Guangqi Chen

Modeling of landslide generated impulsive waves considering complex topography in reservoir area

The impulsive wave is considered as one of the most notably secondary hazards induced by landslides in reservoir areas. The impulsive wave with considerable wave amplitude is able to cause serious damage to the dam body, shoreline properties and lives. To investigate and predict the wave characteristics, many experimental studies employed the generalized channels rather than the realistic topography. Deviation from the idealized geometries may result in non-negligible effects due to the wave refraction or reflection with complex topography. To consider the topography effect, a prototype scaled experiment was conducted. A series of tests with different collocation of parameters were performed. The experimental results were then summarized to propose empirical equations to predict the maximum wave amplitude, and wave decay in channel direction. The generalized empirical equations can obtain better results for wave features prediction by compared with those derived from the idealized models. Furthermore, a 3D numerical modeling corresponding to the physical experiment was conducted based on the SPH method. The wave characteristics in the sliding and channel directions were investigated in detail including the maximum wave amplitude, wave run-up, wave arrival time and wave crest amplitude decay. The comparison between the simulation and experiment indicates the promising accuracy of the SPH simulation in determining the general features even with complex river topography. Finally, the limitation and applicability of both the experimental and numerical methods in analyzing the practical engineering problems were discussed. Combination of the both methods can benefit the hazard prevention and reduction for landslide generated impulsive waves in reservoir area.

Numerical Simulation in Rockfall Analysis: A Close Comparison of 2-D and 3-D DDA

Accurate estimation of rockfall trajectory and motion behaviors is essential for rockfall risk assessment and the design and performance evaluation of preventive structures. Numerical simulation using discontinuous deformation analysis (DDA) is effective and helpful in rockfall analysis. Up to now, there have been many reports on application of two-dimensional (2-D) DDA programs. In this paper, the major advantages of rockfall analysis using 2-D and extensions to three-dimensional (3-D) analysis are presented. A practical 3-D DDA code is demonstrated to be capable of simulating free falling, rolling, sliding, and bouncing with high accuracy. Because rockfall trajectories and motion behaviors can be described as combinations of these four types, this demonstration indicates that the implemented code is capable of providing reliable rockfall analysis. Finally, specific tests are conducted to compare 2-D and 3-D DDA rockfall analysis in predicting trajectory and dynamic behavior. The results indicate that 3-D DDA simulations are more appropriate for rough tree-laden inclined slopes in providing detailed spatial distribution, whereas 2-D DDA simulations have better efficiency for slopes dominated by valleys and ravines. These results can help in selecting the appropriate DDA simulation for rockfall analysis.

A new approach for analyzing the velocity distribution of debris flows at typical cross-sections

The asymmetrical distribution of debris-flow velocity in a cross-section has long been observed and is currently regarded as one of the most essential issues in debris-flow research. Due to a lack of quantitative models for the velocity distributions of debris flows, most studies consider only the mean velocity. However, to optimize countermeasure structures, to estimate the erosion rate, or to evaluate the constitutive equations for shear behavior, it is beneficial to know the velocity profile in a cross-section. In this paper, a generalized model of typical channel geometries (e.g., rectangular, trapezoid, or V-shape) is proposed. A description of the velocity distribution that optimizes the Manning–Strickler velocity equation for transverse distribution and Egashira’s velocity equation for vertical distribution is presented; thus, the debris-flow velocity at any point in the cross-section can be calculated and the distribution profile therefore obtained. A well-documented debris-flow reference case and the Jiasikou debris flow in the high-seismic-intensity zone of the Wenchuan earthquake are selected as case studies to demonstrate the model. Analyses of both cases confirm the asymmetrical distribution of debris-flow velocity in cross-section, as originally expected. This shows that the velocity at the top surface in the middle of the channel is much larger than that at each sidewall and than the mean value calculated by former equations. The obtained velocity distribution profile is a better approximation of the observed field profiles.

