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Seattle, WA 98104

+ 47.59909 N
- 122.33136 W

+ 47°  35' 56.72"
- 122° 19' 52.90"


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

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Subject category:
Title: Landslide Hazards along River Corridors - Shallow Debris Slides Severe Hazard Potential
Feature Count: 0
Feature Type: Polygon
Abstract: 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.
Purpose: Suitable for general reference and analyses. Identifies only areas of potential shallow debris landsliding along the river corridor within the study limits in King County.
Status: Complete
Updated: None planned
Time period of content: (single date,as YYYYMMDD):: 20160620
Time period of content: (date range, as YYYYMMDD):: -
Spatial reference: WA State Plane North
FGDC Documentation: Classic or FAQ
Open Data Availability: Yes
Place Keywords: Pacific NW, King County, WA, Washington state,
Theme Keywords: enviro, Pacific NW, King County WA, Washington state, landslide, slide, mass movement, landslide hazard mapping, debris slides, shallow landslides, shallow landsliding, debris sliding,
Lineage: Available
Supplemental Information: The stability of slopes with respect to shallow debris failures is often evaluated using a simple limit equilibrium analysis known as the infinite slope model. The infinite slope model is the only commonly used quantitative slope stability model applicable across an entire landscape. For this DNRP mapping project, an infinite slope analysis was used to generally identify slopes potentially subject to shallow debris slides. Soil strength parameters were estimated by measuring the inclination of 45 valley-wall slopes in the study limits that were relatively straight in profile. Following the DOGAMI protocol (Burns et al. 2012), slopes of 24 degrees to 28 degrees (44 to 54 percent) were identified as having a FOS between 1.25 and 1.5 and having a moderate hazard of being subject to shallow debris slides. Slopes steeper than 28 degrees (54 percent) were identified as having a FOS of less than 1.25 and having a severe hazard of being subject to shallow debris slides.


Maintained by:
King County King County Dept of Natural Resources and Parks, Water and Lands Resource Division
Primary: Ken Rauscher, GIS Analyst, 206-477-4413
(How to e-mail a County contact)
Secondary: KCGIS Center; This mailbox is monitored during business hours.

Attribute Information

_MapBy_Author of mapped shapes
_MapMethod_Method used to identify moderate and severe potential for shallow debris slides
Infinite Slope and OR DOGAMI Spec. Paper 45
_LiDARSrc_Source LiDAR imagery and date
KC GIS 07/2016
_TypeHaz_Identifies severe potential for shallow debris slides


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