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An Ms 6.5 earthquake shocked the Ludian County, Yunnan Province, China, on 3 August 2014 and triggered the Hongshiyan landslide dam. The dam, with a height of 83 m and a lake capacity of 260 × 10 m, threatened more than 10,000 people. A unique feature of this landslide dam was that it formed between a man-made dam and a hydropower plant. An existing drainage tunnel connecting the lake and the hydropower plant became a natural drainage conduit for the landslide dam, which played an important role in the mitigation of the landslide dam risks. This paper reports a quantitative risk assessment for the Hongshiyan landslide dam considering both engineering and non-engineering risk mitigation measures. The risk assessment is divided into three stages according to the implementation of two engineering measures: construction of a diversion channel and excavation of a branch drainage tunnel. The dam breaching hydrographs, flood zones, population at risk, and likely fatalities in each of the three stages are analysed. The optimum evacuation strategy in each stage is also studied based on the principle of minimum total consequence. It is found that the diversion channel decreases the dam breaching peak discharge and the associated risks significantly. The branch drainage tunnel prevent the landslide dam from overtopping failure in non-flooded period; however, the landslide dam may fail by overtopping in a future flood if the inflow rate is larger than the outflow rate through the drainage tunnels, resulting in serious losses of lives and properties. The dam breaching risks in all the three stages could be largely reduced by the optimal evacuation decision, which shows that timely evacuation is vital to save life and properties. The study provides a scientific basis for decision making in landslide dam risk management. [ABSTRACT FROM AUTHOR]

The Hongshiyan rock avalanche is a remarkable landslide disaster with approximately volume of 12 × 106 m3, triggered by the 2014 Ms. 6.5 Ludian earthquake in Yunnan Province, China. This study conducted a comprehensive analysis by the model based on discrete element (DEM-based) numerical simulation to understand the transport process and mechanism for this rock avalanche. The simulation results showed that the transport process of the rock avalanche depends on the input seismic duration and motion. The average velocity of the rock avalanche sharply increases to peak value of 27 m/s and then gradually decreases to zero, and 64% and 36% of the total energy are dissipated by collision and friction, respectively. In this process, the progressions from simple disintegration along pre-existing discontinuities to fragmentation that creates new fracture surface are documented, and gradual increase of the fragmentation degree over time results in the decrease of fragment size and the formation of well-graded and narrower-interval gradation. This fragmentation evolution creates a conductive condition to the development of internal shear, and is closely associated with the dense flow regime that dominates the main body of the rock avalanche but presents discontinuous distribution along the flow thickness direction. In addition, further analyzing the simulated results indicates that more likely effects of fragmentation on mobility of rock avalanches depend on fragmentation-induced special flow structure, which makes a rock avalanche in a flow state with lower friction and lower energy consumption. [ABSTRACT FROM AUTHOR]