47.5 MB - The Whole Building Design Guide

The following characteristics that may add to the flood depthshould be taken into consideration.Small waves: In Coastal A Zones (see Section2.3.2), the DFE shown on FEMA’s maps does Waves and storm-induced erosion are mostnot include the wave height. Coastal A Zones common in coastal areas. However, wideare characterized by 1.5- to 3-foot high waves. rivers and lakes may experience winddrivenwaves and erodible soils are foundThe flood depth should be increased by 3 feetfor sites close to the V Zone boundary or the throughout the United States. For moreshoreline. For sites farther inland, where the information about waves and erosion, referflood depth is at least 3 feet, it should be increasedby 1.5 feet. Interpolation may be usedto FEMA 55, Coastal Construction Manual.to determine the amount that should be addedto the flood depth to account for waves in the Coastal A Zone.Erosion and scour: Flood depths in areas with erodible soils shouldconsider the effects of erosion where floodwaters lower theground surface or cause local scour around foundation elements.In these areas, the flood depth determined using the design floodelevation should be increased to account for changes in conditionsduring a flood event. Not only does lowering the groundsurface effectively result in deeper water against the foundation, itmay also remove supporting soil from the foundation, which mustbe accounted for in the foundation design.2.1.2.2 Design Flood Velocity—RiverineThere are few sources of information that are readily available forestimating design flood velocities at specific locations along riverinebodies of water. If a riverine source has been studied usingdetailed hydraulic methods, some information may be availablein summary form in published studies. Studies prepared for theNFIP (see Section 2.1.1.2) contain tables of data for waterwaysfor which floodways were delineated. For specified cross-sectionsalong the waterway, the Floodway Data Table includes a meanvelocity expressed in feet per second. This value is the averageof all velocities across the floodway. Generally, velocities in theflood fringe (landward of the floodway) will be lower than in thefloodway.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-19