Resolution No. 4814 - Laserfiche WebLink

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For Lane County as a whole, we note that 2-year and 25-year 24-hour precipitation totals are 4" or more and 5" to 8+", respectively in the Coast Range, with values almost as high in the Cascades. Such totals are high enough to generate significant potential localized flooding problems. However, whether or not localized flooding does occur depends on specific local drainage conditions. For example, 5" of rain in one area may cause no damage at all, while 5" of rain in a nearby area may cause road washouts and flooding of buildings. The rainfall data shown in Table 7.1 give general overview of the potential for winter storm flooding in the Eugene/Springfield Metro Area and elsewhere in Lane County, but whether or not flooding occurs at specific sites depends heavily on specific local drainage conditions. For the Eugene/Springfield Metro Area, identification of specific sites subject to localized flooding during winter storms is based on historical occurrences of repetitive flooding events. Such sites in the area, where localized storm water drainage flooding has been repetitive and problematic, were tabulated in Chapter 6 Floods (see Table 6.2). 7.2.2 Wind Hazard Data Wind speeds associated with winter storms vary depending on meteorological conditions, but also vary spatially depending on local topography. For Lane County, the wind hazard levels are highest directly at the coast and then fairly uniform across most of the rest of the county. In the hilly areas, however, the level of wind hazard is strongly determined by local specific conditions of topography and vegetation cover. A regional overview of wind hazards is shown by the data in Figures 7.2 and 7.3 which show contours of wind speed (in kilometers per hour) for western Oregon (Wantz and Sinclair, Distribution of Extreme Wind Speeds in the Bonneville Power Administration Service Area, Journal of Applied Meteorology, Volume 20, 1400~1411, 1981). These data are for the standard meteorological data height of 10 meters (about 39 feet) above ground level. Figures 7.2 and 7.3 show wind speed contours for recurrence intervals of 2-years and 50-years, respectively. These data are for sustained wind speeds. Peak gusts are commonly 30% or so higher than the sustained wind speeds. These wind-speed data are fairly old, but still representative of overall wind storm conditions in Oregon. For the Eugene/Springfield Metro area, the 2-year and 50-year sustained wind speeds are about 60 km/hour and 100 km/hr, respectively. These values correspond to 2- and 50-year wind speeds of about 37 miles/hr and 62 miles/hr, respectively. These 2- year wind speeds are too Iow to cause widespread substantial wind damage. However, there may be significant local wind damage at sites where local wind speeds are higher or where there are especially exposed locations, such at the boundary between clear cut and forested areas. 50-year recurrence interval wind speeds are high enough to cause widespread wind damage. Damage may be severe at particularly exposed sites. Thus, for the Eugene/ Springfield Metro Area, winter storms with significant direct wind damage are not likely Public Review Draft: August 6, 2004 7-5