Y.C. Tai

• Doctor of Engineering
• Professor (Full) at National Cheng Kung University

Granular media are ubiquitous in geophysics and industrial applications and, hence, have attracted increasing interest in the scientific community. Yet, their dynamics is far from being completely understood and exhibits a rich variety of flow regimes, ranging from solid-like to gas-like behaviours. In the process of understanding these complex sys...

Debris flows and avalanches are dangerous natural phenomena, characterized by the gravity-driven motion of granular media immersed in a fluid. For an appropriate hazard assessment or disaster mitigation by scenario investigation, it is crucial to capture the underlying dynamics of the granular solid phase. For this purpose, a multilayer depth-avera...

Herewith we present a multi-phase model for debris flows, of which the flow body is supposed to be composed of water, fine sediment (clay/silt) and grains. The rheology of debris flows varies due to the dynamical variation of the composition concentrations. In the present study the component of silt/clay is an individual phase, and its concentratio...

A rapid change in the pore water pressure of unsaturated soil due to a wetting front is a crucial factor and may result in instabilities in layered slopes. This study presents preliminary research on such a change, which we define as the seepage hammer effect. Vertical infiltration with multiple soil layers by column test was implemented to investi...

We present a grain-fluid mixture for debris flows moving on a rugged (non-trivial) topography, where entrainment and deposition may take place. The model equations are derived with respect to a terrain-following coordinate system, which is constructed based on the topographic surface. The coordinates are fixed in space, and a “subtopography” is add...

We report on the role of density ratio for granular column collapse experimentally and numerically by demonstrating the fundamental changes using a two-particle approach. The influence of the density ratio for the different profile arrangements on the velocity distribution, interface, and energy transition is described using the Moving Particle Sem...

Granular avalanches are dangerous phenomena characterized by the rapid gravity-driven motion of granular solids. The complex dynamics of these flows can be effectively modeled by a multilayer approach, which, however, requires particular attention to the derivation of the model equations in order to allow stable solutions. In this work, we use a we...

For the investigation of landslide mass movement scenarios through numerical simulation, a well-defined released mass and a precise initial source area are required as prerequisites. In the present study, we present a genetic algorithm-based approach for preliminarily assessing the landslide scarp when the local field data are limited, using an ell...

The description of geophysical granular flows, like avalanches and debris flows, is a challenging open problem due to the high complexity of the granular dynamics, which is characterized by various momentum exchange mechanisms and is strongly coupled with the solid volume fraction field. In order to capture the rich variability of the granular dyna...

In this study, a coupled model of Peridynamics into the mesh-free method is extended to simulate the impacting collapse of a granular column with various suspended heights. Experiments on the impacting collapse were conducted to validate the numerical model. It is found that the simulated free surface profiles have a good agreement with the experim...

The importance of scenario investigation in landslide-related hazard mitigation planning has long been recognized, where numerical simulation with physics-based models plays a crucial role because of its quantitative information. However, a plausible failure surface is a prerequisite in conducting the numerical simulation, but it often has a high d...

Two-layer and multi-layer depth-averaged models have become popular for simulating exchange flows, seawater currents and geophysical flows. The partial differential equation systems associated with these models are similar to the single-layer shallow-water model. Yet, their eigenstructures are more complex owing to the pressure coupling between the...

We introduce a GPU-accelerated simulation tool, named Modeling on Shallow Flows with Efficient Simulation for Two-Phase Debris Flows (MoSES_2PDF), of which the input and output data can be linked to the GIS system for engineering application. MoSES_2PDF is developed based on the CUDA structure so that it can well run with different NVIDIA GPU cards...

The Weakly-Compressible Moving Particle Semi-implicit (WC-MPS) approach is applied to investigate the wave propagation and the flow fields induced by sliding blocks. Since there are already studies focusing on an inclination slope of 45°, two slope angles, 40° and 50°, are used to obtain the additional experimental results. Sliding blocks are relea...

Numerical scenario simulation may serve as an efficient and powerful tool for hazard assessment, but it often suffers from the lack of a definite failure surface before the occurrence of failure. In the present study, an idealized curved surface (ICS) is proposed for mimicking the sliding surface in the numerical simulation. Different from the conv...

