On October 10th and November 3rd, 2018, two successive landslides occurred at the same place of Baige village, the border of Sichuan Province and Tibet Autonomous Region, China, which blocked Jinsha River twice. Owing to the rapid rising of water level of the "10·10" dammed lake, on October 12th, the landslide dam breached naturally. Then, the flow channel formed after the breaching of "10·10" dammed lake was blocked by the "11·3" landslide, resulting in an even larger dammed lake. The measures of excavation of drainage channel were taken to decrease the water level of the dammed lake. On November 12th, the dammed lake breached with the peak flow rate of 31 000 m3/s. For the "11·3" Baige dammed lake, the detailed hydrological data were documented, which provided valuable basic data for the study of the breaching process of dammed lake. Based on the breaching mechanism of landslide-formed dam, a numerical model which can consider the hydrodynamic conditions of the lake and the morphological characteristics of dam materials of the landslide dam, was developed. The broad-crested weir equation was adopted to simulate the breach flow discharge, and the variation of water level of the dammed lake was determined according to inflow and breaching flow, as well as the rating curve of water level and surface area of the dammed lake. Based on the shear stress of water flow and the critical shear stress of dam materials, the erosion process was simulated by using the erosion formula for wide-graded dam materials. Under the assumption that the breach slope angles remain unchanged during the longitudinal cutting and transverse broadening processes, the limit equilibrium method is used to analyze the slope instability during the breach evolution process. The coupling process of soil and water during dam breaching was simulated by the time step iterative numerical method. Then, the back analysis of the "11·3" Baige dammed lake was conducted, and the comparison showed that the breach hydrograph, water level variation of dammed lake, and breach morphology evolution calculated by the proposed model are consistent with the measured data. The sensitivity analysis showed that soil erodibility had significant influence on the breaching process, and residual dam height also played a role in the breaching process by affecting the discharged water storage. In addition, the excavation of spillway can reduce the reservoir capacity when the dammed lake breached, thus affecting the breach hydrograph, which is an effective measure to reduce disaster losses.
