THE EVOLVING TYPES OF MASS FAILURE AFTER A WILDFIRE

Wildfire changes soil and hydrologic processes in natural forests, and this results in increased runoff and erosion over time. At a study site in the San Gabriel Mountains, California, USA we observed a switch in the dominate type of mass-failure response following the San Gabriel Complex wildfire (2016) from primarily runoff-generated debris flows in the first year after fire to landsliding in the third year after wildfire. Runoff generated debris flows mobilized material from hillslopes and primarily traveled through channel networks. We found that landslides occurred during a rainstorm in 2019 primarily on south-facing hillslopes in areas burned at moderate/high severity three years prior. Landsliding in the neighboring Colby wildfire (2014) that burned five years prior, had a much lower density, similar to unburned regions suggesting that vegetation in our study area recovers within five years. From these site observations, we propose a conceptual model with three distinct time periods governing mass-failure (no recovery, initial recovery, and fully recovered). During the no-recovery period after wildfire, the water repellency is high, delaying infiltration-related landsliding, and making runoff-generated debris flows more prevalent. Once infiltration rates recover, rainstorms can transmit water to the subsurface, creating the positive pore-water pressure needed to induce landsliding. Simultaneously, roots decay, which reduces the apparent root cohesion, making landsliding more likely during the initial recovery period. Finally, in the fully recovered phase new vegetation roots are established, and hillslopes stabilize again. The proposed three-phased conceptual model, therefore, provides a framework for surface process response for the initial several years following a wildfire.