IPET Report 3 Vol VIII

New Orleans is limited. For example, we do not know with certainty the properties of foundation
soils underlying every section of the extensive levee and floodwall system, or even the frequency
with which hurricanes will occur in the future. Hurricane models can predict winds, waves and
surges only with limited precision, and the reliability models used to predict levee performance
when subjected to hurricane forces are similarly limited. Hence, the risks of hurricane-induced
flooding cannot be established with certainty. Therefore a risk analysis must include not just a
best estimate of risk, but also an estimate of the uncertainty in that best estimate. By identifying
the sources of uncertainty in the analysis, measures such as gathering additional data was taken
to reduce the uncertainty and improve the risk estimates.
The IPET risk analysis considers two states that represent the condition of the New Orleans
HPS.
• The system as it existed before the arrival of Hurricane Katrina. This state is the baseline
for estimating risk.
• The system following Hurricane Katrina with repairs made prior to the 2006 hurricane
season...
Information collected and studies conducted during the IPET studies were used in the analysis
of these two conditions. It is also important to note that the risk analysis of the pre-Katrina
system is not intended to be a hind cast of the risks that the original design posed. The Katrina
experience provides the best hind cast of risks associated with the HPS as it existed prior to
Katrina, i.e., we know what occurred so the risks were high. The pre-Katrina analyses are used
as the base case against which HPS improvements are measured.
Reliability analyses evaluate the performance of the various elements of the system by predicting
the probabilities of failure of the elements under expected loads. The consequences
associated with that performance when coupled with the probabilities of failure provides a measure
of the corresponding risks. The reliability of the various elements of the protection system is
determined using analytical and expert elicitation methods.
Several key considerations and characteristics of the risk studies are important to note:
• Defining the physical features of the system required an accurate inventory of all components
that provide protection against storm surge and waves. It was important to model
not only the cross sections and strength parameters of these components but also transitions
between elements, differences in the crest elevations along a reach of similar components
and varying foundation conditions. The characterization of the physical features
of the protection system was, however, limited by the available information and the
resources available to conduct field surveys, and process the information under IPET.
These limitations are expressed in the analyses as uncertainties that are characterized and
communicated so that they were accounted for in decisions making.
• The hurricane modeling and reliability analyses required an accurate depiction of the
elevations of the tops of levees and walls that make up the HPS. The Risk Team developed
this information from various sources as discussed herein.
VIII-6 Volume VIII Engineering and Operational Risk and Reliability Analysis
This is a preliminary report subject to revision; it does not contain final conclusions of the United States Army Corps of Engineers.