Washington Mudslide: What Happened?

            Residents of the Oso, Washington area were always aware of the possibility of a mudslide, but they could never have imagined the devastation that it would ultimately cause. The Washington mudslide is one of the largest and most destructive mudslides the U.S. has seen in a long time. It has been determined that extreme weather, in the form of heavy, sustained rain, was an essential factor leading up to the event.  The mudslide was 4,400 ft. long, 4,400 ft. wide and up to 40 feet deep. To put that into perspective that's as high as four stories. 

One of the many questions people are asking is what’s the difference between a mudslide and a landslide? A mudslide occurs when the rapid movement of a large mass of mud (formed from loose earth heavy with rain) moves destructively downhill. A landslide is the downward movement of the ground, such as rockfalls.

          Initial news coverage reported up to 120 people missing, but as of now there are 30 confirmed dead and 13 people are still missing.

Setting the Scene

The Soil: Twenty thousand years ago glaciers covered the impacted area.  As the glaciers gradually retreated they left behind very lose soil, sand and glacial rock. This loose layer of soil sits on top of the much harder bedrock. Soil erosion was a significant factor of the impacted ground area. Recent logging in the area also contributed to the erosion problem.  Soil that lacks vegetation and a complex root system is always more susceptible to erosion; be it from wind or rain.  

Rainfall: The Oso area usually receives on the average 6 inches of rain for the month of March. However, in the three weeks leading up to the mudslide the area had already received double the normal amount.  A long soaking rain is a far more serious concern because the ground will eventually become saturated, where a quick rain, even if it is heavy will runoff and quickly evaporate back into the atmosphere.   

Topography: Washington state is characterized by many steep slopes and hills. Areas and communities that are located at the bottom of slopes are more at risk to be impacted by slides. Also located at the bottom of the impacted slope is the Stillaguamish River. The Stillaguamish River played a significant role in the Washington mudslide because it had gradually eroded the bottom of the hill, which supported the higher ground.

           
            As you can imagine the soil and earth can only absorb so much water. When the soil becomes saturated, adhesion between the top layer of soil and the bedrock decreases. The bedrock becomes slick and slippery and eventually the top layer will separate. Therefore, when the rainfall is double the monthly average, the Stillaguamish River is going to run faster, rise above its banks, flood and erode away even more earth at the bottom of the hill at a quicker rate. When all of these factors align, the base of the hill will give out and a mudslide will occur.  

In closing, it is worth mentioning that most people think a mudslide runs from the top down, when in reality, a slide happens because the earth gives out at the bottom of the hill.  When the earth at the bottom can no longer anchor and support the earth on top, then a slide will occur.  The steeper the gradient or higher the hill, the more critical it is for the earth at the bottom of the hill to remain stable.  So, factors such as heavy rains, saturated ground, lack of a significant root system, past erosion and a racing/cresting river at the base of a steep gradient are all key factors for a mudslide to occur.

Graphic: Courtesy of The Washington Post