Washington

» Introduction of Landslide Activities in Washington

Pe Ell Landslide

The Pe Ell landslide occurred on December 3rd, 2007 - It caused a blockage of State Highway 6 and destruction of two houses and a garage.

We are currently in the middle of a statewide landslide hazard zonation mapping project, wherein all state and privately owned timber lands are inventoried (mapped) for landslides and areas of landslide hazard, using an established mapping protocol and resolution. These maps are made available to the public for free via web download. The data is compiled into two GIS databases (landslide inventory and landslide hazard zones) that are also made freely available to the public via web download. In order to create a landslide data repository for the state, these data were compiled with landslide inventories and landform maps that were created by other sources, in some cases for other purposes. It is not known if the local government entities that issue development permits are using these data, but have been encouraged to do so, as many of them have critical areas ordinances requiring geotechnical input for developing on unstable or potentially unstable ground.

The state has requirements to avoid harvesting timber on unstable slopes if doing so would increase the likelihood of failure and delivery of debris to public resources or would threaten public safety. As such, these maps are used by land managers, regulators, and the geotechnical community in developing acceptable harvest plans. The utility of this mapping project was identified during a set of negotiations that led to new unstable slopes rules.

Magnolia Bluff

Landslides along Magnolia Bluff in Seattle during the winter of 1996. Photo courtesy Washington State Department of Ecology.

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There is a new project starting in the Division of Geology and Earth Resources to map landslides along the Puget Sound coastal bluffs as well as provide hazard maps. This information will be primarily used by county planners for critical areas ordinance and growth management. We are also starting a new project mapping landslides along tsunami evacuation routes to determine what hazard they present to the evacuation areas and to determine whether the evacuation route needs to be changed.

» Inventory Status for Washington

Washington has an inventory for both landslides and landforms. We keep these data in a GIS-based database (ArcGIS-SDE-Oracle). The Division of Geology and Earth Resources has a prototype database for tracking losses and other information. Our databases are stored in an ArcGIS format and are accessible via web download and ArcIMS. The Washington State Department of Transportation maintains a separate database for landslides occurring along transportation infrastructure.

North Fork Stillaguamish River

Photo of a late January, 2006 landslide that blocked the North Fork Stillaguamish River located midway between Darrington and Arlington, Washington. 300 yards of steep hillside collapsed, sending several hundred thousand cubic yards of soil/mud/trees into the river. (Photo courtesy of Snohomish County, WA Search and Rescue)

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