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In some cases, debris flow consists of low-permeability soils in which run-out distances, velocities and lateral spreading are highly influenced by pore-water pressures. Basal screens are energy dissipation structures designed to reduce these velocities and run-out distances. These structures consist of grids built on horizontal decks where basal pore pressures of debris flows are made equal to the atmospheric pressure, which increases basal friction. Once the debris flows exit the structure, basal pore pressure increases as the basal surface is impermeable again, but the values have been reduced. In order to model these phenomena, it is necessary to use debris flow simulation models considering two phases and pore-water pressures. Here, a depth-integrated two-phase smooth particle hydrodynamics (SPH) model, previously developed by the authors, is improved by including the vertical structure of pore-water pressure. This is done by incorporating finite-differences meshes associated to each SPH node that represents a solid particle. In this way, a higher precision is obtained in describing excess pore pressure along depth, including the influence of vertical consolidation, height variation and changes in basal permeability. The proposed model is applied to describe debris flows with two different ranges of soil permeability propagating over grid structures. The authors have used laboratory tests run at the Norwegian University of Science and Technology and studied the efficiency of the basal grids by comparing cases equipped by them and cases without any obstacle. The results obtained from the proposed numerical experiments suggest that the proposed two-phase coupled model is able to properly reproduce the behaviour of debris flows, describing more accurately the time–space evolution of pore-water pressures during the whole propagation stage, considering both propagation impermeable beds and pore pressure dissipation structures such as grids.

Rapid flow-like landslides, particularly debris flows and debris avalanches, cause significant economic damage and many victims worldwide every year. They are usually extremely fast with the capability of travelling long distances in short times, sweeping away everything in their path. The principal objective of this paper is to test the ability of the ‘GeoFlow-SPH’ two-phase model developed by the authors, to reproduce the complex behaviour of natural debris avalanches where pore-water pressure evolution plays a key role. To reach this goal, the model is applied to reproduce the complex dynamic behaviour observed in Johnsons Landing debris avalanche including the observed bifurcation caused by the flowing out of part of the moving mass from the mid-channel. Initial thickness deposit trim-line, distribution of deposit volume, and the average velocities were provided for this real case, making it an appropriate case to validate the developed model. The paper also contributes to evaluate the SPH-FD model’s potentialities to simulate the structural countermeasure, like bottom drainage screens, used to reduce the impact of debris flows. The analysis of the results shows the adequacy of the proposed model to solve this complicated geophysical problem.

Debris flows are a type of fast landslides where a mixture of soil and water propagates along narrow channels. The main characteristics are (1) important relative displacements between the solid and fluid phases, and (2) development of pore-water pressures in excess to hydrostatic. The ratios between vertical and horizontal displacements of the flow, from the triggering point to the deposition, indicate that friction angles are much smaller than those measured in laboratories. Debris flows are modeled as two phases flow, but implementing pore-water pressure is an important issue. The purpose of this paper is to improve the existing two phases debris flow models by implementing pore-water pressures in excess to hydrostatic. It is found that pore pressure evolution depends on consolidation, changes in the flow depth, and changes and gradients of porosity. The proposed depth integrated mathematical model is discretized using two sets of SPH nodes (solid and fluid), with a set of finite difference meshes associated with each solid material SPH point. The paper presents two examples from where it is possible to gain insight into the differences between the models (with and without excess pore water pressure).

This paper presents a depth integrated, two-layer SPH new model for debris flows with finite differences meshes associated to nodes to describe pore pressure evolution. The proposed model is applied to describe how the flow evolves when arriving at a grid where pore pressure is made zero.

A debris flow is one of the most dangerous types of geophysical mass flows moving downward mountain slopes and consist of a mixture of soil and rocks with a significant quantity of interstitial fluid. They can travel at extremely rapid velocities affecting long distances due to the significant influence of pore-water pressures on the propagation stage. In this paper, a novel two-phase depth-integrated SPH numerical model is applied to simulate a debris flow in which the effects of bed entrainment and pore-water pressure evolution, essential components to performed debris flows risk assessment, are included. The model was applied to simulate the dynamics of Yu Tung debris flow which is an appropriate benchmark case to examine the applicability of the developed model, as bed entrainment and pore-water pressure evolution were key aspects that affected the moving mass dynamics. The developed two-phase depth-integrated SPH-FD model is applied to assess the structural countermeasure of the bottom drainage screen, used to reduce the impact of debris flows. The reasonable results obtained from the analysis indicate that the model is capable to properly reproduce the propagation of the debris flow and more importantly to correctly perform the time-space evolution of pore water pressures during the whole deformation process from initiation through propagation, over an impermeable and permeable bottom boundary, up to deposition.

