Micro-mechanical controls on entrainment and depositional patterns in wet granular debris flows: Insights from flume experiments and particle-scale characterization

Entrainment significantly modifies the dynamics and runout of debris flows, yet the combined influence of water content, bed properties, and particle-scale characteristics remains poorly constrained. Building on our previous flume-based framework, this study integrates mesoscale flow experiments with micromechanical analysis of debris materials including grain shape, roughness, and fragmentation using X-ray micro-CT imaging. A series of controlled flume experiments were performed using erodible sand beds (4 cm thick) prepared via mist pluviation to minimize segregation. Sixteen flow tests were conducted across a range of volumetric water contents (20–50%), capturing high-speed flow kinematics, entrainment depth, and deposit morphology. Complementary micro-CT imaging of fluvial and colluvial grains enabled quantification of particle shape (sphericity, aspect ratio, surface irregularity) and its potential role in erosion thresholds.

Results show distinct morphological transitions with increasing water content. At low w/c (20–24%), flows exhibited limited mobility and formed short, conical lobes with minimal scouring. Around intermediate w/c (~28%), reduced bed dilatancy and moderate pore pressure generated thicker but shorter deposits, indicating partial suppression of entrainment. At higher w/c (30–50%), enhanced lubrication and basal shear promoted deeper scouring, larger entrainment volumes, and substantially longer runouts with wide, flattened deposits. A parabolic relationship emerged between bed water content and entrainment rate, highlighting the nonlinear coupling between fluid fraction, granular collisions, and bed resistance. Deposits exhibited poor sorting and layered structures similar to natural debris flows, confirming dynamic similarity. Preliminary micro-CT analyses suggest that more angular and elongated grains exhibit larger contact stresses and higher resistance to dislodgement, whereas smoother grains mobilize earlier potentially explaining material-dependent variability in erosion observed across tests. Ongoing work aims to link shape descriptors directly with measured entrainment rates. This combined experimental–micromechanical approach advances our understanding of debris-flow erosion by bridging particle-scale processes and mesoscale dynamics. The results provide new insights for improving entrainment parameterization in debris-flow models and for developing more reliable runout predictions in geophysical hazard assessments.