Efficient Simulation Tools (EST) for sediment transport in geomorphological shallow flows

Entre los fenómenos superficiales geofísicos y medioambientales, los flujos rápidos de mezclas de agua y sedimentos son probablemente los más exigentes y desconocidos de los procesos movidos por gravedad. El transporte de sedimentos es ubicuo en los cuerpos de agua naturales, como ríos, crecidas, costas o estuarios, además de ser el principal proceso en deslizamientos, flujos de detritos y coladas barro. En este tipo de flujos, el material fluidificado en movimiento consiste en una mezcla de agua y múltiples fases sólidas, que pueden ser de distinta naturaleza como diferentes clases de sedimento, materiales orgánicos, solutos químicos o metales pesados en lodos mineros. El modelado del transporte de sedimentos involucra una alta complejidad debido a las propiedades variables de la mezcla agua-sólidos, el acoplamiento de procesos físicos y la presencia de fenómenos multicapa. Los modelos matemáticos bidimensionales promediados en la vertical ('shallow-type') se construyen en el contexto de flujos superficiales y son aplicables a un amplio rango de estos procesos geofísicos que involucran transporte de sedimentos. Su resolución numérica en el marco de los métodos de Volúmenes Finitos (VF) está controlada por el conjunto de ecuaciones escogido, las propiedades dinámicas del sistema, el acoplamiento entre las variables del flujo y la malla computacional seleccionada. Además, la estimación de los términos fuente de masa y momento puede también afectar la robustez y precisión de la solución. La complejidad de la resolución numérica y el coste computacional de simulación crecen considerablemente con el número de ecuaciones involucradas. Además, la mayor parte de estos flujos son altamente transitorios y ocurren en terrenos irregulares con altas pendientes, requiriendo el uso de una discretización espacial no-estructurada refinada para capturar la complejidad del terreno e incrementando exponencialmente el tiempo computacional. Por tanto, el esfuerzo computacional es uno de los grandes retos para la aplicación de modelos promediados 2D en flujos realistas con grandes escalas espaciales y largas duraciones de evento. En esta tesis, modelos matemáticos superficiales 2D apropiados, algoritmos numéricos de VF robustos y precisos, y códigos eficientes de computación de alto rendimiento son combinados para desarrollar Herramientas Eficientes de Simulación (HES) para procesos medioambientales superficiales involucrando transporte de sedimentos con escalas temporales y espaciales realistas. Nuevas HES capaces de trabajar en mallas estructuradas y no-estructuradas son propuestas para el flujos de lodo/detritos con densidad variable, transporte pasivo en suspensión y transporte de fondo generalizado. Una atención especial es puesta en el acoplamiento entre las variables del sistema y en la integración de los términos fuente de masa y momento. Las propiedades de cada HES han sido cuidadosamente analizadas y sus capacidades demostradas usando tests de validación analíticos y experimentales, así como mediciones en eventos reales.Among the geophysical and environmental surface phenomena, rapid flows of water and sediment mixtures are probably the most challenging and unknown gravity-driven processes. Sediment transport is ubiquitous in environmental water bodies such as rivers, floods, coasts and estuaries, but also is the main process in wet landslides, debris flows and muddy slurries. In this kind of flows, the fluidized material in motion consists of a mixture of water and multiple solid phases which might be of different nature, such as different sediment size-classes, organic materials, chemical solutes or heavy metals in mine tailings. Modeling sediment transport involves an increasing complexity due to the variable bulk properties in the sediment-water mixture, the coupling of physical processes and the presence of multiple layers phenomena. Two-dimensional shallow-type mathematical models are built in the context of free surface flows and are applicable to a large number of these geophysical surface processes involving sediment transport. Their numerical solution in the Finite Volume (FV) framework is governed by the particular set of equations chosen, by the dynamical properties of the system, by the coupling between flow variables and by the computational grid choice. Moreover, the estimation of the mass and momentum source terms can also affect the robustness and accuracy of the solution. The complexity of the numerical resolution and the computational cost of simulation tools increase considerably with the number of equations involved. Furthermore, most of these highly unsteady flows usually occur along very steep and irregular terrains which require to use a refined non-structured spatial discretization in order to capture the terrain complexity, increasing exponentially the computational times. So that, the computational effort required is one of the biggest challenges for the application of depth-averaged 2D models to realistic large-scale long-term flows. Throughout this thesis, proper 2D shallow-type mathematical models, robust and accurate FV numerical algorithms and efficient high-performance computational codes are combined to develop Efficient Simulation Tools (EST's) for environmental surface processes involving sediment transport with realistic temporal and spatial scales. New EST's able to deal with structured and unstructured meshes are proposed for variable-density mud/debris flows, passive suspended transport and generalized bedload transport. Special attention is paid to the coupling between system variables and to the integration of mass and momentum source terms. The features of each EST have been carefully analyzed and their capabilities have been demonstrated using analytical and experimental benchmark tests, as well as observations in real events.<br /

