Landuse Planning in Landslide Country: The Washington State Oso Slide

Rebekah Green, Environmental Studies, Western Washington University


On March 22, 2014 a massive section of steep slopes above the Stilliguamish River in Snohomish County, Wash, slid downward in a lahar-like mudflow. The area, called the Steelhead Haven Hill, flow crossed the river and stretched for over 1 square mile, covering a small development and a major rural highway with tens of feet of mud. It was the largest recorded slide in Washington State history, though LIDAR imagery and geological studies suggest other similar-sized slides have occurred along the same stretch of river. As of April 2014, 41 victims have been identified and a temporary road constructed around the slide.

Landslide risk is a complex function of underlying topographic and geological conditions, combined with triggering conditions such as heavy precipitation, logging, development, and/or seismic activity. Coastal bluffs along the Puget Sound regularly sluff off, threatening bluff top homes. Transportation routes along steep slopes are also regularly cut off by avalanche, mud and rock slides. Since 1980, landslide or mudslide have been listed in 12 out of the 33 federally declared disasters in the state, often as a secondary event triggered by severe winter storm or flooding. At the Steelhead Haven Hill, the location of the 2014 massive landslide, the bluff had sluffed away in large landslides in 1949, 1951 and in 2006.

In Snohomish County, the state's Department of Natural Resources had conducted several studies of the underlying geology and hydrology to assess the impact of proposed logging on landslide risk near the Steelhead Haven area. Based upon these studies, twice in the 1980s the state restricted or required modification of timber harvesting proposed on the plateau above the area that eventually slide. Later in the 2000s they required a private timber harvester to halve its proposed harvesting to avoid the landslide sensitive area around the river bluff. Yet, in 2006 a large portion of the bluff adjacent to the harvested area collapsed 600 feet into the river, diverting the flow and coming perilously close to the Steelhead development.

While the state assessed the hazard and managed timber harvesting in the area, the local county also considered restricting community exposure to landslide risk under the 1990 Washington State Growth Management Act. The act attempts to reduce sprawl and protect agricultural and natural resource by requiring most local jurisdictions to create comprehensive plans and support their implementation through infrastructure investment and regulation. Integral to comprehensive planning was the required identification of "critical areas" unsuitable for residential, commercial or industrial development. One such critical area jurisdictions were required to identify was geologically hazardous areas susceptible to landslides and erosion where development may lead to public safety concerns.

Since its enactment, cities and counties required to plan under the GMA, like Snohomish County, have had to use the best available science in identifying geologically hazardous areas prone to land and mudslides and protecting such land from development. The state's Department of Natural Resources is tasked with mapping geologically hazardous zones, including areas susceptible to land and mudslides. The locations of these geologically hazardous areas are provided to local jurisdictions for regulation under GMA, though identification and characterization is incomplete. The department provides open access to these maps through its geologic information portal.

The local county knew of the landslide hazard and past landslide record in the Steelhead Haven Hill region. In the early 2000s the county commissioned an engineering report on the natural hazard risk to the Steelhead Drive community. The report suggested several options including a hoe buyout program, moving the river away from the toe of the hill and slope stablization. The expensive buyout option -- the only option that would have likley significantly reduced loss of life from the 2014 event -- was never pursued, not even partially. Instead, the county opted to attempt to stablize the toe of the Steelhead Haven Hill and permitted new construction even after the 2006 slide.

In 2006 it became clear that an active growth management framework and significant geological and hydrological investigation into the area, was not enough to prevent heavy losses from a predictable landslide. The county, the state and professionals within the planning and emergency management arena are now critically examining what more is needed to protect life and property in a landslide-prone state.

Individuals with expertise/responsibilities in the following areas have helped create the case study:
  • Washington State Legislature
  • Washington State Department of Natural Resources
  • Local planning departments of cities and counties
  • Local emergency management
  • Private developers and private logging , especially logging companies
  • Residents

Key teaching points:

How this example is used in the classroom:
I am currently developing the Oso Landslide case study into an assignment to be accompanied by a lecture discussing the development and management of the area. If possible, the assignment may be coordinated with a field trip to the site.

Read The Growth Management Act of 1990 and summarize how the GMA addresses land and mudslide hazard in 250 words, citing specific passages and paragraphs. (Hint: search for "critical areas" and "geologically hazardous.")
Using the state's online geological map viewer, identify three near population centers -- the Oso slide area and two other landslide risk areas. Review the natural hazard mitigation plans for the corresponding jurisdictions to determine whether these areas are considered geologically hazardous for the purposes of the GMA. For each area, provide a brief summary and answer the following questions:

  1. Based upon your identification of community infrastructure, what specific impacts might land or mudslides have on populations in these areas?
  2. Based upon census data from 1980, 1990, 2000, and 2010, how has populations changed in the areas in and around the landslide-prone zones over time?
  3. Based upon the provided archives of Oso area reports and news coverage of the March 22, 2014 slide, what were the complex causes of the high loss of life and infrastructure impacts from this event?
  4. What changes, if any, would you suggest to the GMA to better protect public safety in regards to landslide events?


The Seattle Times has a significant archive of articles and interactive maps on the event.

The state online geological map viewer can be found here:

The Growth Management Act of 1990 can be viewed here:

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