I think with Isabel the angle at which it hit the coast was significant. Hurricanes very rarely take dead aim on the coast in those regions (NC, VA, MD). They are usually in the process of recurving. Isabel plowed straight into the coast and up the chesapeake making the surge worse. I also think Katrina was a good example of how powerful hurricanes can "bank up" water in addition to the predicted storm surge simply by prolonged high wind speed in the same direction as the coast. Of course, thats my observation...I'm no expert