• provide direct engineering- and reliability-based assessment of the performance of flood protection systems in the evaluation of flood hazards in protected and unprotected areas;6
• account for the performance of risk reduction that is attributable to nonstructural measures;
• provide a means to establish an insurance program that is based on a continuum of flood risks as opposed to the current approach that is partially deterministic;
• offer a clear and informed basis for risk communication;
• provide a reliability-based means for the assessment of levees and flood protection systems and their integrity;
• improve the long-term stability of flood hazard and risk estimates that are a basis for insurance pricing, presuming that uncertainty and changing conditions are accounted for properly throughout the analyses;
• require that levees and appurtenant structures and subsystems be evaluated and modeled as part of flood protection “systems” whose reliability has a significant impact on the flood risks that communities face;
• afford the opportunity for communities and others to understand their flood risks at the community level and to assess the economic benefit of alternative floodplain management options; and
• provide risk information at a community as well as an individual property scale, which would offer the opportunity for communities to more clearly understand the value of community participation in the NFIP.
Such an analysis would provide an in-depth technical evaluation of flood hazards and directly account for the performance of levee flood protection systems, whether accredited within the NFIP or located within an NFIP community (i.e., levee systems that protect against the one percent annual chance flood or not). It would account for all elements of the flood protection system (levees, gates, other structures, etc.) that significantly affect flood risk, as long as they meet minimum design, operation, and maintenance standards. Hence credit could be given when establishing flood insurance rates for flood protection systems that do not protect to the one percent annual chance standard (or any new standard), but at the same time provide some level of protection. Risk analysis results would also differentiate between protection provided by levees that protect to the one percent annual chance flood and higher, for example, 0.2 percent levees. It would be able to recognize the difference in risk between low areas behind levees and higher ground.
In addition to the foregoing, a risk-based analysis offers other advantages. A modern risk-based analysis will account for the uncertainties that affect the quantitative representation of the flood hazard and the estimated performance of flood protection systems. The current analysis for flood risk estimation neglects significant sources of uncertainty in the hydrologic and hydraulic modeling and it does not account for any aspect concerning the performance of flood protection systems other than the binary aspect (meets or fails to meet criteria). And it does not account for uncertainties in the consequences of flooding. The proposed modern flood risk analysis directly accounts for these uncertainties, building on techniques that have been developed and applied in the evaluation of risks associated with other natural phenomena (e.g., earthquakes) and that have been used in the flood and levee areas (USACE, 2006; URS/JBA, 2008; IPET, 2009).
In a broader context, utilizing a modern risk analysis will provide a sound foundation for transitioning to risk-informed floodplain management. For example, it will provide flood hazard and risk information (at the individual property and at the community level) to communities in areas protected by levees and will identify their risk. The present system has implied to communities that live behind accredited levees that they are “safe.” This implication and the lack of specific risk information can be addressed with risk analysis products that are designed, with the support of risk communication experts, to better inform communities and decision makers about flood risks. Also, a modern risk analysis would provide guidance to the communities for development of new strategies that would encourage wise floodplain management.
In addition to advancing the technical basis for flood risk analysis, the approach described here is, in many ways consistent with the methods being developed and implemented by USACE (Box 3-1). However,
6 The terms “reliability” and “performance” are used differently across the flood risk community. For the purposes of this report, the geotechnical behavior of levees is referred to as performance. Reliability is used to refer to the likelihood under a given condition that the levee or levee system will perform in a way for which it was designed (IPET, 2009).