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West China experienced an extremely wet autumn in 2017, and the amount of September–October precipitation is the greatest during 1961–2018. This study explored the physical processes responsible for two primary intraseasonal precipitation events which contribute greatly to such an extremely wet season. The first event peaking in early September is induced by the southward movement of a frontal system from the northern China, which is attributed to the southward displacement of an intraseasonal anticyclonic anomaly. The northerly anomaly associated with the anticyclone advects cold air into West China and thus triggers frontal precipitation. The second event peaking in early October is related to the northward shift and intensification of an intraseasonal anticyclonic anomaly centred over the northwestern Pacific Ocean. The southerly anomaly along the western flank of the anticyclone transports moisture to West China and thus triggers precipitation. The different initiation processes for the two precipitation events may be due to the distinct dynamical and thermal‐dynamical conditions in September and October. West China has sufficient moisture but less dynamical disturbances in September, while it has adequate dynamical disturbances but less moisture in October. [ABSTRACT FROM AUTHOR]

Enhanced debris flow activity observed after the 2008 Wenchuan earthquake, Sichuan Province, SW China, is still intense more than a decade since the earthquake. A heavy rainstorm on 20 August 2019 caused catastrophic debris flows in the epicentral area of the earthquake. By remote sensing imagery interpretation and field investigation, 13 debris flows were identified. An empirical rainfall threshold I-D model (intensity–duration) of post-seismic debris flows is established. Changing rainfall thresholds, characteristics, and initiation mechanisms of the 2019 debris flows were analyzed, which are different from the events that occurred in previous years. We find that the 2019 debris flows were mainly initiated by concentrated runoff erosion of sediments stored in tributary channels of catchments, with little contribution from the coseismic landslides and new hillslope failures. Differently, the major sediment supply of debris flows in previous years was mainly from the reactivation of coseismic landslide deposits. By analyzing the triggering rainfall of all recorded debris flows after the Wenchuan earthquake, we find that the rainfall threshold in 2019 is about 1/2–4/5 of the pre-earthquake value, and the I-D model we built can predict the 2019 debris flows successfully. Considering that there are still a large number of coseismic landslide deposits in the mountain catchments and the rainfall thresholds are slowly recovering, the post-seismic debris flows may last for a long period. [ABSTRACT FROM AUTHOR]

On May 12, 2019, a landslide occurred near a construction site of a reservoir dam along the left bank of the Wulong River, Qinghai Province, China. Video footage showed that the entire failure process of the landslide lasted for 15 min. Although an excavator that parked on the foundation pit was buried and the Wulong River was blocked, this landslide did not cause any casualties. Based on field investigations, 3D laser scanning, unmanned aerial vehicle observations, and borehole surveys, this study aims to reveal the cause and initiation mechanisms of the landslide. The landslide developed on a typical dip slope with thick-bedded conglomerates intercalated with a few mudstone layers. The slide mass mainly consisted of conglomerates with a dip angle varying from 20 to 40°, failing along bedding plane contacts with the underlying muddy intercalation. Numerous tension cracks had developed in the upper part of the slope due to stress relaxation during the process of bank slope evolution associated with rapid river incision. These cracks damaged the integrity of the rock mass and promoted rapid rainfall infiltration. The water infiltrated into the slope and deteriorated the mechanical properties of the rock mass, which resulted in long-term deterioration of the slope stability conditions. In addition, the temperature of Tongren County was below zero for 5 months in a year, hence the freeze–thaw cycle of the water filled in the cracks would have reduced the shear strength further. There were 10 days of continuous rainfall before the landslide occurred, and the cumulative precipitation in 2018 and 2019 was also significantly more than that of previous years. Three primary factors are involved in the landslide event: (1) thick-bedded conglomerates intercalated with a few mudstone layers with contrasts in strength, (2) development of numerous relaxation cracks, and (3) long-term rainfall accumulation. [ABSTRACT FROM AUTHOR]

This paper provides newly found and profound insight into the initiation mechanism of the first Baige landslide occurred on the upper reaches of the Jinsha River in October 10, 2018. The detailed geological characteristics are interpreted by comprehensive field surveys in the past year, which indicate that the Baige landslide developed in the Jinsha River tectonic suture zone, and the active tectogenesis significantly contributes to broken stratigraphic structures and complex spatial distribution of lithologies. The initiation is considered to be long-term creep under the exogenic and endogenic integration, while the active tectogenesis and the influence of serpentinite and foliation, respectively, are the primary exogenic and endogenic factors. In addition, this creep process can be analyzed within three stages: evolution and formation of failure-prone geological structures, progressive deformation and fracturing, and shear failure of the locking section. The distribution and easily degraded behavior of the serpentinite are the fundamental, enabling the formation of failure-prone structures and also responsible for the subsequent deformation evolution. The foliation controls the toppling deformation-failure mode and direction. Furthermore, this catastrophic landslide further reminds us to pay attention to the landslides along the upper reaches of the Jinsha River, and our preliminary results indicate that the distribution characteristics of landslides significantly depend on the Jinsha River tectonic suture zone and the serpentinite mélange belt and thus are always associated with tectonically induced damage. [ABSTRACT FROM AUTHOR]