Propagation analysis of flow like-mass movements to evaluate the effectiveness of passive control works

2016 - 2017Flow-like mass movements are catastrophic events occurring all over the world and may result in a great number of casualties and widespread damages. The analysis of the time-space evolution of the kinematic quantities is a useful tool to understand the propagation stage of these phenomena as well as for control works design. The thesis deals with study of flow regime of Newtonian and non-Newtonian fluids and provides a contribution to this topic through the use of numerical procedures based on FV (finite volume) scheme and SPH (smoothed particle hydrodynamics) method. The FV model, developed by Rendina et al., 2017, is a single phase equivalent model, while the Geoflow-SPH, developed by Pastor et al.2009, considers the propagating mass with an average behavior of solid skeleton and pore water pressure. The flow kinematics are analyzed through the Froude number, widely used in hydraulic engineering, discriminates two different kinematical features i.e. subcritical (slow) or supercritical (rapid) flows. The analysis concern a 1D/2D dam break of Newtonian (water flow) and non-Newtonian flows (in particular based on a viscoplastic and frictional laws). The numerical results highlighted flows are supercritical even in areas far from trigger zones and Froude numbers of viscoplastic flows are higher than frictional flows. Later, the Froude number is used as a quantitative descriptor of the control works response and, more generally, as an useful tool to estimate the efficiency of existing storage basins. The first case study regards Cancia, in the Dolomite Alps, where two storage basins dramatically failed on 2009 due to a short-time sequence of rainfall-induced debris flows and flash floods. The kinematic analysis highlighted that debris flow can be associated to a subcritical flow while flash flood is similar to a supercritical flow and for latter lower is the potential efficacy of control works. The second case study regards Sarno, in the Campania region, where one of the most complex systems of passive control works was built after the 1998 events. The performance of the protection system is analyzed referring to Froude number again which highlighted the importance of planning the emergency/ordinary maintenance of control works. Finally, a new type of passive control work is described, i.e. the permeable rack that has the function of decrease the pore water pressures at the base and inside the propagating mass, thus causing the landslide body to brake and stop. The rack performance is tested as adaptation structure in existing protection systems also.[edited by Author]XXX cicl

Development of robust, physically-based numerical models for transport processes and geomorphodynamics changes

