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
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.