On 2 September 2018, an intense rainstorm swept Mengdong Town in Yunnan Province of China, inducing a serious landslide and debris flow disaster with 10 deaths and 11 missing. Image interpretation, field survey, and slope stability analysis were used to examine the characteristics and initiation mechanism of this hazard. A total of 1774 landslide scars were identified, the area of which occupied 8.26% of the study region. Due to the spatial inhomogeneity of precipitation, these scars mainly concentrated along valleys in the lower part of the study area. Besides, well-forested hillslopes were more prone to landslides, indicating the limited role of trees in stabilizing slopes in extreme rainfall events. The initiation of landslides is mainly attributed to the week cohesion of the saturated clayey sand beneath the root zone, where the tensile resistance of roots was absent, and the increase of positive pore water pressure. Additionally, 288 landslide scars were situated adjacent to roads, demonstrating that the road construction activity had intensified the landslide disaster. Owing to the relatively low mobility of landslides, a considerable portion of landslide debris deposited on the valley floor in smaller watersheds (< 6.03 hm2), while the remaining portion entered high-order channels. In these channels, where stream power was relatively large, the woody debris and soil carried by landslides were entrained by streamflow, and debris flows were formed. Moreover, the magnitude of debris flow was amplified by the vertical and lateral erosion of the stream channel. [ABSTRACT FROM AUTHOR]

Continuous use of diversion-based irrigation has been associated with an increase in the frequency of loess landslides on the South Jingyang Platform, Shaanxi, China. A loess landslide event with a maximum sliding distance of 278 m occurred near the village of Miaodian on May 26, 2015. This landslide event was characterized by four individual landslides. Field investigations, geological exploration, numerical simulation, isotropically consolidated undrained (ICU) triaxial tests, and ring shear tests were conducted to identify its initiation and movement mechanisms. The ICU tests revealed that saturated loess samples were highly liquefiable. High pore water pressure was quickly produced and deviation stress increased the highest value even at low values of axial strain. Geological investigations revealed that cracks penetrated into the saturated zone from the ground surface, and simulation results revealed that these cracks played a dominant role in the infiltration of surface water and led to a rise in the groundwater table. When the infiltration recharge exceeds the holding capacity of the paleosol, the latter behaves as aquifuge under relatively undrained conditions. This process results in the accumulation of water at the bottom of the loess layer, thereby contributing to soil liquefaction and landslide initiation. The ring shear tests revealed that the saturated sand layer of the landslide substrates was subjected to easily inducible high pore water pressure under undrained conditions which led to the thrusting of the sand layer onto the deposit surface and explains the high speed and long runout distance of this landslide. [ABSTRACT FROM AUTHOR]

Debris flows usually occur abruptly and rapidly in mountainous areas and it is difficult to observe their occurrence progress. Therefore, an experiment has become an important method to study debris flow initiation mechanism in recent years. In this paper, taking Xiongjia Gully in SW China as an example and on the basis of artificial rainfall experiment, the author has studied the initiation of gully-type debris flow. Experiment results indicate some relation between rainfall intensity and gully erosion, failure mode of soil mass, initiation mechanism and characteristics of debris flow. Based on several groups of experiments, conclusions were drawn as follows: (1) Under strong precipitation, the infiltration rate and soil water content in different depths are inversely proportional to the rainfall intensity. Intense rainfall favors the overland flow, gully runoff and erosion, but is not conducive to water infiltration. (2) Slope failure modes and initiation mechanism of debris flow vary with different rainfall and runoff conditions. Under the condition with the rainfall intensity of 55 mm/h, the slope failure mode presents a soil liquefaction and landslide. Accordingly, the debris flow initiation mechanism belongs to landslide transformation. However, under the condition of intense rainfall and runoff, gully beds are easy to be eroded and slopes are prone to collapse. Then, debris flows occurred with initiation mechanism of entrainment. (3) In terms of debris flow characteristics, the debris flow occurrence process consists of several intermittent flows. In addition, the debris flow magnitude and flow viscosity are not consistent with rainfall intensity. On the contrary, under condition of intense rainfall of 65 and 75 mm/h, debris flows tend to be watery. However, under the rainfall condition of 55 mm/h, the flow viscosity is higher. The experiment results are consistent well with the natural debris flow occurrence from Xioangjia Gully. [ABSTRACT FROM AUTHOR]

The Daguangbao landslide, with a volume of 1.16 × 109 m3 and an area of 7.2 km2, is the largest landslide triggered by the 2008 Wenchuan earthquake in China. To examine the initiation mechanism of this landslide, a detailed field investigation and a series of saturated triaxial tests were carried out. The field investigation revealed that the Daguangbao landslide initiated along a mylonite zone, the soil of which was mainly sand. Abundant groundwater was observed in the tunnel and the mylonite zone soils had high water contents. In addition, the experimental results showed that the soils from the mylonite zone have a high liquefaction potential. The pore water pressures in the soils under cyclic triaxial tests increased rapidly, which could have caused the soils failure. Therefore, the mechanism by which pore water pressure within the mylonite zone accumulates under seismic loading, which promotes a continuous decrease in the shear strength of soils, ultimately led to the rapid initiation of the Daguangbao landslide. • The DGB landslide initiated along a mylonite zone, the soil of which was mainly sand. The soils had high water contents. • The soils have a high liquefaction potential and that liquefaction can be triggered under VCP tests. • The increase in pore water pressure during the cyclic triaxial tests is affected by grain movement and crushing. • The pore water pressure increases under seismic loading. Ultimately, rapid shear failure of the DGB landslide occurred. [ABSTRACT FROM AUTHOR]