Bed changes in rivers may occur under several morphodynamics and hydrodynamics conditions. The modeling of this type of phenomena can be performed coupling the Shallow Water Equations (SWE) for the hydrodynamic part and the Exner equation for the morphodynamic part. The Exner equation states that the time variation of the sediment layer is due to the sediment transport discharge through the boundaries of the volume. Considering that sediment transport discharge are computed by means of sediment capacity formulae based on 1D experimental steady flows, the assessment of these empirical relations under unsteady 1D and 2D situations must be studied. In order to ensure the reliability of the numerical experimentation, the numerical scheme must handle correctly the coupling between the 2D SWE and the Exner equation under any condition. If possible, it is convenient to express the formulation of different empirical laws under a general framework. In consequence, a finite-volume numerical scheme that includes these two main features has been chosen as a benchmark for comparing the 1D and 2D results obtained when using several well known sediment transport formulae: Meyer-Peter and M\"uller, Ashida and Michiue, Engelund and Fredsoe, Fernandez Luque and Van Beek, Parker, Smart, Nielsen, Wong and Camenen and Larson. In addition, a new interpretation of the Smart empirical law is presented in order to cope with bed load transport over irregular beds of changing slope. Detailed results for this new modified empirical law together with the ones obtained with Meyer-Peter and M\"uller (which is the sediment capacity formula more used in hydraulic engineering) are provided for every test case analyzed. Furthermore, the Root Mean Square Error (RMSE) associated to every formula at each experimental condition is calculated with the purpose of evaluating quantitatively the overall behavior of each one. The results point out that the new interpretation of the Smart formula reaches the most accurate results in all cases, but in a genuinely 2D flow, that is, a situation involving more than one flow direction, the differences among sediment transport formulae are not as noticeable as in the 1D studied situations. Once the forecasting capacity of each sediment transport formula has been studied, another concern is the computational cost. The coupling between the SWE and the Exner equation by means of an augmented Jacobian matrix involves a high number of algebraic operations for computing the eigenvalues and the eigenvectors. Therefore, the computational cost is increased significantly, limiting the applicability of the numerical scheme to realistic situations where large domains are involved. In order to improve the computational efficiency, the coupling technique is modified, not decreasing the number of waves involved in the Riemann Problem but simplifying their definitions. The approach proposed in this thesis is a new strategy to combine concepts from hyperbolic conservation laws and conservative finite volume schemes. With the aim to control numerical stability in the most efficient form possible, a numerical eigenvalue is defined to control the discrete Exner equation in the explicit scheme. This bed wave celerity helps mainly to ensure conservation and to control automatically the numerical stability of the explicit scheme. The effects of the numerical coupling strategy proposed in this thesis are tested against exact solutions and 1D and 2D experimental data. The results emerging from this analysis show that efficiency and accuracy can be obtained when choosing an adequate sediment transport law and the stability condition is augmented by including a new celerity associated to the bed changes. On the other hand, in environmental and civil engineering applications, geomorphological changes are not only present in rivers but also in steep areas where massive mobilizations of poorly sorted material can occur. This sliding material is usually composed by a mixture of sand and water. For simplifying the phenomenon, dry granular flows have been considered as a starting point for the understanding of the physics involved within the landslides. The hypothesis of Saint-Venant equations are considered valid for modeling these land movements. Taking advantage of this approach, in this thesis approximate augmented Riemann solvers are formulated providing appropriate numerical schemes for mathematical models of granular flow on irregular steep slopes. Fluxes and source terms are discretized to ensure steady state configurations including correct modeling of start/stop flow conditions, both in a global and a local system of coordinates. The weak solutions presented involve the effect of bed slope in pressure distribution and frictional effects by means of the adequate gravity acceleration components. The numerical solvers proposed are first tested against 1D cases with exact solution and then are compared with 2D experimental data in order to check the suitability of the mathematical models described in this thesis. Comparisons between results provided when using global and local system of coordinates are presented. Both the global and the local system of coordinates can be used to predict faithfully the overall behavior of the landslides. The performance of the numerical scheme has been studied using novel experimental situations. These laboratory works include bidimensional configurations, the inclusion of obstacles in the flow path and a variable slope in the domain. Hence, a further step in mimicking realistic situations is obtained, since the behavior of the granular flow is affected by the presence of natural elements such as boulders or trees. Three situations have been considered. The first experiment is based on a single obstacle, the second one is performed against multiple obstacles and the third one study the influence of a dike when an overtopping situation takes place. Due to the impact of the flow against the obstacles, fast moving shocks appear, and a variety of secondary waves emerge. Comparisons between computed and experimental data are presented for the three cases. The computed results show that the numerical tool previously developed is able to predict faithfully the overall behavior of this type of complex dense granular flow

