Kinematics of post-wildfire debris flow initiation mechanism: impact of hydrophobic layer spatial variability and particle size

Post-wildfire debris flow events have been more frequent due to climate change and increased wildfire adversity. Changes in noncohesive soil hydrophobicity, burn, and vegetation removal during wildfires dramatically enhance slopes’ vulnerability to rainfall-induced failure in the arid Southwest USA and similar areas worldwide. This paper contributes to a better understanding the hydrophobic sand slope failure mechanism using experiments and theory. In total, 36 indoor raining experiments use different configurations of fine, medium, and coarse hydrophobic sands subjected to various rain intensities on different slope inclinations. Specific findings contribute to understanding how the spatial variability of the post-wildfire hydrophobic layer, concerning depth and the sand grain size variations, affects the slope failure mechanisms. Clarifying slope failure mechanisms in different layouts is crucial for understanding the initiation and further spatiotemporal debris flow evolution. Results indicate that the surficial hydrophobic layer fails because erosion develops simultaneously from many small failure patches. Erosion pattern, time to failure, water, and sediment overflow temporal dynamics correlate to the dominant sand grain size. Furthermore, seepage-induced shallow infinite slope occurs when the hydrophobic sand layer develops below the surface. Finally, this study identifies the fine sand as the most vulnerable to failure in both configurations.