FIGURE 2.1 Extracted image from a paper map (FIRMette) for a riverine area in Greenville, South Carolina. The left side shows an approximate A zone (SFHA, shaded dark gray), where no elevation or floodway information is provided. The right side of the image shows an AE zone (SFHA, shaded dark gray) with lettered cross sections, base flood elevations (wavy lines with elevation), and floodway (hatched area bounded by heavy dashed lines), and a shaded Zone X (moderate flood hazard area, shaded light gray). The other areas are classified as unshaded Zone X (minimal flood hazard). SOURCE: FEMAs Map Service Center, <http://msc.fema.gov/>.

FIGURE 2.1 Extracted image from a paper map (FIRMette) for a riverine area in Greenville, South Carolina. The left side shows an approximate A zone (SFHA, shaded dark gray), where no elevation or floodway information is provided. The right side of the image shows an AE zone (SFHA, shaded dark gray) with lettered cross sections, base flood elevations (wavy lines with elevation), and floodway (hatched area bounded by heavy dashed lines), and a shaded Zone X (moderate flood hazard area, shaded light gray). The other areas are classified as unshaded Zone X (minimal flood hazard). SOURCE: FEMA’s Map Service Center, <http://msc.fema.gov/>.

Base Flood Elevations

The base flood elevation (BFE) is the computed elevation of a flood having a 1 percent chance of being equaled or exceeded in a given year (base flood). It accounts for the volume and velocity of water moving through the watershed and reflects the cumulative effects of topography, soils, vegetation, surface permeability, and other factors. The BFE is the regulatory standard for the elevation or floodproofing of structures, and the relationship between the BFE and the elevation of a structure also determines the flood insurance premium. In general, the higher the first floor elevation, the lower the insurance premium. Consequently, the accuracy of BFEs on the flood maps is important for

FIGURE 2.2 Example of a FIRMette for a coastal area near Myrtle Beach, South Carolina. The figure shows VE zones (SFHAs subject to coastal wave action) and associated elevations at the point on the ground to which the wave runs up during the 1 percent annual chance flood. Landward, the flood zones transition to Zone AE with their associated base flood elevations. SOURCE: FEMAs Map Service Center, <http://msc.fema.gov/>.

FIGURE 2.2 Example of a FIRMette for a coastal area near Myrtle Beach, South Carolina. The figure shows VE zones (SFHAs subject to coastal wave action) and associated elevations at the point on the ground to which the wave runs up during the 1 percent annual chance flood. Landward, the flood zones transition to Zone AE with their associated base flood elevations. SOURCE: FEMA’s Map Service Center, <http://msc.fema.gov/>.

both regulating and insuring properties commensurate with the true risk of flooding.

Despite the importance of accurate BFEs in Special Flood Hazard Areas, in unnumbered A and V zones they are generally only estimated using approximate methods (see “Types of Flood Studies” below), which estimate key variables such as water volume. The determination of flood risk is less certain in these areas, so local communities may require a safety factor (known as freeboard) above the estimated BFE for additional financial protection. However, even where BFEs are established with more certainty, communities may impose freeboard to help protect against damage resulting from multiple 1 percent annual chance floods in a given year or higher than expected flood waters.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement