Vertical Velocity Profiles in Erosive Landslides

Vertical velocity profiles in erosive multiphase mass flows control how momentum is transferred from a moving landslide to the underlying bed and therefore govern erosion, entrainment, and mass enhancement. Although erosion is known to increase landslide mobility, the particle-scale mechanisms by which internal shear drives sediment mobilisation remain poorly constrained. In particular, the vertical distribution of velocity in erosive granular flows is largely unknown, despite providing the critical link between flow dynamics and bed response. Field measurements document a wide range of velocity profile shapes but lack the spatial resolution required to quantify shear close to the bed. By contrast, previous laboratory studies either failed to resolve internal kinematics under erosive conditions or relied on artificial, rounded particles that suppress the frictional interactions characteristic of natural sediments. Consequently, differences between landslide velocity and the velocity of the eroded bed, as well as the vertical shear rates underpinning erosion-entrainment-mobility formulations, remain largely unconstrained by empirical data.

Here we present a new experimental dataset that directly addresses this gap. We conducted controlled laboratory experiments on landslide-like granular flows moving over an erodible bed composed of naturally crushed sand-gravel mixtures. A measurement approach based on Particle Image Velocimetry combines lateral imaging with plan-view observations, allowing continuous vertical velocity profiles to be reconstructed across the full flow depth during active erosion and entrainment. The experiments include dry granular flows and flows with varying water content, two representative grain-size classes, and systematic comparisons between erosive runs and reference cases over a rigid bed.

The results show that both the inertia of the erodible material and the time-dependent erosion rate fundamentally alter the vertical velocity profile. The velocity of the moving landslide and that of the erodible bed can now be clearly distinguished, enabling direct calculation of entrainment velocity and erosion drift. Shear mainly occurs near the bed-flow interface, evolving dynamically as material is entrained and creating velocity gradients that cannot be captured by depth-averaged approximations. These measurements provide the first quantitative characterisation of vertical shear under fully erosive conditions using realistic sediment properties. By resolving particle-scale velocity gradients, this study establishes the experimental basis required to calibrate and verify erosion-mobility models that explicitly depend on shear-rate-controlled entrainment, thereby advancing the predictive modelling of erosive landslides.