Evaluating effectiveness of mitigation measures for large debris flows in Wenchuan, China

To cope with frequent debris flow hazards, many cascade check dams and diversion channels have been adopted in Wenchuan, Sichuan, China, on the eastern margin of the Tibetan plateau. Yet severe damage to local infrastructure still occurred upon large-scale debris flows, and the most common cascading effect of debris flows, river blockage, is often overlooked in conventional mitigation design. This study aims to investigate the effectiveness of mitigation measures against the large-scale debris flows on 20 August 2019 considering both the direct impact of debris flows and the cascading effect and to propose a numerical scheme to simulate serial check dams in a two-dimensional debris flow simulation model. Numerical simulation is conducted to reproduce the debris flow process and the cascading effect and to evaluate the effectiveness of improved mitigation measures. According to field investigations and the numerical analysis, all check dams in the study area were filled, indicating that the existing mitigation measures were inadequate. Simulation results further suggest that serial check dams are effective in reducing downstream deposition and the size of river debris barriers upon large debris flows. In some cases, risks induced by the cascading effect remain high even after check dams have been constructed, suggesting the need for additional mitigation measures. This study contributes to a robust design of debris flow mitigation measures against large-scale debris flows.