Resolution No. 4814 - Laserfiche WebLink

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The water content of soils/rock is a major factor in determining the likelihood of sliding for any given slide-prone location. Thus, most landslides happen during rainy months, when soils are saturated with water. However, landslides may happen at any time of the year. In addition to landslides triggered by a combination of slope stability and water content, landslides may also be triggered by earthquakes. Areas prone to seismically triggered landslides are exactly the same as those prone to ordinary (i.e., non-seismic) landslides. As with ordinary landslides, seismically triggered landslides are more likely for earthquakes that occur when soils are saturated with water. Debris flows and landslides are a very common occurrence in Oregon. DOGAMI conducted a statewide survey of landslides arising from the winter storms in February 1996, November 1996, December 1996 and January 1997 and found 9,582 documented landslide locations. The actual number of landslides was estimated to be many times the documented number. 8.2 Landslide Hazard Assessment for the Eugene/Springfield l~etro Area Areas with high landslide potential within the Eugene/Springfield Metro Area's urban growth boundary are shown on Maps 3E, 3S, 4E, and 4S. Maps 3E and 3S show localities in the Eugene/Springfield Metro Area with high potential for surficial debris flows. Concentrations of high potential areas include the hilly regions northeast of the Eugene/Springfield Metro Area and hilly regions south of the Eugene/Springfield Metro Area. Fortunately, most of these high hazard areas are largely undeveloped and most of these areas are outside of the Eugene/Springfield Metro Area Planning Boundary and the Urban Growth Boundaries. However, there are small pockets of high debris flow hazard within the developed areas, primarily in the southern hilly portions of both Eugene and Springfield. Maps 4E and 4S show DOGAMI's classification of landslide hazard areas for the overall Eugene/Springfield Metro Area (Relative Slope Instability maps). The high landslide potential areas on Maps 4E and 4S represent areas with high potential for earthquake induced landslides which are also representative of high potential for rockfalls, rotational slides, and translational slides from non-earthquake events (such as heavy rainfalls). The geographic distribution of these landslide hazard areas within the Eugene/Springfield Metro Area is generally similar to that shown on Maps 3E and 3S, although with some differences in details. Maps 3E, 3S, 4E and 4S should be interpreted cautiously. These maps provide a regional overview of areas with generally high potential for debris flows or slope instabilities. However, such regional maps have limited spatial resolution and thus may not represent the specific landslide risk for any particular parcel in the Eugene/Springfield Metro Area. Thus, these maps are useful for generally hazard awareness and mitigation planning purposes, but should not be used for regulatory purposes. Specific areas that have had historical problems with debris flows and/or landslides within the Eugene/Springfield Metro Area are summarized below in Table 8.5 (which follows the maps). More detailed landslide hazard assessment requires a site-specific Public Review Draft: August 6, 2004 8-3