Using Risk Analysis for Flood Protection Assessment
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Darker elements have a greater probability of inundation; lighter elements have less. The A transect illustrates a cross section of the channel at which the flow moves from the channel onto the floodplain. This illustrates the shading with a fixed-grid two-dimensional floodplain flow model, coupled with a one-dimensional channel flood model. Thus, the probability of inundation for any location in the floodplain behind a levee could be computed and mapped, considering the natural hazard and the performance of the levee.
This would enable quick inspection to determine areas of greater hazard. This report does not address these issues specifically, but does discuss steps toward the development and implementation of a risk-based program. The following tasks are necessary in order for FEMA to implement a modern risk-based approach successfully:. 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.
This procedure should be consistent in concept with that described in the Summary Description of the Risk-Based Approach section of this chapter, including all components shown. This includes algorithms and computational methods that consider the likely users of the risk analysis procedures.
Develop, test, and deploy a consistent procedure for assessing levee systems rather than individual levee segments and for describing levee performance with a fragility curve. NOTE: Risk analysis products are results of or intermediate products of the risk analysis described in this chapter. Depth-damage models commonly used for assessing consequence as a component of risk analysis are best calibrated and verified with actual damage data. Completing this task will ensure that models for consequence estimates are accurate and reliable. Develop, test, and deploy consistent procedures for describing the uncertainty in all inputs to the risk analysis, include uncertainty about flow frequencies, water surface elevations, levee performance models, and consequence estimates.
Refine and enhance the methods of communication of information on flood risk, including now information on consequence. With the revised risk analysis approach proposed here, the information to be communicated offers an estimate of direct, tangible, economic impact of flooding—the cost to a property owner and information on potential life-safety consequences. The revised analysis will also offer information about the probability that water will exceed specified depths in any year; this can be expanded easily to describe the probability that flood depths will exceed specified depths over the life of a mortgage or another informative duration.
Work closely with other federal agencies to ensure that techniques are consistent across agencies so that communities are not faced with using multiple approaches to evaluate a levee system. See Chapter 8 for further discussion. FEMA can leverage new technology to share the information now available. For example, in the state of North Carolina was designated a Cooperating Technical State by FEMA, a decision largely driven by the impact of Hurricane Floyd in highlighting the need for accurate flood maps for the state.
The cost to FEMA of developing procedures for and implementing a program based on modern risk analysis cannot be predicted with certainty without information from pilot applications, which are a valuable pursuit. However, in the absence of pilot studies, a comparison between the costs for a modern risk analysis that would be above that incurred for a Flood Insurance Study FIS using current methods i. Current FIS procedures require many of the same hydrologic and hydraulic inputs required for a modern risk-based analysis, which indicates that the incremental cost for pilot studies should not be significant.
Topographic data are necessary for establishing the relationship between flood hazard and consequence; these are collected for current FIS studies, and so no additional cost will be incurred for this portion of the modern risk analyses. The greatest cost, not currently incurred for a FIS, is the cost of developing system fragility curves—the functions that represent the likelihood that levees and appurtenant structures will provide the protection for which they were designed.
Developing the curves requires information about the materials, methods of construction, operation and maintenance, performance history, and so on. However, the manner of interpretation of the information is different when compared with the proposed modern risk analysis approach. No longer can an engineer compare the system or levee state to a standard, finding that the standard is or is not met. Instead, a qualified geotechnical engineer will evaluate the integrity of a levee system and incorporate this quantitatively in the conditional performance of the system. For risk analyses to support that study, engineers developed fragility curves at approximately locations in approximately 1 year, following an extensive data collection effort CA DWR, Similarly, USACE has developed levee fragility curves for use in ongoing planning studies, all of which now employ a risk analysis along with procedures and guidance for the development of appropriate levee fragility curves USACE, ; Schultz et al.
A flood hazard study, which is discussed in this chapter, is a consideration of the frequency of flooding included in the FIS. The menu bar at the top of the image allows the user to explore exposure of an individual structures to the flood hazard as well as other options such as a DFRIM export function.
Pink depicts areas subject to one percent annual chance flooding AE zone, floodway , blue areas are subject to 1 percent annual chance flooding AE zone, has base flood elevation established , and green areas are subject to the 0. The modern risk-based approach will be an integral part of all aspects of the NFIP: mapping, floodplain management, portfolio analysis, and insurance rate assessment.
As such, the risk-based analysis will be a living, integral part of the NFIP; something that is used regularly by the program actuaries and analysts.
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By its very nature, the modern risk-based analysis will be a regularly updated, preferably online, resource for FEMA. When a modern risk analysis is performed for a community, it would be based on information that is available at the time. Following its completion, there are factors that may change the risk profile of a community, and therefore require maintenance updating of certain data and possibly revision of hazard, performance, and consequence components and the resulting risk analysis products shown in Table Given the role the risk analysis will play in the NFIP, it is important that FEMA develop and implement procedures to periodically review, maintain, and update the risk-based analysis, as necessary.
