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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 145 <br />4.6.6.3. Hazard Extent/Intensity <br />In the United States, landslides cause billions of dollars in damages and kill between 25 and 50 people <br />annually. Casualties are primarily caused by rockfalls, rockslides, and debris flows. Worldwide, landslides <br />cause thousands of casualties and billions of dollars in damages.170F <br />170 <br /> <br />Landslides can occur suddenly or slowly. The velocity of mass movement depends on slope angle, <br />material, and water content. Some methods used to monitor mass movements can provide an idea of <br />the type of movement and the amount of time prior to failure. It is also possible to determine what areas <br />are at risk during general time periods. Assessing the geology, vegetation, and amount of predicted <br />precipitation for an area can help in these predictions. However, there is no practical warning system for <br />individual landslides. <br /> <br />USGS provides a system of notification for potential landslides, listed in Table 4-87. Watches and <br />warnings may be issued for discrete areas and include advice about contacting the area's local <br />emergency management. Additionally, rainfall induced debris flows watches and warning are weather <br />dependent and will closely track NWS advisories for flash flooding.171F <br />171 <br />Table 4-87. USGS Landslide Advisories <br />Type Description <br />Landslide Advisory <br />A general statement about the potential of landslide activity in a given region relative to <br />developing rainfall predictions. An advisory may include general statements about <br />rainfall conditions that can lead to debris flow activity, and list precautions to be taken in <br />the event of heavy rainfall. <br />Landslide Watch <br />Means that landslide activity will be possible but is not imminent. People in, or planning <br />to travel through, a watch area should know landslide preparedness and stay informed <br />about developing weather patterns. <br />Landslide Warning Indicate that landslide activity is presently occurring, and extreme caution should be <br />taken. <br />4.6.6.4. Probability and Frequency <br />The probability of occurrence for landslides in Kittitas County is medium because significant occurrences <br />of this hazard have occasionally impacted the County and will likely occur again within 25 years. <br />According to the 2023 Washington State Enhanced Hazard Mitigation Plan, the probability of future <br />landslide related disaster declarations each year is 44% in the State. <br /> <br />Landslides are often triggered by other natural hazards such as earthquakes, heavy rainfall, floods, or <br />wildfires; as a result, landslide frequency is often related to the frequency of these other hazards. In <br />Kittitas County, landslides typically occur during and/or after heavy rainfall events, so the potential for <br />landslides largely coincides with the potential for consecutive heavy rainfall events that saturate steep, <br />vulnerable soils. However, the ground must be saturated prior to the onset of a heavy rainfall event for <br />significant land sliding to occur. Most local landslides occur in January when the water table is higher <br />after the wetter months of November and December. Water is involved in nearly all cases and human <br />influence has been identified in more than 80% of reported landslides. Furthermore, as a result of climate <br /> <br />170 United States Geological Survey. (2004). Landslide Types and Processes. Retrieved from <br />https://pubs.usgs.gov/fs/2004/3072/pdf/fs2004-3072.pdf. <br />171 United States Geological Survey. (n.d.). What is the Difference Between a Landslide Advisory, a Landslide <br />Watch, and a Landslide Warning? Retrieved from https://www.usgs.gov/faqs/what-difference-between-landslide- <br />advisory-landslide-watch-and-landslide-warning.