More than 9,000 potential deep-seated landslide sites in the mountain ranges of Taiwan have been identified by a series of renewed governmental hazard mitigation initiatives after the 2009 Morakot typhoon. Among these sites, 186 sites have protection targets where thorough mitigation strategies are to be implemented. One of the important tasks in t...

Mesh-free methods have recently been coupled with constitutive rheological models to model dynamics in dry granular flows. However, this approach has not yet been comprehensively validated in different configurations with regard to the pressure, velocity, shear stress, free surface, and friction factor. Therefore, this study applied the weakly comp...

When water penetrates into soil, interstitial air can become trapped by the infiltrating water. Neglecting the effect of air ventilation could cause deviations in the predicted pore water pressure and the associated effective stress. This study aims at the effect of air ventilation on the coupled hydromechanical responses in homogeneous soil during...

The combined knowledge of the velocity and volume fraction fields is crucial for investigating the dynamics of granular flows, especially in the dense-collisional regime where both frictional and collisional dissipation mechanisms are significant. A laboratory investigation on steady dry granular flows in a straight channel is reported, where slip...

Granular flow usually occurs with strong interaction to the boundary, deposition, or other flow events. In particular, granular flows can frequently interact with each other, resulting in different behaviors. This paper employs a mesh-free numerical method coupled with μ(I) rheology to investigate interaction of granular surface waves. The waves ar...

The continuum deformation of gravity-driven dense granular materials on steep inclined planes exhibit similar flow phenomenon to natural and industrial occurrences. This kind of granular flow dynamics corresponds to unsteady flows which currently lack any theoretical formulations for its kinematic properties. The implementation of the µ(I) rheology...

The dynamic behavior of debris flows features the interplay of a non-hydrostatic pore-fluid pressure with the non-linear deformational behavior of the granular skeleton and the internal contact stress between grains. This complex physical background is considered by amending the classical depth-integrated modeling for granular-fluid flows by two ad...

The particle image velocimetry (PIV) is a well-established non-invasive optical technique for measuring the velocity field in fluids. Recently, the PIV approach has been extended to granular flows, where the medium under investigation is composed of a discrete number of grains that are typically non-transparent and of super-millimetric size. Granul...

Granular flows are involved in geophysical phenomena and industrial applications. The knowledge of the volume fraction is essential for better understanding their dynamics. Indeed, this quantity is highly coupled with the rheology of granular media. Here, we investigated the performance of the stochastic-optical method (SOM), proposed by [Sarno et...

Landslides or debris flows usually arise in mountainous areas and slide over a rough topography. As the flow body is typically a mixture of particles and interstitial fluid, this paper aims at developing a binary mixture model and its numerical implementation for shallow debris flows on rugged topography. The mathematical formulation of this satura...

An extension and numerical approximation of the shear shallow water equations model, recently proposed in [25], is considered in this work. The model equations are able to describe the oscillatory nature of turbulent hydraulic jumps and as such correct the deficiency of the classical non-linear shallow water equations in describing such phenomena....

Measuring velocity fields plays a crucial role in investigating the dynamics of granular flows, which can improve the modeling of hazardous geophysical flows (e.g. avalanches and debris flows) and the control of powder flows in industrial applications. Non-invasive optical methods are invaluable tools for estimating this physical quantity at the la...

To describe granular flows in the dense regime, the μ(I) rheology model was proposed. It has been proven to be effective in reproducing flow dynamics in the dense regime caused by dry granular materials. For continuum modeling, the mesh-free method can easily handle flows with interface such as free surface, which commonly exists in most granular f...

Multi-layer depth-averaged models are widely employed in various hydraulic engineering applications. Yet, such models are not strictly hyperbolic. Their equation systems typically lose hyperbolicity when the relative velocities between layers become too large, which is associated with Kelvin-Helmholtz instabilities involving turbulent momentum exch...

The volume fraction plays a crucial role in the dynamics of granular flows. This work is devoted to developing a novel cost-effective optical method for determining the near-wall volume fraction. By means of a numerical investigation, performed by Monte Carlo generations of mono-disperse sphere distributions under controlled illumination conditions...

Landslide induced waves have been the focus of attention in recent years. The majority of the research has mainly focused on wave generation but few investigations on the flow field. This paper examined the wave propagation and the corresponding flow fields in open channels caused by a wedge sliding along a slope to simulate a non-deformable landsl...

