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On July 20, 2013, a landslide-induced debris flow occurred in Dujiangyan city, China. Eleven houses were destroyed with more than 166 fatalities. Previous literature indicated that the 2008 Ms 8.0 earthquake and a long period of heavy rain contributed to this landslide. The rock slide was initiated and 21% of the rupture mass fell from the cliff. These rocks fragmented when impacting the ground and then entrained the base soil. Subsequently, the volume of rockfall increased by 28%. In an area of flow convergence, a short-lived blockage occurred, and the following outburst triggered a debris flow. Previous field investigations have shown that both topography and bed entrainment affect the landslide-debris flow transformation. However, the dynamic process has not been quantified. In this study, the discrete element method (DEM) considering the dragging and rolling effects was used to perform a back numerical analysis to capture the dynamic processes. The computational results indicate two different entrainment scenarios, plowing and shearing. The former occurred when the rock slide detached from the cliff, and the latter occurred when debris material moved above the colluvium. The landslide transformed into a debris flow during a short-term blockage that occurred in the gradually narrowing channel. During flow convergence, the inertial shearing stress developed due to the inter-particle collision. [ABSTRACT FROM AUTHOR]

The fluctuation of the water level in a reservoir may induce various types of slope movements. Some of these movements are new, whereas others are old but reactivated. Many ancient landslide accumulations are distributed in the deep valleys of the eastern Qinghai-Tibet Plateau margin in China and will likely be reactivated after the completion of reservoirs and pose a risk to reservoirs, dams, facilities, and towns near mountainous areas. The Shuangjiaping ancient landslide, located in the Dadu River, Sichuan Province, China, is an example of this case. Since August 2010, the western part of the accumulation body has seen the gradual appearance of deformations. The Pubugou reservoir water level rises and inundates the front edge of the accumulation body, thus threatening the safety of the national highway G108 and 37 houses on it. This study is based on field work investigation, drilling work, aerial photography, and profile survey. The site-scale investigation shows that the deformation area is only a part of an ancient landslide accumulation, with an area of approximately 50.4 × 10 m and a volume of ca. 956 × 10 m. Boundary and deposit characteristics of landslide accumulation are specified, which could be divided into four zones, namely, zones A and C, which are an avalanche accumulation area mainly composed of large blocks with diameter ranging from 1 to 3 m (some blocks have a diameter ranging from 5 to 10 m), zone B, a residual integrated rock mass accumulation area with 'fake bedrock,' and zone D, a fine material accumulation area. A conceptual model is proposed to explain the mechanism and sliding process of this ancient landslide. The model includes translational sliding, stopping and hanging in air, avalanche accumulating, and transforming. The saltation of topography, material structure, and kinematic characteristics is the evidence used to identify the ancient landslide in deeply incised mountain areas. The current activity is found to be a surficial deposit displacement, and the whole landslide accumulation is stable or quasi-stable. [ABSTRACT FROM AUTHOR]