Not only not "unimaginable" but in fact predicted in great detail; LA times article
A warning sent but left unheeded - Los Angeles Times: "A warning sent but left unheeded
As commentators and public officials survey the morass of loss and desolation that once was a great American city called New Orleans, one of the words we hear and read over and over again is 'unimaginable.'

In fact, the tragedy that this week destroyed a vibrant metropolitan area that was home to 1.4 million people and the city proper that was a national cultural treasure was not simply imagined but foreseen with a prescience that now seems eerily precise."