Impact force of post-fire debris flows over erodible beds

Department of Civil and Environmental Engineering of The Hong Kong University of Science and Technology, Hong Kong SAR, China

^* Corresponding author: mzhengaj@connect.ust.hk

Abstract

After wildfire events, water repellent soil is often found in the subsurface layer of channel bed in the burnt area. Debris flows generated from burnt basins and ensuing entrainment of the channel bed pose imminent threat to infrastructure and human lives. However, the fundamental interaction mechanisms of debris flow overriding water repellent bed and resulting impact force on debris-resisting barriers have yet to be elucidated. In this study, physical flume experiments are conducted to simulate post-fire debris flows overriding and entraining a sand bed with varied wettability. Compared to a wettable bed, water repellent sediment exhibits a tremendous increase in the erosion depth and subsequent impact force on the barrier. The test results demonstrate that debris flows overriding water repellent sediment can be particularly hazardous and the effects of water repellency need to be captured by the design criteria of debris resisting barriers in burnt basins.