Runout mechanism of landslides in alluvial basins with emphasis on the impact and erosion effects

Abstract. Landslide runout is a critical factor in risk assessment, and runout distance is the most widely used indicator of mobility. Runout distance is determined both by the landslide's initial conditions and through interactions with erodible substrates, which can affect momentum by altering basal friction or by increasing overall flow volume, generally increasing runout distance. After initiation, landslide processes can be separated into two phases: an impact phase and a runout phase. While erosion during the runout phase has been considered in prior studies, impact forces themselves have been overlooked. Here, we combine fieldwork in SE Tibet, laboratory tests, and numerical modelling to resolve the dynamics and effect of impact loading on landslides in alluvial basins. Impact-loading ring-shear tests and numerical simulations, backed up by field evidence, indicate that impact forces can near-instantaneously generate high excess pore water pressure within a saturated substrate, reducing basal friction of the landslide mass and extending runout. Both impactor and substrate properties, including stiffness and compressibility, control the impact load and duration, leading to different runout patterns and landslide mobilities. We find that the farthest runout occurs at an intermediate impact level, when the normal component of peak impact stress matches the self-weighted stress of the final deposits, as this condition most effectively liquefies the substrate. The findings highlight the importance of considering substrate properties for both erosion and impact during landslide runout analyses, particularly those occurring in alluvial basins.