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Appendix I: Risk Analysis Background
Pages 243-258

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From page 243... ... Although a fundamental description of economic consequences and an associated probability of occurrence did play a limited role in flood insurance rate setting and management planning (e.g., in the computation of expected inundation reduction benefit attributable to a flood management project) , a detailed understanding and description of risk were not widespread in the floodplain management community, and procedures and tools for risk analysis were not readily available. Read the entire page →
From page 244... ... · Account for the performance of flood protection systems throughout the range of possible hazards. The analyses consider, for example, that a levee designed for the 10 percent chance event may reduce adverse economic consequence due to the one percent chance event, even though it will not eliminate all damage due to that event. Read the entire page →
From page 245... ... 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. Read the entire page →
From page 246... ... . In this context, risk is a function of the flood hazard a community is exposed to, the vulnerability of flood protection systems and the potential that their failure will contribute to flooding, and the consequences associated with system failures and the damage to a community exposed to flooding, including economic impact and life safety. Read the entire page →
From page 247... ... ; r = probability as a measure of the confidence to which an estimate of n is the true value, or the epistemic uncertainty in the estimate of n. For purposes of a flood risk analysis, the consequences of flooding are measured in terms of the economic damages. Read the entire page →
From page 248... ... However, a property that is located at or near a breach may be completely destroyed as a result of the depth of water, the high velocities of flow that will occur, and/or the fact the unit is located in the scour zone of the breach which undermines the foundation of the unit, etc. A primary product of the risk analysis for an individual insured property or for an entire portfolio is an assessment of the frequency of economic consequences, which will vary in scale depending on the size of the flood events and the flooding mechanisms that can occur. Read the entire page →
From page 249... ... . Therefore, as part of the flood risk analysis, an epistemic uncertainty model needs to be developed for the flood hazard analysis, the fragility analysis, and the flood damage model. Read the entire page →
From page 250... ... FEMA generally follows the standard of practice for this distribution fitting with historical flow observations. In cases for which data available are inadequate for distribution fitting, FEMA guidance suggests regional methods, transfer methods, empirical equations, and watershed modeling methods for estimating discharge of specified probability.4 While certain refinements to both distribution fitting and alternative methods 4 See http://training.fema.gov/EMIWeb/edu/docs/fmc/Chapter%204%20-%20Flood%20Risk%20Assessment.pdf. Read the entire page →
From page 251... ... A key to success of the risk analysis proposed herein is the capability to model the behavior of the riverine, coastal, or floodplain system with sufficient detail and sufficient accuracy to predict water surface elevations and velocities throughout the area of interest, given the discharge values estimated with frequency fitting or alternative methods. Capabilities for such modeling are readily available and well understood in the engineering and floodplain management community. Read the entire page →
From page 252... ... to close gates or other closures, operation drainage systems, etc. For purposes of the risk analysis, the reliability of structures is characterized in terms of the conditional probability of failure expressed as a function of an appropriate metric of the flood hazard (i.e., peak flood elevation) Read the entire page →
From page 253... ... Observations may be obtained systematically through controlled experiments or may be collected in an opportunistic fashion from flood events. Analytical methods that can be used to develop fragility curves are based on structural models that evaluate the likelihood that performance limits of the structure are exceeded. Read the entire page →
From page 254... ... FIGURE I-4 Uncertainty in the estimate of levee fragility for a levee in New Orleans. Dashed lines correspond to the 0.05 and 0.95 uncertainty levels in the estimate of the conditional probability of failure. Read the entire page →
From page 255... ... Figure I-6 illustrates conceptually such an integrated model of channel and floodplain flow, as would be required for a complete risk analysis as proposed herein. A 1-D model represents the channel flow regime, and a water-surface elevation is computed for a specified exceedence probability. Read the entire page →
From page 256... ... Such analysis can be completed for any flow condition in the channel, including the one percent chance event and larger or smaller events. For risk analysis, properties of the levee breach are described for modeling the breach. Read the entire page →
From page 257... ... The quantification also includes the propagation of epistemic uncertainties in the flood hazard, levee system performance and economic damage assessment, to derive the uncertainty in the risk analysis results. With current analytical and computational capabilities the estimation of flood risks can be carried out at the individual property level, the community level, and for the NFIP portfolio. Read the entire page →
From page 258... ... 2012. 2012 Central Valley Flood Protection Plan: A Path for Improving Public Safety, Environmental Stewardship, and Long-Term Economic Stability. Read the entire page →

From page 243...

... Although a fundamental description of economic consequences and an associated probability of occurrence did play a limited role in flood insurance rate setting and management planning (e.g., in the computation of expected inundation reduction benefit attributable to a flood management project) , a detailed understanding and description of risk were not widespread in the floodplain management community, and procedures and tools for risk analysis were not readily available.

Appendix I: Risk Analysis Background Pages 243-258

Appendix I: Risk Analysis Background
Pages 243-258