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

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 and inertia by altering basal friction and by increasing overall flow volume, generally increasing runout distance. During erosion, the landslide mass can exert sudden impact forces on the substrate at points where the slope changes abruptly. While erosion effects have been extensively discussed 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. As density and geometry of the impactor determine a landslide impact force, both the impactor and substrate properties, including stiffness and compressibility, control the impact duration. The interplay of these two factors can lead to different landslide runout patterns and mobilities. The findings indicate that the farthest runout does not necessarily occur under the highest impact force; rather, an appropriate impact level exists that most effectively liquefies the substrate. The findings highlight the importance of considering substrate properties for both impact and erosion during landslide runout analyses, particularly those occurring in alluvial basins.