Introduction to the Discussion:
On March 22, 2014, a large landslide occurred that destroyed 49 homes in the Steelhead neighborhood east of Oso, Washington, Snohomish County. This slide was the single deadliest landslide in the history of the United States with 43 people who lost their lives. This was a politically charged event as John Pennington, Director of Snohomish County’s Department of Emergency Management, and Snohomish County Public Works Director, Steve Thomsen, stated this was a completely unforeseen event. Meanwhile, the Seattle Times reported that this was not completely unforeseen and that geological studies of the area reported frequent landslides in the past 100 years. I want you to make your own assessment of the situation by answering the questions using the data/articles provided.
see attachment file preslide photo.pdf
Look at the above aerial photographs of the Steelhead neighborhood and the hillside before and after the landslide occurred. How is river action contributing to the probability of a landslide here, based on what you observe in the “before” photo. (Hint: Think about how meandering streams erode and deposit sediment).
1. Identify and describe one feature on the “before” map would be a possible indicator that a slide is impending, or that slide activity has occurred in the recent past?
A study conducted by GEER (a team for Geotechnical Extreme Events Reconnaissance sponsored by the National Science Foundation) has written a thorough report on the Oso landslide. Use the report to answer the following questions (The GEER Report:
SEE ATTACHED FILE (GEER OSO LANSLIDE REPORT)
note that important text is highlighted for you!)
1. Explain how precipitation and pre-slide topography may have contributed to the failure. Refer to page 140/129 in the GEER Report. (This notation refers you to page 140 of the pdf document or page 129 of the report itself, note that they are different! I will refer to page numbers this same way for future questions).
2. Has this area (the North Fork Stallaguamish River Valley) experienced slides previous to the 2014 slide? Explain. Use the map below. (also, page 58/48, GEER report).
3. The 2014 Oso slide is indicated with red hatchures on the map above. Has the 2014 Oso landslide site experienced slope failures in the past? Explain. (page 59/49, GEER report)
4. Read the eyewitness accounts of the landslide survivors (pages 99-101/88-90, GEER report). In your opinion, what 2 observations were most striking or surprising to you?
5. Snohomish County is responsible for managing development in the valley. (pages 63-66/53-56, GEER Report). Did Snohomish County have a set of land-use restrictions for the Oso landslide area?
Through your research you have idenitified the Stallaguamish River Valley as a very active area for landslides. In fact, a slide occurred in 2006 in the same location as the 2014 Oso landslide. Furthermore, a 1999 geologic report
SEE ATTACHED FILE (HAZEL GOLD BASIN LANDSLIDES PDF)
by Seattle geomorphologist, David Miller, written for the Army Corps of Engineers, identifies at least 6 slides that occurred at this site in 1937, 1942, 1951, 1952, 1967 and 1988. Snohomish County Officials had access to this report. Even so, people decided to build homes after the 2006 event, at the base of the 2006 landslide using the setbacks required by Snohomish County. A setback is a minimum distance homeowners may build from the base of the slide area.
1. Even though the houses built after the 2006 slide were not in violation of set back rules, do you think the county is culpable for the loss of life and property in this case? Why? Use your answers from above, the landslide map above, and read this article:
(scroll through the dates to visualize and read the history of the area) and this article to help you answer: URL:
2. What solution would you recommend to the local city council or county commissioners to reduce the risk of future landslide events? Consider structural solutions, national risk plans and/or land-use restrictions.