That information from tonight's CBS television 60 Minutes show. As of this time, the transcript/video are not posted on the 60 Minutes website; clearly the segment, hosted by Scott Pelley, was put together shortly before air time. For readers who are located in time zones where the show airs later than EDT, the discussion about the floodwalls comes near the end of the first segment.
The correction about the floodwalls will surely be all over the news within the coming days and adds another layer of complexity to the situation.
The problem with the floodwalls, according the expert who spoke to 60 Minutes, is that either they work or they don't. The expert said categorically that the floodwalls protecting New Orleans were only built to withstand a Category 3 storm.
If that information holds up, it had to be known to the state and local officials in Louisiana. This would mean they knew for absolute certain that the floodwalls would be breached. They would know as soon they received news of a Category 4 or 5 headed for the New Orleans portion of the Gulf.
That would mean they absolutely knew that without evacuation of the low-lying regions of the city, virtually all the people in those regions would be drowned or stranded on rooftops. They would also know that search and extraction from rooftops would be a slow, painstaking effort requiring specialized equipment.
Again, if it is true that it was the floodwalls that were breached, there would be no question that as soon as they got word of Katrina's track, the New Orleans officials and the state government would have known that the city had to be fully evacuated as a precautionary measure.
In other words, with the knowledge that Cat 4 or 5 would breach the floodwalls, they would have evacuate the city even if they were not 100% certain that the storm would make landfall exactly in the New Orleans region. Even a close call from a Cat 4 or 5 would flood the bowl in which New Orleans sits.
And keep in mind that New Orleans is already 6 feet below sea level. Thus, a 20 foot storm surge actually works out to a 26 foot surge.