47.5 MB - The Whole Building Design Guide

Site modifications are not appropriatein floodways along riverine waterways,where obstructions to flows can increaseflood elevations. Engineering analyses arerequired to determine the impact of suchmodifications.creased level of protection to buildings. Theevaluations involve engineering analyses todetermine whether the desired level of protectionis cost-effective, and whether theproposed site modifications alter the floodplainin ways that could increase flooding.The effectiveness of typical site modificationsand their ramifications must be examined foreach specific site.Earthen fill: Fill can be placed in the flood hazard area for the purposeof elevating a site above the design flood elevation. If the fillis placed and compacted so as to be stable during the rise and fallof floodwaters, and if the fill is protected from erosion, then modifyinga site with fill to elevate a facility is preferred over othermethods of elevation. Not only will buildings be less exposed toflood forces, but, under some circumstances (such as long durationfloods), critical facilities may be able to continue to function.Whether nonstructural fill is placed solely to modify the site, orstructural fill is placed for the purpose of elevating buildings,placement of fill can change flooding characteristics, includingincreased flooding on other properties. Engineering analysescan be conducted to determine whether eliminating floodplainstorage by filling will change the direction of the flow of water,create higher flow velocities, or increase the water surface elevationin other parts of the floodplain. Fill is a less effective elevationmethod in flood hazard areas exposed to wave action, such as thebanks of wide rivers, back bays, or Coastal A Zones, because waveaction may erode the fill and adequate armoring or other protectionmethods can be expensive.Excavation: Excavation alone rarely results in significantly alteringthe floodplain on a given parcel of land. Excavation that modifiesa site is more commonly used in conjunction with fill in order tooffset or compensate for the adverse impacts of fill.Earthen levee: A levee is a specially designed barrier that modifiesthe floodplain by keeping the water away from certain areas (seeFigure 2-23). Levees are significant structures that require detailed,site-specific geotechnical investigations; engineering analyses toidentify whether flooding will be made worse on other properties;structural and site design to suit existing constraints; design of in-2-56 MAKING CRITICAL FACILITIES SAFE FROM Flooding