Reservoir-induced landslides and risk control in Three Gorges Project on Yangtze River,China

The Three Gorges region in China was basically a geohazard-prone area prior to construction of the Three Gorges Reservoir （TGR）. After construction of the TGR, the water level was raised from 70 m to 175 m above sea level （ASL）, and annual reservoir regulation has caused a 30-m water level difference after impoundment of the TGR since September 2008. This paper first presents the spatiotemporal distribution of landslides in six periods of 175 m ASL trial impoundments from 2008 to 2014. The results show that the number of landslides sharply decreased from 273 at the initial stage to less than ten at the second stage of impoundment. Based on this, the reservoir-induced landslides in the TGR region can be roughly classified into five failure patterns, i.e. accumulation landslide, dip-slope landslide, reversed bedding landslide, rockfall, and karst breccia landslide. The accumulation landslides and dip-slope landslides account for more than 90%. Taking the Shuping accumulation landslide （a sliding mass volume of 20.7 × 106 m^3） in Zigui County and the Outang dip-slope landslide （a sliding mass volume of about 90 × 106 m^3） in Fengjie County as two typical cases, the mechanisms of reactivation of the two landslides are analyzed. The monitoring data and factor of safety （FOS） calculation show that the accumulation landslide is dominated by water level variation in the reservoir as most part of the mass body is under 175 m ASL, and the dip-slope landslide is controlled by the coupling effect of reservoir water level variation and precipitation as an extensive recharge area of rainfall from the rear and the front mass is below 175 m ASL. The characteristics of landslide-induced impulsive wave hazards after and before reservoir impoundment are studied, and the probability of occurrence of a landslide-induced impulsive wave hazard has increased in the reservoir region. Simulation results of the Ganjingzi landslide in Wushan County indicate the strong relationship between landslide-induced surge and water variation with high potential risk to shipping and residential areas. Regarding reservoir regulation in TGR when using a single index, i.e. 1-d water level variation, water resources are not well utilized, and there is also potential risk of disasters since 2008. In addition, various indices such as 1-d, 5-d, and 10-d water level variations are proposed for reservoir regulation. Finally, taking reservoir-induced landslides in June 2015 for example, the feasibility of the optimizing indices of water level variations is verified.

Interaction among controlling factors for landslides triggered by the 2008 Wenchuan, China Mw 7.9 earthquake

The 12 May 2008 Mw 7.9 Wenchuan, China earthquake triggered about 200,000 landslides, which were controlled by a number of factors. This study examines five factors： slope angle, slope aspect, lithology, peak ground acceleration （PGA）, and fault side （relative position on the seismogenic fault, i.e., hanging wall or footwall）, to determine how these factors control the co-seismic land- slide occurrence and whether one or more factors, acting alone or in concert, are involved in promoting or suppressing landslides. We performed a multi-factor statistical analysis using data from the 2008 Wenchuan earthquake. The results show that in the areas characterized by steep topography or where strong ground shaking occurred during the earthquake, there is a closer relation- ship between slope aspect and landslide number density （LND） than other areas. The relationship between lithology and LND values depends on PGA. In turn, the relationship between LND values and PGA is also influenced by lithology. In addition, the controlling effect of lithology on co-seismic landslides on the hanging wall of the seismogenic fault is greater than that on the footwall. Examining interactions among these factors can improve understanding of the mechanisms of co-seismic landslide occurrence.