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. Within its current program, FEMA has requirements for archiving hydrologic and hydraulic models used in the FIS FEMA, and requiring that information on levees be maintained and certification be periodically updated.
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. As noted in Chapter 1 , the requirements that determine whether FEMA recognizes a levee system on NFIP maps include 1 design criteria, 2 operation plan criteria, and 3 maintenance plan criteria. If the levee system does not meet the criteria, the levee system is assumed to provide no protection.
Within the context of a risk-based analysis to evaluate the floodplain and flood protection systems in particular, the analysis would quantify the residual risk in the area behind every flood protection system, and develop insurance pricing based on the level of protection that is provided. For instance, communities will be given credit for the level of protection that is provided, even if this level is below the base flood standard—the estimated risk analysis and corresponding rates will recognize this change.
AIR Models Overview. Accessed November 28, DOE Department of Energy. Accessed December 12, Flood Risk Management. Accessed August 14, Accessed August 8, Accessed December 4, Kaplan, S. On the quantitative definition of risk. Risk Analysis 1 1 Morgan, G. Moser, D. Bridges, S. Cone, Y Haimes, B. Harper, L.
Shabman, and C. Transforming the Corps into a Risk Managing Organization. White Paper. Accessed February 5, Accessed February 27, North Carolina Floodplain Mapping Program. Fact Sheet. Accessed February 21, Disaster Resilience: A National Imperative. RMS Models.
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Rogers, J. Accessed November 15, Sayers, P. Galloway, E. Penning-Rowsell, F. Shen, K. Wang, Y. Chen, and T. Le Quesne. Schultz, M. Gouldby, J. Simm, and J. Washington, DC: U. Army Corps of Engineers. Rockville, MD: U. Nuclear Regulatory Commission. EM Design and Construction of Levees. ER Letter to J. EC Accessed November 27, The NFIP was established by Congress with passage of the National Flood Insurance Act in , to help reduce future flood damages through NFIP community floodplain regulation that would control development in flood hazard areas, provide insurance for a premium to property owners, and reduce federal expenditures for disaster assistance.
The flood insurance is available only to owners of insurable property located in communities that participate in the NFIP. Currently, the program has 5,, million policies in 21, communities3 across the United States. Property owners with a federally back mortgage within the SFHAs are required to purchase and retain flood insurance, called the mandatory flood insurance purchase requirement MPR. Levees and floodwalls, hereafter referred to as levees, have been part of flood management in the United States since the late 's because they are relatively easy to build and a reasonable infrastructure investment.
A levee is a man-made structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control, or divert the flow of water so as to provide protection from temporary flooding. A levee system is a flood protection system which consists of a levee, or levees, and associated structures, such as closure and drainage devices, which are constructed and operated in accordance with sound engineering practices.
The study addressed four broad areas, risk analysis, flood insurance, risk reduction, and risk communication, regarding how levees are considered in the NFIP. Specific issues within these areas include current risk analysis and mapping procedures behind accredited and non-accredited levees, flood insurance pricing and the mandatory flood insurance purchase requirement, mitigation options to reduce risk for communities with levees, flood risk communication efforts, and the concept of shared responsibility.
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Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released. Get This Book. Visit NAP. Looking for other ways to read this? No thanks. Page 34 Share Cite. Page 35 Share Cite. A risk analysis is a detailed examination: performed to understand the nature of adverse consequences from a particular event to human life, property, or the environmental; an analytical process that provides information about or quantifies probabilities and consequences of an unwanted event.
Using Risk Analysis for Flood Protection Assessment | Martina Zeleňáková | Springer
NRC, A subset of a risk analysis, a flood risk analysis is an analytical process that provides information about or quantifies probabilities and consequences of a flood event. Page 36 Share Cite. Page 37 Share Cite. Page 38 Share Cite. This is more precisely defined as an analysis where the likelihood of adverse consequence loss, personal injury is quantitatively evaluated, taking into account 1.
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Page 39 Share Cite. Page 40 Share Cite. In , in the wake of Hurricane Katrina, USACE issued Risk Analysis for Flood Damage Reduction Studies, to update the 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. Page 41 Share Cite. The hazard level that causes failure of a structure or component is not exactly known; therefore, the conditional prob- FIGURE Elements of the modern flood risk analysis.
Page 42 Share Cite. Page 43 Share Cite. Page 44 Share Cite. However, the FIGURE Estimate of the frequency of levee system failure as a single or best estimate, aleatory uncertainty top , and an estimate that includes the quantification of both the aleatory and epistemic uncertainty bottom of one result from a flood risk analysis; the frequency of occurrence per year that a levee system would fail. Page 45 Share Cite. Page 46 Share Cite. Page 47 Share Cite.
