USACE Risk-Related Guidance
USACE began to develop an approach to using modern risk concepts in the evaluation of flood management projects in the early 1990s. In 1996, the agency published its first formal guidance document on risk, Risk-Based Analysis for Flood Damage Reduction Studies (USACE, 1996). This manual describes how the risk concepts would be used in USACE flood damage reduction studies in the conduct of hydrologic, hydraulic, geotechnical, and other analyses. The manual became the standard for conduct of hydrologic and hydraulic analyses for planning studies but was not adopted by the geotechnical and other communities within USACE. In 2000, USACE issued Design and Construction of Levees to promulgate the basic principles that should be used in design and construction of levees (USACE, 2000). That manual noted the use of a risk-based approach and hydrologic and hydraulic design, but indicated that geotechnical would continue to use a deterministic methodology.
In 2006, in the wake of Hurricane Katrina, USACE issued Risk Analysis for Flood Damage Reduction Studies, to update the 1996 manual on risk-based analysis, noting that
The ultimate goal is a comprehensive approach in which the values of all key variables, parameters, and components of flood damage reduction studies are subject to probabilistic analysis. Not all variables are critical to project justification in every instance. In progressing toward the ultimate goal, the risk analysis and study effort should concentrate on the uncertainties of the variables having a significant impact on study conclusions. (USACE, 2006).
Structural and geotechnical analyses were to be included in overall risk-based analyses.
In 2006, in the wake of levee failures during Hurricane Katrina, USACE directed the conduct of an evaluation of the performance of the New Orleans hurricane protection system. The results of this evaluation, which was carried out by the Interagency Performance Evaluation Task Force, highlighted the need for the use of modern risk analyses in the design and construction of levees and related flood damage reduction structures. This evaluation led to the establishment of the USACE Levee Safety Program with the mission “to assess the integrity and viability of levees and recommend courses of action to make sure that levee systems do not present unacceptable risks to the public, property, and environment.” In response to congressional and administration direction, it also initiated a levee inventory project that has been transformed into the National Levee Database (NLD) (see Chapter 6).
Faced with the need to evaluate many levees that were part of both the USACE and NFIP programs, in 2010, USACE issued the USACE Process for the National Flood Insurance Program (NFIP) Levee System Evaluation. This document provided guidance to field activities on the evaluation of NFIP levees to determine if USACE would certify that the levees met the standards of 44 CFR §65.10. The document indicated that it was “USACE policy to apply a probability and uncertainty analysis framework to NFIP levee system evaluations.” It noted however that, “Probability of exceedance and uncertainty-based methodologies are under development and emerging for structural and geotechnical engineering elements but are not yet sufficiently mature for direct application in NFIP levee system” (USACE, 2010).
In 2008, USACE established a Risk Management Center to develop policies, methods, tools and systems to enhance its risk management activities overall, including as this applied to levees and dams. As part of its tool and policy development activity, the Risk Management Center is developing processes by which levee safety can be analyzed in the field and levee safety classifications determined for use in levee management.
USACE is currently using risk analysis in its programs and continues to develop, along multiple paths, its approach to use of these concepts in levee planning, design, and evaluation. No single approach has been promulgated, but all are founded on the use of risk-based methodologies. Although USACE’s goal is to ultimately develop an integrated approach to risk analysis across its flood risk reduction portfolio, the agency has not yet issued fully coordinated policy guidance and technical instructions that would define the approach and how it will be implemented.