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CHAPTER 12. LANDSLIDE <br />12-4 <br /> <br />Figure 12-5. Kittitas County Landslide Hazard Areas <br />12.2.2 Frequency <br />Landslides are often triggered by other natural hazards such as earthquakes, heavy rain, floods or wildfires, <br />so landslide frequency is often related to the frequency of these other hazards. In Kittitas County, landslides <br />typically occur during and after major storms, so the potential for landslides largely coincides with the <br />potential for sequential severe storms that saturate steep, vulnerable soils. Landslide events occurred during <br />the winter storms of 2009 and 2011. According to SHELDUS records, the planning area has been impacted <br />by severe storms at least once every other year since 1960. Until better data is generated specifically for <br />landslide hazards, this severe storm frequency is appropriate for the purpose of ranking risk associated with <br />the landslide hazard. <br />In general, landslides are most likely during periods of higher than average rainfall. The ground must be <br />saturated prior to the onset of a major storm for significant landsliding to occur. Most local landslides occur <br />in January after the water table has risen during the wet months of November and December. Water is <br />involved in nearly all cases; and human influence has been identified in more than 8 0 percent of reported <br />slides.