Multiphase Landslide Runout Modeling of Rainfall-Induced Debris Flow Event in the Western Ghats

Debris flow is a catastrophic form of landslide, which exhibits extreme kinetic energy and high velocity. Runout modeling is necessary to estimate the runout distance, inundated area, volume of debris mass, velocity, and energy associated with the debris flow. Also, studying the flow behavior of such past events will contribute toward the prediction of future potential inundation area, runout distance, debris volume, and resulting kinetic and pressure energy. Kerala, continuously being one of the most landslide-affected state in India for the last 5 years, loses hundreds of lives every year due to debris flow events. The current study makes use of r.avaflow program, which is based on the general multiphase mass flow model and its applicability in runout analyses of rainfall-triggered debris flow events in the Western Ghats region, Kerala. In the study, fluid phase and solid phase are modeled separately using shear rate-dependent Herschel-Bulkley viscoplastic and shear rate-independent Mohr-Coulomb plastic rheology models, respectively. Two recent debris flow events in the Western Ghats region were selected from the Wayanad districts of Kerala, namely, Puthumala and Pancharakolly. In these case studies, the post-event data were used to calibrate the rheological input parameters for the runout modeling based on the area under the receiver operating curve. Additionally, the effect of spatial resolution of the digital elevation model on the prediction accuracy is tested for a coarser and finer resolution of 30 m and 10 m. The results are compared against a single-phase runout modeling software, RAMMS (rapid mass movement simulation) as well.