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<br />CHAPTER FOUR <br />ASSESSING CHARACTERISTICS OF FLOOD SITUATIONS <br /> <br />When evaluating the possibility of flood proofing, consideration must be given to the characteristics of <br />the flooding that affect the building. These characteristics, along with the characteristics of the site and <br />the building, suggest the most effective and feasible flood proofing options. <br /> <br />8 ,"00.'" c,,'^~'"="" <br /> <br />Flood Depth <br /> <br />A building is susceptible to floods of various depths, with floods of greater depth occurring less fre- <br />quently than floods of lesser depths. Potential flood elevations from significant flooding sources are <br />shown in flood insurance studies for communities participating in the NFIP. For the purpose of assessing <br />the depth of flooding likely to impact a structure, it is convenient to use the flood levels shown in FISs, <br />historical flood levels, and/or flood information from other studies and reports. The depth of flooding af- <br />fecting a structure can be calculated by determining the height of the flood above the ground elevation at <br />the site of the structure. <br /> <br />If a building is subject to flooding depths greater than three feet, elevating or relocating the structure are <br />the most effective measures of flood proofing. Other measures may not be as appropriate because water <br />depths greater than three feet may cause hydrostatic forces high enough to render structural damage or <br />cause wall collapse. Flood proofing with levees and flood walls for depths greater than three feet can be <br />undertaken, but may require structural devices to control flow under the levee or flood wall as well as <br />other expensive methods. <br /> <br />If a well constructed building is subject to flooding depths less than three feet, hydrostatic forces are not <br />as significant. Therefore, consideration can be given to using barriers, sealants, and closures for flood <br />proofing. If shallow flooding causes a basement to fill with water, wet flood proofing can be used to <br />reduce flood damage. Special devices are available to prevent basement flooding due to water back-up <br />through sewers. <br /> <br />Flood Velocity <br /> <br />The speed at which flood waters move-the flood flow velocity-is normally expressed in terms of feet <br />per second (fps). As floodwater velocity increases, hydrodynamic forces are added to the hydrostatic <br />forces from the depth of still water, significantly increasing the possibility of building failure. Greater <br />velocities can quickly erode or scour the soil surrounding buildings. These fast-moving waters can also <br />result in structural failure by erosion and their impact may move a building from its foundation. When <br />floodwater velocities exceed three fps, it becomes difficult, if not impossible, for adults to maintain their <br />balance while walking through a flooded area. Unfortunately. there is usually no readily available source <br />of information to determine potential flood velocities in the vicinity of specific buildings. Historical <br />information from past flood events is probably the most reliable source. If personal knowledge of past <br />flood erosion and/or movement of structures is not available, others in the neighborhood may be able to <br />provide this type of information. If specific information on flood velocities is available and indicates that <br />the building is subject to flood waters with velocities greater than three fps, professional advice is critical <br />in the selection of an appropriate flood proofing measure. <br /> <br />14 <br />