2014/05/28 Council Agenda Packet

Contributing Factors to Landslides The occurrence of a landslide is dependent on a combination of site -specific conditions and influencing factors. Common factors that contribute to landslides fall into four broad categories: 1. Climatic/hydrologic (rainfall or precipitation) 2. Geomorphic (slope form and conditions - i.e., slope shape, height, steepness, vegetation and underlying geology) 3. Geologic/geotechnical/hydrogeological (groundwater) 4. Human activity. Climatic Climatic factors influencing landslides include the duration of rainfall events, intensity of rainfall, and type of precipitation (i.e., snow or rain), as well as rainfall conditions over a period of time (antecedent conditions). Typically, numerous landslide events are associated with intense and/or prolonged periods of rain (Baum et. al., 2000). Recorded landslides impacting the corridor largely occurred during the winter wet season between October and April. An example of an unusually large, deep-seated landslide occurred in January 1997 south of Edmonds in the town of Woodway (railroad milepost 14.80) following a two -week period of heavy precipitation (Figure 4). Some episodes of widespread landsliding corresponded with storms involving the rapid melting of previously accumulated snow by wind and warm rain, which is referred to as "rain -on -snow" storm event. The landslide cut 50 feet into the property above, passed over the railroad tracks and knocked a freight train into the Puget Sound.1 Many of the shallow landslides prevalent along the corridor have occurred during a single storm event involving one or more days of intense rainfall (Baum et al, 2000). Antecedent conditions Refers to the amount of rainfall that has fallen in previous weeks, months or even years. Figure 4: 1997 Woodway landslide. t www.ecy.wa.gov/programs/sea/landslides/show/woodway.html 11