Flexible barriers undergo large deformation to extend the impact duration, and thereby reduce the impact load of geophysical flows. The performance of flexible barriers remains a crucial challenge ...because there currently lacks a comprehensive criterion for estimating impact load. In this study, a series of centrifuge tests were carried out to investigate different geophysical flow types impacting an instrumented flexible barrier. The geophysical flows modelled include covered in this study include flood, hyperconcentrated flow, debris flow, and dry debris avalanche. Results reveal that the relationship between the Froude number,
Fr
, and the pressure coefficient
α
strongly depends on the formation of static deposits called dead zones which induce static loads and whether a run-up or pile-up impact mechanism develops. Test results demonstrate that flexible barriers can attenuate peak impact loads of flood, hyperconcentrated flow, and debris flow by up to 50% compared to rigid barriers. Furthermore, flexible barriers attenuate the impact load of dry debris avalanche by enabling the dry debris to reach an active failure state through large deformation. Examination of the state of static debris deposits behind the barriers indicates that hyperconcentrated and debris flows are strongly influenced by whether excessive pore water pressures regulate the depositional process of particles during the impact process. This results in significant particle rearrangement and similar state of static debris behind rigid barrier and the deformed full-retention flexible barrier, and thus the static loads on both barriers converge.
Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent collapses a series of landslide dams, large debris flows can form in a ...very short period. Moreover, the torrent can amplify the scale of the debris flow in the flow direction. The catastrophic debris flows that occurred in Zhouqu, China, on 8 August 2010 were caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies. In the wake of the incident, a field study was conducted to better understand the process of cascading landslide dam failures and the formation of debris flows. This paper looks at the geomorphic properties of the debris-flow gullies, estimates the peak flow discharges at different locations using three different methods, and analyzes the key modes (i.e., different landslide dam types and their combinations) of cascading landslide dam failures and their effect on the scale amplification of debris flows. The results show that five key modes in Luojiayu gully and two modes in Sanyanyu gully accounted for the scale amplification of downstream debris flows in the Zhouqu event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream as a complex cascade of geomorphic events (failure of landslide dams and erosion of the sloping bed) can scale to become catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in a high cost.
This study presents a field-scale simulation of the Hongshiyan landslide in China. It uses an advanced numerical approach (material point method (MPM)) and a constitutive model (the Drucker- Prager ...model + mu(I) rheological relation) for the three-dimensional (3D) simulation. The performance of the developed MPM model is validated with laboratory-scale experimental data on granular collapse before being applied to field-scale analyses. ArcGIS data are used to create a 3D MPM model of the soil body with complicated geometry. Although the developed model can describe the multiple phases of granular flow, it focuses on the runout behavior of the landslide in this work. The landslide is assumed to have occurred suddenly due to an earthquake, and global sudden failure rather than progressive failure is modeled. The MPM simulation results match reasonably well with the measured post-earthquake topography (e.g., deposit height of about 120 m and stretch length of about 900 m in the river) and landslide duration of about 1 min. The velocity of the sliding mass increases rapidly during flow, especially in the first 20 s. The velocity profiles along the depth direction at different locations of the sliding body exhibit an exponential distribution similar to that of a Bagnold-type profile, indicating that the sliding body is fully mobilized. The rate-dependent dissipation parameter j3 used in the model significantly influences the runout behavior (e.g., flow speed, velocity distribution, and deposit shape).
Summary
Granular debris flows are composed of coarse solid particles, which may be from disaggregated landslides or well‐weathered rocks on a hill surface. The estimation of agitation and the flow ...process of granular debris flows are of great importance in the prevention of disasters. In this work, we conduct physical experiments of sandpile collapse, impacting 3 packed wooden blocks. The flow profile, run‐out distance, and rotation of blocks are measured. To simulate the process, we adopt a material point method (MPM) to model granular flows and a deformable discrete element method (DEM) to model blocks. Each block is treated as comprising 9 material points to couple the MPM and DEM, and the acceleration of grid nodes arising from the contacts between granular material and blocks is projected to the discrete element nodes working as body forces. The contacts between blocks are detected using the shrunken point method. The simulation results agree well with the experimental results. Thus, the coupling method of MPM and DEM developed in this work would be helpful in the damage analysis of buildings under impact from the debris flows.
