Correlating grain-size distributions, transport mechanism, and runout distance of debris flow deposits in the Himalayas

Debris flows are rapid mass movements with great potential energy to move and are among the most dangerous natural hazards due to their high velocities and longer runout distances. For hazard assessment, early warning systems, and to construct structural mitigation measures in mountainous catchments, it is crucial to study the origin, initiation, and dynamics of debris flows as well as the characterisation of the associated erosion and deposition processes. Debris flow deposit grain-size distributions (GSD) reflect the source properties and the transport and deposition mechanisms and control the sediment transport rates in fluvial systems. In this study, we characterise deposits of ~120 debris flows that occurred in Kedarnath, Mandakini valley, India, during the 2013 North India Floods and find the relation between GSDs and runout distances. Here, we use an approach that combines two methods of measuring GSDs, i.e., volumetric sieving and pebble count. Volumetric sieving can measure grain size only up to 80 mm and takes much fieldwork, while the pebble count method can only measure surface grain sizes but can measure all three axes of grains which is useful in the case of non-spherical grains. We measure surface and subsurface grain sizes and large boulders using this approach. After obtaining the GSDs for the number of debris flows, we do a statistical study on the relationship between GSDs, transport mechanism and runout length. Debris material characterisation is crucial, and the approach has large potential applications in understanding the initiation, failure, and transport mechanisms of extreme-precipitation induced sediment disasters.