Simulation of energy transfers in waves generated by granular slides

International audienceThis paper presents a multi-fluid Navier-Stokes modelling of the waves generated by two granular slides (subaerial and submarine) which were previously studied experimentally and a pure synthetic submarine case used for results interpretation. In the numerical model, air and water are considered as Newtonian fluids. The slide is modelled as a Newtonian fluid whose viscosity is adjusted to fit the experimental results. Once the viscosity is adjusted, the first and the second waves are shown to be accurately reproduced by the model even though the computed slide is slower. For the subaerial case, the viscosity value found is shown to be consistent with the granular µ(I) rheological law. The second part of this work focuses on the energy transfers between a slide and its generated waves. Energy balance is computed in each phase. The wave energy is evaluated in the wave propagation zone. Energy dissipation, kinematic and potential energies are taken into account in the computation of energy transfer ratio allowing for a better understanding of the phenomena. In light of these results, the wave train generation process is discussed as well as the importance of the slide dynamics in the wave generation stage. The amount of energy transferred to wave is not constant with time and the transfer rate depends strongly on the definition of this rate as well as the case considered. For instance, in the subaerial case simulated, the energy transferred to surface waves is 30 % of the energy transferred to water at the time the transfer stops, but this conversion rate is only equal to 4 % of the overall available potential slide energy at the end of the process. For the two submarine cases simulated, the corresponding values, equal in both cases, are 2 % and 1 %, respectively. The simulation results also show that the slide energy is transferred to the water in a short period of time at the beginning whatever the case considered. This observation may be related to the initial nil slide velocity (subaerial case) and the relatively large slope values considered (both cases). Nevertheless, the results illustrate the importance of accurate simulation of the slide dynamics within the wave generation process

Floating solar panels on reservoirs impact phytoplankton populations:A modelling experiment

Floating solar photovoltaic (FPV) deployments are increasing globally as the switch to renewable energy intensifies, representing a considerable water surface transformation. FPV installations can potentially impact aquatic ecosystem function, either positively or negatively. However, these impacts are poorly resolved given the challenges of collecting empirical data for field or modelling experiments. In particular, there is limited evidence on the response of phytoplankton to changes in water body thermal dynamics and light climate with FPV. Given the importance of understanding phytoplankton biomass and species composition for managing ecosystem services, we use an uncertainty estimation approach to simulate the effect of FPV coverage and array siting location on a UK reservoir. FPV coverage was modified in 10 % increments from a baseline with 0 % coverage to 100 % coverage for three different FPV array siting locations based on reservoir circulation patterns. Results showed that FPV coverage significantly impacted thermal properties, resulting in highly variable impacts on phytoplankton biomass and species composition. The impacts on phytoplankton were often dependent on array siting location as well as surface coverage. Changes to phytoplankton species composition were offset by the decrease in phytoplankton biomass associated with increasing FPV coverage. We identified that similar phytoplankton biomass reductions could be achieved with less FPV coverage by deploying the FPV array on the water body's faster-flowing area than the central or slower flowing areas. The difference in response dependent on siting location could be used to tailor phytoplankton management in water bodies. Simulation of water body-FPV interactions efficiently using an uncertainty approach is an essential tool to rapidly develop understanding and ultimately inform FPV developers and water body managers looking to minimise negative impacts and maximise co-benefits

Floating solar panels on reservoirs impact phytoplankton populations: a modelling experiment

Floating solar photovoltaic (FPV) deployments are increasing globally as the switch to renewable energy intensifies, representing a considerable water surface transformation. FPV installations can potentially impact aquatic ecosystem function, either positively or negatively. However, these impacts are poorly resolved given the challenges of collecting empirical data for field or modelling experiments. In particular, there is limited evidence on the response of phytoplankton to changes in water body thermal dynamics and light climate with FPV. Given the importance of understanding phytoplankton biomass and species composition for managing ecosystem services, we use an uncertainty estimation approach to simulate the effect of FPV coverage and array siting location on a UK reservoir. FPV coverage was modified in 10% increments from a baseline with 0% coverage to 100% coverage for three different FPV array siting locations based on reservoir circulation patterns. Results showed that FPV coverage significantly impacted thermal properties, resulting in highly variable impacts on phytoplankton biomass and species composition. The impacts on phytoplankton were often dependent on array siting location as well as surface coverage. Changes to phytoplankton species composition were offset by the decrease in phytoplankton biomass associated with increasing FPV coverage. We identified that similar phytoplankton biomass reductions could be achieved with less FPV coverage by deploying the FPV array on the water body's faster-flowing area than the central or slower flowing areas. The difference in response dependent on siting location could be used to tailor phytoplankton management in water bodies. Simulation of water body-FPV interactions efficiently using an uncertainty approach is an essential tool to rapidly develop understanding and ultimately inform FPV developers and water body managers looking to minimise negative impacts and maximise co-benefits

