That's true, but Katrina was an anomoly. It weakened quite a bit at the last minute, but was still pushing water from it's more powerful phase. The second largest recorded storm surge was around 23' -- associated with Cat 5 Hurricane Camille in 1969.
"Irene's storm surge potentially extremely dangerous for the mid-Atlantic coast
Irene's large size, slow motion, arrival at high tide, and Category 3 strength at landfall in North Carolina will likely drive a storm surge of 8 - 10 feet into the heads of bays in Pamlico Sound, and 3 - 6 feet in Albemarle Sound. As the storm progresses northwards, potential storm surge heights grow due to the shape of the coast and depth of the ocean, though the storm will be weakening. If Irene is a Category 1 storm as it crosses into Virginia, it can send a storm surge of 4 - 8 feet into Chesapeake Bay and Norfolk. I give a 50% chance that the surge from Irene in those locations will exceed the record surges observed in 2003 during Hurricane Isabel. The region I am most concerned about, though, is the stretch of coast running from southern Maryland to Central New Jersey, including Delaware and the cities of Ocean City and Atlantic City. A Category 1 hurricane can bring a storm surge of 5 - 9 feet here. Irene's large size, slow movement, and arrival at the highest tide of the month could easily bring a surge one Category higher than the storm's winds might suggest, resulting in a Category 2 type inundation along the coast, near 10 - 15 feet. This portion of the coast has no hurricane experience, and loss of life could be heavy if evacuation orders are not heeded. I give a 30% chance that the storm surge from Irene will bring water depths in excess of 10 feet to the coasts of Maryland, Delaware, and New Jersey."
Dr. Jeff Masters' WunderBlog : Irene an extremely dangerous storm surge threat to the mid-Atlantic and New England : Weather Underground