Bushfire effects on soil properties and post-fire slope stability: the case of the 2015 Wye River-Jamieson Track bushfire

Abstract. Bushfire is a destructive natural disaster that leads to vegetation loss and increased soil infiltration. Over a long post-fire period, root death and reduced reinforcement decrease soil shear strength. During rainfall, shallow landslides in burned areas become more frequent and widespread. This study focused on Wye River and Separation Creek in Australia, affected by the 2015 Wye River-Jamieson Track bushfire. Ten months after the bushfire, multiple slope failures, including the Paddy’s Path landslide, occurred during heavy rains from 12 to 14 September 2016, disrupting the Great Ocean Road connecting towns. This study aims to assess changes in slope stability during rainfall before and after the bushfire. Controlled laboratory burning tests simulated bushfire effects on soil, resulting in changed soil properties after the fire: increased permeability due to soil particle coarsening and reduced soil shear strength, especially cohesion. Considering the changes in soil properties before and after the fire, a simplified hydrological numerical model for infiltration calculation was employed to analyze time-dependent changes in groundwater level depth, surface water depth, and safety factor during rainfall. Comparing pre- and post-fire results indicated higher susceptibility to shallow slope failures in burned areas, with rapid rises in groundwater level and surface water acting as triggers. These findings enhance the understanding of landslide triggering mechanisms in post-fire slopes and provide insights for mapping landslide susceptibility in bushfire-prone regions.