At about 11 am a large landslide occurred in northwest Washington State last Saturday. According to the U. S Geological Survey, USGS, the recent heavy rain conditions and soil saturation of the glacial deposits in that area led to the landslide. This was a falling rock, mud and debris flow-the most common type and often the most deadly type of landslide. The hillside collapsed at a speed that caught the local community unaware. Landslide debris covered about 30 houses and 0.8 miles of State Route 530. Flow also dammed and partially blocked the North Fork Stillaguamish River, creating a potential for flooding at the blockage. A pool of water currently 20-30 feet deep has formed behind the blockage a naturally formed dam. There is danger of flooding when this dam gives. There are still over 100 people reported missing the number is reported to be up to 176, but that number contains many duplicates from consolidating all the lists of the missing. It is unlikely any will be found alive. According to the Snohomish County Twitter feed, 16 bodies have been recovered and they believe that they have located an additional 8 bodies.
There are many types of landslides, and this event in Washington was a “debris flow,” also commonly referred to as a “mud slide” or “mud flow.” A debris flow is a flowing mixture of water-saturated debris that moves downslope under the force of gravity. Debris flows consist of material varying in size from clay to boulders that are tens of meters in size. When moving, they resemble masses of wet concrete and tend to flow downslope along channels or stream valleys. These mud slides can flow at up to 30 miles an hour and gives people little time or warning to get out of the way. These types of landslides occur most frequently in California, but the glacial deposits of sand and silt and weeks of rain created the conditions for a landslide. The hillside that collapsed had a history of slides and a study performed for the Washington Department of Ecology in 1997 identified the potential for a large catastrophic failure of the slope. Nonetheless, development of the area proceeded.
Hundreds of thousands of landslides of some scale occur in the United States each year from the tiny to the massive. Landslides occur in all 50 states and U.S. territories, and cause $1-2 billion in damages and more than 25 fatalities on average each year. Falling rocks, mud, and debris flows are the most common and deadly of the landslides, and yet there is still much to learn about how and why they happen. Any area composed of very weak or fractured materials resting on a steep slope can have a landslide when the conditions are right. Those conditions are predominately excess weight from heavy rains and melting snow that causes the slopes to fail. The rock and soil slopes are weakened though saturation by snowmelt and of heavy rains and literally begin to flow. Earthquakes of magnitude 4.0 and greater have been known to trigger landslides.
There is so much we do not know about our earth. The USGS science is helping answer questions such as where, when and how often landslides occur, and how fast and far they might move. USGS scientists produce maps of areas susceptible to landslides and identify what sort of rainfall conditions will lead to such events. You can watch the video about the USGS Landslide Program and check the maps to know if your area is suspect able to landslides.