fairly deep below the surface.
The core of levees should be clay, in order to stop water migration.
Everything in LA and East Texas is Clay Clay Clay, in varying degrees of "stiffness".  Sometimes you do see a pretty stiff layer that you'd almost think is rock, but usually its just clay with a lot of micro coral shells well embedded.