ASSESSMENT OF POST-FIRE LANDSLIDE PROCESSES IN THE PACIFIC NORTHWEST, USA USING A THREE-DIMENSIONAL, REGIONAL-SCALE, LANDSLIDE INITIATION MODEL

Wildfire is an environmental disturbance known to exacerbate landslide and debris flow susceptibility in steep terrain. The landslide-related processes tied to post-fire disturbance in drier climates (i.e., Southern California, Colorado) have been studied extensively, but less is understood about how post-fire processes impact landslide susceptibility in wetter regions such as the Pacific Northwest (PNW), USA. This is increasingly concerning as wildfires throughout the PNW, and other humid regions, continue to grow in both extent and frequency. We use RegionGrow3D (RG3D) – a three-dimensional, deterministic landslide initiation model for regional-scale simulations – and a case study in the 2017 Eagle Creek Fire to assess the relative influence of three post-fire processes influencing slope stability: root reinforcement decay, loss of rainfall interception by the forest canopy, and changes in evapotranspiration. These processes are also integrated into a water balance model to assess associated changes in soil moisture stemming from antecedent and event rainfall. Through a comparison of pre- and post-fire environmental conditions, we estimate the influence of potential mechanical and hydrological changes induced by wildfire on landslide susceptibility. Further, parameters related to post-fire response are constrained to estimate a time history of landslide susceptibility induced by vegetation damage and subsequent regrowth. Finally, we use RG3D to estimate potential volumes of source material from modeled shallow landslides. This is an especially important component of post-fire debris flow modeling in the PNW, where shallow landslides initiated by soil saturation and root reinforcement decay are the primary trigger of postfire debris flows, as opposed to erosion from surface runoff.