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contribution or detraction from stability needs to be evaluated on a site -specific basis by <br />qualified professionals. <br />Implications <br />While a landslide on an adjacent slope does not always impact the rail line, about 80 percent of <br />the documented landslides between 1914 and 2001 generated debris that reached one or both <br />railroad tracks. Despite the investigation bias of this data (landslides are generally only <br />investigated when they might affect the tracks), the close proximity of the tracks to the base of <br />the steep slopes and the very limited area available for debris containment is a primary reason for <br />the apparent high likelihood of impact to the tracks when a landslide does occur. The volume of <br />debris, material and transport characteristics [i.e., material composition, velocity, viscosity <br />(thickness), path of travel, etc.], location of landslide initiation, and the potential to gather <br />additional material during transport (bulking) further influence the potential for debris run -out <br />onto the tracks and the extent of impacts. <br />Given the wide range of potential factors that influence landslide initiation characteristics, it is <br />virtually impossible to predict the location and impacts of a single event within such a long <br />landslide -prone corridor. However, of all the potential influencing factors, five factors were <br />judged by Shannon & Wilson (2001) to be the most differentiating in quantifying risk of <br />landslide -related impacts to the tracks: <br />1. Density of slides — Number of historic landslides per quarter mile of track. <br />2. Catchment area — Available area between the base of the slope and tracks to contain <br />debris. <br />3. Slope height — Influences both debris volume and impact/run-out characteristics. <br />4. Geology — Tendency of specific geologic units to experience landslides. <br />5. Line closures — Percentage of total number of landslides per quarter mile of track that <br />impacts tracks. <br />Such experience is invaluable for prioritizing where and what type of future mitigation should be <br />considered when funding for capital improvements is available. There is ongoing research to <br />develop better understanding of the precise climatic conditions that have a high potential of <br />generating shallow landslides. <br />15 <br />