Nikhil Kumar Pandey

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.

Accurately modeling debris flow behavior remains challenging, particularly for highly viscous flows, due to limitations in conventional rheological models and uncalibrated field data. This study integrates the Herschel-Bulkley-Papanastasiou (HBP) model with Smoothed Particle Hydrodynamics (SPH) using the DualSPHysics framework, combining numerical simulations with experimental and field investigations to improve debris flow modeling. A flume-based experimental setup was used to analyze flow initiation, velocity evolution, and deposition patterns under controlled conditions, providing crucial calibration data for the numerical model. Geotechnical and geophysical site investigations further refined key rheological parameters, ensuring accurate representation of material behavior. The calibrated model was then applied to the 2020 Pettimudi debris flow in Kerala, India, capturing key flow characteristics such as a peak velocity of 16 m/s, hydrodynamic pressures of 80–200 kPa, and a deposition width of 110 m. These predictions were validated through field surveys and historical data, demonstrating the model’s reliability in replicating real-world debris flow behavior. The study highlights the effectiveness of SPH combined with the HBP model in addressing limitations in traditional methods, providing a flexible and scalable framework for simulating debris flows. The methodology offers valuable insights into flow mechanics and has significant potential for hazard assessment, mitigation planning, and back-analysis of past debris flows in vulnerable regions worldwide This study highlights the novelty of integrating experimental, numerical, and field-based approaches to improve debris flow modeling. The findings demonstrate that SPH, combined with the HBP model, provides a flexible framework for simulating viscous debris flows, addressing limitations in traditional methods.