Design of a stability assessment approach for rainfall-induced debris slides using physical modelling and multi-scale numerical simulations

Rainfall-induced debris slides pose a significant threat to lives and infrastructure in mountainous terrains. Recent increases in the frequency and intensity of debris slides in the Indian Himalayas have been attributed to both human activities and extreme rainfall. Although debris slides are generally shallow and of lesser volume, they result in substantial impacts, including road blockages, river aggradation, infrastructure damage, and considerable economic losses due to their significant numbers during extreme rainfall events. The failure mechanisms of rainfall-induced debris slides are complex, largely due to the involvement of soil–rock mixtures rather than pure soil or intact rock. This variability challenges conventional slope stability analysis and necessitates more refined, material-specific modelling approaches to accurately forecast and mitigate debris slide hazards. In this study, we present a slope stability assessment model developed based on laboratory-based reduced-scale flume experiments. We also design the model to simulate the relationship between rainfall input and the initiation of debris instabilities. To evaluate and calibrate the model, we simulate various debris slide scenarios at different scales: a laboratory-based reduced-scale flume experiment, a single debris slide event at the slope scale, and a group of rainfall-induced debris slides at the catchment scale, all from the Lesser Himalayan catchment in India. The calibrated model was then used to conduct a parametric study assessing the influence of various controlling parameters on debris slide initiation. Furthermore, the model also establishes critical rainfall Intensity–Duration (ID) thresholds for debris slide initiation, thereby strengthening the existing meteorological threshold-based early warning system in India.