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2025 Hazard Mitigation Plan Kittitas Gounty, Washington ffitffigci 4.6,6.3. Hazard ExtenUlntensity ln the United States, landslides cause billions of dollars in damages and kill between 25 and 50 people annually. Casualties are primarily caused by rockfalls, rockslides, and debris flows. Worldwide, landslides cause thousands of casualties and bitlions of dollars in damages.-170 Landslides can occur suddenly or slowly. The velocity of mass movement depends on slope angle, material, and water content. Some methods used to monitor mass movements can provide an idea of the type of movement and the amount of time prior to failure. lt is also possible to determine what areas are at risk during general time periods. Assessing the geology, vegetation, and amount of predicted precipitation for an area can help in these predictions. However, there is no practicalwarning system for individual landslides. USGS provides a system of notification for potential landslides, listed in Table 4-87. Watches and warnings may be issued for discrete areas and include advice about contacting the area's local emergency management. Additionally, rainfall induced debris flows watches and warning are weather dependent and will closely track NWS advisories for flash flooding.-171 Table 4-87. USGS Landslide Advisories 4.6.6.4. Probability and Frequency The probability of occurrence for landslides in Kittitas County is medium because significant occurrences of this hazard have occasionally impacted the County and will likely occur again within 25 years. According to the 2023 Washington State Enhanced Hazard Mitigation Plan, the probability of future landslide related disaster declarations each year is 44% in the State. Landslides are often triggered by other natural hazards such as earthquakes, heavy rainfall, floods, or wildfires; as a result, landslide frequency is often related to the frequency of these other hazards. ln Kittitas County, landslides typically occur during and/or after heavy rainfall events, so the potential for landslides largely coincides with the potential for consecutive heavy rainfall events that saturate steep, vulnerable soils. However, the ground must be saturated prior to the onset of a heavy rainfall event for significant land sliding to occur. Most local landslides occur in January when the water table is higher after the wetter months of November and December. Water is involved in nearly all cases and human influence has been identified in more than 80% of reported landslides. Furthermore, as a result of climate 170 United States Geological Survey. (2004). Landslide Types and Processes. Retrieved from https ://pu bs. usqs. qov/fs/2004/3072/pdf/fs2004-3072. pdf . 171 United States Geological Survey. (n.d.). What is the Difference Between a Landslide Advisory, a Landslide Watch, and a Landslide Warning? Retrieved from https:/iwww.usqs.qov/faqs/what-difference-between-landslide- -and-landslide-wa Landslide Advisory A general statement about the potential of landslide activity in a given region relative to developing rainfall predictions. An advisory may include general statements about rainfallconditions that can lead to debris flow activity, and list precautions to be taken in the event of heavy rainfall. Means that landslide activity will be possible but is not imminent. People in, or planning to travelthrough, a watch area should know landslide preparedness and stay informed about developinq weather patterns. Landslide Watch Landslide Warning lndicate that landslide activity is presently occurring, and extreme caution should be taken. DescriptionType Chapter 4: Hazard ldentification and Risk Assessment 145