Chaojun Ouyang

Entrainment of bed material by Earth-surface mass flows: Review and reformulation of depth-integrated theory

Earth-surface mass flows such as debris flows, rock avalanches, and dam-break floods can grow greatly in size and destructive potential by entraining bed material they encounter. Increasing use of depth-integrated mass and momentum conservation equations to model these erosive flows motivates a review of the underlying theory. Our review indicates that many existing models apply depth-integrated conservation principles incorrectly, leading to spurious inferences about the role of mass and momentum exchanges at flow-bed boundaries. Model discrepancies can be rectified by analyzing conservation of mass and momentum in a two-layer system consisting of amoving upper layer and static lower layer. Our analysis shows that erosion or deposition rates at the interface between layers must, in general, satisfy three jump conditions. These conditions impose constraints on valid erosion formulas, and they help determine the correct forms of depth-integrated conservation equations. Two of the three jump conditions are closely analogous to Rankine-Hugoniot conditions that describe the behavior of shocks in compressible gasses, and the third jump condition describes shear traction discontinuities that necessarily exist across eroding boundaries. Grain-fluid mixtures commonly behave as compressible materials as they undergo entrainment, because changes in bulk density occur as the mixtures mobilize and merge with an overriding flow. If no bulk density change occurs, then only the shear traction jump condition applies. Even for this special case, however, accurate formulation of depth-integrated momentum equations requires a clear distinction between boundary shear tractions that exist in the presence or absence of bed erosion.

A MacCormack-TVD finite difference method to simulate the mass flow in mountainous terrain with variable computational domain

A two-dimensional mountainous mass flow dynamic procedure solver (Massflow-2D) using the MacCormack-TVD finite difference scheme is proposed. The solver is implemented in Matlab on structured meshes with variable computational domain. To verify the model, a variety of numerical test scenarios, namely, the classical one-dimensional and two-dimensional dam break, the landslide in Hong Kong in 1993 and the Nora debris flow in the Italian Alps in 2000, are executed, and the model outputs are compared with published results. It is established that the model predictions agree well with both the analytical solution as well as the field observations.

MacCormack-TVD Finite Difference Solution for Dam Break Hydraulics over Erodible Sediment Beds

AbstractCoupled shallow water equations integrated with sediment transport and morphological evolution are presented in this paper. The momentum exchange terms that originated from the interaction between flow and sediment, which were ignored by several researchers, are taken into account. The time and space second-order, MacCormack total variation diminishing (TVD) finite difference method is used to solve these equations. A series of numerical simulations compared with laboratory dam-break experiments were carried out. The simulated results are in good agreement with experimental measured results, which demonstrates that the current computational framework is able to determine the dam-break hydraulics over erodible sediment.

Seismic stability analysis of soil nail reinforced slope using kinematic approach of limit analysis

Prediction of the critical seismic yield acceleration coefficient and the seismic permanent displacement of soil nail reinforced slope under seismic loading has been playing an important role in helping design in the earthquake-prone areas. In this paper, the seismic stability of soil nail reinforced slope is analyzed using the kinematic theorem of limit analysis. The log-spiral failure mechanism is considered and the corresponding analytical expressions are derived to calculate the critical seismic yield acceleration coefficient and the permanent displacement of slope subjected to earthquake loading. A series of calculations are carried out to illustrate the influence of inertial force on the stability of a nail-reinforced slope. Parametric studies indicate that the strength and geometry of slope as well as characteristic parameters of soil nail have a significant effect on the critical seismic yield acceleration coefficient and the permanent displacement of soil nail reinforced slope.

Numerical modeling and dynamic analysis of the 2017 Xinmo landslide in Maoxian County, China

A catastrophic landslide occurred at Xinmo village in Maoxian County, Sichuan Province, China, on June 24, 2017. A 2.87×106 m3 rock mass collapsed and entrained the surface soil layer along the landslide path. Eighty-three people were killed or went missing and more than 103 houses were destroyed. In this paper, the geological conditions of the landslide are analyzed via field investigation and high-resolution imagery. The dynamic process and runout characteristics of the landslide are numerically analyzed using a depth-integrated continuum method and MacCormack-TVD finite difference algorithm. Computational results show that the evaluated area of the danger zone matchs well with the results of field investigation. It is worth noting that soil sprayed by the high-speed blast needs to be taken into account for such kind of large high-locality landslide. The maximum velocity is about 55 m/s, which is consistent with most cases. In addition, the potential danger zone of an unstable block is evaluated. The potential risk area evaluated by the efficient depth-integrated continuum method could play a significant role in disaster prevention and secondary hazard avoidance during rescue operations.

