47.5 MB - The Whole Building Design Guide

valley or the presence of obstructing bridges, riverine floodingmay rise just a few feet or tens of feet above normal levels. Thedepth of coastal flooding is influenced by such factors as the tidalcycle, the duration of the storm, the elevation of the land, andthe presence of waves. Depth is a critical factor in building design,because the hydrostatic forces on a vertical surface (such asa foundation wall) are directly related to depth, and because costsassociated with protecting buildings from flooding increase withdepth. Under certain conditions, hurricanes can produce stormsurge flooding that is 20 to 30 feet above mean sea level or, in extremecases such as reported during Hurricane Katrina, as muchas 35 feet above mean sea level.Duration: Duration is the measure of how long the water remainsabove normal levels. The duration of riverine flooding is primarilya function of watershed size and the longitudinal slope ofthe valley (which influences how fast water drains away). Smallwatersheds are more likely to be “flashy,” which refers to the rapiditywith which floodwaters rise and fall. Areas adjacent to largerivers may be flooded for weeks or months. Most coastal floodingis influenced by the normal tidal cycle, as well as how fast coastalstorms move through the region. Areas subject to coastal floodingcan experience long duration flooding where drainage is pooror slow as a result of topography or the presence of flood controlstructures. For example, there may be depressionsin the land that would hold water,or water may be trapped behind a floodwallor levee with inadequate drainage. Morecommonly, coastal flooding is of shorterduration, on the order of 12 to 24 hours, especiallyif storms move rapidly. Floodingalong large lakes, including those behinddams, can be of very long duration becausethe large volume of water takes longer todrain. For building design, duration is importantbecause it affects access, buildingusability, and saturation and stability of soilsand building materials. Information aboutflood duration is sometimes available as partof a flood study, or could be developed by aqualified engineer.Local drainage problems create pondingand local flooding that often is not directlyassociated with a body of water such asa creek or river. Although such flooding isrelatively shallow and not characterizedby high velocity flows, considerabledamage may result. Areas with poordrainage frequently experience repetitivedamage. Some local drainage problemsare exacerbated by old or undersizeddrainage system infrastructure. Floodingcaused by drainage problems typicallyoccurs as sheetflow or along waterwayswith small drainage areas. This type offlooding is often not mapped or regulated.MAKING CRITICAL FACILITIES SAFE FROM Flooding2-