Testimony of
Raymond B. Seed, Ph.D.
Professor of Civil and Environmental Engineering
University of California at Berkeley
On behalf of the
NSF-Sponsored Levee Investigation Team
Before the Committee on Homeland Security and Government Affairs
U.S. Senate
November 2, 2005
II. Why Did the Levees and Floodwalls Fail?
This is a map of the central New Orleans region, prepared initially by the U.S. Army Corps of Engineers and then modified to reflect additional findings of our investigation teams. It shows the locations of many of the levee breaches that occurred, and serves as a good base map for our discussions today. Not shown on this map are the additional flood protection levee systems that extend down the lower reaches on the Mississippi River, providing a narrow additional protected corridor down to the Gulf.
The storm surges produced by Hurricane Katrina resulted in numerous breaches, and consequent flooding of approximately 75 percent of the metropolitan areas of New Orleans. Most of the levee and floodwall failures were caused by overtopping, as the storm surge rose over the tops of the levees and their floodwalls and produced erosion that subsequently led to failures and breaches.
Overtopping was most severe at the east end of the flood protection system, as the waters of Lake Borgne were driven west producing a storm surge on the order of 18 to 25 feet that massively overtopped levees immediately to the west of this lake. This photo shows one piece of a six mile section of levees at the northeast corner of the MRGO channel that were massively overtopped and eroded by this storm surge, which then sent floodwaters racing towards St. Bernard Parish. There is virtually nothing left of these levees along some parts of this stretch.
A very severe storm surge also occurred farther to the south, along the lower reaches of the Mississippi River, and significant overtopping produced additional breaches in this region as well. This photo shows homes that were carried across the narrow protected corridor in southern Plaquemines Parish by a breach on the west levee, and then thrown astride the crest of the Mississippi Riverfront levee.
Overtopping was lesser in magnitude along the Inner Harbor Navigation Channel and along the western portion of the MRGO channel, but the consequences of this overtopping were again severe. This overtopping again produced erosion and caused numerous additional levee failures. This photo shows the well known breach at the west end of the Ninth Ward. We spent some time figuring out the answer to the chicken and the egg question, and it is our preliminary opinion that the infamous large barge was drawn in through a breach that was already open.
Most of the failures in this central New Orleans area were the result of overtopping, and one of the common failure modes was simply water cascading over concrete floodwalls and then carving sharply etched trenches at the back sides of these walls. This reduced the lateral support at the back sides of the walls, and left them vulnerable to the high water forces on their outboard faces.