explained and integrated into the report, especially with regard to levee design. The report discussed the issues of potential failure mechanisms at levee breach sites, concluding that the explanation of the failure mechanism for the 17th Street Canal breach, while plausible, was not fully convincing, and that alternative failure mechanisms should be assessed. It recommended that special emphasis be given to gathering data at areas of the protection system that were loaded to near capacity by storm surges, but did not fail. It also was noted that the IPET faced a challenge in developing a robust and defensible assessment of the risk and reliability of the hurricane protection system, and it was recommended that IPET provide a thorough and understandable explanation for the method being used in its assessment of risk and reliability of the hurricane protection system.
Third report—October 2006
This report (NRC, 2006c) reviewed the IPET draft final report, which was issued in June 2006. The report noted the prominence of risk and uncertainty within the IPET evaluations and reports, and recommended a stronger emphasis on explaining key uncertainties and implications for decision making. It recommended there be more substantial documentation to support the hypothesis that breaches along the Inner Harbor Navigation Canal (IHNC) were caused by overtopping and erosion, or foundation failure. It also recommended that the report examine other possible failure modes. Further advice on soils sampling and testing measures was included: it recommended that the IPET present site plans and soil profiles at breach sites, that clay and marsh deposit strengths be estimated at locations other than the 17th Street Canal, and that IPET provide additional guidance for identifying erodible soils, quantifying the degree of soil resistance to erosion, and selecting and placing soils to resist erosion. The report noted that although the IPET risk analysis approach was coherent and logical, that the levels of uncertainty in estimates of risks in flooding were expected to be large. It also recommended that as a complement to its joint probability methods results, that the IPET create a set of hurricane scenarios that simulate a variety of possible, future storm conditions for the New Orleans region.
Fourth report—February 2008
This report (NRC, 2008a) reviewed a single volume of the IPET report—Draft Volume VIII on Engineering and Operational Risk and Reliability Analysis. That volume was released in October 2007. A key finding from this report was that the overall risk assessment method used by the IPET seemed appropriate for evaluating risks associated with the New Orleans hurricane protection system, but that the interim draft Volume VIII “does not provide sufficient presentation and explanation of the methods employed or results obtained to allow this to be clearly determined.” The report concluded that further information was necessary in order to fully explain and validate the method used and results that were obtained. The report recommended that the IPET more carefully document the data, assumptions, and models being used in its risk analysis, and that it present intermediate results and note that component models being used would evolve and improve over