3D numerical simulation of debris-flow motion using SPH method incorporating non-Newtonian fluid behavior

Flow-type landslide, such as debris-flow, often exhibits high velocity and long run-out distance. Simulation on it benefits the propagation analysis and provides solution for risk assessment and mitigation design. Previous studies commonly used shallow water assumption to simulate this phenomenon, ignoring the information in vertical direction, and the Bingham model to describe constitutive law of non-Newtonian fluid can cause numerical divergence unless necessary parameter is defined. To address the issue, the full Navier–Stokes equations are adopted to describe the dynamics of the flow-type landslides. Additionally, the general Cross model is employed as the constitutive model, which ensures the numerical convergence. Rheological parameters are introduced from the Bingham model and the Mohr–Coulomb yield criterion. Subsequently, the governing equations incorporating the modified rheological model are numerically built in the smoothed particle hydrodynamics (SPH) framework and implemented into the open-source DualSPHysics code. To illustrate its performance, the 2010 Yohutagawa debris-flow event in Japan is selected as a case study. Parameters regarding the debris magnitude, i.e., the front velocity and section discharge, were also well analyzed. Simulated mass volume and deposition depth at the alluvial fan are in good agreements with the in situ observation. On the basis of the results, the developed method performs well to reproduce the debris-flow process and also benefits the analysis of flow characteristics, affected area for risk assessment and mitigation design.

Elementary analysis on the bed-sediment entrainment by debris flow and its application using the TopFlowDF model

The Geographic Information System (GIS)-based TopFlowDF model is an effective simulation tool to analyse the debris-flow propagation on the fan. It has been validated in the previous research works and shows a satisfactory accuracy. We review the framework of the model in this paper and discuss that the simulation results are rather sensitive to the input conditions (e.g. the user-defined start point and debris-flow volume). In fact, previous studies have elucidated that sediment entrainment by debris flow conspicuously influences these input conditions. In this paper, we develop an elementary static approach with a three-layer model to estimate the entrainment and determine the amplification of total mass volume and peak discharge. Subsequently a new concept of “critical line” is proposed to detect the erodible reaches in the channel. Two cases in Japan and Norway are selected to illustrate the approach; analytical results show a good agreement with the in situ survey. The presented approach can provide a better solution to the input conditions as required by TopFlowDF model.

The slope modeling method with GIS support for rockfall analysis using 3D DDA

Rockfall is the most frequent major hazard in mountainous areas. For hazard assessment and further countermeasure design, realistic and accurate prediction of rockfall trajectory is an important requirement. Thus, a modeling method to represent both geometrical parameters of slope and falling rock mass is required. This study, suggests taking the advantages of discontinues deformation analysis (DDA) and geographical information system (GIS). In this study, after developing a three dimensional (3D) DDA program, firstly a special element named contact face element (CFE) was introduced into 3D DDA; secondly, effectively modeling tools with GIS support were developed. The implementation of CFE also improves the efficiency of both the contact searching and solution process. Then a simple impact model was devised to compare the 3D DDA implemented directly with a sliding model with theoretical analysis to verify the reliability of the modified 3D DDA program and investigate the parameter settings. Finally, simulations concerning rock shapes and multi-rocks were carried out to show the applicable functions and advantages of the newly developed rockfall analysis code. It has been shown that the newly developed 3D DDA program with GIS support is applicable and effective.

Distribution Pattern of Landslides Triggered by the 2014 Ludian Earthquake of China: Implications for Regional Threshold Topography and the Seismogenic Fault Identification

