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CHAPTER 12. LANDSLIDE <br />12-3 <br />are no records in the county of fatalities attributed to mass movement. However, deaths have occurred <br />across the west coast as a result of slides and slope collapses. The Oso Landslide of 2014 in Snohomish <br />County, WA provides context for the worst case scenario. This event caused 43 fatalities, and destroyed 40 <br />homes as well as nearly a mile of State Route 530. The landslide moved about 18 million tons of sand, till, <br />and clay at an average speed of 40 mph. <br />Location <br />The best available predictor of where movement of slides and earth flows might occur is the location of <br />past movements. Past landslides can be recognized by their distinctive topographic shapes, which can <br />remain in place for thousands of years. Most landslides recognizable in this fashion range from a few acres <br />to several square miles. Most show no evidence of recent movement and are not currently active. A small <br />proportion of them may become active in any given year, with movements concentrated within all or part <br />of the landslide masses or around their edges. <br />The recognition of ancient dormant mass movement sites is important in the identification of areas <br />susceptible to flows and slides because they can be reactivated by earthquakes or by exceptionally wet <br />weather. Also, because they consist of broken materials and frequently involve disruption of groundwater <br />flow, these dormant sites are vulnerable to construction-triggered sliding. <br />The basis for the mapping for this risk assessment is the Landslide Hazard Zonation Project prepared by <br />the Forest Practices Division of the Washington Department of Natural Resources. Identification of <br />unstable slopes to aid in mitigation of landslide hazards is now an integral part of land management and <br />regulation in Washington. Permanent rules adopted by the Washington Forest Practices Board in 2001 <br />address landslide hazards from specific landforms across the state (WAC 222-16-050 (1)(d)). This <br />methodology was developed to provide standardized methods for landslide inventories and for producing <br />hazard maps to identify unstable slopes in support of forest practices rules. It also provides a framework <br />for monitoring the success of new forest practices related to unstable slopes. The Landslide Hazard Areas <br />for the planning area are shown in Figure 12-5. All buildings within the hazard areas are located in <br />Unincorporated Kittitas County. For additional information on exposure, see Section 12.5.