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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 142 <br />4.6.6. Landslide <br />4.6.6.1. Description <br />A landslide is the downslope movement of a mass of rock, debris, or earth at rates that range from inches <br />per year to tens of miles per hour. Landslides are a type of "mass wasting," which denotes any downslope <br />movement of soil and rock under the direct influence of gravity. The landslide term is comprised of five <br />(5) types of slope movement – falls, topples, slides, spreads, and flows – which are further subdivided by <br />the type of geologic material (i.e., bedrock, debris, or earth).167F <br />167 A major difference between the types of <br />landslides is their depth and the amount of water they contain. Slope movement occurs when forces <br />acting downslope exceed the strength of the earth’s material that composes the slope. The modes of <br />slope movement are listed in Table 4-86.168F <br />168 <br /> <br />Landslides are an important natural process because they shape the Earth’s landscape and contribute <br />to the overall environmental quality of a region. However, when landslides impact people, property, or <br />infrastructure it is considered a natural hazard. <br />Table 4-86. Types of Slope Movement <br />Type Description <br />Falls and Topples <br />Usually rapid, downward movement of large pieces of rock or debris. Sometimes this is <br />enough rock to cover a road or block a stream or river. Rockfalls and topples are <br />common in Washington’s mountain highways. <br />Slides <br />Downslope movements of soil or rock along a discrete or narrow failure surface and can <br />be deep-seated or shallow. The initiation of slides, like flows or rockfalls, is sensitive to <br />steep slopes, the additional weight of water or other loads, and friction along their base. <br /> <br />• Translational Slides: Usually fail along geologic discontinuities such as faults, <br />joints, bedding surfaces, or the contact between two rock types. They move out <br />or down along a planar surface with little tilting and can travel great distances. <br />Translational slides can contain loose sediments or large slabs of bedrock. <br />• Block Slides: A type of translational slide that occurs when large and relatively <br />intact slabs of rock or earth are rapidly transported downslope. These type of <br />landslides can be large and damaging and occur where alternating layers of <br />strong and weak rock slope downhill. <br />• Rotational Slides (Slumps): occur along a curved or spoon-shaped surface. <br />Back-tilting may occur near the scarp of the landslide and there is often a toe of <br />displaced material. Rotational slides often occur because the internal strength <br />of the material is overcome by its own weight. They are usually composed of <br />relatively loose or unconsolidated material. <br /> <br />167 United States Geological Survey. (n.d.). What is a Landslide and What Causes One? Retrieved from <br />https://www.usgs.gov/faqs/what-landslide-and-what-causes-one. <br />168 Washington State Department of Natural Resources. (n.d.). Landslides. Retrieved from <br />https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/landslides.