Mapping the Zone Improving Flood Map Accuracy (2009) / Chapter Skim
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5 Coastal Flooding
5 Coastal Flooding
Pages 67-78
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From page 67... ...
The end result is that coastal models that approximate the processes and phenomena flood mapping is much more complex and uncertain that lead to coastal floods. The predictions yield base than riverine flood mapping, and its accuracy is less flood elevations (BFEs)
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From page 68... ...
and wave setup (due to breaking from the wave and surge models is used to calculate waves, Figure 5.2) and a fluctuating water level caused wave crest values using erosion and wave calculations by wave runup (the maximum extent of high-velocity through the Coastal Hazards Analysis and Modeling uprush of individual waves above the average water Program (CHAMP)
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From page 69... ...
Evolution of Coastal Flood Models and Mapping FEMA has also made many incremental improve ments in probabilistic methods for selecting an ensem Prior to 1975, coastal BFEs for Flood Insurance ble of hurricane and storm parameters and return Rate Maps (FIRMs) were calculated using limited periods; storm surge modeling; and calculation of historical records and an early storm surge model, wave setup, wave runup, wave crest, erosion, and the but without consideration of waves.
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From page 70... ...
TABLE 5.1 Elements of FEMA's Current Coastal Flood Mapping Process Coupled Surge and Wave Models for Empirical and Probabilistic Input Data SWEL Calculation Wave Crest and BFE Calculation • Hurricane data • 2-D storm surge model • CHAMP/WHAFIS erosion and wave calculations along • Probabilistic hurricane wind model data • 2-D wave model one-dimensional (1-D) transects • Hurricane ensemble and return period data • Post-storm topographic data to verify CHAMP/WHAFIS • Bathymetric data results • Pre- and post-storm topographic data
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From page 71... ...
states in the future. • WHAFIS wave crests and BFEs are not 1 percent FEMA's guidelines for coastal flood mapping have annual chance values (i.e., probabilistic wave conditions also evolved. Policies and procedures were established are not incorporated in the WHAFIS calculations)
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From page 72... ...
This section identifies opportunities 2-D surge and wave models have demonstrated their to improve the accuracy of coastal flood models and ability to calculate wave setup and wave crest (Sheng recommends ways to guide the development of the next and Alymov, 2002; IPET, 2008)
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From page 73... ...
The impact of uncertainties in SWAN. Although FEMA has not fully embraced the these factors on the accuracy of calculated storm surge use of coupled 2-D surge and wave models to calculate and coastal inundation has not been examined, but may BFEs and wave crests, the successful use of this method need to be quantified to make significant improvements by the Interagency Performance Evaluation Task Force in coastal models and maps.
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From page 74... ...
area and bays behind the topographic features could be In addition to developing new capabilities, the underestimated. Hence, it is important to incorporate next generation of coastal flood models can take better the effect of topographic features on coastal flood- advantage of the capabilities of existing 2-D and 3-D ing in 2-D or 3-D storm surge and wave models, as models.
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From page 75... ...
FEMA should expand collection trated in Figure 5.6, would significantly advance of high-resolution topographic data to all coastal FEMA's coastal models by yielding more accurate counties and require collection of post-storm topoestimates of the SWEL, wave crest, and BFE. graphic data to validate storm surge and wave models and improve their accuracy.
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From page 76... ...
and shallower. The extent to which perturbations in A Comprehensive Coastal Flood Mapping the bathymetry affect storm surges or waves modeled Uncertainty Study using FEMA's flood mapping methods is unknown, although preliminary tests suggest that surge and FEMA has overseen many incremental improvewaves are more sensitive to nearshore bathymetry than ments to the basic CHAMP/WHAFIS model structure.
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From page 77... ...
For example, the test bed led and increased computing power, and sped by the need by the University of Florida is comparing four research to better understand and represent coastal flood prostorm surge and inundation models as well as the flood cesses in the wake of Hurricane Katrina. maps produced using the models. A comprehensive The key to improving coastal flood maps lies in uncertainty study could help identify opportunities to improving the coastal flood models that are used to increase the accuracy of coastal flood studies and priori- calculate the BFE, improving estimates of hurricane ties for improving FEMA's coastal flood modeling and return period, and gathering more accurate pre- and mapping methods.
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