Page 48 Share Cite. Page 49 Share Cite. Page 50 Share Cite. Information Category Risk Analysis Product Use Description Flood hazard floodplain Floodplain stage frequency distributions for locations in the floodplain; including at the individual property level. This result will also include an estimate of the uncertainty in the flood frequency estimate. Risk communication Report on likelihood of inundation to various depths for individual parcels, census tracts, or other subdivisions, better communicating risk to occupants, owners. Flood hazard floodplain Flood stage frequency distributions for locations in the floodplain; including at the individual property level.
Mitigation Provide information on depths, velocities, and other flood properties throughout floodplain, for specified events, to inform emergency evacuation and safe shelter planning by government agencies, industry, and public. Risk communication, mitigation Support wise land-use decision making by identifying annual exceedence probability of a parcel, tract, etc. Flood hazard floodplain Flood stage frequency distributions for locations in the floodplain, including at the individual property level.
Risk communication, mitigation Similar to above, but conditional flooding can be displayed to answer question: Given channel flood flow or stage of specified annual exceedence probability, will a parcel, tract, etc. For convenience, this answer too can be mapped. Deaggregation of the flood hazard floodplain Conditional probability distribution on the relative contribution of different flood scenario types levee overtopping, levee breach, breach locations, interior drainage and ponding, etc.
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Risk mitigation, emergency preparedness Report on the relative contribution of different events flood events, levee failure events, etc. This detailed information provides insight into the hazards to which a community is exposed and can guide the selection of mitigation measures and emergency preparedness and support risk communication.
Page 51 Share Cite. Information Category Risk Analysis Product Use Description Levee system performance Levee system fragility estimates, including an estimate of the uncertainty in the system fragility. Risk communication A levee system fragility curve provides quantitative information on the performance of a levee system, which may comprise multiple levee reaches, gate structures, etc. It provides direct information on the reliability of infrastructure performance and the protection it is intended to provide for a community. This can be integrated with hazard information to provide information on combined probability that a certain flood level will occur and that the levee will protect from interior area flooding.
Levee system performance Levee system fragility estimates, including an estimate of the uncertainty in the system fragility Mitigation Provides information useful for identification of deficiencies and for setting priorities on repairs or upgrades to existing infrastructure, especially when integrated with hazard information. Frequency distribution on consequences Frequency distribution on the consequences of flooding. This will also include an estimate of the uncertainty in the frequency of exceeding levels of consequences.
Insurance Enables computation of expected annual damage for use with premium determination procedures described in Chapter 5. This information can be provided at the individual structure level and community level. Mitigation Enables computation of expected annual damage for comparison with cost estimated for flood management alternatives to assess feasibility of investment for federal, state, and local mitigation planning. Risk communication Provides property owner or floodplain occupant with information about flood losses that they may expect on average for any year or over a longer duration.
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Floodplain stage frequency distributions for locations in the floodplain; including at the individual property level. Report on likelihood of inundation to various depths for individual parcels, census tracts, or other subdivisions, better communicating risk to occupants, owners. Flood stage frequency distributions for locations in the floodplain; including at the individual property level.
Provide information on depths, velocities, and other flood properties throughout floodplain, for specified events, to inform emergency evacuation and safe shelter planning by government agencies, industry, and public. Support wise land-use decision making by identifying annual exceedence probability of a parcel, tract, etc.
For convenience, and in keeping with tradition, this information can be displayed on a map similar to existing FEMA maps Figure Flood stage frequency distributions for locations in the floodplain, including at the individual property level. Similar to above, but conditional flooding can be displayed to answer question: Given channel flood flow or stage of specified annual exceedence probability, will a parcel, tract, etc. Conditional probability distribution on the relative contribution of different flood scenario types levee overtopping, levee breach, breach locations, interior drainage and ponding, etc.
Report on the relative contribution of different events flood events, levee failure events, etc. Levee system fragility estimates, including an estimate of the uncertainty in the system fragility. A levee system fragility curve provides quantitative information on the performance of a levee system, which may comprise multiple levee reaches, gate structures, etc. Provides information useful for identification of deficiencies and for setting priorities on repairs or upgrades to existing infrastructure, especially when integrated with hazard information.
Frequency distribution on the consequences of flooding. Enables computation of expected annual damage for use with premium determination procedures described in Chapter 5. Enables computation of expected annual damage for comparison with cost estimated for flood management alternatives to assess feasibility of investment for federal, state, and local mitigation planning.
Provides property owner or floodplain occupant with information about flood losses that they may expect on average for any year or over a longer duration. Registered in England as a limited company No. Connect with WIT Press:.
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