Understanding the characteristics and mechanics of granular flows along sloping channels is fundamental and vital for the study of different in situ geophysical flows. Using the discrete element ...method (DEM), three-dimensional (3-D) dry granular flows are numerically modeled to study the contact behavior of solid particles along sloping channels, determine the 3-D velocity profiles along the flow height, and computer the corresponding shear rate. The channel confinement effect on granular flows is also investigated. By capturing the flow height and utilizing the definition of the Savage number, the variation in flow regimes inside a granular body along a sloping channel is researched. Finally, the fluctuation of solid particles in the flow is investigated with the use of granular temperature, and is proven to be influenced by the Savage number. Analysis shows that the combination of granular temperature and the Savage number is a good way to identify the flow regimes of granular flows.
Rheological characteristics of unsteady dry granular flows along a sloping channel are investigated by discrete element method in this paper. 3-D velocity profiles, the pertinent flow regimes, and then the fluctuation of solid particles in the flowing granular bodies are systematically studied, respectively. Display omitted
► Unsteady channelized granular flows are simulated by discrete element method. ► Rheological characteristics of dry granular flows are investigated here. ► 3-D velocity profiles and the solids fluctuation in granular flows are researched. ► Flow regimes of channelized granular flows are identified.
Run-up of granular debris flows against slit dams on slopes is a complex process that involves deceleration, deposition, and discharge. It is imperative to understand the run-up mechanism and to ...predict the maximum run-up height for the engineering design and hazard mitigation. However, the interaction between granular flows and slit dams, which significantly affects the run-up height, is still not well understood. In this study, an analytical model based on the momentum approach was derived to predict the run-up heights of granular debris flows. A numerical investigation of granular debris flow impacting slit dams using the discrete element method (DEM) was then conducted. The influence of the Froude number (
N
Fr
) and the relative post spacing (
R
) on run-up height were studied. This study illustrates that the analytical model based on the momentum approach can predict the run-up heights well within a certain range of Froude numbers. There is a critical value of relative post spacing (
R
C
): within the critical value, the maximum run-up height is insensitive to the relative post spacing; once
R
exceeds the critical value, the maximum run-up height decreases rapidly as the relative post spacing increases.
Slit dam is an open-check barrier structure widely used in mountainous regions to resist the destructive impacts of granular flows. To examine the dynamics of granular flow impact on slit dams, a ...numerical study by discrete element method (DEM) is presented in this article. The study considers dry granular materials flowing down a flume channel and interacts with slit dams installed at the lower section of the flume. The particle shape is explicitly considered by particle clumps of various aspect ratios. The slit dams are modeled as rigid and smooth rectangular prisms uniformly spaced at in the flume. Four key stages of granular flow impact on the slit dams have been identified, namely, the frontal impact, run up, pile up, and static deposition stages. In the impact process, the kinetic energy of the granular flow is dissipated primarily by interparticle friction and damping. The trapping efficiency of the slit dams decreases exponentially with the relative post spacing, while it increases with the particle clump aspect ratio. The numerical results can provide new insights into the optimization of relative post spacing for slit dam design.
