By Hugh Shipman
Twenty years ago this month, the Puget Sound region was entering its second wet winter in a row. Western Washington had seen serious flooding and landslides in 1995 and 1996, and the wet fall of 1996 had also left soils saturated. Then, during the last week of December, heavy snow blanketed much of the central Puget Lowland. On New Year’s Eve, rain began to pour. Over the following two days, the rain and the rapidly melting snow led to widespread landslides throughout the region, particularly along Puget Sound’s steep coastal bluffs.
Hundreds of landslides were documented, ranging from relatively small shallow debris avalanches to reactivation of deeper slides. More than one hundred slides occurred along the railroad between Seattle and Everett. Slides impacted developed neighborhoods in Seattle, such as West Seattle and Magnolia. Slides damaged residential communities built below high bluffs in Kitsap and Island Counties and undermined bluff-top homes throughout the area.
The wet conditions left slopes primed for continued slides. On January 9th 1997, a deep landslide occurred in Woodway, south of Edmonds, blocking the railroad and knocking a train onto the beach. Fortunately, no one was hurt by this event. But during renewed rains less than two weeks later, a slide occurred on Rolling Bay walk on Bainbridge Island, destroying a house and killing four members of the Herren family.
The landslides, combined with damage from floods and snow-loading, led to a federal disaster declaration and subsequent involvement of FEMA. This resulted in a productive collaboration between the USGS, FEMA, and the City of Seattle to document the event, inventory and analyze historic slide patterns and precipitation thresholds, and make recommendations (later adopted) to the state legislature to require licensing of geologists.
The timing and the geography of this hazard event was not a big surprise. The event followed patterns observed more than twenty years earlier after a similar event in the winter of 1972 to 1973, when very wet conditions led to widespread sliding in many of the same places. Puget Sound is surrounded by high bluffs of poorly consolidated sediments that are prone to failure when they get wet. Scars of past slides, both shallow and deep, are clearly visible on the landscape (and even more so now with the availability of LIDAR data). Significant development has occurred above, below, and on many of these historic slides, and pressure to develop these waterfront and water-view properties remains high.
Landslides are a major coastal hazard on Puget Sound, and we can be confident that they will continue to occur during wet weather, following large earthquakes, and perhaps, as coastal erosion rates increase with rising sea levels. Recent new work by DNR, prompted in part by the tragic 2014 SR530 Landslide in Oso, will be updating landslide maps for the entire region, but much work will be needed to make sure this information gets to local agencies and to the public with guidance that effectively reduces the potential risks from future events.
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