Modélisation des vagues générées par les glissements de terrain. Approches discontinues et continues et transferts d'énergie

Tsunamis generated by landslides can be sub-aerial or underwater. Few real case data are available on the size, composition and speed of the slide at the time of wave generation. To study wave generation processes, it may be interesting to use laboratory experiments or numerical simulations. Many numerical landslide and flow models are used in the literature. In this thesis, two approaches to landslide modelling are considered. The flows are solved by the THETIS model developed at the University of Bordeaux. This model solves the multi-fluid Navier-Stokes equations for incompressible and immiscible fluids. The thesis is divided into three parts.A discontinuous approach is presented in the first part of the thesis. This method consists in modelling a landslide by a set of solid discs in the THETIS computational fluid mechanics code. Solid discs are modelled by a fluid with a very high viscosity. This fluid deforms very little but the collision of two discs is plastic, i.e. if two discs collide, they merge. To avoid this phenomenon, a method based on volume forces managing solid/solid collisions is developed. Simulations to validate this method are carried out from experiments on stacking cylinders dropped on a slope.The second part deals with the modelling of the landslide by a Newtonian fluid or µ(I) rheology for two log release experiments on the slope of a channel. A study of energy transfers from slide to wave is presented. It highlights the speed of transfers and the importance of slide modelling at the beginning of its movement for wave generation. This study made it possible to make more realistic simulations of waves generated by the fall of a slope part of the Cumbre Vieja volcano in the Canary Islands by Abadie et al (article in revision) by modelling the slide with a high viscosity Newtonian fluid.In the framework of the RAVEX project (Risks Associated with Explosive Volcanism), the last part of the thesis presents simulations of waves generated by the release of a very low viscosity fluid on a slope. The objective is to validate the modeling of wave generation by pyroclastic flow with a low viscosity Newtonian fluid. Indeed, pyroclastic flows are granular flows that have high mobility due to pore gas pressure and are therefore close to Newtonian fluid flows. The influence of the physical parameters of the fluid (density and viscosity) and the opening height of the door on the wave height generated is finally studied.Les tsunamis générés par des glissements de terrain peuvent être subaériens ou sous-marins. Peu de données de cas réels sont disponibles sur la taille, composition et vitesse du glissement au moment de la génération des vagues. Pour étudier les processus de génération des vagues, il peut être intéressant de passer par des expériences en laboratoire ou des simulations numériques. De nombreux modèles numériques de glissements de terrain et d'écoulement sont utilisés dans la littérature. Dans cette thèse, deux approches de modélisation du glissement de terrain sont considérées. Les écoulements sont résolus par le modèle THETIS développé à l'université de Bordeaux. Ce modèle résout les équations de Navier-Stokes multifluide pour des fluides incompressibles et immiscibles. La thèse est divisée en trois parties.Une approche discontinue est présentée dans la première partie de la thèse. Cette méthode consiste à modéliser un glissement de terrain par un ensemble de disques solides dans le code de mécanique des fluides numérique THETIS. Les disques solides sont modélisés par un fluide dont la viscosité est très élevée. Ce fluide se déforme très peu mais la collision de deux disques est plastique, c'est à dire que si deux disques entrent en collision, ils fusionnent. Pour éviter ce phénomène, une méthode basée sur des forces volumiques gérant les collisions solides/solides est développée. Des simulations visant à valider cette méthode sont réalisées à partir d'expériences d’empilement de cylindres lâché sur une pente.La deuxième partie traite de la modélisation du glissement de terrain par un fluide newtonien ou de rhéologie µ(I) pour deux expériences de lâcher de billes sur la pente d'un canal. Une étude des transferts d'énergie du glissement aux vagues est présentée. Elle permet de mettre en avant la rapidité des transferts et l'importance de la modélisation du glissement au début de sa mise en mouvement pour la génération des vagues. Cette étude a permis de faire des simulations plus réalistes de vagues générées par la chute d'un flanc du volcan Cumbre Vieja aux Canaries réalisées par Abadie et al. (article en révision) en modélisant le glissement par un fluide newtonien à forte viscosité.Dans le cadre du projet RAVEX (Risques Associés au Volcanisme EXplosif), la dernière partie de la thèse présente des simulations de vagues générées par le lâcher d’un fluide très peu visqueux sur une pente. L’objectif est de valider la modélisation de la génération de vagues par un écoulement pyroclastique par un fluide newtonien à faible viscosité . En effet, les écoulements pyroclastiques sont des écoulements granulaires qui ont une grande mobilité dû à la pression du gaz interstitiel et se rapprochent donc d'écoulements de fluide newtonien. L'influence des paramètres physiques du fluide (densité et viscosité) et la hauteur d'ouverture de la porte sur la hauteur de vague générée est enfin étudiée

