Landslide Hazards along River Corridors - Shallow Debris Slides Moderate Hazard Potential

Shallow debris slides are a common style of slope movement both in the Puget Lowland and Cascade Mountains. Shallow debris slides are characterized by failure of a relatively shallow layer of soil typically sliding on a surface of more competent material, either bedrock or dense glacial sediments. Shallow debris slides are typically 3 to 6 feet (1 to 2 meters) and translational. Shallow colluvial soils on slopes are formed through a variety of processes, including breaking up of the underlying in-place substrate (either bedrock or Quaternary sediments) by freeze/thaw, wetting/drying, bioturbation, and chemical weathering. Soils on steep slopes in King County vary significantly with respect to soil thickness, soil strength, and hydraulic properties; this variability presents the central challenge in assessing their stability across a landscape. Shallow debris slides are widespread across the landscape and are common occurrences during periods of high rainfall in both mountain and lowland settings. Shallow debris slides are often triggered by intense rainfall or, in developed areas, discharge of stormwater onto a steep slope. The potential for shallow debris slides is often increased by other human activities including vegetation removal (including logging) and inappropriate fill placement. Although comparatively small in scale, shallow debris slides can occur on almost any steep slopes anywhere on the landscape. Because this type of landslide is shallow in depth and small in lateral extent, the debris often does not travel a long distance — unless it transitions into a debris flow. But the debris is typically saturated, can move quickly, and often carrying logs or entire trees. Impact and inundation by such debris can be highly destructive in the area of deposition.