47.5 MB - The Whole Building Design Guide

have been prepared by the U.S. Army Corpsof Engineers for most reaches of the Atlantic Hurricanes can produce storm surgeand Gulf coasts. The maps combine the resultsof many scenarios to show the maximum than the BFE shown on the FIRMs.flooding and waves that rise much higherpotential surge inundation associated withdifferent categories of hurricanes. State andlocal emergency management offices use the maps for evacuationplanning.2.1.1.3 Design Flood ElevationThe DFE establishes the minimum level of flood protection thatmust be provided. The DFE, as used in the model building codes,is defined as either the BFE determined by the NFIP and shown onFIRMs, or the elevation of a design flood designated by the community,whichever is higher. The DFE will always be at least as highas the BFE. Communities may use a design flood that is higherthan the base flood for a number of reasons.For example, a design flood may be used toaccount for future upland development, torecognize a historic flood, or to incorporate afactor of safety, known as freeboard.Facility owners, planners, and designersshould check with the appropriate regulatoryauthority to determine the minimumflood elevation to be used in site planningand design. Although the NFIP minimumis the BFE, State or local regulations commonlycite the 0.2-percent-annual-chance flood (500-year flood)as the design requirement for critical facilities, or the regulationsmay call for added freeboard above the minimum flood elevation.Even if there is no specific requirement to use the 0.2-percent-annual-chanceflood for siting and design purposes, it is stronglyrecommended that decisionmakers take into consideration theflood conditions associated with this lower probability event orfrom other floods of record.“Freeboard” is a factor of safety usuallyexpressed in feet above a flood level. Freeboardcompensates for the many unknownfactors that could contribute to floodheights, such as wave action, constrictingbridge openings, and the hydrologicaleffect of urbanization of the watershed. Afreeboard from 1 to 3 feet is often appliedto critical facilities.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-15