Numerical Simulation of the Natural Erosion and Breaching Process of the “10.11” Baige Landslide Dam on the Jinsha River

Abstract

Outburst flood of the dammed lake poses a great threat to the down-stream residents and properties, in addition, the process of dam erosion and flood routing is very complicated. Thus, the research which using numerical method to simulate and repeat the typical landslide dam breaching, has a great significance for disaster prevention and mitigation in the downstream. The Jinsha River, was dammed twice recently at Baige, Tibet, one on 10 October 2018 and the other on 3 November 2018. Accordingly two large landslide dams were formed in a three-week interval, and breached subsequently, causing a major loss of property and damage in the downstream. This study focuses on the “10.11” Baige landslide-dammed lake. A three-dimensional numerical model of “10.11” Baige landslide dam is established with the terrain data acquired by UAV, and the actual process of dam breaching is simulated and reproduced using the Flow-3D software. The Flow-3D software could be powerful, which has the ability to simulate the phenomenon of turbulence flow and sediment movement under complex terrain conditions. The parameters required for this study, such as hydrological data and particle composition of dam body, are directly referred to the field data. A flow monitoring section is set up near the downstream of the dam, and the simulated peak discharge process is in good agreement with the measured values. Furthermore, the velocity distribution and sediment scour in the natural spillway are analyzed. The results show that: overtopping scour of dam can be divided into four phases, i.e. before the outburst, rapid expansion phase, peak discharge phase and phase with breach develops steadily. In second phase, the sediment erosion rate is large and the terrain changes fast, so the breach expands rapidly. After the flood peak, the upstream water level and discharge decrease gradually, and the development speed of breach becomes slow, finally the terrain tends to be stable. The velocity of flow on the ramp of spillway is large during the releasing period, which leads to the maximum scour depth. On the contrary, the flow velocity in the front and middle of the spillway is small and the scour depth is relatively small. The terrain becomes flat at the downstream of the dam body, so the sediment sinks under the influence of weight and friction with the riverbed, then siltation occurs. There was obvious headward erosion in the process of first dam breaking: with the undercutting of spillway, the bed drop-off moved to the upstream continuously. Headward erosion is an important scour mode of dam breaching and the dominant force of channel expansion, which can produce strong scour effect in a short period of time. This study is significant for understanding the process and mechanism of the “10.11” Baige landslide dam breaching, which could provide technical reference to the management and accommodation of emergency.