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Abstract: Understanding the mechanisms of landslide failure and resulting damming processes is crucial for constraining the hazard chain associated with landslide damming events. On May 12, 2008, the Wenchuan earthquake caused a large number of secondary hazardous events such as landslides and collapses, most of which occurred in high mountain gorges. In these areas, the sliding distance of landsides is often very short, and the landslides are often characterized by high speed, short distance, and damming. Using the Tangjiashan landslide damming events induced by the Wenchuan Earthquake as a model, this paper reconstructs the three dimensional failure and associated damming caused by the landslide based on a detailed geological investigation of the landslide area and using the dynamic finite element technique. The study proposes a four-stage model for the failure and damming process of the Tangjiashan landslide: earthquake induction and progressive destruction of the slide body, destruction and high-speed sliding of the slide body, slide body impact, disintegration, and river blockage for the formation of a damming body, and vibration compression. The average speed of the Tangjiashan landslide reached a maximum value of 22m/s about 30s after the earthquake, and the landslide reached stability about 60s later. [Copyright &y& Elsevier]

Abstract: The 2008.5.12 Wenchuan Earthquake (seismic magnitude Mw 7.9 according to the USGS) triggered a great number of landslides. A spatial database of landslides is built by interpreting the remote sensing (RS) data which covers 11 counties severely damaged by the earthquake with area of about 31,686.12km2. The geological settings of the study area are detailed mapped including geologic structure and lithology. The digital elevation model (DEM) of the study area with resolution of 30m×30m is presented to address the topographic characteristics. Correlations between the occurrence of landslides with geological settings i.e. active faults (the coseismic surface ruptures), geological unit, slope gradient, slope elevation as well as slope aspect are analyzed using spatial modeling in GIS with the aids of field investigations. It can be concluded that the distance to the causative faults and slope gradient are much more important than other factors. Landslides are clustered distributed along the causative faults of F2 and F3, and landslides incidence are dramatically decreasing with the increasing of the distance to the causative faults. The landslides incidence has an obvious increase near the active faults i.e. F1. Landslide concentration (LC, landslides/km2) has an obvious exponent increase with slope gradient. However, LC has no obvious relationship with geological unit and slope elevation; the difference of LC value in each geological unit (each slope elevation category) maybe have been the result affected by slope gradient for each geological unit (each slope elevation category) and distance of each geological unit (each slope elevation category) to causative fault zone. Isoseismal map reflects the incidence of landslides in some degree, and LC increases dramatically with Intensity increasing, almost all landslides occurred in the region above VI degree. Statistics result also indicates that in the near field of causative faults, landslides tend to have the initial sliding direction similar to the movement of the causative faults. And preliminary study shows that landslides incidence varies in different slopes with different structure, and consequent slopes and obsequent slopes have a higher landslides incidence than other layered slopes. [Copyright &y& Elsevier]

The catastrophic Tangjiawan landslide, triggered by the 2008 Wenchuan earthquake, blocked the Duba River, impounding one of the most dangerous coseismic barrier lakes in Beichuan, China. The lake was drained by an artificial spillway within one month after the earthquake to minimize the potential dam-breach flooding risk. However, on September 5, 2016, this landslide was reactivated and dammed the river again, creating a 20-m high dam at the same location and resulting in the formation of a barrier lake with a volume of 0.6 million m 3 . The day after the event we carried out a field investigation of the landslide and obtained a high-resolution image and DEM using UAV. The satellite images from 2005, 2008, 2010 and 2015 were also collected to analyze the evolution of the landslide. Together with multi-temporal DEMs, the geometry and volumes of the displaced mass and the landslide dams and barrier lakes were calculated using image interpretation and 3D spatial analysis with GIS. This landslide is of great scientific interest, as it presents a good example of multiple reactivation of a pre-historical giant landslide under different triggering conditions: a reactivation of an older landslide during the Wenchuan earthquake, and a second reactivation during a rainfall event several years later. Meanwhile from the hazard assessment and prevention perspective, it is also representative as it dammed the river twice in 2008 and 2016, posing threats to both upstream and downstream areas. We infer that the successive landslides in this region could be caused by the strong tectonic activities: including earthquakes and high average uplifting rate. The results of landslide volume analysis using multi-temporal DEMs, contribute to the landslide mechanism analysis, and suggest that the landslide volume estimation is effected by the landslide type, landslide rupture surface location, and resolution of DEM. We also compare the performance of different empirical models of landslide stability and dam-breach flood parameters and discuss their application during the quick assessment of the potential hazard of the landslide dams. Generally, the successive landslide dams at the Tangjiawan site are caused by the successive landslide reactivations on an anti-dip slope controlled by strong tectonic activity and river erosion, involve with a mass of loose materials of previous landslide deposition, and possess high flood risk to the downstream area. The experience gained in this work can be used to assist the hazard assessment and the planning of the emergency measures for similar landslide dams in the future. [ABSTRACT FROM AUTHOR]

