47.5 MB - The Whole Building Design Guide

m Land subsidence, which increases flood depthsm Failure of dams (resulting from seismic activity, lack of maintenance,flows that exceed the design, or destructive acts), whichmay suddenly and unexpectedly release large volumes of waterm Failure of levees (associated with flows that exceed thedesign, weakening by seismic activity, lack of maintenance, ordestructive acts), which may result in sudden flooding of areasbehind leveesEach type of flooding has characteristics that represent importantaspects of the hazard. These characteristics should be consideredin the selection of critical facility sites, the design of new facilities,and the expansion or rehabilitation of existing flood-pronefacilities.Riverine flooding results from the accumulation of runoff fromrainfall or snowmelt, such that the volume of flow exceeds thecapacity of waterway channels and spreads out over the adjacentland. Riverine flooding flows downstream under the forceof gravity. Its depth, duration, and velocity are functions of manyfactors, including watershed size and slope, degree of upstreamdevelopment, soil types and nature of vegetation, topography,and characteristics of storms (or depth of snowpack and rate ofmelting). Figure 2-1 illustrates a cross-section of the generic riverinefloodplain.Flood Hazard Area(100-Year Floodplain)Figure 2-1:The riverine floodplainFringeFloodway*StreamChannelFringeFloodLevel* Floodway is defined in Section 2.1.1.2Normal WaterLevelMAKING CRITICAL FACILITIES SAFE FROM Flooding2-