WHEN AVOIDANCE IS NOT AN OPTION: LESSONS FROM MONITORING THE OSO LANDSLIDE DURING RECOVERY OPERATIONS

On a sunny morning in March 2014, the Oso (SR 530) landslide suddenly collapsed and swept away a neighborhood located on the adjacent flat alluvial floor, dammed the North Fork Stillaguamish River, and buried WA State Route 530 in a debris avalanche and debris flow. Although some survivors were initially rescued, 43 people perished. After a major landslide, expert advice often calls for avoiding the site until ground conditions, groundwater pressures, and weather conditions stabilize. For five weeks following the Oso slide, avoidance was not an option. Here, up to 600 workers, equipped with hand tools and heavy excavation machinery, toiled beneath the slide scar in a search, rescue, and recovery operation. To help detect additional massive sliding that might threaten on-going operations, multiple government agencies (including Snohomish County, WA State Dept. of Natural Resources, WA Dept. of Transportation, and the USGS) implemented a multi-faceted near-real time monitoring system, augmented by periodic surveys. During working hours, numerous geologists and engineers helped monitor and interpret data from this system. In addition to daily landslide-monitoring discussions, we developed clear and simple protocols for providing activity updates to the incident management team and for issuing warnings.

Monitoring focused on detecting movement or acceleration of ground upslope and adjacent to the slide scar. Near-real time components included upslope extensometers and precipitation gauges, as well as spider units deployed by helicopter. Spider units contained onboard high-precision GPS receivers, seismometers, and radio telemetry equipment. These data were uploaded to a web-based dashboard for easy viewing. Additional periodic surveys focused on upslope stake lines and survey reflectors at the slide margin, as well as change detection from repeat aerial orthophotographs and lidar, from terrestrial laser scans, and from ground reconnaissance. Lessons learned include the need for: rapid deployment of instrumentation and telemetry, redundant sensors (both type and location), clear lines of communication, and staff familiarity with all data and its interpretation. Although no major additional slope failure occurred during the monitoring period, these lessons are applicable to other landslide crises.