Rapid episodic erosion of a cohesionless landslide dam: Insights from loss to scour of Yangjia Gully check dams and from flume experiments

Abstract

An underestimation of local scouring in loose cohesionless landslide deposits may cause rapid and episodic failure of a hazard mitigation structure. A rock avalanche triggered by the 2008 Wenchuan earthquake generated an abundance of erosion-sensitive sediment in Yangjia Gully, resulting in several debris flows. Twelve concrete pile check dams were constructed along the gully to mitigate debris flow hazard in 2017. However, three of the downstream check dams progressive collapsed during the next two years. Field investigations revealed at the downstream of most check dams an accelerating scour, which quickly led to their instability and failure. Energy dissipation structures built at the toe of the dams moderated the local scour but did not prevent it. Repeated field investigations and flume experiments were carried out to better understand the process of bed erosion. The erosion rate is substantially high, and appears to be irreversible without artificial intervention once a hydraulic drop is formed. The presented case in Yangjia Gully shows the consequence of underestimating rapid erosion. Furthermore, several suggestions are proposed for hazard mitigation engineering in erosion-sensitive deposit.