[H-DS07] Landslides and related phenomena

[HDS07-20] Rainfall-induced debris slides in the Indian Himalayas: mechanisms, risk assessments and forecast potential

Manish Dewrari¹, *Srikrishnan Siva Subramanian¹ (1.Centre of Excellence in Disaster Mitigation and Management Indian Institute of Technology Roorkee, Roorkee - 247 667, Uttarakhand, India)

Keywords:Debris slides, Rainfall, Flume experiments, Failure mechanisms, Numerical modelling, Risk assessment

Rainfall-induced debris slides are common geological hazards in mountainous terrains like the Himalayas. In India, the Uttarakhand state experiences many rainfall-induced landslides, i.e., debris slides and debris flows over hillslopes of different catchments across the Himalayan terrain during the Indian Summer Monsoon (ISM). Although studies have extensively studied rainfall-induced landslides in the Himalayas, the mechanisms, risk assessments and forecast potential of debris slides still need to be explored explicitly. For improved risk assessments, we approach this multi-scale problem at different scales, i.e., lab, slope, and catchment scales. First, at the lab scale, we performed reduced-scale flume experiments to understand the hydrological and geomechanical behaviour of debris slopes having grain sizes of soil-rock mixtures ranging from fine silts to coarse gravels. Further, debris material characterisation was done to identify the index, hydrological and geotechnical properties. In the flume experiments, we observed that as rainfall infiltrated, the volumetric water content gradually increased while matric suction decreased, decreasing the apparent cohesion. The results also showed a correlation between slope failure, pore water pressure, matric suction, and moisture content. We identify a preliminary mechanism possible over debris slides under various rainfall intensities. Second, we numerically simulate the reduced-scale flume experiments (lab scale) and upscale the numerical model to analyse an actual case study of debris slide (slope scale) that occurred in the Himalayas. We find a satisfactory representation of the experiments and actual slope failures through the numerical model. Third, we simulate debris slides triggered during the 2013 North India floods over a selected catchment in Uttarakhand, India. The numerical model we developed in this study based on the lab-scale and slope-scale studies could be used for debris flow risk assessments at the catchment scale. Finally, we correlate a meteorological threshold, i.e., the Intensity-Duration (ID) curve, to understand the forecast potential of debris slides.