The continuum, shallow-water type landslide models, derived in the previous chapters are applied to two devastating landslides in Taiwan for back-analysis of the landslide motion. The two landslides are the Tsaoling and Hsiaoling landslides, which were large-scale rapid avalanche landslides triggered by the Chi-Chi earthquake, 1999, and the excessi...

In this chapter we apply the numerical implementation introduced in Chap. 7 to investigate and illustrate the key features of the models in the topography-fitted coordinate system.

The aim in Part III of the book was to present a way to deduce depth-averaged model equations for shallow geophysical mass flows down arbitrary topographies, be they active or not. To make the presentation accessible we considered thin one-layer films of a one-component (or single-phase) material.

The ultimate goal of this chapter is to deduce the mass and linear momentum balance laws in the topography-fitted coordinates introduced in Chap. 2, and to be prepared with some mathematical prerequisites when we formulate constitutive equations for the flowing material down arbitrary topography.

In this chapter we deduce depth-averaged model equations for thin mass flows down arbitrary topographies, be they stationary or active, when the flowing material is assumed a single-phase (or one-component) continuum body.

The depth-averaged modelling equations in Sects. 4. 5, 4. 6 apply to flowing materials for which the constitutive properties are only required to satisfy ordering approximations on the components of the stress tensor and on the erosion/deposition rate.

In this chapter we present a numerical implementation of the model equations. In general, the numerical schemes for shallow-water-like equation systems are suitable for applications to the proposed equation systems.

This chapter is devoted to the presentation of the curvilinear coordinates that we use for the description of a shallow mass flow down arbitrary topography.

Geophysical mass flows, such as landslides, avalanches or debris flows, are frequent mass movement processes in mountain areas and often cause disastrous damage. This book lays a foundation for formulating the depth-averaged equations describing the shallow geophysical mass flows over non-trivial topography. It consists of the detailed derivation o...

This is a pilot study on the occurrence mechanism of large-scale landslide. In the studying, we can find the phenomenon that pore water pressure is rising rapidly during heavy rainfall event. The phenomena is usually induced while wetting front contacts the plane of weakness, interface of multi-material or groundwater surface in the process of infi...

Earthquakes can generate seismic disturbances that propagate vast distances and trigger landslides that can achieve high-speeds. It remains difficult to identify the co-seismic ground motion of these landslides and their triggering earthquakes. In this paper, we report on the analysis of co-seismic ground motions generated by the initiating fractur...

We describe the experimental measurements and discrete element simulations of steady rapid granular chute flows disturbed by a small fixed semi-cylindrical obstacle. The flow chute has a width of 5 cm, a length of 150 cm, an inclination of 30°, with an obstacle installed at approximately the center of the chute. The steady disturbed flow has a regi...

The dynamics of dry granular flows is still insufficiently understood. Several depth-averaged approaches, where the flow motion is described through hydrodynamic-like models with suitable resistance laws, have been proposed in the last decades to describe the propagation of avalanches and debris flows. Yet, some important features of the granular f...

SUMMARY The flow of water–kaolinite mixtures exhibits a non-Newtonian nature that differs from the flow of Newtonian fluid. The varying viscosities and shear history of non-Newtonian fluid flows necessitate the use of a rheology model in moving particle semi-implicit (MPS) for the numerical studies. On the other hand, the Lagrangian method has the...

The propagation of dry granular flows, such as rock and snow avalanches, can be described by depth-averaged models. Different from classical shallow-water equations, these models take into account the anisotropy of normal stresses inside the flowing pile through using an earth pressure coefficient in the pressure term. A new regularization function...

A granular flow is normally comprised of a mixture of grain-particles (such as sand, gravel or rocks) of different sizes. In this study, dry granular flows are modeled utilizing a set of equations akin to a two-phase mixture system, in which the interstitial fluid is air. The resultant system of equations for a two-dimensional configuration include...

An alternative formulation is proposed for deriving depth-integrated equations for gravity-driven granular avalanches over a non-trivial topography with small curvature. The coordinate system of Bouchut and Westdickenberg (20045. Bouchut , F and Westdickenberg , M . 2004. Gravity driven shallow water models for arbitrary topography. Commun. Math....

