Resolution No. 5303 - Laserfiche WebLink

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Eugene -Springfield Area Natural Hazards Mitigation Plan 2. Hazard Descriptions 2.6.6 Probability of Future Occurrence The probability of riverine flooding in Eugene -Springfield is moderate and the probability of stormwater flooding is high. A moderate probability indicates one riverine flooding incident is likely in the neat 35 to 75 years. A high probability indicates one stormwater flooding incident is likely within the neat 35 years. 2.6.6 Vulnerability Assessment and Capacity 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 base floodplain (also referred to as the 100-year floodplain). A common error is to assume structures within the base floodplain are at risk of flooding while structures outside of the base floodplain are not. Some important guidance for interpreting flood hazard is given below. Being in the 100-year (or base) floodplain does not mean floods happen once every 100 years. Rather, it means the probability of a flood in the 100- year base flood level or higher has a 1% chance of happening each year. Much flooding happens outside of the mapped base 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 base flood. Second, many flood prone areas flood because of local stormwater drainage conditions. Such flood prone areas may have nothing to do with the base floodplain boundaries. The key determinant of a structure's flood hazard is the relationship of the structure's elevation to the flood elevations for various flood incidents. Thus, homes with first floor elevations below or near the 10-year flood elevation have drastically higher probabilities of flooding than other structures with first floor elevations near the 50-year or 100-year flood elevation. Areas protected by flood control levees, such as Springfield's 42nd Street Levee, were originally mapped as being protected from the 100-year flood incident. However, in response to numerous levee failures during Hurricane Katrina, levees now must also be certified as being structurally adequate to retain their accreditation as flood control structures. If the City of Springfield is unable to obtain certification for the 42nd Street Levee, the neat update of the flood control maps for the section of the McKenzie River paralleled by the levee may be prepared as if the levee was not in place. This would greatly increase the area of the City within the mapped 100-year floodplain. In Oregon, Oregon Administrative Rule 660-008 requires local governments, when planning for needed housing, ensure it is located on buildable land "...suitable, available and necessary for residential uses." 2-36 January 2020