The 3 August 2014 Ludian earthquake with a moment magnitude scale (Mw) of 6.1 induced widespread landslides in the Ludian County and its vicinity. This paper presents a preliminary analysis of the distribution patterns and characteristics of these co-seismic landslides. In total, 1826 landslides with a total area of 19.12 km2 triggered by the 3 August 2014 Ludian earthquake were visually interpreted using high-resolution aerial photos and Landsat-8 images. The sizes of the landslides were, in general, much smaller than those triggered by the 2008 Wenchuan earthquake. The main types of landslides were rock falls and shallow, disrupted landslides from steep slopes. These landslides were unevenly distributed within the study area and concentrated within an elliptical area with a 25-km NW–SE striking long axis and a 15-km NW–SE striking short axis. Three indexes including landslides number (LN), landslide area ratio (LAR), and landslide density (LD) were employed to analyze the relation between the landslide distribution and several factors, including lithology, elevation, slope, aspect, distance to epicenter and distance to the active fault. The results show that slopes consisting of deeply weathered and fractured sandstones and mudstones were the more susceptible to co-seismic landslides. The elevation range of high landslide susceptibility was between 900–1300 m and 1800–2000 m. There was a generally positive correlation between co-seismic landslides and slope angle, until a maximum for the slope class 40°–50°. The co-seismic landslides occurred preferably on Southeast (SE), South (S) and Southwest (SW) oriented slopes. Results also show that the landslide concentration tends to decrease with distance from the surface projection of the epicenter rather than the seismogenic fault, and the highest landslide concentration is located within a 5–6 km distance of the seismogenic fault. Regarding the epicenter, the largest landslide clusters were found on the SE, northeast by east (NEE) and nearly West (W) of the epicenter. In addition, we also suggest that statistical results of slope gradients of landslides might imply a threshold topography of the study area within a tectonically active background. By analogy with other events, the statistical results of landslides aspects also imply the seismogenic fault of the Ludian earthquake might have been the Northwest (NW)-trending fault, which is consistent with other studies.

Numerical Analysis of the Largest Landslide Induced by the Wenchuan Earthquake, May 12, 2008 Using DDA

The Daguangbao landslide, with an estimated affected area of about 7.3–10 million m2 and a volume of 750–840 million m3, is the largest landslide induced by the 2008 Wenchuan earthquake. The sliding mass travelled about 4.5 km and blocked the Huangdongzi valley, forming a landslide dam nearly 600 m high. In order to investigate the landslide progression and reproduce the post-failure configuration, the kinematic behavior of sliding mass was simulated by a dynamic discrete numerical analysis method called DDA that has been widely applied for geotechnical engineering problems due to its superiority in modeling the discontinuous material. In this simulation, based on the shape of failure surface and the character of slope topography, the whole slope was divided into three parts: base block, upper sliding mass, and lower sliding mass. Then two sliding masses were divided into the smaller discrete deformable blocks based on pre-existing discontinuities. Corrected real horizontal and vertical ground motion records were applied as volume forces act to the base block. The simulation results of landslide progression, sliding distance, and shape of post-failure were in good agreement with those obtained from post-earthquake investigation, description from the survivors. Therefore, the methodology applied in this paper is able to capture essential characteristics of the landslide and give a post-failure configuration.

GIS-based Numerical Modelling of Debris Flow Motion across Three-dimensional Terrain

The objective of this study is to incorporate a numerical model with GIS to simulate the movement, erosion and deposition of debris flow across the three dimensional complex terrain. In light of the importance of erosion and deposition processes during debris flow movement, no entrainment assumption is unreasonable. The numerical model considering these processes is used for simulating debris flow. Raster grid networks of a digital elevation model in GIS provide a uniform grid system to describe complex topography. As the raster grid can be used as the finite difference mesh, the numerical model is solved numerically using the Leap-frog finite difference method. Finally, the simulation results can be displayed by GIS easily and used to debris flow evaluation. To illustrate this approach, the proposed methodology is applied to the Yohutagawa debris flow that occurred on 20th October 2010, in Amami-Oshima area, Japan. The simulation results that reproduced the movement, erosion and deposition are in good agreement with the field investigation. The effectiveness of the dam in this real-case is also verified by this approach. Comparison with the results were simulated by other models, shows that the present coupled model is more rational and effective.

Earthquake Induced a Chain Disasters

A strong earthquake not only cause directly damage on constructs but also can result in a series of natural disasters such as landslide, debris flow and flooding. These secondary disasters occurs as a chain disasters as shown in Fig. 1. A strong earthquake can induce a large amount of landslides. And then, a large scale landslide can create a landslide dam when its debris fill into and stop a river. The water impounded by a landslide dam may create a dam reservoir (lake). While the dam is being filled, the surrounding groundwater level rises and causes back-flooding (upstream flooding). And because of its rather loose nature and absence of controlled spillway, a landslide dam is easy to fail catastrophically