In this paper a system of depth-integrated equations for over-saturated debris flows on three-dimensional topography is derived. The lower layer is a saturated mixture of density preserving solid and fluid constituents, where the pore fluid is in excess, so that an upper fluid layer develops above the mixture layer. At the layer interface fluid mas...

We propose a saturated binary mixture model for debris flows of the Coulomb-mixture type over temporally varying topography, where the effects of erosion and deposition are considered. Due to the deposition or erosion processes, the interface between the moving material and the stagnant base is a non-material singular surface. The motion of this si...

In this study, we detail a method for estimating the flux-averaged solid fraction of a steady granular flows moving down an inclined rectangular chute using velocity measurements from along the perimeter cross section, combined with knowledge of the mass flow rate through the cross section. The chute is 5cm wide and 150cm long with an adjustable in...

1] Typhoon Morakot struck southern Taiwan in the summer of 2009, causing the most severe flooding since the 1950s. In the early morning of August 9, rainfall triggered the Hsiaolin landslide, and the resulting debris avalanche covered the township of Hsiaolin Village, Kaohsiung. Around five hundred people were buried alive. Reconstruction of the ru...

We consider a one‐sided collapse flow from the view point of a model with a deforming topography. This is done by treating the collapsing flow into two layers: a rapid surface flowing layer and a stagnant base. A time‐varying interface, the deforming topography, separates the two layers. The evolution of the topography is described by a moving coor...

Based on the "shallow water models over arbitrary topography" by Bouchut and Westdickenberg [2004], and the "Coulomb-mixture theory" by Iversion and Denlinger [2001], we propose a saturated binary mixture model over temporally varying topography, where the effects of the entrainment and deposition are considered. Due to the deposition or erosion pr...

This study investigates the microstretch continuum modeling of granular assemblies while accounting for both the dilatant and rotational degrees of freedom of a macroelement. By introducing the solid volume fraction and the gyration radius of a granular system, the balance equations of the microstretch continuum are transformed into a new formulati...

This paper presents a three-dimensional, two-layer model for shallow geophysical mass flows, such as debris flows, hydraulic sediment transport, or sub-aquatic turbidity currents down arbitrary natural topographic terrains. The bottom layer is a dense granular fluid which interacts with the stagnant basal topography through an erosion/deposition me...

We use the non-Cartesian, topography-based equations of mass and momentum balance for gravity driven frictional flows of Luca etal. (Math. Mod. Meth. Appl. Sci. 19, 127–171 (2009)) to motivate a study on various approximations of avalanche models for single-phase granular materials. By introducing scaling approximations we develop a hierarchy of mo...

The 1999 Chi-Chi earthquake triggered the catastrophic Tsaoling landslide in Taiwan. The geomorphological change measured from the data of the 1989 and 2000 aerial photos reveals that the scar and deposit volumes are about 0.126 km3 and 0.15 km3 respectively. The debris material ran over a distance of 1.6 km with 500 m descent in elevation. In this...

The purpose of this paper is to derive modeling equations for debris flows on real terrain. Thus, we use curvilinear coordinates adapted to the topography as introduced, e.g., by Bouchut and Westdickenberg, and develop depth-averaged models of gravity-driven saturated mixtures of solid grains and pore fluid on an arbitrary rigid basal surface. Firs...

When dealing with geophysical flows across three-dimensional topography or other thin layer flows, for the physical modelling and for computational reasons, it is more convenient to use curvilinear coordinates adapted to the basal solid surface, instead of the Cartesian coordinates. Using such curvilinear coordinates, e.g. introduced by Bouchut and...

We use the erodible model proposed by Tai and Kuo to investigate complex granular flows in which deposition and erosion are significant. The initial motivation comes from experiments of granular collapse which exhibit both phenomena. A numerical model with a flux balanced scheme is developed, and the eigenstructure of its quasilinear form as well a...

In this paper a detailed approach is proposed for the behavior of two-dimensional cohesionless granular materials moving down a confined inclined plane chute into the horizontal run-out zone, where the upslope propagating bore is treated as a growing deposition heap. It deals with the theoretical-numerical and experimental treatments. The depth-ave...

