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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]

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, we carried out a series of physical experiments using a submersed flume model to investigate how sand/clay content influences the depositional mechanism of submarine debris flows. A three-dimensional biphasic numerical model, with a Herschel-Bulkley rheology, was used to back-analyze the physical experiments. The calibrated numerical model was then used in a back-calculation to investigate the effects of viscosity on the deposition process. The results show that as submarine debris flows mix with water during the deposition process, shear stress at the slurry-water interface generates a vortex that leads to a swirl-wedge front head. High-viscosity slurry flows with a swirl-wedge front head travel at a higher aspect ratio and with a greater radius of rotation. Hydroplaning was observed when the front head was lifted by water during flow. The lifting height increased with flow depth fluctuation. Higher viscosity slurry was found to lift more rapidly due to its larger vortex and the decrease in density at the front head over time, both of which promote fluctuation. Although a high-density slurry has a greater lifting height, the ratio of lifting height to front head height is lower, indicating a smaller lifting force influence. Lower density flows have higher kinetic energy as the transfer of potential energy into kinetic energy is more efficient. Kinetic energy dissipation comprises three stages: (1) gravity-dominated coherent flow and hydroplaning lead to a rapid increase in kinetic energy; (2) sharp reduction in kinetic energy as slurry mixes with water, coherence and hydroplaning are reduced, and the influence of the shear stress increases; (3) slurry mixed very well with water, turbidity current dominates the kinetic energy dissipation. High-density slurry dissipates quicker in the last two stages. In stage 3, which dominates the temporal evolution of the debris flow, the Froude number is lower than 1, the flow thins and elongates, and the deposition process of submarine debris flows is dominated by gravity, and the difference of morphology of the different cases become clear. [ABSTRACT FROM AUTHOR]

A systematic study of the physical and mechanical processes of landslide development and evolution is important for forecasting, early warning, and prevention of landslide hazards. In the absence of on-site monitoring data, seismic networks can be employed to continuously record ground seismicity generated during landslides. However, landslide seismic signals are relatively weak and inevitably affected by noise interference. Furthermore, systematic characterization and reconstruction of the landslide evolution process remain poorly reported. An evaluation method to recognize landslide events based on seismic signal characteristics is therefore important. This study analyzes the 2019 "7.23" Shuicheng landslide based on data from nearby seismic stations. A landslide seismic signal recognition method is developed based on short-time Fourier transform (STFT) and band-pass filter (BP-filter) analysis. Data from 14 stations near the landslide were reviewed and the landslide data from one station was selected for analysis. The landslide seismic signal was noise-attenuated by using the empirical mode decomposition (EMD) and BP-filter methods. Fast Fourier transform (FFT), STFT, and power spectral density analyses were applied to the landslide seismic signal with higher signal-to-noise ratio (SNR) to obtain the time–frequency signal characteristics of the landslide process. Finally, combined with landslide field survey data, the dynamic process of the landslide was reconstructed based on the seismic signal, and the landslide was divided into four stages: the fracture-transition stage, the accelerated initiation stage, the bifurcation-scraping stage, and the deposition stage. The dynamic characteristics of each stage of the landslide are presented. The results indicate that the initial fracture point of the landslide is located between the bottom of the sliding source area and the top of the acceleration zone, not as traditionally thought, at the top of the sliding source area; this would be difficult to determine through field survey and analysis only. These results provide theoretical guidance for the study of seismic signal extraction, identification of landslide dynamic parameters, and characterization and reconstruction of landslide processes. [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]

In this paper, a modified finite difference model is proposed to simulate the propagation of flowslides. Modifications of the new model are conducted by calculating the lateral pressure coefficient k in the sliding mass and the entrainment and centrifugal effect during the transport process. The strength parameters are modified based on the size of the entrainment to consider the change in the landslide strength due to material mixing. Two dam break problems are simulated to test the accuracy and stability of the numerical scheme, and the results show good agreement with the analytical solutions and the measured data. Then, the model is used to analyze a typical flowslide: the Dagou landslide in Gansu Province, China. The model can accurately predict the details of the motion of the landslide, especially behaviors such as turning along the meandering gully and thrusting on the gully slopes due to centrifugal force. [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]