Los flujos de derrubios son un fenómeno geológico que tiene lugar con cierta regularidad en regiones montañosas del planeta causando cada año cuantiosas pérdidas económicas y un elevado número de víctimas en todo el mundo. El desarrollo de modelos continuos que permitan representar la propagación de este tipo de flujos másico geofísicos, desde su desencadenamiento hasta su deposición, no es una tarea sencilla debido a la complejidad del fenómeno, conduciendo a un sistema de ecuaciones en derivadas parciales hiperbólico, dependiente del tiempo y no lineales. Los flujos de derrubios son uno de los tipos de deslizamiento que causan mayor número de daños, presentando una naturaleza multifísica en el que el comportamiento acoplado esqueleto sólido-fluido intersticial juega un papel fundamental en la fase de propagación. En esta Tesis Doctoral se presenta un modelo matemático integrado en profundidad para flujos de derrubios basado en las ecuaciones de Biot-Zienkiewicz incluyendo leyes de arrastre para tener en cuenta la interacción entre el esqueleto sólido y el fluido intersticial. Este modelo incorpora la influencia de la disipación de excesos de presión de poros, aspecto crucial para la evaluación de riesgos asociada a esta tipología de flujos gravitatorios. El modelo matemático de dos fases (esqueleto sólido y fluido intersticial) que se presenta en esta Tesis Doctoral extiende el modelo propuesto por Pastor y colaboradores en 2015 ya que permite una descripción más precisa de la evolución de las presiones intersticiales al incorporar no sólo los cambios de tensión total debidos a la variación de la altura del flujo, sino a cambios en la porosidad. Este nuevo modelo matemático puede ser empleado en la propagación de flujos de derrubios con distintas permeabilidades permitiendo, permitiendo reproducir la dispersión lateral observada en este tipo de flujos de derrubios la cual depende de la presión intersticial. El modelo matemático ha sido resuelto numéricamente a través del método Smoothed Particle Hydrodynamics (SPH) con un doble juego de partículas, una para representar el movimiento del esqueleto y otra para representar el movimiento del agua intersticial. Como novedad un esquema de diferencias finitas ha sido incluido a las partículas de sólido para incorporar la evolución de las presiones de poros a lo largo de la fase de propagación del flujo de derrubios. Hasta donde llega el conocimiento del autor, esta es la principal novedad de la presente Tesis Doctoral, ya que es la primera vez en la que un modelo continúo integrado en profundidad desarrollado para la propagación de flujos de derrubios es resuelto numéricamente con la técnica SPH con dos juegos de partículas incluyendo la disipación de los excesos de presión de poros (modelo SPH-DF). De esta forma esta Tesis Doctoral extiende el trabajo de 2017 de Manuel Pastor y colaboradores. El funcionamiento así como las limitaciones del presente modelo han sido evaluadas a través de una serie de simulaciones de verificación que incluyen 1) problemas de rotura de presa, 2) ensayos a pequeña escala incorporando rejillas permeables en la superficie de deslizamiento y 3) dos casos reales de flujos de derrubios acaecidos en Hong Kong. Los buenos resultados obtenidos a través de los análisis de validación indican que el modelo matemático propuesto es capaz de reproducir adecuadamente la velocidad de propagación así como la extensión y profundidad de la zona afectada por el deslizamiento. Más aún, el modelo representa adecuadamente la evolución de las presiones intersticiales a lo largo de la fase de propagación considerando una superficie basal tanto permeable como impermeable. ----------ABSTRACT---------- Landslides are a geological phenomenon which occur frequently in mountainous regions or along rivers, causing major economic damage and a large number of casualties around the world. Development of continuous based models to represent the propagation of these geophysical mass flows, from initiation to deposit in the run-out zones, is not an easy task due to the complexity of the phenomenon. Moreover, many of the interesting fast catastrophic landslides, such as debris flows, involve more than one phase where the coupling between the phases plays a fundamental role. Such multiphase materials can be properly described by using a multiple set of nodes for different phases, and a suitable drag law to take into account the interaction between particles. The principal objective of this Doctoral Thesis is to develop mathematical and numerical modelling for simulating debris flows in which considering the effect of pore-water pressure dissipation is essential for risk analysis. A debris flow consists of low to high permeable soil in which lateral spreading could be highly affected by excess pore-water pressure. The numerical modeling of these types of debris flow has not been investigated until now. In this study, the two-phase model proposed by Pastor et al. (2017) is extended and also an improvement in description of pore pressure evolution (Pastor et al., 2015) is presented in order to take into account with more precision changes caused not only by vertical consolidation and height variations, but also by new considerations which include changes of basal surface permeability and porosity variations. The new mathematical approach is based on the depth integrated mathematical model of Zienkiewicz and Shiomi (1984), and is capable to reproduce the propagation of debris flows with soil permeability ranging from high to low. These mathematical equations are discretized by using Smooth Particle Hydrodynamic (SPH) technique where a double set of nodes, one to represent the movement of the solid particles and another to represent the movement of the fluid particles, is defined. As a novelty, this Doctoral Thesis provides a contribution to enhance the two-phase numerical model with adding a 1D finite difference grid to each SPH node that represents a solid particle in order to improve the description of pore-water pressure (SPH-FD model). The performance and limitations of the model is assessed using a series of benchmark exercises, including (i) dam break problem to show the main features of the model, (ii) flume tests equipped with a (permeable) rack which performed in Trondheim, and (iii) two real cases for which we have had access to their reliable information. The good results obtained from the validation analysis of the mentioned benchmarks indicated that the proposed model is capable to properly reproduce the propagation velocity, runout distance and deposit thickness of the debris flows, and more importantly to correctly performs the time-space evolution of pore-water pressures during the whole propagation stage from initiation to propagation over an impermeable and permeable bottom boundary, and up to deposition.

