FLUVIAL EROSION AND SEDIMENT DELIVERY AFTER THE 2014 OSO LANDSLIDE BASED ON REPEAT STRUCTURE-FROM-MOTION PHOTOGRAMMETRY

On March 22, 2014, the Oso Landslide formed a valley-spanning deposit that impounded the North Fork Stillaguamish River to a height of 8 meters (m). The river overtopped the deposit in about 24 hours, initiating a process of erosion and re-channelization. As part of a larger study of the downstream geomorphic response of the landslide, regular Structure-from-Motion (SfM) surveys of the deposit were acquired to track the erosion of that new channel. The knickpoint that formed at the downstream edge of the deposit migrated upstream about 800 meters, nearly the entire width of the deposit, in about a week. Within several months, the river had incised 7 m into the deposit and largely re-obtained pre-landslide channel grade. Larger flood events in the fall of 2014 resulted in lateral erosion, but cohesive, clay-rich banks prevented rapid erosion and sediment delivery. Relatively little erosion occurred during two exceptional floods in the fall of 2015, indicating the channel had re-obtained a quasi-equilibrium form. Total erosion through the end of the 2015 water year was 820,000 ± 100,000 tonnes, representing about two times the ambient sediment load in the river. Over 75 percent of this eroded sediment was silt and clay, 20 percent was sand, and 4 percent was gravel. Fluvial sediment monitoring indicated that the majority of the eroded sediment was rapidly mobilized in suspension through the entire basin, exiting into Puget Sound within weeks of initial entrainment. The downstream geomorphic response to the landslide was generally subdued, and the influence on basin-scale sediment loads was modest. This limited impact is attributed to both the fine-grained nature of the deposit as well as the high ambient sediment supply of the river system. These results highlight the utility of SfM as a rapid, accurate and affordable survey method for areas with limited vegetation.