THE SWIFT CREEK LANDSLIDE IN NW WASHINGTON STATE: CONSTRAINING MOTION OF 40M M3 OF SERPENTINITE VIA GPS, PHOTOGRAMMETRY, AND REPEAT LIDAR

Intense sedimentation in Swift Creek and Sumas River, located in the Cascades foothills in NW Washington results from erosion of the oversteepened, unvegetated toe of a large (0.55 km²) active landslide. The peculiar nature of the bedload and suspended sediment in the creek are a direct result of the weathering of the serpentinitic bedrock in which the landslide is rooted.

The Swift Creek landslide, located on the west slope of Sumas Mountain, is a large, complex earth slide-earth flow. We use terrestrial laser scanning (TLS) and airborne LiDAR, repeat photography, and campaign GPS to record surface displacement and to quantify the volumetric transfer of material. Motion analysis over ten years reveals a significant mass-transfer of material to the toe; short-term analysis shows that a significant amount of material is removed from the toe during the wet NW winters. Surface displacement of the forested, main mass is relative constant at 2-4 m/yr, but more variable over the toe (3-40 m/yr).

Secondary hazards include proximal debris flows, downstream flooding, and streamside deposition of asbestos-rich sediment.