Changing Riverine Environment
Author: Casey Kramer (WSDOT)
At approximately 10:37 a.m. on Saturday, March 22, 2014, a major mudslide occurred four miles east of the town of Oso, Washington; when a portion of a hillside collapsed, sending mud and debris over a half mile across the North Fork of the Stillaguamish River Valley. The incident devastated a neighborhood of over 40 homes and covered over 4000 feet of State Route (SR) 530 with upwards of 20 feet of mud and debris. The highway was closed from MP 36.41 to 37.85 which disconnected the travelling public on this major east-west transportation route for over two months. The event dammed the North Fork of the Stillaguamish River for a period of approximately 24 hours, causing flooding upstream and making conditions in the vicinity of the slide extremely difficult to traverse.
To assist in rescue and recovery efforts a multi-agency technical team was organized consisting of technical experts in the fields of hydrology, hydraulics, and fluvial geomorphology. The lake level team was comprised of members from the USGS, WSDOT, DNR, USACE, FEMA, NOAA, and Snohomish County. The team worked in unison with a parallel team who focused on the geology and stability of the landslide. Within days of the mudslide, the USGS deployed several real-time stream gages and rapid deployment buoys to monitor water surface elevations upstream of the slide. In addition to the real-time gages, the NOAA Northwest River Forecast Center developed flow and stage forecasts downstream, at and upstream of the mudslide. These forecasts allowed emergency operations and rescue workers knowledge of potential water levels to assist in worker safety.
The mudslide initiated within an approximately 650 foot hillslope comprised of unconsolidated glacial and colluvial deposits, overall displacing an estimated 10 million cubic yards of material which covered upwards of one square mile of the valley floor (approximately 150,000 cubic yards on SR 530).
To understand potential risks to rescue and recovery efforts and infrastructure, a hydraulic model of the reach was needed to assess forecasted hydrologic events throughout the basin. WSDOT flew aerials and LiDAR shortly after the mudslide capturing ground elevations which was combined and compared with historical LiDAR data. The data was extremely useful to understand depths of debris above historical ground levels; however the water which was impounded due to the mudslide inundated areas upstream of the slide for several thousands of feet, resulting in elevations of water surface from the LiDAR rather than ground elevations. To supplement the LiDAR data in the inundated areas, Snohomish County contracted with a local consulting firm to collect bathymetric data which ultimately resulted in a combined surface which was used to develop a baseline hydraulic model of the site.
Two main hydraulic models were developed to assess hydraulic conditions through the reach; 1) a 1-D HEC-RAS model developed by FEMA and the USACE which covered the reach from the confluence with the South Fork of the Stillaguamish to upstream of the project site and 2) a 2-D SRH model developed by WSDOT and FHWA which focused on determining potential flow paths through the mudslide deposits.
This presentation will summarize the evaluation and analyses conducted to assist in rescue and recovery efforts and ultimately the design and reconstruction of SR 530.
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How to Cite: Kramer, C. (2014) “Washington State Mud Slide”, National Hydraulic Engineering Conference 2014. 1(2014).