Landslide mobilized debris flow at Kalli village in Achham, Nepal: A case study

Debris flows are fast moving gravity-driven mass flows that carry significant momentum, posing a considerable threat to human lives, infrastructure and environments. With its complex geomorphology, diverse landforms, harsh climates, and fragile geologic conditions, the Himalayas, one of the youngest mountain ranges with some of the highest peaks of the world, often experiences extreme mass movements along the downslopes. The present study is focused on a debris flow event that started in 1983 and is still active in Kalli village of Achham district in the lesser Himalayas of Nepal. The runout distance of large boulders in this event reached nearly 3 kilometers and shows that the area is highly venerable to debris flow damage. A field study was conducted to assess the causes of the landslide which mobilized the debris flow. The key material parameters, debris material distribution, and rainfall patterns were analyzed. The field study shows that the major factor for the repeated landslides was the local geology which consists of inter-bedding of shale and sandstone with different deformability, competency, and permeability along with the orientation of joints. In addition, several topographical factors also contributed, such as the rice fields with flat land holding water at the upper part of the crown, causing accumulation and continuous supply of the water and increasing the pore water pressure which exacerbated the stability problem. Subsequently, numerical simulations were performed to evaluate the extremity of potential debris flows in this region. A multi-phase mass flow model was employed in the numerical modeling to calibrate the relevant parameters in the studied debris flow. The result shows that the internal angle of friction of solids and fine solids decreases, and basal friction decreases dramatically during the debris flow. The simulation results fairly match the field observations in terms of run out distance and deposition pattern, and demonstrate that the multi-phase mass flow model can be of promising potential for assessing and predicting future debris flow hazards in this region.