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

For more information about this layer please see the GIS Data Catalog. 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.如需了解该图层的更多信息，请参阅地理信息系统（GIS）数据目录。浅层碎屑滑坡是皮吉特低地（Puget Lowland）与喀斯喀特山脉（Cascade Mountains）范围内常见的斜坡运动类型，其典型特征为相对浅层的土体发生破坏，通常沿更坚固的介质（基岩或密实冰川沉积物）表面滑动，厚度通常为3至6英尺（1至2米），且属于平移式滑坡。斜坡上的浅层坡积土（colluvial soils）由多种作用形成，包括通过冻融、干湿循环、生物扰动与化学风化作用分解下伏原位基质——基岩或第四纪沉积物（Quaternary sediments）。金县（King County）陡坡上的土体在厚度、强度与水力特性方面存在显著差异，这种空间异质性是全域斜坡稳定性评估面临的核心挑战。浅层碎屑滑坡广泛分布于各类地形中，在山地与低地的强降雨时段均为常见灾害，通常由强降雨诱发；在开发区域，暴雨径流冲刷陡坡也可触发滑坡。其他人类活动，如植被移除（含伐木作业）与不当填土作业，通常会加剧浅层碎屑滑坡的发生风险。尽管此类滑坡规模相对较小，但几乎可发生于全域范围内的任意陡坡之上。由于该类滑坡深度较浅且波及范围有限，滑坡碎屑通常不会远距离运动，除非其演变为碎屑流。但滑坡碎屑通常处于饱和状态，运动速度快，且常裹挟原木乃至整棵树木，此类碎屑在堆积区造成的冲击与淹没效应，往往具有极强的破坏性。