risks and the performance of levees and flood protection systems in general. As a response to the damage caused by Katrina and the levee failures, the USACE formed a team to conduct a detailed risk and reliability analysis of the New Orleans hurricane protection system, including an assessment of economic consequences as part of the Interagency Performance Evaluation Task Force study (IPET, 2009). During the same period and partly in response to Hurricane Katrina, the California Department of Water Resources carried out a comprehensive risk analysis on the more than 1,200 miles of levees in the Sacramento-San Joaquin Delta (URS/JBA, 2008). This analysis evaluated flood and seismic risks to Delta levees (which are more like dams because they have water against them year round) and the economic risks to the state. Following the Delta risk study, California DWR is conducting flood risk studies in the Central Valley, including the extensive development of levee fragility curves. As part of the California Central Valley Flood Protection Plan, engineers have developed levee fragility curves at approximately 300 locations in approximately 1 year, following an extensive data collection effort (CA DWR, 2012). Similarly, USACE has developed levee fragility curves for use in ongoing planning studies, all of which now employ complete risk analysis (USACE, 2006).
The events of Hurricane Katrina led to significant changes at USACE in terms of their use of risk analysis methods in its levee and dam safety programs. In fact, the IPET evaluation led to USACE making a major paradigm shift, transforming USACE to a risk-informed agency (USACE, 2008). Whereas USACE had been using risk analysis in limited ways prior to Katrina, the impact of failure of USACE-designed structures led to a wholesale move to implementation of risk analysis for the management of the nation’s flood risk. As part of these changes, USACE is advancing its risk analysis methods for levees and developing new tools to improve the assessment of flood protection systems (USACE, 2008).
DETAILED DICUSSION OF A MODERN RISK-BASED ANALYSIS
As described in the report, there are a number of key elements to conducting a modern risk analysis. These include:
• the requirement that uncertainties, specifically epistemic uncertainties, be identified and evaluated as an integral part of the analysis;
• modeling and considering the performance of flood protection systems, including the potential for failure and breaching, misoperation, overtopping, and flooding of areas protected by these systems, in the assessment of the potential for flooding and damage; and
• a systems-based approach for the modeling of the hydrologic and hydraulic systems and the flood protection systems that provide protection to a community.
As it pertains to the NFIP, the purpose of a risk-based approach to the analysis of flood risks, including the performance of flood protection systems, is multifold. It is intended to provide a quantitative measure of the flood damages associated with flood hazards that can be used to support the NFIP determination of flood insurance rates. This assessment needs to consider the performance of levee systems and, given the language in 44 CFR §65.10, all elements of flood protection systems and their role in the potential for flooding. In addition to insurance-motivated needs of the NFIP, the risk analysis and, more specifically, the products of the risk analysis will support other floodplain management aspects of the program (risk communication, planning, etc.).
The analysis of flood risks is a multidisciplinary evaluation that is made on the basis of available information. As such, the recognition and evaluation of these epistemic uncertainties is a key concept in modern risk analysis. As a starting point for the development of an NFIP risk analysis methodology, it is important that a clear definition of uncertainty and risk be established. Furthermore, as procedures for the implementation of an NFIP risk analysis method are developed, clear definitions of the responsibilities of the professionals involved in the study, the evaluations they are expected to perform, and the results they generate are critical. Experience suggests that in complex, multidisciplinary evaluations it is important that responsibilities are clearly defined and the technical interface between different parts of the analysis are laid out.