Resolution_2025 Kittitas County Hazard Mitigation Plan

Home Browse Search

2025 Hazard Mitigation Plan Kittitas County, Washington Chapter 4: Hazard Identification and Risk Assessment 143 Type Description Flows It is generally a slurry mixture of water, soil, rock and (or) debris that moves rapidly downslope. Flows may or may not be confined to a channel. • Earthflows: Have a characteristic 'hourglass' shape. The slope material loses strength and runs out, often forming a bowl or depression at the head. Flows usually occur in fine-grained material on moderate, water-saturated slopes. • Debris Flows: Usually occur in steep gullies, move very rapidly, and can travel for many miles. They may contain more coarse material than mudflow when channelized. Slopes where vegetation has been removed by fire or humans are at greater risk for debris flows and many other types of landslides. • Debris Avalanches: Unchanneled debris flows that move very rapidly. They typically do not mobilize far and sometimes move like a snow avalanche. • Lahars: debris flows that originate on volcanoes. A volcanic eruption can rapidly melt snow and ice, causing a deluge of rock, soil, ash, and water that accelerates down the slopes of a volcano, devastating anything in its path. They can travel great distances and damage structures in flat areas far from their source. • Lateral Spreads: occur on very low-angle slopes toward a free face such as a cliff or embankment. Movement is accompanied by cracking of the ground. Failure is often caused by liquefaction (when soil is transformed from a solid to a liquid), usually because of an earthquake. • Soil Creep: is the very slow (inches/year), steady, downward movement of soil or rock. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. Complex Occurs when more than one (1) type of movement occurs sequentially during a failure. Composite Occurs when more than one (1) type of movement does not occur sequentially during a failure. 4.6.6.2. Hazard Location The best available predictor of where movement of slides and earth flows might occur is the location of past movements. Past landslides can be recognized by their distinctive topographic shapes, which can remain in place for thousands of years. Most landslides recognizable in this fashion range from a few acres to several square miles and many show no evidence of recent movement and are not currently active. A small proportion of them may become active in any given year, with movements concentrated within all or part of the landslide masses or around their edges. The recognition of ancient dormant mass movement sites is important in the identification of areas susceptible to flows and slides because they can be reactivated by earthquakes or by exceptionally wet weather. Also, because they consist of broken materials and frequently involve disruption of groundwater flow, these dormant sites are vulnerable to construction-triggered sliding. Some areas are more likely to experience landslides, such as: 169F 169 • Areas where wildfires or human modification of the land have destroyed vegetation. 169 Centers for Disease Control and Prevention. (2024). Landslides and Mudslides and Your Safety. Retrieved from https://www.cdc.gov/landslides-and-mudslides/about/index.html.