Quantifying the risk to life posed by hyperconcetrated flows

2012-2013In recent years, the disasters caused by landslides tragically increased due to the demographic growth and the indiscriminate use of land. Among the different types of landslides, flow-like phenomena - often simultaneously affecting large areas - are associated with the most catastrophic consequences in terms of loss of human life and economic damage. Understanding, forecasting and controlling the risk posed by flow-like phenomena are now recognised to be a priority for the safety of human life. As a result, a growing interest of both technical and scientific Communities, in performing risk analyses aimed at estimating the risk in a quantitative way has been recorded (Corominas et al., 2013). This PhD Thesis focus on the use of the quantitative risk analysis (QRA) procedures, specifically aimed at estimating the risk to life posed by flow-like phenomena. The use of QRA can allow the overcoming of some limits inherent to qualitative risk analyses in addressing practical problems (i.e. the prioritisation of management and mitigation actions as well as the allocation of associated resources). However, mainly theoretical contributions are provided on the topic at the international level. This can be due to the complexity of the procedures to be adopted for QRA purposes as well as to the significant amount of required input data (of both technical and socio-economic nature). The main goal of this research is to fill this gap by applying, improving and optimising the use of the QRA as a formal and structured tool for professionals involved in the management of the risk posed by flow-like phenomena. In this regard, the research activities focus on the quantitative estimation of the risk for loss of life, at medium and site-specific scale, posed by the occurrence of hyperconcentrated flows. The Thesis preliminarily provides a description of the main features of the flow-like phenomena, with an emphasis to those dealing with debris flows and hyperconcentrated flows. Then, the basic concepts and methodological approaches (with their limits and potentialities) of both qualitative and quantitative risk analysis and zoning are discussed. An overview of the current risk zoning in Italy, performed via qualitative risk analyses, is thus presented. On the basis of the above premises, the relevant benefits that, at regional and at site-specific scales, can be achieved passing from a qualitative to a quantitative risk analysis are highlighted. At medium scale, the analysis of historical records of landslide events in the Campania region (southern Italy) allows the identification and the characterisation of the different flow-like phenomena that may occur. In particular, these latter are individuated within a homogeneous geological context where carbonate slopes are covered by pyroclastic soils systematically affected by rainfall-induced slope instabilities later propagating as – often catastrophic in terms of life and properties losses – debris flows or hyperconcentrated flows. The thorough studies and researches carried out as well as the original results achieved allow to make relevant considerations – from both technical and scientific points of view – concerning both their spatial and temporal distribution (in terms of frequency) and the initial and boundary conditions which influence their occurrence. At detailed scale, the research activity focus on the quantitative estimate of the risk to life loss with reference to residents at the toe of Monte Albino (located in the Municipality of Nocera Inferiore (SA), Campania region), posed by the occurrence hyperconcentrated flows. The novelty of the proposed procedure consists in conjugating the fundaments of the risk theory with the geotechnical approach, providing a deeper understanding of the mechanisms that leads to the different and complex stages of movement. To this aim, a thorough in-situ investigations (with the purpose of framing the geological and geomorphological characteristics and to identify the 'hillslope' proneness to different slope instabilities, to characterise the spatial distribution of soil pyroclastic covers and their litho-stratigraphic structure) and laboratory tests (in order to have a complete physical and mechanical characterisation of the involved soils) are carried out. This study represents the indispensable prerequisite for the correct engineering modelling of phenomena at detailed scale - from the triggering stage to the propagation stage - obtaining in this way the definition of different hazard scenarios. The obtained results are used to estimate the expected consequences in terms of loss of human life with reference both to the most exposed persons within each of the impacted houses (to which corresponds the highest temporal-spatial probability) and to the average exposed person in open space (representing the average behaviour of a group of people). Finally, the procedure of quantitative risk estimate has allow to rank the portions of the urbanised territory at risk and, consequently, to provide a prioritisation of the areas needing structural mitigation measures. [edited by author]XII n.s

