Realistic, reliable, and effective modeling of cross-shore sediment transport is not present in the current literature. Building that model requires the accurate recreation of breaking wave processes in the nearshore. To develop that first step for an as-yet-to-be-designed model, multiple phase-resolving wave transformation algorithms are reviewed for in-depth investigation. The COULWAVE model is selected for robust testing. Testing of the COULWAVE model shows that, although capable of recreating realistic results, it does not adequately describe major wave characteristics in the surf zone, across a wide range of conditions, to warrant use in a future cross-shore sediment transport model.

The production and transportation of hydrocarbon fluids in multiphase pipelines could be severely hindered by particulate solids deposit such as produced sand particles which accompany hydrocarbon production. Knowledge of the flow characteristics of solid particles in fluids transported in pipelines is important in order to accurately predict solid particles deposition in pipelines. This research thesis presents the development of a three-dimensional (3D) Computational Fluids Dynamics (CFD) modelling technique for the prediction of liquid-solids multiphase flow in pipes, with special emphasis on the flow in V-inclined pipe bends. The Euler-Euler (two-fluid) multiphase modelling methodology has been adopted and the multiphase model equations and closure models describing the liquid-solids flow have been implemented and calculated using the finite volume method in a CFD code software. The liquid phase turbulence has been modelled using a two-equation k−ε turbulence model which contains additional terms to account for the effects of the solid-particles phase on the multiphase turbulence structure. The developed CFD numerical framework has been verified for the relevant forces and all the possible interaction mechanisms of the liquid-solids multiphase flow by investigating four different numerical frameworks, in order to determine the optimum numerical framework that captures the underlying physics and covers the interaction mechanisms that lead to sand deposition and the range of sand transport flow regimes in pipes. The flow of liquid-sand in pipe has been studied extensively and the numerical results of sand concentration distribution across pipe and other flow properties are in good agreement with published experimental data on validation. The numerical framework has been employed to investigate the multiphase flow in V-inclined pipe bends of ±4o−6o, seemingly small inclined bend angles. The predicted results which include the sand segregation, deposition velocity and flow turbulence modulation in the pipe bend show that the seemingly small pipe bends have significant effect on the flow differently from that of horizontal pipes. The pipe bend causes abrupt local change in the multiphase flow characteristic and formation of stationary sand deposit in the pipe at a relatively high flow velocity. The threshold velocity to keep sand entrained in liquid in pipe bends is significantly higher than that required for flow horizontal pipes. A critical implication of this is that the correlations for predicting sand deposition in pipelines must account for the effect of pipe bend on flow characteristics in order to provide accurate predictions of the critical sand transport velocity (MTV) in subsea petroleum flowlines, which V-inclined pipe bends are inevitable due to seabed topology.

Les environnements arides végétalisés seront parmi les environnements les plus impactés par la désertification dans le cadre du changement climatique. Ces environnements légèrement végétalisés sont caractérisés par une balance précaire entre un état de résilience et de vulnérabilité qui est intrinsèquement menacé par la désertification pouvant potentiellement mener à une augmentation du transport des sédiments éolien et une dégradation des environnements. Le nombre d’interactions présentes entre la végétation, la pluie, le transport des sédiments et la présence d’herbivore en milieu aride, ainsi que leur nature non-linéaire rend difficile de représenter ces interactions à l’aide de modèle physique et mathématique. La modélisation complexe est mieux adaptée à la représentation des interactions complexes entre la végétation, la pluie, le transport des sédiments et la présence d’herbivores dans les systèmes arides. Un nombre considérable d’études ont utilisées les modèles complexes pour étudier l’effet de la végétation sur le transport des sédiments ou l’effet de la présence d’herbivore sur la végétation, mais peu d’études ont utilisées une approche intégrant ces trois composantes en un même modèle. Un nouveau modèle d’herbivorie basé sur l’agent (GrAM) est présenté sous forme d’extension du modèle ViSTA_M17 et permet une meilleure représentation de l’impact des régimes de pâturage en environnement aride végétalisé. Cet ajout ayant un modèle complexe de transport des sédiments et de végétation déjà établit vise présenter un modèle hybride pouvant représenter l’impact de l’herbivorie sur la composition végétale et le transport des sédiments en environnement aride à l’échelle du paysage. Le développement du nouveau module à l’intérieur de la structure du modèle ViSTA original a souligné certaines limites de ce dernier, notamment une sensitivité importante de la végétation et de la force de cisaillement du vent. Le modèle ViSTA_GrAM répond à certaines limites du modèle original par l’intégration d’un nouveau module d’herbivorie et présente une avancée vers une modélisation environnementale englobante permettant une meilleure compréhension des dynamiques spatiales et temporelles des environnements arides. L’approche englobante utilisée par le modèle ViSTA_GrAM est bénéfique à la prise de décision, puisqu’elle offre un outil permettant d’explorer les réponses des environnements arides à un changement de leur végétation, leur régime de pluie, leur régime de transport des sédiments ou leur régime d’herbivorie. Les modèles complexes et l’exploration de scénarios futurs des environnements arides peuvent permettre d’améliorer la gestion de ces mêmes environnements. / Vegetated arid environments will be among one of the most affected by desertification as a result of climate change. These sparsely vegetated regions exhibit a delicate balance of resilience and vulnerability that are profoundly challenged by desertification, potentially producing an important positive feedback leading to increased aeolian activity and therefore land degradation. The high level of interaction between rainfall, vegetation, sediment transport and grazing in these arid environments and the non-linear nature of these interactions make them difficult to predict by traditional mathematical modeling mean. Complex modeling, on the other hand, offer better representation of the intricate relation between vegetation, rainfall, sediment transport and grazing in an arid environment system. A sizable amount of studies has been conducted with complex models to explore the effect of vegetation on sediment transport or grazing effect on vegetation, but few have used a truly integrative approach where all tree components were represented in a complex model. This research present a novel agent-based model (GrAM) integrated as an extension to already complete sediment transport-vegetation complex model (ViSTA) allowing a more refined representation of grazer’s impact in vegetated arid environments. This addition to the ViSTA model is aimed to combine a land management and systematic approach in a coupled model, to represent, at a landscape level, the impact of grazing on the composition of vegetation and sediment movement in arid environments. The development of this new module within the original ViSTA model, has highlighted some limitations of this model, most notably concerning its sensitivity to vegetation and wind shear. The ViSTA_GrAM model addresses these limitations through integrating a new module of grazing as the next step toward an integrated modelling effort that permits models to effectively increase our spatial and temporal understanding of arid environments vegetation, sediment transport and grazing dynamics. Integrative approach, like the one provided by the ViSTA_GrAM model, is beneficial to decision making by providing tools to investigate the response of an arid environment to different state of their vegetation, rainfall regime, wind stress and grazing regime. By developing complex modeling in arid environment and exploring various future scenarios for arid environment, we hope to lead to better management plan of those same environment.