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Vegetation is widely recognized as a key factor controlling the occurrence of shallow landslides in vegetation-covered areas. In such areas, the root system plays a critical role both in enhancing root-soil mechanical properties and in changing soil hydrological properties. However, owing to its complexity and nonuniformity, the root system is always neglected or simplified in existing infiltration process models, making it difficult for such models to reflect the influence of root systems on shallow landslides. Considering the shallow landslide cluster that happened in Mengdong (Yunnan Province, Southwest China) in 2018, this study quantitatively investigated the root distribution and obtained the prevailing physical and hydraulic properties through density tests, shear strength tests, and saturated seepage tests. Field investigation indicated that the root system distribution obeys an exponentially decayed polynomial model. In the entire profile, the maximum root area density was 0.145 mm2 cm−2 at depth of 20–40 cm, which comprised 483 roots, and 80% of the roots were distributed above the slip surface. Laboratory test results indicated that root-soil above the slip surface had lower density (minimum density: 1.04 g cm−3) and higher porosity (maximum porosity: 61.23%) than soil below, which induced permeability 10–17 times higher above the slip surface. A potential relationship was found between slip surface location and root system distribution. Differences in root distribution and resultant changes in the hydrological properties of soil might reduce slope stability during extreme rainfall, which could induce shallow landslides. This research could be used as reference for slope stability and hydraulic process analysis in forested areas. [ABSTRACT FROM AUTHOR]

The northeastern Tibetan Plateau exhibits steep topography and strong internal or external dynamic geological effect and is frequently subjected to strong earthquakes and heavy rainfall. The geological evolution has resulted in a wide distribution of ancient landslides, which has become a hotspot for studying ancient landslide formation and reactivation. In recent decades, several ancient landslides on both banks of the Longwu River, Qinghai Province, China were reactivated, causing serious economic losses and casualties. This study conducted remote sensing interpretation and ground surveys on these ancient landslides. Totally 59 ancient landslides were identified, and the formation mechanism, evolution process, and resurrection mechanism of the Longwu Xishan No.2 ancient landslide were analyzed by means of a detailed field geological survey, drilling, and series of experimental tests such as the particle size distribution test, the X-ray diffraction test and the mechanical properties test. The results show that the formation of these ancient landslides is closely associated with the uplift of the Tibetan Plateau and the erosion of the Longwu River. Firstly, the intermittent uplift of the Tibetan Plateau lead to the diversion and downcutting of the Longwu River basin, which forms the alternate slope topography with steep and slow slopes, thereby providing favourable topography and slope structure conditions for the formation of landslides. Secondly, 34.5% clay-mineral content in the Neoproterozoic mudstone with 32.7% particle size less than 0.005 mm, and the corrosion and softening effects of the Neogene mudstone with high clay mineral content under the erosion of water provides favourable material conditions for the formation of landslides. Thirdly, rainfall and human activities are the primary triggering factors for the revival of this ancient landslide group. It is revealed that the evolution process of the ancient landslides on both banks of the Longwu River can be divided into five stages namely tectonic rapid uplift slope formation, river erosion creep-sliding deformation, slope instability critical status, landslide failure-movement-accumulation, and slope reactivation under rainfall erosion and engineering excavation. [ABSTRACT FROM AUTHOR]

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]

On August 14, 2020, it occurred a landslide-induced debris flow in Wangcang County, Sichuan Province, Southwest China, three people killed, and two houses destroyed. Behaved as a typical geohazard chain, it originated from a rockslide in marlstone, controlled initially by structural planes in rock mass, and evolved into a debris flow through successive geophysical process. This debris flow reached a run-out distance of 2050 m in the horizontal and 628 m in vertical direction. Remote sensing analysis, field survey, and laboratory tests were conducted to investigate its failure mechanism and dynamic characteristics. The results found that the source zone in the gully was 16.32 × 103 m2, with a volume of 317.52 × 103 m3. Interbedded marlstone with dense structural planes was one of controlling factors for the landslide failure. The continuous rainfall up to 315 mm during the previous 24 days and 32 mm at the date before the incident directly motivated rock slope failure. The contributions of topography, rock fragmentation, entrainment, and flow discharge for this process were discussed. It is found that changes in slope from 11° to 40° provided tremendous potential energy for the rock mass sliding with high speed and then disintegrating in traveling. Entrainment in the channel increased the fine content of the moving rocky mixture. In addition, the flow discharge changed the rheological properties of the mixture by adding seepage. These factors significantly affected the transformation process. Notably, the turning relief or confluence site may be accelerating this process. This research may provide references for further research on geohazard chains. [ABSTRACT FROM AUTHOR]

