Risks of Future Landslides – and Actual Past Landslides – Ignored

By ThinkReliability Staff

Risk is determined by both the probability of a given issue occurring, and the consequence (impact) if it does. In the case of the mudslide that struck Oso, Washington on March 22, 2014, both the probability and consequence were unacceptably high.

The probability of a landslide happening in the area had not only been well-documented in reports as far back as 1951, the same area where dozens were killed on March 22 had experienced 5 prior landslides since 1949. The consequences of these prior landslides were less than the 2014 landslide because of the severity of the landslide, and because increased residential development meant more people were in harm’s way.

While the search for victims is still ongoing, the causes and impacts of the landslide are mostly known. This incident can be analyzed using a Cause Map, or visual root cause analysis, to show the cause-and-effect relationships that led to the tragic landslide.

First, we capture the background information and the impact to the goals in the problem outline, thereby defining the problem. The landslide (actually a reactivation of an existing landslide, according to Professor Dave Petley, in his blog) occurred around 10:40 a.m. on March 22, 2014 in an Oso, Washington residential area. As previously noted, there had been prior landslides in the area, and there were outdated boundaries used for logging permissions (which we’ll talk more about later). The safety goal was impacted due to the 30 known deaths, 15 and people missing. (Not all of the 27 have been identified, so the known dead and missing numbers may overlap. However, at this point, there is little hope that any additional rescues will take place.) The environmental goal was impacted due to the landslide and the customer service goal (insofar as the residents can be considered customers of their local area) was impacted due to the displacement of 30 families. Logging in an area that should have been protected impacts the regulatory goal. The estimated losses (of residences and belongings) are approximately $10 million, impacting the property goal and the massive search and a recovery effort impacts the labor goal.

Beginning with these impacted goals, asking ‘why” questions allows us to develop cause-and-effect relationships showing how the incident occurred. The safety goal was impacted because of the deaths and missing, which resulted from people being overcome by a landslide. In order for this to occur, the landslide had to occur, and the people had to be in the vicinity of the landslide.

As is known from history (see the timeline on the downloadable PDF), this area is prone to landslides. Previous reports identified the erosion of the area due to the proximity of the river as a cause of these landslides. An additional cause is water seepage in the area. Water seepage is increased when the water table rises from overly wet weather (as is typically found at the end of winter). Trees can help reduce water seepage by absorbing the water. When trees are removed, water seepage in an area can increase significantly. Because of this, removal of trees (for logging or other purposes) is generally restricted near areas prone to landslides. However, for reasons yet unknown, logging was permitted in what should have been a restricted area, because the maps used to allow it were outdated. Says the geologist who developed the new maps, “I suspect it just got lost in the shuffle somewhere.” Additionally, analysis by the Seattle Times, the logging went into the “old” restricted area as well. The State Forester is investigating the allegations and whether the logging played a role in the landslide.

Regardless of the magnitude of the impact of the logging and weather, the area was prone to landslides. Yet it was allowed to be developed, despite multiple reports warning of danger and five previous landslides. In fact, construction in the area resumed just three days after the last landslide in 2006. The 2006 landslide also interrupted a plan to divert the river farther from the landslide area. Despite all of this, the area built up (with houses built as recently as 2009) and those residents were allowed to stay. (While buying out the residents was under consideration, it was apparently dismissed because the residents did not want to move.) While officials in the area maintain that they thought it was safe, a long history of reports and landslides suggest otherwise.

If a lack of knowledge of the risk of the area continues to be a concern, aerial scanning with advanced technology (lidar) could help. Use of lidar in nearby Seattle identified four times the number of landslide zones that were spotted with aerial surveying, which is more typically used.

To view a summary of the investigation, including a timeline, problem outline and Cause Map, please click “Download PDF” above.