Oso Landslide

Oso, Washington Landslide
Photo:  Washington State Patrol via Wikipedia
I am reminded once again that I live in a geologically active corner of the planet.  The Oso, Washington landslide has become an international news event.  In loss of life terms, authorities are telling us it will exceed the Mount St. Helens eruption.  As of this morning, there are 26 confirmed deaths with 90 still listed as missing.  The New York Times has one of the best stories of the event I have found.

The slide crossed State Road 530 and the Stillaguamish River.  Mud and debris is at least 40-50 feet/12-15 meters deep in some places.  The lake in the foreground of the photo began forming when the river was dammed by the slide.  The river is now cutting a new channel through the debris field.  This is lessening fears of a catastrophic release and flash flood downstream.

The view in the photo is looking down the Stillaguamish Valley towards Arlington.  Turn 180° towards Darrington and the jagged, snow-capped peaks of the North Cascades come into view.  It is little wonder people want to live in such a beautiful place.  Governor Inslee commented that ice age glaciers "carved a very beautiful state," but left behind earth that can be very dangerous.

I work in Arlington downstream from the event.  Driving past the river into work this past week, I noticed it had an odd light gray color.  A coworker and his family who live in Oso were evacuated due to concern about flash flooding.  Those who live in Darrington faced a 2.5 hour commute.  They must first drive north to Concrete, then to Burlington where they can pick up I-5 to Arlington.  The Mountain Loop Highway between Darrington and Granite Falls, closed during winters, has been reopened early.  This is a big help, but some of it is unpaved, one-lane Forest Service road.  People are asked to stay clear of this roadway leaving it available for responders and residents.

Mudslide Diagram, besafenet.net
I have also been reminded of my own landslide experience here on South Fidalgo Island.  The Gibralter Road Landslide of 1990 was a slow, rotational slide that took place over several weeks.  There was no loss of life in that event and relatively minor property damage.  The Oso slide was a sudden, cataclysmic mud slide.  The diagram to the left from Be Safe Net illustrates what happened.

There are similarities between the two events.  Both occurred in deep, unstable glacial soils consisting of sand, rocks and clay.  Both were precipitated by heavy rainfall and excessive water saturation of this unstable soil.  The Oso slide occurred on a higher, much steeper embankment possibly undercut by the river.  Along with a greater water content, that might account for the difference.  A curved vs. planar failure surface also distinguishes the two events.

There is no doubt the Oso landslide will become a subject of intense scientific study.  Geology textbooks will include chapters on the event.  My blog friend Dan who is a professional geologist has already posted several articles.  Some county building departments have focused on the interests of developers and tax bases when issuing construction permits.  It is likely they will now be paying more attention to science and geologists' reports before issuing those permits.