In this study, the experimental investigation on whether the landslide surge waves can influence the dam failure process caused by flood overtopping was presented. The experiments were conducted in a large width flume with length of 50 m, width of 4 m and height of 2 m. The time between the start of the earth dam break and the surge wave reaching the dam site was set to 80 s. During this period, the water depth of the reservoir constantly decreased. The experimental results showed that the surge wave height decreased while the propagation velocity increased slightly as the surge moved to the dam site. Several surge waves could overtop the dam crest and the shear height of the dam crest caused by each surge wave was a function of the surge wave height, which could be described non-dimensionally by Δz* = 0.946 Hs*2.113. The surge wave influenced the water level falling velocity and breach width expanding speed from the time of the overlay of the breaking dam. The surge wave could lead to a large instantaneous peak discharge that exceeded the dam failure peak discharge of the experiments with no surge wave. [ABSTRACT FROM AUTHOR]

Due to the complex geological processes of Qinghai-Tibet Plateau, numerous deposits, especially the large-scale ancient landslide deposits, are characteristic features of the valleys incised in southwestern China. Intense water level fluctuations since 2011 in Maoergai Reservoir, China, registered the reactivation of Xierguazi ancient landslide, and presented a significant risk to neighboring facilities. Based on detailed field survey and drilling exploration, the landslide was divided into Zone A and Zone B, and other characterizations of landslide were studied as well. To precisely measure the extent of landslide displacement during filling and drawdown stage, surface displacement monitoring system was deployed on the landslide. The monitoring analyses data reveal that reservoir fluctuation is the dominant factor influencing landslide displacement, especially during drawdown stage. Moreover, a future sliding is anticipated in Zone A, while a creep had already existed in Zone B. A reservoir regulation was then established using the lead-lag correlation between reservoir fluctuation and landslide displacement and landslide stability analysis. In the end, the follow-up deformation monitoring demonstrates that the reservoir regulation controlled the landslide effectively. Landslide control by reservoir regulation in Maoergai can serve as a case study for other settlements involved in similar construction activities. [ABSTRACT FROM AUTHOR]

On July 23, 2019, a large-scale landslide occurred in Jichang town, Shuicheng County, Liupanshui City, Guizhou Province in China. The landslide, which moved along two gullies, resulted in strong punching-shear, induced scarping on vegetation and large destruction of houses, and finally formed a deposit with a volume of 2 × 106 m3. This research aims to understand the effect of topography on landslide kinematics. To achieve this aim, a detailed field investigation was first carried out with an unmanned aerial vehicle (UAV) aerial photography survey, resident interviews, and field sampling. The rainfall analysis indicates the effective rainfall within 7 days before landslides was 70.14 mm which exceeded the rainfall threshold of 54.3 mm in this region, which finally triggered the landslide. Traditional soil mechanic tests were then performed to identify the soil properties of the source material. Combined with numerical simulation using the nonlinear shallow water equation, the whole process of landslides was divided into four stages: instability stage, acceleration stage, transformation stage, and impact and accumulation stage. The simulations results show the landslide block slid with a low velocity of 8 m/s for about 100 m. Then, Froude number of landslide increases from 2 to 3 when passing the high and steep terrain, indicating that landslide change to inertial dominated with potential same Froude behavior of classic debris flow. The rupture mass slid with the peak velocity of 23 m/s and diverged in two gullies and ran out for about 600 m. The maximum velocity is 23 m/s in east gully while only 15 m/s in west gully. Compared with deep and incised valleys in the west, shallow and straight valley in the east decreases the deposit depth and further increases the velocity of landslide material with increased runout distance. This research may provide a fast flow path of back analyzing geo-hazards on complex terrain and serve as a basis for future research on long runout landslides. [ABSTRACT FROM AUTHOR]

Bed entrainment changes the rheology of the sliding mass and the topography of the sliding surface, finally influencing the propagation of flow-like landslides. In previous studies, both empirical methods and physically based methods have been used to simulate bed entrainment. However, the influences of bed entrainment on the rheology and topography of flow-like landslides were not deeply explored. In this paper, the physically based model proposed by Fraccarollo and Capart (J Fluid Mech 461:183–228, 2002) is adopted to calculate the bed entrainment rate, and a new method is proposed to consider the rheology change associated with bed entrainment in flow-like landslides. The new rheology change method and the Fraccarollo and Capart model are incorporated into a quasi-3D finite difference code to analyze an ideal case and two typical flow-like landslides. The two real landslides are the Dabaozi landslide and the Dagou landslide in the Chinese Loess Plateau. They represent two different bed entrainment scenarios: the erodible mass is relatively thick in the Dabaozi landslide, while that of the Dagou landslide is relatively thin. The results show that both the topography and rheology changes have a significant influence on the propagation of flow-like landslides: (1) the rheology change mainly influences the run-out distance of a landslide, while the topography change mainly impacts the lateral spreading; (2) entraining soft materials can significantly increase the run-out distance of a flow-like landslide; (3) the topography change can obviously constrain the lateral spreading of those landslides when the erodible mass is relatively thick. In addition, it shows that the rheology change and topography change influence each other in the propagation of a flow-like landslide, and the proposed rheology change method in this paper can properly reflect this interactive process. [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]