Assisted Island County with emergency-response actions and long-term planning related to a major landslide. The Ledgewood Landslide that occurred on March 27, on the west side of Whidbey Island was one of the largest shoreline landslides in recorded Puget Sound history. The slide created a near-vertical headscarp at the crest of the slide area that was approximately feet wide. The slope failure pushed debris approximately feet into Admiralty Inlet, with an estimated 20 to 30 feet of uplift at the toe.
The Ledgewood Landslide occurred within an older landslide complex approximately one mile long, and destroyed one residence, put several others at risk, and cut off access to approximately 14 homes. GeoEngineers provided a rapid site assessment the day of the slide, followed by an extensive, short-turnaround subsurface exploration and instrumentation program.
Within hours after the hillside collapsed, GeoEngineers visited the site with ICPW personnel and began a comprehensive geologic reconnaissance of the landslide area. In consultation with ICPW, we recommended which residences in the slide area should be temporarily or permanently abandoned and assessed when temporarily evacuated residents could return to their homes. We also completed a multi-phase geotechnical investigation of the landslide area and conducted stability analyses to evaluate risks to public safety and infrastructure in the vicinity of the slide.
At one point during the field investigation phase, we had five drilling rigs and crews working simultaneously at the site to obtain critical subsurface information. GeoEngineers also contracted for additional LiDAR imagery of the slide area and directed the installation of instrumentation to monitor ground movements and subsurface water levels near the landslide mass.
However, the very steep crest of the landslide has retreated episodically since the primary event and represents an ongoing hazard to structures and people in the area. GeoEngineers continues to support ICPW by evaluating the landslide monitoring data and by considering options for improving roadway access to homes that were isolated by the slope failure. The landslide occurred suddenly and catastrophically during the early morning hours.
Clinton resident Eugene Elfrank has had two landslides occur near his home during the last two weeks because of rain saturating the bluff uphill of where he lives.
Since then, another one occurred Jan. Deputy Chief Wendy Moffat said the Dec. A culvert pipe came down with the slide. Elfrank said the second slide added about eight or nine feet to the debris piled up already and happened while he was standing outside his home. He compared the sound of the landslide to a freight train.
The previous owner of the home, knowing that the area was prone to landslides, had it built. During the nearly six years Elfrank has owned the Hastings Road residence, he recalled there being four other slides in addition to the two that just happened recently. Some have been going on for years. Since Jan. Fire crews responded Monday to a tree that had fallen into a power pole near Double Bluff, which blocked access to the park for a few hours while PSE, Puget Sound Energy, addressed the issue.
Common types of slides are shown on Figure through Figure The most common is the shallow colluvial slide, occurring particularly in response to intense, short-duration storms, where antecedent conditions are prevalent Baum, et. The largest and most destructive are deep-seated slides, although they are less common. All mass movements are caused by a combination of geological and climate conditions, as well as the encroaching influence of urbanization.
Vulnerable natural conditions are affected by human residential, agricultural, commercial and industrial development and the infrastructure that supports it. The occurrence of a landslide is dependent on a combination of site-specific conditions and influencing factors.
Most commonly, the factors that contribute to landslides fall into four broad categories:. Change in slope of the terrain, increased load on the land, shocks and vibrations, change in water content, groundwater movement, frost action, weathering of rocks, and removing or changing the type of vegetation covering slopes are all contributing factors. In general, landslide hazard areas are where the land has characteristics that contribute to the risk of the downhill movement of material, such as the following:.
Erosion is the process by which material is removed from a region of the earth's surface. It can occur by weathering and transport of solids sediment, soil, rock, and other particles in the natural environment.
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