storm surge; it was primarily Katrina’s large size that contributed to the highest storm surge ever measured in North America. The IPET also implemented and calibrated state-of-the-art models for coastal wave and storm surge response.
The IPET conducted a detailed evaluation in this area and provided explanations of the HPS performance during Hurricane Katrina. As explained in the Executive Summary of the draft final report, IPET concluded, “With the exception of four foundation design failures, all of the major breaches were caused by overtopping and subsequent erosion” (IPET, 2008, p. I-2). The report further states that “The levee-floodwall designs for the 17th Street Canal and London Avenue Outfall Canals and the IHNC were inadequate for the complex and challenging environment.” In a September 3, 2008, letter to Corps of Engineers Chief Robert van Antwerp, the American Society of Civil Engineers (ASCE) External Review Panel complimented the Corps for its acknowledgement that designs were inadequate for extreme hurricane conditions, but also noted that “engineers routinely are expected to design for such conditions” (ASCE, 2008). This report concurs with the ASCE team on this issue.
The nature of the performance of the hurricane protection system during Hurricane Katrina was an important area of investigation in the IPET studies, especially the geotechnical assessments of the four sites of foundation failures in the HPS. Special explorations were conducted in the field, which were complemented by laboratory centrifuge studies and analytical investigations using numerical modeling and limit analysis. The IPET team concluded that a singular driving mechanism was a key factor affecting each of the failures; however, alternative factors contributing to failure were proposed by others, notably by a research team that was working through a grant from the National Science Foundation (NSF). An earlier report from this NAE/NRC committee drew attention to the complex soil conditions and the number of unknowns still associated with these sites despite the extensive work conducted (NRC, 2006b). In the end, that report advised the IPET to “be aware of alternative failure mechanism and assess the potential for instability at other locations along the levee system” and that “The explanation of the failure mechanism for the 17th Canal Street breach, while plausible, is not fully convincing, and alternative failure mechanisms should be more rigorously assessed” (NRC, 2006b). These issues likely will continue to be debated, with a gradual professional consensus developing about appropriate means to incorporate these findings into future design. For the time being, all reasonable possible failure modes in designs for levees and floodwalls should be considered and examined, and attention should be given to ongoing professional discussion about the issues in order to facilitate design improvements.