Levees and the National Flood Insurance Program Improving Policies and Practices (2013) / Chapter Skim
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3 Moving to Flood Risk Management and a Modern Flood Risk Analysis
3 Moving to Flood Risk Management and a Modern Flood Risk Analysis
Pages 33-56
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From page 33... ...
Flood risk management represents comprehensive efforts, to continuously carry out analyses, assessments, and related mitigation implementation activities to reduce flood risk (FLOODsite, 2004; Sayers et al., 2012)
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. FIGURE 3-2 The evolution of flood management practice through history.
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. FIGURE 3-3 The flood risk management cycle illustrates the multistep flood risk management process.
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The flood hazard analysis currently used within the NFIP to assess flood risk establishes a legislated level of protection (the one percent annual chance flood) , using standard hydrologic methods that compute the expected elevation of that flood at a given location and assumes that any flood protection structures at that location designed to pass the one percent annual chance flood will in fact do so.
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. Economic and life-safety risks associated with accredited levee systems are not accounted for in the current NFIP policy, which generally means that flood risks have not been effectively evaluated and, as a result, not effectively managed.
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4 ADVANTAGES OF A MODERN RISK-BASED APPROACH TO FLOOD RISK ANALYSIS AND INSURANCE In this report, a modern flood risk analysis is defined as a risk analysis using the best available science and analytical methods to evaluate flood risk. This is more precisely defined as an analysis where the likelihood of adverse consequence (loss, personal injury)
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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)
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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 evalu ation, 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.
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. The hazard level that causes failure of a structure or component is not exactly known; therefore, the conditional prob FIGURE 3-5 Elements of the modern flood risk analysis.
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A MODERN RISK-BASED ANALYSIS: THE FOUNDATIONAL BASIS As a starting point for the development of an NFIP flood risk analysis, a conceptual framework for a flood risk analysis can be defined as R = f(H,V,C)
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From page 43... ...
But most importantly, it establishes the quantitative representation of risk. As mathematically depicted above, a flood risk analysis is an evaluation that is made on the basis of available information, in which all aspects of the analysis are subject to uncertainties: the flood hazard analysis, the assessment of levee system performance, the assessment of flooding/inundation, and the estimate of consequences.
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. One result from a flood risk analysis is the likelihood that a levee system will fail or a measure of the randomness of the flood hazard and the future performance of the levee system, that is, aleatory uncertainty (Figure 3-6, top)
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However, because of epistemic uncertainties in each of the elements of the risk analysis -- in the NFIP flood hazard analysis, the evaluation of levee system performance, and the assessment of consequences -- there is epistemic uncertainty in the distribution of economic consequences. A more modern flood risk analysis captures how epistemic uncertainty manifests in the estimate of the expected losses (Figure 3-7, bottom)
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FIGURE 3-7 (Top) F/N curve illustrating the estimate of the frequency distribution on economic consequences without epistemic uncertainty .
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Both aleatory and epistemic uncertainty are shown on the right, giving a more complete estimate of the likelihood of inundation.
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For example, a local or state agency seeking to reduce flood risk could use information on levee performance to make investment decisions for strengthening levees or implementing a property buy-out program in areas of greatest potential damage. STEPS TO DEVELOPMENT AND IMPLEMENTATION OF A RISK-BASED APPROACH Realigning the NFIP to undertake a modern risk analysis, use products of such an analysis, and account for uncertainty in the inputs is a shift from "business as usual." Current NFIP analyses focus on hazard -- primarily on the 1 percent and 0.2 percent annual chance exceedance events.
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Implementing a Risk-Based Approach The following tasks are necessary in order for FEMA to implement a modern risk-based approach successfully: 1. Develop, test, and deploy -- with the same care and rigor with which standard procedures for hydrologic and hydraulic analysis and mapping have been developed, tested, and deployed -- a procedure for the complete risk analysis.
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. Deaggregation of Conditional probability Risk mitigation, Report on the relative contribution of different the flood hazard distribution on the relative emergency events (flood events, levee failure events, etc.)
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6. Refine and enhance the methods of communication of information on flood risk, including now information on consequence.
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. This information is available on the Web through North Carolina's Floodplain Mapping Information System, which includes access to DFIRMs and other features such as data export capability and a property address search function to enable viewing of flood risk to a specific property (Figure 3-10)
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There are a number of agents of change that may affect future flood risks: climate change, community development and changes in land use, changes in population, condition of flood mitigation systems (success of system maintenance, changes in system configuration, etc.) , changes in the watershed, and so on.
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From page 54... ...
With the development and implementation of a risk-based analysis to evaluate community flood risks, FEMA needs to extend these practices to a community risk-based approach and faithfully maintain them. FEMA policy with regard to levees is spelled out in 44 CFR §65.10.
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From page 55... ...
2012. Flood risk management: A Strategic Approach -- Consultation Draft.
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