FIGURE 5.2 Schematic of wave setup (·; rise in the water surface caused by breaking waves of height H) and wave runup (R(t); the rush of wave water up a slope or structure). Wave setup and wave runup raise water elevations above the stillwater level (SWL). SOURCE: U.S. Geological Survey, <http://coastal.er.usgs.gov/hurricanes/impact-scale/water-level.html#runup>.

FIGURE 5.2 Schematic of wave setup (η; rise in the water surface caused by breaking waves of height H) and wave runup (R(t); the rush of wave water up a slope or structure). Wave setup and wave runup raise water elevations above the stillwater level (SWL). SOURCE: U.S. Geological Survey, <http://coastal.er.usgs.gov/hurricanes/impact-scale/water-level.html#runup>.

for Flood Insurance Studies (WHAFIS) program. The recent Mississippi study used the SWEL and wave setup calculated by the Advanced Circulation (ADCIRC) and Simulating WAves Nearshore (SWAN) models to calculate the wave crest in CHAMP. The wave crest is combined with the SWEL and wave setup to yield the BFE. Depending on the region, wave runup and overtopping may have to be calculated and added to the wave crest.

Evolution of Coastal Flood Models and Mapping

Prior to 1975, coastal BFEs for Flood Insurance Rate Maps (FIRMs) were calculated using limited historical records and an early storm surge model, but without consideration of waves. In the late 1970s, FEMA supported the development of a 2-D storm surge model (FEMASURGE) for calculating the SWEL caused by storm surge, again without consideration of wave effects on the storm surge or BFEs. These early models used simplified assumptions, coarse grid resolutions, and a simple parametric hurricane model to minimize computational effort.

In 1977, FEMA asked the National Research Council (NRC) to determine how to incorporate calculations of wave height and runup in flood map projects for Atlantic and Gulf coast communities. The NRC (1977) concluded that wave height predictions should be included in coastal flood mapping and provided a methodology to account for varying fetch lengths (length of water over which a given wind has blown), barriers to wave transmission, and regeneration of waves likely to occur over flooded land areas. Based on the NRC (1977) recommendations, FEMA developed WHAFIS to provide wave heights for the BFEs.

FEMA has also made many incremental improvements in probabilistic methods for selecting an ensemble of hurricane and storm parameters and return periods; storm surge modeling; and calculation of wave setup, wave runup, wave crest, erosion, and the effects of structures on surge and waves. For example, the Joint Probability Method (JPM), introduced in 1981, was used to determine the hurricane ensemble and return period in coastal regions based on available hurricane data and statistical properties of hurricane wind parameters at landfall. The catastrophic flooding in Louisiana and Mississippi during Hurricane Katrina in 2005 triggered new interest in developing more advanced models. JPM has been improved, and the Interagency Performance Evaluation Task Force



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