Hydrological Controls on the 30 July 2024 Wayanad Debris-Flow Disaster: Rainfall Extremes, Catchment Response, and Runout Dynamics

In late July 2024, a prolonged spell of extreme monsoon rainfall led to progressive slope saturation in the upper Punnapuzha catchment, culminating in a catastrophic landslide and debris-flow disaster in Meppadi Grama Panchayat, Wayanad District, Kerala, India, which then resulted in widespread loss of life and severe geomorphic alteration of the Punnapuzha river corridor. Understanding the hydrological processes that governed initiation of slope failure, debris mobilization, and long runout is critical for improving landslide hazard assessment in steep, monsoon-dominated terrains. This study presents an integrated, event-based reconstruction of the disaster, focusing on the role of rainfall characteristics, catchment-scale hydrological response, and debris-flow dynamics. Rainfall analysis was carried out using data from several raingauge stations surrounding the landslide crown, with particular emphasis on spatial representativeness and consistency during extreme events. These rainauges recorded more than 570 mm of rainfall over 29–30 July 2024, indicating rapid slope saturation and exceptional hydrological loading. Catchment response was simulated using the SWAT+ hydrological model, calibrated and validated against observed discharge records. The model reproduces daily runoff dynamics reasonably well and provides insight into the antecedent moisture conditions and runoff generation that preceded slope failure. To capture terrain modification caused by the event, post-landslide LiDAR-derived elevation data (0.1 m resolution) were compared with pre-event satellite-based DEMs. This analysis reveals extensive aggradation, channel widening, and reorganization of flow paths along an approximately 8 km debris-flow corridor. Two-dimensional debris-flow simulations were then performed using the non-Newtonian module in HEC-RAS, adopting Bingham rheology to represent high-concentration sediment–water mixtures. Simulations on pre-event terrain show strong agreement with observed runout extent and deposition patterns, with maximum flow depths exceeding 40 m near the landslide crown and progressively decreasing downstream. The results demonstrate that the disaster was controlled not by rainfall magnitude alone, but by the combined effects of intense short-duration rainfall, rapid catchment response, and efficient debris routing along confined valley geometry. By explicitly linking rainfall variability, hydrological response, and debris-flow propagation, this study provides a process-based framework for interpreting extreme landslide events in tropical mountain regions and highlights the importance of integrating hydrological understanding into landslide hazard analysis.