A fundamental issue for describing gravity-driven flows over general topography is the search for an “optimal” coordinate. Bouchut and Westdickenberg [1] proposed an arbitrary coordinate system (BW) for general topography. The unified coordinate (UC) system (e.g., [2], [3]), which was developed for computational fluid dynamics, combines the advanta...

In the present work we treat granular materials as mixtures composed of a solid and a surrounding void continuum, proposing then a continuum thermodynamic theory for it. In contrast to the common mass-weighted balance equations of mass, momentum, energy and entropy for mixtures, the volume-weighted balance equations and the associated jump conditio...

Snow avalanches, landslides, rock falls and debris flows are extremely dangerous and destructive natural phenomena. The frequency of occurrence and amplitudes of these disastrous events appear to have increased in recent years perhaps due to recent climate warming. The events endanger the personal property and infra-structure in mountainous regions...

Shock waves, dead zones and particle-free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography. Understanding and modelling these flows is of considerable practical interest for industrial processes, as well as for the design of defences to protect buildings, structures and...

Shock formations are observed in granular avalanches when supercritical flow merges into a region of subcritical flow. In this paper we employ a shock-capturing numerical scheme for the one-dimensional Savage–Hutter theory of granular flow to describe this phenomenon. A Lagrangian moving mesh scheme applied to the nonconservative form of the equati...

The Savage-Hutter equations of granular avalanche flows are a hyperbolic system of equations for the distribution of depth and depth-averaged velocity components tangential to the sliding bed. They involve two phenomenological parameters, the internal and the bed friction angles, which together define the earth pressure coefficient which assumes di...

One means of preventing areas from being hit by avalanches is to divert the flow by straight or curved walls or tetrahedral or cylindrical-type structures. Thus, there arises the question how a given avalanche flow is changed regarding the diverted-flow depth and flow direction. In this paper a report is given on laboratory experiments performed fo...

The Savage-Hutter (SH) theory (1989) of dense granular avalanche flow uses an earth pressure coefficient Kx which depends on the internal angle of friction and the bed friction angle but assumes different values in diverging and converging flows. So the earth pressure coefficient is undefined when the strain rate ∂u/∂x changes sign. Steady plane fl...

Rapid shallow granular free surface flows occur when a surface layer of static granular material becomes unstable or when granular material is released by some mechanism onto an inclined surface. They are abundant all around us from salt in a salt cellar and flow at. the free surface of stockpiles, to landslides, rock-falls and snow slab avalanches...

Snow avalanches are relatively dry and dense granular flows for which the Savage-Hutter (SH) equations have been demonstrated to be an adequate mathematical model. We review these equations and point out for which cases the equations have been tested against laboratory experiments. Since the equations are scale invariant and because agreement with...

The Savage-Hunter theory for granular avalanches assumes that the granular material is in either of two limiting stress states, depending on whether the motion is convergent or divergent. At transitions between convergent and divergent regions, a jump in stress occurs, which necessarily implies that there is a jump in the avalanche velocity and/or...

The Savage-Hutter model is generalized by including a velocity-dependent drag in addition to the usual Coulomb dry friction at the base of the avalanche. Both linear and quadratic velocity dependencies are considered, with either constant or asymptotically constant drag coefficients for large thickness h. The singular nature of the constant coeffic...

Large scale stratification patterns [1] are formed when a mixture of two grain sizes, or more, repeatedly avalanche downslope and are brought to rest by upslope shock wave propagation. The avalanches are generated by either surface deposition, basal erosion or rotation of the free surface. Provided each of these processes take place at sufficiently...

The Savage-Hutter theory for granular avalanches assumes that the granular material is in either of two limiting stress states, depending on whether the motion is convergent or divergent. At transitions between convergent and divergent regions, a jump in stress occurs, which necessarily implies that there is a jump in the avalanche velocity and/or...

The Savage-Hutter model is generalized by including a velocity-dependent drag in addition to the usual Coulomb dry friction at the base of the avalanche. Both linear and quadratic velocity dependencies are considered, with either constant or asymptotically constant drag coefficients for large thickness h. The singular nature of the constant coeffic...

Although the Savage-Hutter theory for shallow granular flows has certain similarities with the shallow water equations of hydrodynamics, shock waves have not been observed until relatively recently. In this paper laboratory experiments are presented, in which plane travelling shock waves and station- ary oblique shocks develop. The experiments also...