Seismic signature of landslide dam breaching and it’s possible early warnings.

In eastern Taiwan, a landslide dam formed on February 4, 2021 in the Danan River. The lithology of the collapsed material is composed of schist and meta-sandstone with a thickness of a few centimeters to tens of meters. The landslide dam partially breached on August 7, 2021, and completely broke in October 2021. In the event of a dam failure, the downstream influence area includes residents along the river of Tongxin Village, the power plant, and an important artificial channel to transport farmland around Tongxin Village for irrigation. Therefore, real-time monitoring of dam failure is needed to provide early warnings of impending floods. The traditional monitoring method is to install a water level gauge behind the landslide dam for emergency response. However, it is impossible to establish a water gauge monitoring system on site because landslides usually occur in rugged mountainous areas. In this study, seismic analysis is adopted to capture seismic signals possibly caused by landslide dam failure, and to track the location of flooding after landslide dam failure. In this study, after the formation of the landslide dam, a real-time broadband velocity-type seismometer station (station code DALB) was deployed on the top of a mountain, and two Geophones stations (station code DALU, DALD) were installed in the midstream and downstream, respectively. Combined the difference between the arrival time of seismic waves and the distance along the river channel, the flow velocity can be measured. Optical satellite images were used to constraint the time range of dam failure based on the change in the highest elevation of the dammed lake surface. Finally, by using the riverine seismic signals, the time point of dam breaching can be determined, and the flow velocity of water and sand in the river channel can be estimated. Our studies can provide an early warning of few minutes to the downstream. The results of the time-frequency analysis also showed that the front reach of the breach is dominated by debris flow.