FIGURE S.2 Inundation maps of the area where the Tar-Pamlico River empties into Pamlico Sound of North Carolina. The figure on the left is based on a digital elevation model (DEM) with 30-meter post spacing created from the USGS NED. The figure on the right is based on a DEM with 3-meter post spacing created from North Carolina Floodplain Mapping Program lidar data. The dark blue tint represents land that would become inundated with 1 foot of storm surge or sea level rise. The light blue area represents uncertainty in the extent of inundation at the 95 percent confidence level. SOURCE: Gesch (2009).

FIGURE S.2 Inundation maps of the area where the Tar-Pamlico River empties into Pamlico Sound of North Carolina. The figure on the left is based on a digital elevation model (DEM) with 30-meter post spacing created from the USGS NED. The figure on the right is based on a DEM with 3-meter post spacing created from North Carolina Floodplain Mapping Program lidar data. The dark blue tint represents land that would become inundated with 1 foot of storm surge or sea level rise. The light blue area represents uncertainty in the extent of inundation at the 95 percent confidence level. SOURCE: Gesch (2009).

order of magnitude and support the need for new topographic surveys, as called for in a National Research Council (NRC, 2007) report Elevation Data for Floodplain Mapping. In two of the study areas, random errors in topographic data produce inaccuracies in floodplain boundaries, but do not significantly alter the total area of the floodplain. In the other study area, in addition to random errors, there is a large systematic difference between the lidar and NED data that results from a misalignment of the stream location between the base map planimetric information and the topographic data. As a result, the total areas of the floodplains defined from lidar and from the NED differ by 20 percent. Because imagery is improving faster than elevation, the misalignment problem is growing more acute.


Finding 2. Coastal flood maps can be improved significantly through use of coupled two-dimensional storm surge and wave models and improved process models, which would yield more accurate base flood elevations.


The science of riverine flooding is reasonably well understood, and improvements to inland flood maps can focus on harnessing available technology. In contrast, advancing understanding of the complex dynamics of the coastal inundation process is necessary for improving the accuracy of coastal flood maps. Coastal flood models are evolving rapidly, but published results suggest that replacing FEMA’s one-dimensional model for calculating wave heights (Wave Height Analysis for Flood Insurance Studies [WHAFIS]), which was introduced in the late 1970s, with a two-dimensional wave model would improve the accuracy of calculated base flood elevations. Coupled two-dimensional surge and wave models, as well as models that account for erosion processes, the effects of structures, and variations in topography, offer the potential for further improvements of coastal flood map accuracy. A comparison of available models, conducted by an independent external advisory group, would help quantify uncertainties and indicate which models should be incorporated into mapping practice.


Finding 3. Flood maps with base flood elevations yield greater net benefits than flood maps without.


Benefit-cost analyses have shown that the greatest benefits of more accurate flood maps are avoided flood losses to planned new buildings and avoided repairs to infrastructure through more accurate base flood elevations and depiction of floodplain boundaries. Producing a more accurate base flood elevation yields the greatest increment of benefits because it enables insurance premiums and building restrictions to be set commensurate with a more realistic profile of the horizontal and vertical extent of flooding. Only the more expensive of FEMA’s flood study methods—detailed studies and most limited detailed studies—yield a base flood elevation. A comparison of study methods in the



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