A large landslide occurred in northwest Washington at about 11:00 am PDT on Saturday, March 22, 2014. Recent rain conditions and soil saturation led to the onset of the landslide.
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 is forming behind the natural dam, also creating a flood hazard downstream if the natural dam is breached. Currently, the pool is approximately 20-30 feet in depth.
USGS scientists are supporting state and county agencies responding to the event. It is a collaborative effort, with many working hard to provide assistance, assess the situation, and alleviate impacts as the hazard isn’t over. For example, future rain conditions or snowmelt from nearby mountain tops could exacerbate the situation. Current research is focused on gathering more LiDAR imagery and aerial photographs to help with mapping the extent of the landslide.
Monitoring Water Levels and Flood Potential
The USGS operates a streamgage to measure water levels about 12 miles downstream from the landslide, on the North Fork Stillaguamish River at Arlington. The river level at the gage dropped suddenly at about 1:30 pm PDT on Saturday. The drop in water level was about 1.2 feet, which is equivalent to a drop in discharge of about 1,200 cubic feet per second. Go online and see near-real-time data.
USGS scientists continue to monitor the streamgage remotely. The streamgage is operating normally (measurements every 15 minutes, transmissions to the web every
hour) but could possibly be damaged if large debris comes down the river. Discussions are underway on whether and how to increase the reporting frequency of the streamgage, but this modification of instrumentation requires reprogramming at the field site. Due to safety concerns, USGS staff are currently not scheduled to go to the field-monitoring site.
Consideration also is currently being given to making frequent streamflow measurements downstream of the slide at auxiliary sites and installing rapid deployment gages and turbidity meters. We are exploring deploying a buoy to measure the elevation of the pool elevation behind the blockage.
The USGS is working with the National Weather Service (NWS) to do preliminary modeling of possible scenarios considering what would happen if the blockage on the Stillaguamish River were to break. This will help estimate the range of potential discharge from the pool that has formed behind the landslide. The NWS will use these data to develop flood inundation maps to estimate where flooding could likely occur.
No Associated Earthquake
Seismograph readings show no indication of an earthquake in association with the landslide near Oso, Washington, on March 22, 2014. The readings show two wave signals, the first is the landslide at 17:37 UTC and a second is a slide that occurred at 17:41 UTC. There are no earthquakes or other local seismic events on the records around the time of the landslide. The seismic signals are of long period surface waves, with no clear high-frequency P or S phases that we would expect to see if a local earthquake occurred at the time of the event. The landslides generated elevated levels of local ground shaking for over an hour. Seismic readings are from the University of Washington Pacific Northwest Seismic Network, operated in cooperation with the USGS.
Coordinated Emergency Response
Snohomish County is the lead responding agency and is coordinating closely with local agencies. The Washington State Department of Natural Resources, Washington State Emergency Management Division, and Washington State Department of Transportation are the primary state staff at the site to help assess the flood hazard and evaluate how the river may rework the landslide and natural dam in the next few days. Many other organizations are playing a supportive role and providing all hands on deck to assist.
Prior Landslides in the Area
Other large, and perhaps sudden, landslides have occurred in this valley. Large landslides are the norm in many parts of the western foothills of the North Cascades. In particular the Nooksack Valley in Whatcom County, from the town of Glacier downstream, has seen at least five large catastrophic landslides in the last 12,000 years.
Type of Landslide
There are many types of landslides, and this event in Washington was a rotational slide complex. The term “slide” refers to mass movements where there is a distinct zone of weakness that separates the slide material from more stable underlying material and movement occurs mainly by slipping or sliding (rather than flowing) along this zone. The two major types of slides are rotational slides and translational slides. A rotational slide is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide.
Material along the lower edges of the slide broke up and transformed to 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 blocks several tens of meters in maximum dimension. When moving, they resemble masses of wet concrete and tend to flow downslope along channels or stream valleys.
USGS: Start with Science
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 one of the most common and sometimes deadly hazards, yet there is still much to learn about how and why they happen.
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. For more information, watch a video about USGS landslide science, and visit the USGS Landslide Hazards Program website.
Scientists at the USGS are also asking you to help by reporting your landslide experiences and sightings at the new USGS “Did You See It?” website.
Further, the USGS is working with the NWS on a Debris Flow Warning System to help provide forecasts and warnings to inform community and emergency managers about areas at imminent risk.
View photographs from a recent aerial survey of the landslide and impacts.
More Info on the Washington Landslide
Read the following reports by the Washington Division of Geology and Earth Resources:
- Geologic map of the Oso 7.5-minute quadrangle, Washington
- Geologic map of the Mount Higgins 7.5-minute quadrangle, Washington