Mitigating the Impact of Debris Flows on the Built Environment: A Case Study of Southern Himalayas

Climate change-induced geohazards pose significant threat to the sustainability and serviceability of built environment. Among such disasters debris flows are prominent in hilly areas and pose threat to life and property all over the world. Debris flows are coupled geo-hydro-mechanical phenomena with high flow velocity and long runout distance, resulting in a large impact force on the associated built environment. For effective hazard mitigation it is crucial to investigate the dynamic impact of debris flows on structures. It has been established that barriers in the way of debris flow helped to reduce the energy of the flow, leading to a lesser impact on the downstream end. As such many studies have focused on installing barriers at varied locations and of various sizes. However, there is still need for innovative research on how to increase the performance of these barriers. In this study an investigation to evaluate the impact on a structure on downstream due to debris flows is carried out. Besides, the implications of introducing a barrier structure with passages on upstream end is also studied. Smoothed Particle Hydrodynamics (SPH), a mesh-free Lagrangian method, is employed to capture the motion of debris flow and its impact on a rigid structure. For this study the authors have considered the Rishiganga river valley of Uttarakhand state in India, where a recent event of debris flow on February 07, 2021, caused large destruction to important facilities including the hydroelectric power plant. Located in the southern part of the Himalayas, this region is geo-morphologically sensitive and seismically active, making it susceptible to frequent events of landslides, debris flows and other mass movements. Three dimensional (3D) analyses are carried out for three different cases: case1, with no barrier structure on the upstream, case 2, where a barrier structure with one large passage has been placed and case 3, where a barrier structure with two passages has been placed on the upstream. Based on the outcomes, it is inferred that the presence of a rigid structure at the upstream end reduces the impact on the downstream structure considerably. The impact is found to be highest for case 1, followed by cases 2 and 3, with impact values which are only 35% and 30% of case 1, respectively. Similar trend is found in the velocity gradient at a location between the barrier and the main structure. After the introduction of barrier structure, there is a decrease of approximately 10% in maximum velocity for case 2 and a drastic decrease of approximately 90% for case 3 as compared to case 1, showing consistency with the impact values. It is established, that the introduction of passages decreases the impact considerably owing to the decrease in velocity as well as the volume of debris reaching the main structure, because of some accumulation behind the barrier. Moreover, increasing the number of passages, while keeping the passage area constant, causes the flow to become more streamlined, hence making the flow more uniform, which leads to a further reduction in impact forces.