Geohazards in the three Gorges Reservoir Area, China – Lessons learned from decades of research

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 × 10 6  m 3 ) in Zigui County and the Outang dip-slope landslide (a sliding mass volume of about 90 × 10 6  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.

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

Water level change upstream of a reservoir highlights the risk of a landslide-prone area on the banks of a reservoir. This paper conducted a study on the deformation mechanism of a selected landslide that occurred in the Three Gorges Reservoir (TGR) after the water level of the reservoir changed. The long-monitored surface deformation of the slide mass revealed that the deformation of the landslide was related to the water level changes in the reservoir, especially of the change between flood and floodless seasons. The measured internal lateral displacements in the landslide showed that such a landslide was characterized by a trail-mode. FLAC3D was adopted to model the landslide by examining the plastic zone, factor of safety, and the displacement in the x-direction in consideration of four conditions: the natural state of a landslide in the TGR, the initial impoundment, the subsequent rise of water level, and the drawdown of water level. The numerical results indicated that the landslide mass tended to be unstable during the initial impoundment; the subsequent rise of water level had a limited effect on the landslide happening, but the drawdown of water level directly triggered the landslide. The landslide changed from push-mode to trail-mode. It is strongly recommended that drawdown of the water level in the reservoir be carefully controlled to mitigate the effect on landslide mass.

Stability analysis of a typical landslide mass in the Three Gorges Reservoir under varying reservoir water levels

Susceptibility of reservoir-induced landslides and strategies for increasing the slope stability in the Three Gorges Reservoir Area: Zigui Basin as an example

Landslide identification and monitoring are two significant research aspects for landslide analysis. In addition, landslide mode deduction is key for the prevention of landslide hazards. Surface deformation results with different scales can serve for different landslide analysis. L-band synthetic aperture radar (SAR) data calculated with Interferometric Point Target Analysis (IPTA) are first employed to detect potential landslides at the catchment-scale Wudongde reservoir area. Twenty-two active landslides are identified and mapped over more than 2500 square kilometers. Then, for one typical landslide, Jinpingzi landslide, its spatiotemporal deformation characteristics are analyzed with the small baseline subsets (SBAS) interferometric synthetic aperture radar (InSAR) technique. High-precision surface deformation results are obtained by comparing with in-situ georobot measurements. The spatial deformation pattern reveals the different stabilities among five different sections of Jinpingzi landslide. InSAR results for Section II of Jinpingzi landslide show that this active landslide is controlled by two boundaries and geological structure, and its different landslide deformation magnitudes at different sections on the surface companying with borehole deformation reveals the pull-type landslide mode. Correlation between time series landslide motion and monthly precipitation, soil moisture inverted from SAR intensity images and water level fluctuations suggests that heavy rainfall is the main trigger factor, and the maximum deformation of the landslide was highly consistent with the peak precipitation with a time lag of about 1 to 2 months, which gives us important guidelines to mitigate and prevent this kind of hazard.

Landslide Identification and Monitoring along the Jinsha River Catchment (Wudongde Reservoir Area), China, Using the InSAR Method

At 4:40p.m. on November 23, 2008, the Gongjiafang slope collapsed on the north bank of Yangtze River in Wu Gorge of Three Gorges Reservoir. The 380,000-m3 sliding mass consisted mainly of cataclastic rock. A video record of the major sliding incident was analyzed using the general laws of physical motion. The analysis indicated that the maximum speed and maximum acceleration of the sliding mass were 11.65 m/s and 2.23 m/s2, respectively, and that the maximum amplitude and the propagation velocity of the water wave near the landslide were 31.8 m and 18.36 m/s, respectively. Wave run-up investigation indicated that the maximum run-up on shore was 13.1 m, which declined to 1.1 m at Wushan dock 4 km away. The incident causes no casualties, but did result in economic losses of RMB five million. The numerical simulation model GEOWAVE was used to simulate and reproduced the impulse wave generated by the landslide; the results were in good agreement with the observed incident. The numerical simulation data were then applied to analyze the decay and amplification effects of the landslide wave in the river course. The field investigations and witness information provide valuable materials for the studies of landslide kinematics and impulse waves generated by landslides. In addition, the research results provide a useful reference for future similar waves generated by landslides in reservoirs.

Analysis of waves generated by Gongjiafang landslide in Wu Gorge, three Gorges reservoir, on November 23, 2008

The Qianjiangping landslide occurred on July 13, 2003 when water level in the Three Gorges Reservoir in China reached 135 m a.s.l. for about 1 month. Thirteen people on the slope and 11 fishermen on boats in the nearby area were killed by this landslide and its wave. As a typical case, the Qianjiangping landslide is researched extensively, but the wave itself was studied much less. In this paper, a fluid–solid coupling model based on general moving objects (GMO) collision model and renormalisation group (RNG) turbulent model were employed, simplifying the sliding motion of the Qianjiangping landslide as a rigid body circular motion whose validity was verified by comparing with actual survey conditions related. Numerical simulation analysis shows that in the Qianjiangping landslide event, the solid–fluid energy-transferring rate was 5.97 %, while the motion of sliding mass could push and raise water body. The maximum-modelled wave run-up generated by the Qianjiangping landslide was 40.4 m (up to 175.4 m a.s.l.), while the maximal observed one was 39 m. It was the solitary wave, and the wave celerity was 32∼36 m/s or so. The hazardous river length of impulse wave was estimated to be 12.6 km, where wave height was more than 1 m high. The fluid–solid coupling numerical model employed by this paper can provide important reference for wave generated by other landslides around the world. Research of the landslide-generated wave is very significant in landslide hazard reduction and prevention.

