Go ahead and imagine it, this is why we're asking the Dutch for technical help.
A storm the size of Katrina would have encompassed all of northwestern Europe from Denmark to Brittainy. The Mississippi Coastline is longer than the joint coastlines of the Netherlands, Belgium and Northern France to Dieppe. It is also flatter. The wall of water would have pushed up the Scheldt to Antwerp and flood The great storms of the north sea are not to be despised, but not even the storm of 1953 was anything like Katrina.