2 April 2014

The Oso (Steelhead) landslide in Washington State – could it have been foreseen?

Posted by Dave Petley

The Oso (Steelhead) landslide in Washington

The best news about the Washington landslide is that, as forecast, the number of victims is likely to be rather lower than was feared at one stage (I noted earlier that this might well be the case).  The official toll has now reached 28 people known to have been killed, with a further 22 remaining missing.  To have recovered 28 bodies to date is an impressive achievement, and this will have been deeply traumatic for those involved in the search and rescue operations.

So now attention, in the media at least, is focused on whether the landslide could have been foreseen.  Opinion on the internet varies greatly (no surprises there!), ranging from the view that the risk was tolerably low to the view that it was completely foreseeable.  I’ll nail my colours to the mast – to my mind this was foreseeable event, and as such the disaster represents a failure of hazard management.

Using the past as a guide to the future

The simplest way to understand the likely future behaviour of a landslide is to examine how similar landslides have behaved in the past.  In the case of the Steelhead landslide, this was very simple.  Immediately to the west of the landslide there is another large failure.  This landslide is very obvious on the LIDAR imagery.  Dan McShane provided an analysis of the Steelhead landslide history from Google Earth images within a day or so of the slide event, and included LIDAR imagery (from before the most recent movement of the Steelhead landslide) that showed this other failure to the west:


In this image the Steelhead landslide is highlighted on the right, and this much larger landslide is highlighted in the centre of the image.  Note that there is another landslide scar between these two landslides.  The key aspect of this larger landslide is the extensive area that the debris has covered.  And look at the morphology of the deposit as shown in the imagery, and in particular the structure of the material that has flowed onto the valley floor, as shown in this BBC/AP image:


So this bluff has a long history of landslides, and the evidence on the ground suggests that they can be very mobile.  Note also from the LIDAR imagery the shape of the land above the old Steelhead landslide scar – this promontory does not look like a stable configuration in such a weak deposit.

The defence of the emergency planners (as reported by Andrew Alden) in Washington seems to be that after the 2001 movement event at Steelhead the landslide was mitigated by protecting the toe from erosion by the river.  There is little doubt that a large amount of work was undertaken in this respect.  This is a Google Earth image of the foot of the landslide in 2003:


And this is the same view in 2007:



Clearly the river was relocated and the toe of the landslide was protected.  Note the houses that are visible in the second image that were not there in 2003.  However, for these very large landslides it is likely that the failure is driven by groundwater driven processes in the upper part of the slope.  Protecting the toe will have done little to prevent this from occurring.  Thus, the mitigation did not address the primary concern.

The 2001 landslide left material high on the hillside that was sitting above a scar that was far too steep (see the image in this post).  The LIDAR data suggests that the runout from such a collapse could be extensive.  In that context I find the decision to build new houses at the foot of the landslide to be very surprising.