Nope.
http://www.csmonitor.com/2005/1103/p02s02-ussc.html
"We found dozens of breaches throughout the levee system," says Peter Nicholson, who leads the American Society of Civil Engineers' levee-assessment team."
ASCE senate testimony: http://www.ewrinstitute.org/files/pdf/katrinalevees.pdf
"Rather than a few breaches through the floodwalls in
the city caused largely by overtopping, we found literally dozens of breaches throughout
the many miles of levee system. A number of different failure mechanisms were
observed, including scour erosion caused by overtopping, seepage, soil failure, and
piping."
"Where the storm surge was most severe, causing massive overtopping, the levees
experienced a range of damage from complete obliteration to intact with no signs of
distress."
Two specific breaches were investigated and failed in the way you describe:
"Finally, three major breaches, and at least one significantly distressed levee-floodwall
section, were investigated at sites along the 17th Street and London Avenue canals
which, as explained before, were clearly not overtopped.
Obvious soil failures within the embankment or foundation soils at or below the bases of
the earthen levees had occurred at two of the breaches. At the distressed section,
seepage and piping were evident. These types of soil instabilities appear likely to have
been responsible for failure of these wall systems."
Because two breaches failed in the way you described doesn't mean there were only two breaches, or that the sewer company ruined everything for everyone. It's possible the root cause was the panels weren't sunk low enough in the first place. It's definitely the case that there were other failures with other failure reasons.
Wikipedia looks like a decent place to start for an overview, but as always you'll need to check their sources. http://en.wikipedia.org/wiki/2005_levee_failures_in_Greater_New_Orleans