Numerical analysis on wave generated by the Qianjiangping landslide in Three Gorges Reservoir, China

On June 24, 2015, Hongyanzi slope located in Wushan County of the Three Gorges Reservoir collapsed, generating 5–6-m-high impulse waves, which overturned 13 boats, killed 2 persons, and injured 4 persons. It is the second incident of landslide-generated impulse waves since the 175-m experimental impoundment in 2008. The emergency investigation shows that Hongyanzi landslide is a bedding soil landslide with a volume of 23 × 104 m3 induced by a series of triggering factors such as rainfall, flooding upstream, and reservoir drawdown. The nonlinear Boussinesq water wave model is used to reproduce the impulse waves generated by the landslide of June 24th. The numerical simulation results suggest that the wave propagation process was influenced by the T-shaped geomorphic conditions of river valley, and the coastal areas in the county seat were the major wave-affected areas, which is opposite to the landslide. The numerical wave process accord well with the observed incident, and the investigation values were in good agreement with the calculated values. Moreover, the worst-case scenario of the 7 × 104 m3 deformation mass beside Hongyanzi landslide is potential to generate impulse waves, which was predicted with the same numerical model. This adjacent deformation mass will probably generate impulse waves with maximum height and run-up of 2.2 and 2.0 m, respectively, and only a very few areas in the water course had waves rising to a height of 1 m or above. The research results provide a technical basis for emergency disposal to Hongyanzi landslide and navigation restriction in Wushan waterway. More importantly, it pushes the risk management of the navigation based on the impulse wave generated by landslide. It is advised that the Three Gorges Reservoir and other reservoirs around the world should put more efforts in performing special surveys and studies on the potential hazards associated with landslide-generated impulse waves.

Risk management study on impulse waves generated by Hongyanzi landslide in Three Gorges Reservoir of China on June 24, 2015

Landslide-related impulse waves are catastrophic but accidental, so limited data on field measurements are available; scaled physical experiment is therefore a functional method to simulate and analyze this phenomenon. A large-scale physical Froude-similar model to produce impulse waves was constructed based on the Chinese Gongjiafang landslide, which occurred on the main stream of Three Gorges after the impounding in the reservoir in China. With a scale of 1:200, the model had the dimensions of 24, 8, and 1.3 m. Four water levels, 145, 156, 172.8, and 175 m, were modeled for the experiments, and marble coarse sands were used to imitate the actual cataclastic rock mass. Wave height gauges, high-speed cameras, and run-up measuring instruments were used to monitor wave fluctuations in the model. Among the experiments, the ones modeled using a water level of 172.8 m best confirmed the actual conditions in the Gongjiafang landslide, representing a good validation of the experiments. This study obtained, for the first time, specific data on the reproduced impulse waves’ convergence and superposition during propagation, and of the energy change between impulse waves and reflected waves. The test data describe a rapid decaying and gradual decaying rule for the wave heights and run-ups. The Froude-similar experiments presented in this article help us to understand the whole procedure of impulsive wave generated by cataclastic rock mass failure, and the results acquired contribute to studies of impulse waves caused by similar bank destabilizations worldwide.

A physical similarity model of an impulsive wave generated by Gongjiafang landslide in Three Gorges Reservoir, China

An impulse wave generated by landslide is a major detriment to waterway safety in reservoirs. Thick-bedded pillar-shaped rockmass is one of the principal potential tsunami generators in the Three Gorges Reservoir and other reservoirs around the world. However, the impulse wave generated by a pillar-shaped rockmass was seldom studied in the past. The Jianchuandong dangerous rockmass lies on the left bank of Yangtze River in the Wu Gorge of the Three Gorges Reservoir with a total volume of 36 × 104 m3 or so, standing as a serious threat to the safety of the waterway, a renowned “expressway”. Along with the deterioration of the base mass, the Jianchuandong rockmass is likely to either slide or topple due to instability. A solid–fluid coupled computational fluid dynamic model is built to calculate the potential impulse wave induced by the Jianchuandong dangerous rockmass. The potential impulse waves under four working conditions with two failure modes and two water levels are discussed. Numerical simulation results show that the potential maximum wave height and maximum run-up are 47.1 and 27.2 m, respectively. Based on the integrated calculation results, we deduce a 15 km channel segment likely to be affected by the Jianchuandong rockmass-induced impulse wave for early warning purpose. By engineering geological analogy, the calculation results are verified with the instance data of two impulse wave events generated by landslides in the Three Gorges Reservoir. The purpose of this study is to provide a technical support for early warning, prevention and control of hazards associated with the Jianchuandong dangerous rockmass and to serve as a reference for researches on impulse wave generated by pillar-shaped rockmass collapse around the world.

Yueping Yin

Papers

Analysis of waves generated by Gongjiafang landslide in Wu Gorge, three Gorges reservoir, on November 23, 2008

Numerical analysis on wave generated by the Qianjiangping landslide in Three Gorges Reservoir, China

Risk management study on impulse waves generated by Hongyanzi landslide in Three Gorges Reservoir of China on June 24, 2015

A physical similarity model of an impulsive wave generated by Gongjiafang landslide in Three Gorges Reservoir, China

Potential risk analysis on a Jianchuandong dangerous rockmass-generated impulse wave in the Three Gorges Reservoir, China