Strong earthquakes can trigger extensive landslides, generating abundant loose deposits that are prone to be remobilized as debris flows by subsequent rainfall events. The magnitude-frequency distribution of post-earthquake debris flows often changes significantly over time. However, the key parameters controlling the post-earthquake debris flow activity and magnitude were poorly understood. In this study, we selected a representative catchment in the Wenchuan earthquake-hit region to study the activity of debris flows from 2008 to 2020, as well as the changes in parameters due to material depletion, grain size coarsening and vegetation recovery. Samples were collected from the field in different years after the earthquake and were tested in the laboratory to obtain their geotechnical properties. A numerical model was improved by considering the changing properties to fit better to dynamic conditions after the earthquake. We found that the particle size of the loose material gradually coarsened, the shear strength showed an increasing trend, while the erodibility coefficient of loose material showed a decreasing trend. The evolution of these parameters controlled the magnitude-frequency of post-earthquake debris flows and also changed their behavior from slope failures in the first few years after the earthquake to concentrated surface runoff erosion (channel erosion) in later years. The results have an important implication for risk assessment and prediction of future debris flows in earthquake-prone regions. • An improved numerical model simulates multiple debris flows after earthquake. • The parameters that control the magnitude-frequency of debris flows chang over time after the earthquake. • The post-earthquake debris flow activity is impacted by source material depletion, grain size coarsening, and revegetation. [ABSTRACT FROM AUTHOR]

The 2008 M w 7.9 Wenchuan Earthquake (Sichuan, China) was possibly the largest and most destructive recent earthquake as far as the geo-hazards are concerned. Of the nearly 200,000 landslides triggered originally, many remobilized within a few years after the initial event by rainfall, which often caused catastrophic debris flows. The cascades of geo-hazards related to the Wenchuan Earthquake motivated research worldwide to investigate the triggering and mechanisms of co-seismic landslides, their rainfall-induced remobilization, the generation of debris flows, the evolution of their controlling factors, and the long-term role of earthquakes in shaping the topography. On the eve of the 10th anniversary of the 2008 Wenchuan Earthquake, we present a short review of the recent advances in these topics, discuss the challenges faced in the earthquake-related geo-hazards mitigation practice, and suggest priorities and guidelines for future research. [ABSTRACT FROM AUTHOR]

The extremely strong Wenchuan earthquake triggered thousands of landslides in Sichuan Province, China. Flow-like landslides, such as the Tangjiashan, Wangjiayan, and Donghekou landslides were among the most destructive, causing many casualties and serious economic damage. It is therefore important to identify the flow mechanisms and investigate the specific characteristics of these flow-like landslides. This paper presents a three-dimensional model based on smoothed particle hydrodynamics (SPH) to simulate rapid landslide motion across 3D terrain. The Navier–Stokes equations in a CFD framework are used as governing equations, and artificial viscosity is incorporated into the pressure terms in the momentum equation to dissipate energy to avoid numerical oscillation and particle penetration, thus improving the stability of the numerical results. A non-Newtonian fluid model, the Bingham model, has proven suitable for describing the relationship between the shear strain rate and the shear stress in highly deformed soil materials. In the proposed model it is used as the constitutive model to describe the fluidization characteristics of flow-like landslides combined with the Mohr–Coulomb yield criterion. The model incorporates a no-slip boundary condition, to consider the effect of a solid boundary on slope movement. Ghost particles are created and assigned an artificial velocity. The viscous force caused by the solid boundary is calculated using the relative velocities between the fluid and the boundary particles. Open Multiprocessing (OpenMP), an API for multi-platform shared-memory parallel programming, is used to improve the efficiency of the SPH code running. To show the validity of the proposed approach, a benchmark problem of 3D dam break was simulated. The calculated distances of the surge front at different times agree well with the test results. Numerical modeling of the propagation of the Tangjiashan, Wangjiayan, and Donghekou landslides was performed by the application of SPH models to real flow-like landslides. The whole flow processes of these flow-like landslides across the 3D terrain are represented. The landslides change direction, split or join in, and spread or contract in their flow path in response to the local topography. Time-history curves of the velocity and displacement were obtained to analyze the movement characteristics of the landslide mass. The shapes of the deposition zones after slide occurrence were investigated. Comparisons of the SPH simulated geometry and the surveyed landslide configurations for the Tangjiashan, Wangjiayan and Donghekou landslides were conducted, and show a high degree of similarity. This indicates that the proposed 3D SPH model can accurately represent the evolution of the final slide shape. The prediction of the fluidization characteristics of earthquake-induced flow-like landslides can notably reduce sudden loss of life, as it provides a means for mapping hazardous areas, for estimating the hazard intensity, and for identification and design of appropriate protective measures. [ABSTRACT FROM AUTHOR]

