Resolution No. 4814 - Laserfiche WebLink

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6.3.3 Interpreting Flood Hazard Data for Mapped Floodplains The level of flood hazard (frequency and severity of flooding) is not determined simply by whether the footprint of a given structure is or is not within the 100-year floodplain, A common error is to assume that structures within the 100-year floodplain are at risk of flooding while structures outside of the 100-year floodplain are not. Some important guidance for interpreting flood hazard is given below, A. Being in the 100-year floodplain does not mean that floods happen once every 100 years. Rather, a 100-year flood simply means that the probability of a flood to the 100-year level or greater has a 1% chance of happening each year. B. Much flooding happens outside of the mapped 100-year floodplain. First, the 100-year flood is by no means the worst possible flood. For flooding along the Willamette River, the 500-year flood is 4 feet higher than the 100-year flood. Thus, floods greater than the 100- year event will flood many areas outside of the mapped 100-year floodplain. Second, many flood prone areas flood because of local storm water drainage conditions. Such flood prone areas have nothing to do with the 100-year floodplain boundaries. C. The key determinant of flood hazard for structure or other facility is the relationship of the elevation of the structure or facility to the flood elevations for various flood events. Thus, homes with first floor elevations below or near the 10-year flood elevation have drastically higher levels of flood hazard than other structures with first floor elevations near the 50-year or 100-year flood elevation. The importance of first floor elevations in determining flood hazard levels is illustrated in the data shown below in Figure 6.2. These data show the statistical return period for flooding reaching the first floor for structures with various first floor elevations near the Willamette River at State Highway 126 in Springfield. Homes with first floor elevations at 434 or 435 feet have return periods for flooding of less then 10 years. As floor elevations increase, the return period for flooding increases markedly, with homes a 440 feet expected to flood only once about every 117 years and homes at 443 feet expected to flood only about once every 376 years, on average. Thus, even in the same neighborhood or the same block, the level of flood hazard for homes varies markedly depending on the specific elevations of each home. Public Review Draft: August 6, 2004 6-7