Numerical Simulation of Post-Fire Debris Flow Hazards Using a Triggering-Propagation Model

Due to a combination of mountainous terrain, Mediterranean climate, and frequent wildfire event, post-fire hazards are inevitable in Southern California. Quantifying the post-fire consequences of a burn event provides the foundation for risk analysis and hazard mitigation planning. This study aims to simulate the hazards after a fire in Montecito using a triggering-propagation model. The model considers varying surface material conditions and erosion effects, which can simulate the initiation and propagation processes of debris flows. The scheme and the parameters are evaluated by the debris-flow event in Montecito following the Thomas Fire. The simulated debris-flows depths basically coincide with the observed ones, which indicate that the model is reliable to be used to predict the post-fire debris flows. Bed erosion mainly occurs on the surface bed of burned areas, where the erosion depths are much larger than those on unburned areas. Wildfires increase the magnitude of erosion. The bed-erosion materials travelling down from hillsides enlarge the debris-flow intensity significantly. Surface-erosion runoff processes are by far the most predominant contributors to debris flows. Though the materials provided by slope failures are much smaller than that from erodible surface beds, the slope failure process still augments erosion entrainment. The entrainment from surface beds significantly increases when there is a sharp increase of slope failures. Once the materials are entrained by surface runoff, the flows gain mass quickly and descend along steep slopes and channels.