Where the storm surge from Hurricane Katrina was most severe, damage to New Orleans levees ranged from obliteration to intact with no signs of distress, a leader of the engineering team assessing the levees told the U.S. Senate.
Much of the difference in the degree of damage can be attributed to the types of levees and the materials used in their construction, said Peter Nicholson, assessment team leader for the American Society of Civil Engineers and associate professor of civil and environmental engineering at the University of Hawaii. The majority of the destroyed or heavily damaged earthen levees were constructed of sand or “shell fill,” which was easily eroded. Soil failures found within embankments or foundation soils at or below the bases of the earthen levees were responsible for the levee system’s collapse.
Technical assessment teams from ASCE and the University of California, Berkeley, jointly released a preliminary report Nov. 2 on the performance of the New Orleans levees.
Nicholson explained the team’s findings to the Senate Committee on Homeland Security and Governmental Affairs.
The team consisted of engineers with a range of geotechnical expertise in the safety and inspection of dams and levees. They examined levee failures as well as distressed and intact portions of the levee system and found dozens of breaches throughout the system, Nicholson said in his testimony before the Senate.
Even though Hurricane Katrina made landfall in the Gulf Coast with wind speeds near 150 mph, the real damage in New Orleans resulted from the flooding.
In his team’s report, Nicholson gives several recommendations for reconstruction of the New Orleans levee system. Specific points include:
· Levees need additional overtopping protection at the inboard sides of the floodwalls to minimize erosion.
· Crest heights of the levees need to be planned in a systematic way, so that if overtopping occurs, it happens at desired locations along the levee’s floodwall frontage where the walls are more robust.
· Transitions should be improved so they are not locations of potential weakness in otherwise contiguous perimeter flood protection systems.
ASCE says in the report that Congress should enact a National Levee Inspection and Safety Program modeled after the National Dam Safety Program. The group also discourages new development in the floodplain and says flood-resistant design, voluntary relocation of homes and businesses and the revitalization of wetlands, which are natural barriers, should be taken into account in the reconstruction plan.