Tying Flood Insurance to Flood Risk for Low-Lying Structures in the Floodplain (2015) / Chapter Skim
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3 Methods for Assessing Flood Risk
3 Methods for Assessing Flood Risk
Pages 23-34
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From page 23... ...
. ASSESSING THE COMPONENTS OF The water surface elevation–exceedance probability FLOOD RISK function may be derived through statistical analysis of observations of annual maximum water surface elevaAssessing Flood Hazard Flood hazard assessment estimates the probability 1 In this report, probability is expressed as a percent chance exceedance, which is probability × 100.
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From page 24... ...
, plain may be represented as a grid of linked cells, with then transform that with a discharge–water surface the movement of water modeled from the channel elevation function (Figure 3.1c) to derive the water to grid cells adjacent to the channel, then from cell surface elevation–exceedance probability function.
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From page 25... ...
SOURCE: Courtesy of David Ford, David Ford Consulting Engineers, Inc. R02820 Fig 3.3.eps FIGURE 3.3 For complex floodplains, water surface elevations on the floodplain may be computed with a two-dimensional model.
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From page 26... ...
Floodplain depth B R02820 Fig 3.5a.eps 1 Damage as % value Probability of levee failure 0 Floodplain depth Channel water surface elevation R02820 Fig 3.5b.eps FIGURE 3.5 Exposure and vulnerability are represented with R02820 Fig 3.4.eps FIGURE 3.4 Variability in the performance of flood protection an inundation depth–damage function (a) , which may be develsystems, such as levees, is represented with a fragility function, oped from a generic inundation depth–percent damage function, which is included in the risk assessment.
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From page 27... ...
In each successively numbered A zone, the difference 1 0 between the 1 percent and 10 percent annual chance Annual exceedance exceedance water surface elevation increases, with dif probability ferences ranging from 0.5 feet for Zone A1 (broad, R02820 Fig 3.6.eps FIGURE 3.6 Flood risk is commonly represented with a dam- shallow floodplains) to 20 feet for Zone A30 (narrow, age–exceedance probability function.
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From page 28... ...
engineering criteria, the NFIP credits is used in the rate formula in Box 2.1.2 In application, the levee with eliminating inundation and correspondNFIP analysts would determine the difference between ing damage caused by events more frequent than the the 1 percent and 10 percent annual chance exceed- 1 percent annual chance exceedance flood. Until 2013, ance water surface elevations for a structure from flood levees that fell short of the standard were considered s tudies of an area, find the corresponding numbered A "non-accredited" and assumed not to reduce flood zone, and then derive the appropriate PELV curve with hazard.
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From page 29... ...
The function includes discharge values for a range of probabilities, commonly Flood Risk between 50 percent and 0.2 percent annual chance exceedance. The discharge–exceedance probability The NFIP expresses flood risk in terms of loss and function is transformed to a water surface levation– e corresponding probability.
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From page 30... ...
The expected transforms a PELV curve with a DELV curve to annual damage (average annual loss) is computed by in- derive a damage–exceedance probability function (as tegrating the damage–exceedance probability function.
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From page 31... ...
In addition, the state has determined water surface elevations and flood depths The North Carolina Floodplain Mapping Program for five flood probabilities: 10 percent (10-year depth) , is assessing certain components of flood risk as a part 4 percent (25-year depth)
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From page 32... ...
.8 Average annual frequency depth grids are intended to provide a platloss is computed for individual properties in floodplains form for NFIP hydrologic and hydraulic analyses and statewide, and publicly available databases and visual- to help individuals better understand and visualize their ization tools provide easy access to these estimates, as flood hazard. The NFIP analyzes flood depth at the well as to the underlying reports of hazard, exposure, same flood probabilities as North Carolina's (10 pervulnerability, and consequence.
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From page 33... ...
The NFIP COMPREHENSIVE RISK ASSESSMENT hydrologic method, the USACE method, and the The 2013 NRC report Levees and the National comprehensive risk assessment recommended in NRC Flood Insurance Program: Improving Policies and Practices (2013) use an inundation depth–exceedance probability recommended and summarized the advantages of the function to describe flood hazard, whereas catastrophe NFIP moving toward a more comprehensive approach models typically use Monte Carlo sampling to generate to risk analysis that builds on current SACE and U a long series of synthetic stream flows or ocean tides catastrophe model methods.
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From page 34... ...
inundation depth–damage functions are developed for In contrast, with the comprehensive risk assessment classes of structures. and, to a lesser degree, the USACE method, epis A significant difference among the risk assessment temic uncertainty about the water surface elevation– methods is that the NFIP method was developed to exceedance probability function is described with a assess flood losses for individual structures, whereas probability distribution about the mean value of elevathe USACE, comprehensive risk assessment, and tion predicted for a specified probability.
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