In this work an approach to representing debris floods and flows is proposed, based on a depth-integrated mathematical model and smooth particle hydrodynamics numerical technique. The main contributions of the present work are twofold: (a) an improved rheological Bingham model where solid fraction concentration can change as solids are entrained by the current and (b) implementation of boundary conditions describing the injection process of water from channel weirs into the domain. The model has been applied to the case of Lo Wai debris flood (Hong Kong, August 2005). Numerical results obtained with the proposed model are in reasonable agreement with field observation. Fil: Lin, Chuan. Hohai University; China Fil: Pastor, Manuel. Universidad Politécnica de Madrid; España Fil: Yague, Angel. Universidad Politécnica de Madrid; España Fil: Tayyebi, Saeid Moussavi. Universidad Politécnica de Madrid; España Fil: Stickle, Miguel Martin. Universidad Politécnica de Madrid; España Fil: Manzanal, Diego. Universidad Politécnica de Madrid; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina Fil: Tongchun Li. National Engineering Research Center Of Water Resource; China Fil: Liu, Xiaoqing. National Engineering Research Center Of Water Resource; China

The choice of a pure cohesive or a pure frictional viscoplastic model to represent the rheological behaviour of a flowslide is of paramount importance in order to obtain accurate results for real cases. The principal goal of the present work is to clarify the influence of the type of viscous model—pure cohesive versus pure frictional—with the numerical reproduction of two different real flowslides that occurred in 1966: the Aberfan flowslide and the Gypsum tailings impoundment flowslide. In the present work, a depth-integrated model based on the v−pw Biot–Zienkiewicz formulation, enhanced with a diffusion-like equation to account for the pore pressure evolution within the soil mass, is applied to both 1966 cases. For the Aberfan flowslide, a frictional viscous model based on Perzyna viscoplasticity is considered, while a pure cohesive viscous model (Bingham model) is considered for the case of the Gypsum flowslide. The numerical approach followed is the SPH method, which has been enriched by adding a 1D finite difference grid to each SPH node in order to improve the description of the pore water evolution in the propagating mixture. The results obtained by the performed simulations are in agreement with the documentation obtained through the UK National Archive (Aberfan flowslide) and the International Commission of large Dams (Gypsum flowslide). Fil: Dutto, Paola. Bundesanstalt für Materialforshung und-prüfung; Alemania Fil: Stickle, Miguel Martín. Universidad Politécnica de Madrid; España Fil: Pastor, Manuel. Universidad Politécnica de Madrid; España Fil: Manzanal, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long". Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías y Ciencias de la Ingeniería "Hilario Fernández Long"; Argentina. Universidad Politécnica de Madrid; España. Universidad Nacional de la Patagonia; Argentina Fil: Yague, Ángel. Universidad Politécnica de Madrid; España Fil: Moussavi Tayyebi , Saeid. Universidad Politécnica de Madrid; España Fil: Lin, Chuan. Hohai University. College of Water Conservancy and Hydropower; China Fil: Elizalde, María Dolores. Consejo Superior de Investigaciones Cientificas; España