From "The Steady Buildup to a City's Chaos" comes this explanation of what I saw on the TV at the time:
Army Corps officials were trying to close the gaps in the levees [on Aug. 30], but their hurried efforts to stem the flow were hampered by a lack of supplies. They could not find 10-ton sandbags or the slings they needed to drop the bags from helicopters; most of their personnel had evacuated, and so had their local contractors. "We didn't expect any breaches," Dan Hitchings of the agency's Mississippi Valley Division later explained. "We didn't think we were going to have a wall down." The corps tried to drop smaller sandbags into the 17th Street breach, but they simply floated away with the current.
Note that, as pointed out here before, what actually failed were the flood walls and not the levees. So, there's a difference between a "wall" and a "levee", but whether the walls were expected to fail or just the levees is not known.