Post-Fire Controls on Landslides in Debris Flows in the Pacific Northwest

Student Investigator: David De Sousa, OSU

Partners: DOGAMI, USGS, University of Oregon, US Forest Service

Wildfire is known to amplify the likelihood and magnitude of debris flows in steep terrain. Post-fire debris flows typically occur during the winter wet season, suggesting that rainfall-driven erosion is a strong control on in-channel preconditioning and triggering of these hazards in the immediate year or two following fire. However, in the cool, wet climates of the Pacific Northwest (PNW), debris flows frequency seems to remain elevated following the first several winters owing to infiltration-driven debris flows controlled by shallow landslides. The longer-term potential for shallow landslides and their associated post-failure mobility appears to be enhanced by the decay of root systems associated with tree mortality. However, the timescales of this amplification or attenuation of post-fire debris flow likelihood and magnitude are poorly-constrained in the PNW.  An interagency team of academic and government researchers are working to evaluate the temporal controls on post-fire debris-flow initiation and their impact on transportation infrastructure. This research focuses on (1) monitoring the hydrological and climatic conditions associated with landslides in several burn scars in western Oregon, (2) physical characterization of soil and root strength following wildfire, and (3) interpretation of topographic change in select areas of extensive debris-flow activity. This recently launched four-year project is intended to better characterize controls on the delayed triggering of post-fire landslides and their associated debris flows and improve existing models that reflect the amplification and subsequent attenuation of this hazard with landscape recovery.