Two Approaches for Numerical Modelling of Waves Generated by Landslides: Macroscopic and Grain Scales

International audienceThe present works aims to show two approaches for the numerical modelling of waves generated by landslides. The first approach is based on a macroscopic view of the landslide. Two cases are introduced : the pyroclastic flow and the generation by a granular flow. Regarding the pyroclastic flow, if we consider that the high interstitial pressure persists during the propagation as showed in some experiments (Roche et al.), the slide has a fluid-like behaviour and therefore can be modelled as a Newtonian fluid. Some experiments are in process to assess this hypothesis. In the case of granular flow, we deal with the experiment of glass beads falling on a slope into water (Viroulet) for two diameters of beads. First, the landslide is modelled as a Newtonian fluid. The aim is to determine the viscosity value for each case and be able to reproduce the first wave. To be closer to the granular media, the mu(I)-rheology is also introduced (GDR MiDi). This rheology has been proposed to model dense granular flow and parameters are defined by the media. The second approach is to model the grain itself in the granular media. It can be done by coupling a DEM code with a Navier-Stokes code for example (Shan and Zhao). However, here, the idea is to compute the slide and the fluids with only a Navier-Stokes (NS) code. To realise that, the solid are modelled using penalised fluid (Ducassou et al.). Yet, the interactions between solid have to be manage by an additional routine in the NS code. A first model has been developed for interaction between discs. Experimental results are expected for the validation of this routine like the fall of several cylinders on a slope into water. References : O. Roche, S. Montserrat, Y. Niño, and A. Tamburrino. Pore fluid pressure and internal kinematics of gravitational laboratory air-particle flows: Insights into the emplacement dynamics of pyroclastic flows. Journal of Geophysical Research, 115(B9), September 2010. Sylvain Viroulet. Simulations de tsunamis générés par glissements de terrains aériens. Thèse de doctorat, Aix-Marseille Université, France, 2013. GDR MiDi. On dense granular flows. The European Physical Journal E, 14(4):341-365, August 2004. Tong Shan and Zidong Zhao. A coupled CFD-DEM analysis of granular flow impacting on a water reservoir. Acta Mechanica, 225(8):2449-2470, August 2014. B. Ducassou, J. Nunez, M. Cruchaga, and S. Abadie. A fictitious domain approach on a viscosity penalty method to simulate wave / structure interaction. To appear in Journal of Hydraulic Research

La Palma landslide tsunami: calibrated wave source and assessment of impact on French territories

International audienceIn this paper, we present new results on the potential La Palma collapse event, previously described and studied in Abadie et al. (2012). Three scenarios (i.e., slide volumes of 20, 40 and 80 km 3) are considered, modeling the initiation of the slide to the water generation using THETIS, a 3D Navier-Stokes model. The slide is a Newtonian fluid whose viscosity is adjusted to approximate a granular behavior. After 5 min of propagation with THETIS, the generated water wave is transferred into FUNWAVE-TVD (Total Variation Diminishing version of FUNWAVE) to build a wave source suitable for propagation models. The results obtained for all the volumes after 15 min of Boussinesq model simulation are made available through a public repository. The signal is then propagated with two different Boussi-nesq models: FUNWAVE-TVD and Calypso. An overall good agreement is found between the two models, which secures the validity of the results. Finally, a detailed impact study is carried out on La Guadeloupe using a refined shallow water model, SCHISM, initiated with the FUNWAVE-TVD solution in the nearshore area. Although the slide modeling approach applied in this study seemingly leads to smaller waves compared to former works, the wave impact is still very significant for the maximum slide volume considered on surrounding islands and coasts, as well as on the most exposed remote coasts such as Guadeloupe. In Europe, the wave impact is significant (for specific areas in Spain and Portugal) to moderate (Atlantic French coast)

Validation and inter-comparison of models for landslide tsunami generation

10.1016/j.ocemod.2021.101943OCEAN MODELLING17

A database of validation cases for tsunami numerical modelling

This work has been performed by a French national consortium within the framework of the nationalproject Tandem, with aim to improve knowledge about tsunami risk on the French coasts. Workpackage#1 of this project was the opportunity to build a database of benchmark cases to assess the capabilitiesof 18 codes, solving various set of equations with different numerical methods. 14 test cases were definedfrom the existing literature with validation data from reference simulations, theoretical solutions or lab experiments.They cover the main stages of tsunami life: 1) generation, 2) propagation, 3) run-up and submersion,and 4) impact. For each case several of the numerical codes were compared in order to identify the forces andweaknesses of the models, to quantify the errors that these models may induce, to compare the various modellingmethods, and to provide users with recommendations for practical studies. In this paper, 3 representativecases are selected and presented with an analysis of the results.Tsunamis en Atlantique et MaNche : Définition des Effets par Modélisatio