Debris flows and general steep slope shallow water flows numerical simulation

The main target of this research is to develop a numerical model for debris flow simulations. As it is known, in general, this kind of flows occur in steeped mountain slopes. When dealing with the complete 3D physic system of equations that model the phenomenon this singular characteristic has not special effect. To simulate large events (typical scales in real world) is not possible to use the complete equations (three dimensional, variable density, non-hydrotactic…) so a spacial dimension reduction is necessary combined with several simplifications to reduce the complexity of the system, along the present text previous works references are introduced to justify the selected simplifying hypothesis. During the mathematical manipulation of the equations, performed in order to reduce the spacial dimension (3D & 2D), the real complexity of the problem emerges, important consequences on the coordinate system appear. This means that, to obtain a simpler version of the physical model of the phenomenon, complex mathematical operations are needed. In the approach presented in this work the complexity of the problem is reduced in to manners: • Applying direct physical hypothesis on the flow characteristics. • Applying mathematical hypothesis. An example of these physical simplifications could be the monophasic fluid hypothesis.An example of the mathematical simplifications could be the fact of the curvature terms neglecting. In this work, the coordinate system selected for the model is named Proposed Coordinates System (PCS), this coordinates system is based on the work of Bouchut and Westdickenberg (2004) and Berger and Carey (1998a). The metric characteristics of the system are calculated and important conclusions are extracted from the analysis of the curvature terms. A link exist between the curvatures and the Christoffel symbols, if the curvatures are discarded also the Christoffel symbols should be discarded for model consistency. In the model governing differential equations, the use of curvilinear coordinates (PCS) provokes the existence of metric source terms defined as a Christoffel symbols functions. So, neglecting these terms the equations become simplified. Strictly talking the model is valid for steep but slowly varying slopes, where the curvatures are very small, although in the real test cases used for validating the model, the curvatures does not fit this condition. The rough approximation to the debris flow process through simplified physics used in the model probably hides the low accuracy of the model in strongly curved areas. Many other different coordinates system choices exist in the scientific literature, in this work some of them are commented and analyzed. Along the development of the model, different problems appear and different strategies and methodologies are proposed in order to overcome them. The first important problem is related to the physics of the process, the debris flows tend to naturally develop flow pulses, the flowing mixture stops and later is remobilized. The model includes what is called ”stop and go” mechanism to capture this behavior. The second problem is related with the boundary conditions, standard debris flow hydrograph includes sharp gradients which can become flow shocks, in this work a new methodology is proposed to introduce the boundary conditions in a way that shocks are correctly solved also in the boundary of the computational domain, contrary to the standard method of characteristics (Henderson, 1966; Abbott, 1966). The general family of the numerical methods selected to solve the resulting system is the Finite Volume Method (FVM) using the Riemann solver. This approach is extensively used in the debris flow simulation framework, so its selection is justified. In order to fit the special requirements of the PCS coordinates system the methodology developed by Rossmanith et al. (2004) and Rossmanith (2006) is used. This approach solves the problem of the non-orthogonal coordinates in the FVM framework. To validate the code analytical, experimental and real test cases are selected. The model validation process is partial because the lack in analytical solutions, especially for complex geometries

The effects of roots on the hydro-mecanical behavior of unsaturated pyroclastic soils

2016 - 2017Pyroclastic soils are widely diffused all over the world and they are characterized by high porosity and an open metastable internal structure. In situ they usually cover the shallowest layers of slopes in unsaturated conditions. As consequence, they are often involved in rainfall induced flow-like landslides triggered, during the rainy season, by water infiltration in unsaturated pyroclastic soils on steep slopes. The rain water infiltration leads to the volumetric collapse of the metastable structure in unsaturated conditions, and to liquefaction in fully saturated conditions. Once triggered, the propagating mass can reach great distances and cause many damages when it impacts with structures or infrastructures. These damages can be count as loss of life and economic damages. As risk mitigation measures for these rainfall induced flow-like landslides, structural and passive control works such as dissipative basins and/or brindles have been usually adopted over the centuries. An alternative sustainable risk mitigation measure can be represented by bio-engineering techniques, since they use natural elements such as woods or vegetation for stabilizing slopes prone to failure. The effectiveness of bio-engineering practices depends firstly on the soil properties. This aspect was investigated by carring out an experimental study on the effect of soil nutrients on the plant growth and how this is reflected on the soil hydraulic response. It was found that nutrient availability in soil enhance the plant growth, particularly the root number, and this increases the effectiveness of the vegetation on induced soil suction during evapotranspiration. After this preliminary study, the hydro-mechanical behavior of pyroclastic soils (widely known as rich in nutrients) permeated by roots of perennial graminae, typically used for controlling surface erosion, was investigated. From drying (Evapotranspiration) and wetting (Infiltration) test results it can be claimed that the presence of roots influences mostly the shallowest layers of the soil (up to 1.2 m). In particular, during drying the effect of roots on induced soil suction is highlighted in dry season, when air temperatures are high and the vegetation is florid. On the other hand, during wetting, the presence of roots tends to delay the water infiltration, even if the magnitude of suction reduction depends on the initial condition. Oedometer tests provided original insigths on the role of roots on the internal structure of these collapsible soils. In particular, it was found that during root growth, the soil structure tends to reduce its porosity and this is reflected into a reduction of the collapsibility of the root permeated soil during wetting in unsaturated condition. Shear strength of rooted soil, performed trough consolidated drained and undrained triaxial tests, show that the presence of roots increases both total cohesion and the internal friction angle, proportionally with the root biomass in the soil. Moreover, consolidate triaxial test results in undrained conditions showed that during post-failure stage the presence of roots reduces drastically the increment of pore water pressures avoiding the probability of static liquefaction of the material. All those insights allow having a basic framework to design further experimental investigations in order to consider this technique a sustainable risk mitigation measure in unsaturated pyroclastic soils of the Campania region. [edited by author]XVI n.s

