Huuricane Katrina - What Went Wrong in New Orleans
Many of the New Orleans levee and floodwall failures in the wake of Hurricane Katrina occurred at weak-link junctions where different levee or wall sections joined together, according to a preliminary report released by independent investigators from the University of California, Berkeley, and the American Society of Civil Engineers (ASCE).
Researchers found significant erosion and inconsistencies in crest heights at "transition" points where the earthen embankment meets up with concrete structural sections in the levee system. The disparities in levee sections exist, the report points out, because multiple authorities are responsible for designing and maintaining different sections of the levee system.
In addition, areas where the perimeter flood protection system was disrupted to allow passage of trains or other surface transit created other points of vulnerability.
"Levees are 'series' systems, where the failure of one component means failure of the whole system," said Seed in his congressional testimony. "They have less redundancy than many other engineered systems, and the consequences of failure are high."
Other findings include:
* While storm surges resulted in water rising over the tops of and overwhelming numerous levees and floodwalls, three major levees along the banks of the 17th Street and London Avenue canals failed at water levels below the tops of the floodwalls. The failures at these levees were not caused by overtopping, but instead were likely caused by weaknesses in the soil underlying the foundation of the levees.
* Piping erosion, characterized by water tunneling through the soil, possibly due to or worsened by the uprooting of trees on or near the levee, may have contributed to the London Avenue Canal failure.
* Many of the levees and floodwalls that failed due to overtopping could have performed better had relatively inexpensive modifications been implemented.
The investigators said the addition of erosion protection -- including concrete splash slabs, paving of the ground surface in key locations, or piles of rocks placed on top of the soil -- on the back or land sides of floodwalls might have helped protect the levees that were overtopped.
"The performance of many of the levees and floodwalls could have been significantly improved, and some of the failures likely prevented" had such erosion prevention measures been taken, Seed testified before the committee, which is chaired by Sen. Susan Collins (R-Maine) and Sen. Joe Lieberman (D-Conn).
The failures and damage observed at the flood protection system's transition points call for an overall review of the design of the entire system, said Seed. As the Army Corps of Engineers works to repair and rebuild the New Orleans levees, the investigators suggest that an independent board of consultants be retained to review the adequacy of the interim and permanent levee repairs now underway. The report notes that such boards are commonly used by public agencies to review the design and construction of major dams.