Empirically derived rainfall thresholds of debris flows are used for regional-scale early warning. However, triggering rainfall intensities of post-seismic debris flows evolve with time, causing high false alarms since thresholds estimated by conventional methods ignore the uncertainty of non-triggering rainfall events. Based on 172 triggering rainfalls and 2396 non-triggering rainfalls from 2008 to 2013 after the Wenchuan earthquake, we analyzed the evolution of probabilistic rainfall thresholds for post-seismic debris flows using a Bayesian technique. We found, rainfall thresholds significantly decrease compared with pre-earthquake events initially and later tend to increase annually. Meanwhile, the triggering rainfall characteristics tend to gradually change from a short-duration high-intensity pattern to a long-duration and low-intensity pattern. We also checked the effect of antecedent precipitation on debris flows by defining an IET (inter-event time). Our results suggest the antecedent precipitation plays an important role in low-intensity long-duration rainfall-induced debris flows and has little effect on the short-duration high-intensity rainfall-induced debris flows. The characteristics of triggering rainfall for debris flows after the Wenchuan earthquake can be best reflected by IET = 7 h. By employing a Naïve Bayes algorithm, uncertainties of three rainfall threshold models, I-D (mean intensity-duration), I T -D (triggering intensity-duration), and I T -E (triggering intensity-cumulative rainfall) were investigated, and the I T -D model was found to perform best. The characteristics and evolution of debris flow rainfall thresholds over the years after the Wenchuan earthquake was best presented by the Bayesian probabilistic rainfall thresholds. We believe the results can be used to improve the precision of an early warning system for post-seismic debris flows. • We present probabilistic rainfall thresholds for debris flows post-Wenchuan earthquake using a Bayesian technique. • Antecedent precipitation is significant to trigger debris flows by low-intensity long-duration rainfall. • Thresholds significantly decreased post-earthquake, then increased annually, not yet returned to the pre-earthquake level. [ABSTRACT FROM AUTHOR]

A large and catastrophic rock avalanche occurred at the Xinmo village in the Maoxian county of the Sichuan province of China on June 24, 2017. The avalanche destroyed 64 houses and killed 10 people, and 73 people were reported to be missing. In this study, we focused on the contributions of six historical earthquakes to the collapse of rocks in the source area and the landslide amplification caused by the entrainment of pre-existing deposits on the landslide path. The historical earthquakes in this area contributed different levels of damage on the source rocks; particularly, the 1933 Diexi earthquake induced a few initial cracks in rocks at the crest of the steep hillslope. Under the combined time-effects of long-term weathering and gravity, the rock blocks in the source area were detached from the crest along the intersecting cracks and over-dip soft layer (namely, the sliding surface). The avalanche was amplified by the entrainment of colluvium on its path. The influence of the colluvium on the movement process of the Xinmo landslide was modelled by the GPU-based parallel discontinuous deformation analysis (DDA) method, and the effect of the impact and entrainment in the debris flow was rigorously analysed. The results indicate that the impact and entrainment can continuously increase the volume of debris flow, and the thickness of the colluvium was established to be in the range of 25.9–33.9 m based on the analysis of the deposit pattern and movement duration of the avalanche. There were numerous unstable rocks induced by historical earthquakes, at the crests of steep hill slopes in the earthquake-stricken area. With long-term damage caused by environmental factors and gravity, the failure of unstable rocks with high potential energy is likely to cause severe disasters. The excess pore water pressure likely to be present in the colluvium or deposits during impact and entrainment can considerably reduce the effective stress and degrade friction resistance along the runout path. As a result, the landslide can be intensively amplified owing to the effect of impact and entrainment under a lower friction resistance • The volume of rock avalanche sharply increased by entrainment effects. • The rocks at the source were damaged by historical earthquakes. • The colluvium exerts different influences on the movement of the avalanche with an increase in its thickness. • Numerous earthquake-damaged rocks at the summit of mountains are hazardous. [ABSTRACT FROM AUTHOR]