A gravel-sand bifurcation:a simple model and the stability of the equilibrium states

A river bifurcation, can be found in, for instance, a river delta, in braided or anabranching reaches, and in manmade side channels in restored river reaches. Depending on the partitioning of water and sediment over the bifurcating branches, the bifurcation develops toward (a) a stable state with two downstream branches or (b) a state in which the water discharge in one of the branches continues to increase at the expense of the other branch (Wang et al., 1995). This may lead to excessive deposition in the latter branch that eventually silts up. For navigation, flood safety, and river restoration purposes, it is important to assess and develop tools to predict such long-term behavior of the bifurcation. A first and highly schematized one-dimensional model describing (the development towards) the equilibrium states of two bifurcating branches was developed by Wang et al (1995). The use of a one-dimensional model implies the need for a nodal point relation that describes the partitioning of sediment over the bifurcating branches. Wang et al (1995) introduce a nodal point relation as a function of the partitioning of the water discharge. They simplify their nodal point relation to the following form: s*=q*k , where s* denotes the ratio of the sediment discharges per unit width in the bifurcating branches, q* denotes the ratio of the water discharges per unit width in the bifurcating branches, and k is a constant. The Wang et al. (1995) model is limited to conditions with unisize sediment and application of the Engelund & Hansen (1967) sediment transport relation. They assume the same constant base level for the two bifurcating branches, and constant water and sediment discharges in the upstream channel. A mathematical stability analysis is conducted to predict the stability of the equilibrium states. Depending on the exponent k they find a stable equilibrium state with two downstream branches or a stable state with one branch only (i.e. the other branch has silted up). Here we extend the Wang et al. (1995) model to conditions with gravel and sand and study the stability of the equilibrium states

Towards a predictive multi-phase model for alpine mass movements and process cascades

Alpine mass movements can generate process cascades involving different materials including rock, ice, snow, and water. Numerical modelling is an essential tool for the quantification of natural hazards. Yet, state-of-the-art operational models are based on parameter back-calculation and thus reach their limits when facing unprecedented or complex events. Here, we advance our predictive capabilities for mass movements and process cascades on the basis of a three-dimensional numerical model, coupling fundamental conservation laws to finite strain elastoplasticity. In this framework, model parameters have a true physical meaning and can be evaluated from material testing, thus conferring to the model a strong predictive nature. Through its hybrid Eulerian–Lagrangian character, our approach naturally reproduces fractures and collisions, erosion/deposition phenomena, and multi-phase interactions, which finally grant accurate simulations of complex dynamics. Four benchmark simulations demonstrate the physical detail of the model and its applicability to real-world full-scale events, including various materials and ranging through five orders of magnitude in volume. In the future, our model can support risk-management strategies through predictions of the impact of potentially catastrophic cascading mass movements at vulnerable sites