The stability of a dense granular assembly can be greatly reduced by a pore pressure of the interstitial fluid, and the body may fail and transit from a solid-like state to a fluid-like state. This ...process involves two major problems: large deformation and hydro-mechanical coupling. In this work, a three-dimensional fully coupled hydro-mechanical model using material point method (MPM) is developed. Darcy's law, considering the inertial effect, is adopted to govern the motion of interstitial water, and the conservation of momentum of the mixture is used to govern the motion of the solid, i.e., granular materials. The spatial discretization schemes for these equations are derived using the generalized integration material point method (GIMP), and the proposed coupled MPM formulation is implemented in a three-dimensional numerical code. The developed model is first quantitatively validated by comparing the simulation results of temporal evolution of spatial distribution of hydraulic pressure in a one-dimensional oedometer test with the analytical results. An experiment is designed to observe the failure of a saturated sand pile, in which the partial-saturated region is avoided by increasing the hydraulic head at the input boundary, and the kinetic energy of water is dissipated by a filtering cloth. The failure process is simulated with the MPM code. It is found that the location of the shear band in the simulation agrees with the location of the sliding surface in the experiment. The temporal evolutions of the spatial distributions of hydraulic pressure and the solid velocity distribution at a specific time are given to provide insight into the mechanism of the failure process. This work would be helpful in understanding the initiation mechanism of debris flows induced by rainfall, and sand production in gas hydrate-bearing sediments due to increasing fluid content associated with hydrate dissociation.
Display omitted
•A generalized interpolation material point method (GIMP) is developed for saturated dense granular materials.•The location of the shear band in the simulation coincides with the sliding surface in the experiment.•The distribution of the solid velocity mimics the location of the sliding surface.
Extreme rainfall events in mountainous environments usually induce significant sediment runoff or mass movements—debris flows, hyperconcentrated flows and stream flows—that pose substantial threats ...to human life and infrastructure. However, understanding of the sediment transport mechanisms that control these torrent processes remains incomplete due to the lack of comprehensive field data. This study uses a unique field data set to investigate the characteristics of the transport mechanisms of different channelized sediment‐laden flows. Results confirm that sediments in hyperconcentrated flows and stream flows are mainly supported by viscous shear and turbulent stresses, while grain collisional stresses dominate debris‐flow dynamics. Lahars, a unique sediment transport process in volcanic environments, exhibit a wide range of transport mechanisms similar to those in the three different flow types. Furthermore, the Einstein number (dimensionless sediment flux) exhibits a power‐law relationship with the dimensionless flow discharge. Machine learning is then used to draw boundaries in the Einstein number‐dimensionless discharge scheme to classify one flow from the other and thereby aid in developing appropriate hazard assessments for torrential processes in mountainous and volcanic environments based on measurable hydrologic and geomorphic parameters. The proposed scheme provides a universal criterion that improves existing classification methods that depend solely on the sediment concentration for quantifying the runoff‐to‐debris flow transition relevant to landscape evolution studies and hazard assessments.
Key Points
Dimensionless analyses reveal distinct transport mechanisms of debris, hyperconcentrated, and stream flows
Boundaries drawn by Support Vector Machines distinguish different flows in the Einstein number and dimensionless flow discharge phase diagram
Lahars exhibit a wide range of flow dynamics and sediment transport mechanisms from stream flow to debris flow
Slit dams are open-type structures used to mitigate debris-flow hazards by constricting the flow and attenuating the kinetic energy. However, slit dams are often filled up as they are designed to ...impede debris volume instead of reducing kinetic energy of debris flows. To better understand the regulation function of slit dams against debris flows, physical model tests were carried out using a 7-m-long flume. The water content and relative post spacing were varied to discern their influence on the regulation function. Results reveal that the velocity attenuation and trapping efficiency is strongly controlled by water content and relative post spacing. Water content fundamentally reflects the degree of liquefaction (effetive grain-contact stress) and capacity of energy dissipation of debris flows. When water content < 26%, relative post spacing has a noticeable effect on velocity attenuation, trapping efficiency, and run-out distance. In contrast, when water content ≥ 26%, the influence relative post spacing is negligible. Furthermore, a new relationship between velocity attenuation and trapping efficiency for the design of slit dams is proposed to avoid the slit dam being easily filled up by sediments contained in debris flows.
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