Cyclic Hardening Behavior of Polycrystals with Penetrable Grain Boundaries: Two-Dimensional Discrete Dislocation Dynamics Simulation

A two-dimensional discrete dislocation dynamics (DDD) technology by Giessen and Needleman (1995), which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocation-penetrable grain boundaries (GBs) on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.

Dynamic process simulation of construction solid waste (CSW) landfill landslide based on SPH considering dilatancy effects

Construction solid waste (CSW) landfill landslides, such as the Guangming New District landslide, which occurred in Shenzhen (hereafter the Shenzhen landslide), occur when the material is loose and saturated. They usually exhibit characteristics such as abrupt failure and whole collapse. During the propagation of landslides, dilatation behavior plays an important role in causing liquefaction, resulting in high velocity and exceptionally long run-out dynamics. We propose a dynamic model for describing fluidized CSW landslides by integrating the dilatancy model into smoothed particle hydrodynamics (SPH). The dilatancy model implies that the occurrence of dilation or the contraction of the granular-fluid mixture depends on the initial solid volume fraction. The dynamic model is used to simulate the Shenzhen landslide, and special attention is paid to the effects of different initial solid volumes on the mobility of the CSW landslide. The results show that when the solid volume fraction is higher than the critical value, contraction occurs, the excess pore water pressure increases, and the basal friction resistance is reduced. CSW landslide mobility is based on the initial solid volume fraction (or initial void ratio) of the granular-fluid mixture; a slight change in the initial volume fraction significantly affects the mobility of the CSW landfill landslide.

Effects of segregation in binary granular mixture avalanches down inclined chutes impinging on defending structures

This study investigates the segregation processes and impact response of binary granular mixtures with identical densities but different sizes particles subjected to gravity. Deposition was compared using discrete element method (DEM) numerical experiment and laboratory experiment to determine the material parameters in the particle flow code in three dimensions (PFC3D). With proper material parameters, many numerical experiments were performed on an idealized binary granular mixture avalanche to reveal its kinetic properties, with a particular focus on the results of the final run-out distance, fluid velocity, and impact force exerted on defending structures. The simulation results show that the energy dissipation in granular avalanches is higher with uniform particle sizes than with mixed particle sizes, indicating lesser energy dissipation in segregation processes. Coarse particles also play an important role in determining the kinetic properties of binary granular mixture avalanches; specifically, they obviously affect the maximum impact force when the storage area length is small. On the other hand, fine particles play an important role when the storage area length is large. These results suggest that the effects of coarse particles in granular avalanches containing more than one particle size may be at least as important.

Coupled Model of Two‐phase Debris Flow, Sediment Transport and Morphological Evolution

The volume fraction of the solid and liquid phase of debris flows, which evolves simultaneously across terrains, largely determines the dynamic property of debris flows. The entrainment process significantly influences the amplitude of the volume fraction. In this paper, we present a depth-averaged two-phase debris-flow model describing the simultaneous evolution of the phase velocity and depth, the solid and fluid volume fractions and the bed morphological evolution. The model employs the Mohr–Coulomb plasticity for the solid stress, and the fluid stress is modeled as a Newtonian viscous stress. The interfacial momentum transfer includes viscous drag and buoyancy. A new extended entrainment rate formula that satisfies the boundary momentum jump condition (Iverson and Ouyang, 2015) is presented. In this formula, the basal traction stress is a function of the solid volume fraction and can take advantage of both the Coulomb and velocity-dependent friction models. A finite volume method using Roes Riemann approximation is suggested to solve the equations. Three computational cases are conducted and compared with experiments or previous results. The results show that the current computational model and framework are robust and suitable for capturing the characteristics of debris flows.

Failure mechanisms and characteristics of the 2016 catastrophic rockslide at Su village, Lishui, China

This paper describes a recent large rockslide, which occurred at Su village in Lishui, Zhejiang Province, China, on September 28, 2016. In the past decade, a vegetation-free deformed surface was clearly visible and frequent rockfalls were noticed. Due to strong sustained rainfall, approximately 0.4 million m3 of granite blocks rapidly descended from the upper part of the hillside. The mass rushed into the V-shaped valley resulting in the formation of a barrier dam and dammed lake. The catastrophic rockslide caused 27 deaths and more than 20 houses were destroyed. The evolutionary process before the rockslide is clearly captured by high-resolution remote sensing images and photos. Video of the rockslide and field investigations show entrainment of superficial material in the middle and lower parts of the slope.