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2025 Hazard Mitigation Plan <br />Kittitas County, Washington <br /> <br /> <br />Chapter 4: Hazard Identification and Risk Assessment 146 <br />change, the frequency of landslides in areas at risk is expected to increase due to the impacts of extreme <br />precipitation and wildfires (post-wildfire debris flows).172F <br />172 <br /> <br />The landslide annualized frequency value represents the average number of recorded landslide hazard <br />occurrences in event days, per year over the period of record (11.8 years). Table 4-88 outlines the <br />annualized frequency for landslides, based on FEMA NRI data, for Kittitas County. <br />Table 4-88. Landslide Annualized Frequency (FEMA National Risk Index) <br />Location Events on Record <br />(2010 – 2021) Annualized Frequency <br />Kittitas County 4 0.1 events per year <br />Annualized frequency is defined as the expected frequency or probability of a hazard occurrence per year. <br />4.6.6.5. Past Events <br />There is little recorded information regarding landslides in Kittitas County. Between 1960 and 2023, there <br />have been nine (9) significant landslide events in Kittitas County most of which have coincided with <br />presidential disaster declarations for severe storms and flooding. The recorded events occurred on <br />January 26, 1965; December 18, 1972; January 17, 1997; January 9, 2009; October 11, 2009; January <br />11, 2011; November 12, 2015; June 1, 2018; and February 2020.173F <br />173 Although there are no recorded <br />fatalities in the County attributed to landslides, deaths have occurred across the west coast. The Oso <br />Landslide of 2014 in Snohomish County, Washington provides context for the worst case scenario. This <br />event caused 43 fatalities, destroyed 40 homes, and buried approximately one (1) mile of State Route <br />530. The landslide moved about 18 million tons of sand, till, and clay at an average speed of 40 miles <br />per hour. <br />4.6.6.6. Vulnerability and Impacts <br />Life Safety and Health: In the United States, landslides result in 25 to 50 deaths each year. Landslides <br />can cause injuries and fatalities to anyone that gets trapped in a vehicle or structure during a landslide, <br />either in transit or if they happen to be standing in the area when the landslide occurs. Buildings, roads, <br />and other structures built on landslide prone slopes can collapse trapping people causing injury or death <br />during a landslide incident. Additionally, landslides can block roads and pathways impacting access to <br />emergency services potentially hindering evacuation efforts.174F <br />174 In general, all of the population exposed <br />to the higher risk landslide areas are considered to be vulnerable. As the population increases and many <br />homes are built on property above or below bluffs and on steep slopes subject to mass movement, <br />increases the number of lives endangered by this hazard. <br /> <br />Property Damage and Critical Facilities: Landslides can severely damage or destroy homes, rendering <br />them uninhabitable. Additionally, property value may decrease in landslide prone areas and the cost of <br />repairs can be significant. Roads, bridges, and utility lines (e.g., power, communication, water, irrigation, <br />and gas) are vulnerable and can be impacted by a landslide incident. Access to major roads is crucial to <br />life safety and response operations after a disaster event and recovery operations. Landslides can block <br />egress and ingress on roads, causing neighborhoods to become isolated, traffic problems, and delays <br /> <br />172 Washington State Emergency Management Division. (2023). Washington State Enhanced Hazard Mitigation <br />Plan. Retrieved from https://mil.wa.gov/asset/651ec296d76a9/2023_WA_SEHMP_final_20231004.pdf . <br />173 Events prior to 2018 were obtained from the Spatial Hazard Events and Losses Database for the United States <br />(SHELDUS) for the 2019 update of the Hazard Mitigation Plan. However, SHELDUS was not utilized the 2024 <br />update and events that on or after 2018 were provided by the Kittitas County Public Works Department. <br />174 Centers for Disease Control and Prevention. (2024). Landslides and Mudslides and Your Safety. Retrieved <br />from https://www.cdc.gov/landslides-and-mudslides/about/index.html.