47.5 MB - The Whole Building Design Guide

For waterways without detailed studies, methods that are commonlyused in civil engineering for estimating open channel flow velocitiescan be applied.Upper bound velocities caused by HurricaneKatrina along the Mississippi coast,where storm surge depths neared 35 feetdeep, have been estimated at nearly 30feet per second (20 miles per hour).2.1.2.3 Design Flood Velocity—CoastalEstimating design flood velocities in coastal flood hazard areas issubject to considerable uncertainty, and there is little reliable historicalinformation or measurements from actual coastal floodevents. In this context, velocity does not refer to the motion associatedwith breaking waves, but the speed of the mass movement offloodwater over an area.The direction and velocity of floodwaters can vary significantlythroughout a coastal flood event. Floodwaters can approach asite from one direction as a storm approaches, then shift to anotherdirection (or through several directions) as the stormmoves through the area. Floodwaters can inundate some low-lyingcoastal sites from both the front (e.g., ocean) and the back (e.g.,bay, sound, or river). In a similar manner, at any given site, flowvelocities can vary from close to zero to very high. For these reasons,when determining flood loads for building design, velocitiesshould be estimated conservatively and it should be assumed thatfloodwaters can approach from the most critical direction andthat flow velocities can be high.Despite the uncertainties, there are methods to approximatecoastal flood velocities. One common method is based on thestillwater depth (flood depth without waves). Designers shouldconsider the topography, the distance from the source of flooding,and the proximity to other buildings and obstructions before selectingthe flood velocity for design. Those factors can direct andconfine floodwaters, with a resulting acceleration of velocities.This increase in velocities is described as the“expected upper bound.” The “expectedlower bound” velocities are experienced inareas where those factors are not expectedto influence the direction and velocity offloodwaters.2-20 MAKING CRITICAL FACILITIES SAFE FROM Flooding