Modélisation de la dynamique océanique barotrope dans l'estuaire et le plateau amazoniens

The vast majority of material flows (solid and dissolved, organic and inorganic) coming from continent into ocean happens in continental margin, as it is the interface between continents and oceans. Continental margin also exchange materials and elements with ocean water. One of the main sources of chemical elements in the ocean is erosion and transport by rivers, material from continent to ocean being promoted by re-suspension of sediments and important water mixtures. The AMANDES project aims to study transports from the Andean mountains to the Atlantic Ocean through the Amazon system. Three complementary disciplines - continental and marine geochemistry, physical oceanography, and hydrodynamic modeling (including data assimilation) -- are combined to achieve these goals. None of the available models of this area can be used in AMANDES project. Therefore, the purpose of the thesis has been to establish a new model, with which it will be possible to explore in depth the problem of transport, i.e. the moving of the whole water column and what it contains, such as sediments. However, establishing a new hydrodynamical model in order to study the transport of materials by the Amazon River in the Atlantic Ocean was a subject too ambitious for just one thesis. As a consequence, we have established the foundations for a model to be extended thereafter. In order to meet its modelling needs, the AMANDES project has chosen the unstructured grids model T-UGOm. Unstructured grids are very flexible, and thus can be closely adapted to the specific geographical area being modeled. In the case of the Amazon estuary, where the geography can be complex, this characteristic is a critical advantage. To apply T-UGOm to the case of the Amazon river, we have clarify some numerical schemes. Also, as the estuary is an area of strong tides, large shallow waters, and subject to strong currents, bottom friction has a major influence. As a consequence, we have made a major effort to refine the inclusion of friction in this model. Moreover, shallow waters and complex geography induce a strong influence of coastlines and bathymetry, so a significant effort has been dedicated to establish a digital coastline and bathymetry database as fine and accurate as possible. As tide is the hydrodynamic phenomenon of greatest amplitude in the area, we focused on its modelling. Comparing our best solution with in situ and satellite data, the developments mentioned above have significantly improved the tidal modeling. Also, this solution is closer to the collected data than those already published. This opens the way for a more complete hydrodynamic modeling, especially transport.À la frontière entre les continents et les océans, les marges continentales sont le siège de la grande majorité des apports de matière - dissoutes et solides, organiques et inorganiques - d'origine continentale vers l'océan. Elles sont aussi directement en contact avec les masses d'eau océaniques qui échangent de la matière et des éléments avec ces marges. L'érosion et le transport par les rivières étant une des sources essentielles des éléments chimiques à l'océan, la remise en suspension de matériel sédimenté et les forts mélanges d'eau favorisent les transferts de matière du continent vers l'océan. Le projet Amandes a pour objectif l'étude de ces échanges entre continents et océans, en étudiant le cas particulier du transport d'éléments en provenance des montagnes andines vers l'océan Atlantique par le système amazonien. Trois disciplines complémentaires - géochimie continentale et marine, océanographie physique et modélisation hydrodynamique (incluant l'assimilation de données) - sont associées pour atteindre ces objectifs. Parmi les modèles de la zone existants, aucun ne répond aux besoins du projet Amandes. Cette thèse a consisté à établir un nouveau modèle, qui permettra d'explorer en profondeur le problème du transport, c'est-à-dire le déplacement de toute la colonne d'eau et ce qu'elle contient, par exemple des sédiments. Cependant, établir un nouveau modèle hydrodynamique dans le but d'étudier le transport de matériaux par le fleuve Amazone dans l'océan Atlantique était un sujet trop ambitieux pour une seule thèse. En conséquence, nous nous sommes attachés à établir les bases d'un modèle qui sera étendu par la suite. Pour répondre à ses besoins de modélisation, le projet Amandes a opté pour le modèle à grilles non-structurées T-UGOm. L'avantage des grilles non-structurées est leur grande souplesse, qui leur permet de s'adapter finement aux spécificités géographiques de la zone modélisée. Dans le cas de l'estuaire de l'Amazone, où la géographie peut être complexe, cette particularité est un avantage critique. Pour appliquer T-UGOm au cas de l'Amazone, il a fallu préciser certains schémas numériques. Également, l'estuaire étant une zone de fortes marées, avec des fonds de faibles profondeurs sur de grandes étendues et soumise à de forts courants, le frottement de fond a une influence primordiale. Nous avons donc apporté un effort important pour affiner la prise en compte de ce frottement par le modèle. D'autre part, tant la faiblesse des fonds marin que la complexité de la géographie induisent également une forte influence du trait de côtes et de la bathymétrie. En conséquence, une part importante des efforts a été dédiée à établir un trait de côtes et une bathymétrie de la zone aussi précis que possible. La marée étant le phénomène hydrodynamique de plus forte amplitude dans la zone, nous nous sommes focalisés sur sa modélisation. Les développements évoqués précédemment ont permis, lorsque l'on compare notre meilleure solution de marée avec les données in situ et satellites, d'améliorer sensiblement la modélisation de la marée. En comparant notre meilleure solution avec celles déjà publiées, celle-ci est plus proche des données collectées. Ceci ouvre la voie à une modélisation hydrodynamique plus complète, en particulier du transport

Modélisation de la dynamique océanique barotrope dans l'estuaire et le plateau amazoniens

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

Unstructured-mesh Modelling of the Hydrodynamics of the Congo River Estuary and Adjacent Coastal Zone

With the second largest river outflow and one of the widest hydrological basins in the world, the Congo River is of major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other major rivers such as the Amazon, Nile, Yangtze or Mississippi. The Congo river-estuary-coastal sea continuum was modelled by means of the unstructured-mesh, finite-element model slim (sites.uclouvain.be/slim). Two-dimensional discontinuous finite-element methods were used, which are similar to those already implemented in the Scheldt (Belgium, The Netherlands) and Mahakam (Indonesia) land-sea continua. In any modelling work, scarcity of data is an issue. But, as the Congo River and Region of freshwater influence is a very remote area, collecting data is even more challenging than usual. For instance, some area are simply unobserved. While applying slim to the Congo River and adjacent coastal zone, we developed tools and procedures to take advantage of as many sources of information as possible. These tools can use any kind of image, georeference it, and then exploit any information it contains. Then, the extracted data can easily be merged with other sources. This model being now up and running, we have made the best use of every remaining data to validate it. The most abundant source of information in this area is satellite tidal analysis. Therefore, the main validation we made is comparing runs of our model with these satellite data and some tidal gauges measurements. As the model results were found to be in fairly good agreement with the available measurements, the hydrodnamics was investigated by computing the age of the water, that is the time elapsed since leaving the upstream, riverine boundary of the domain. The velocity field exhibits quasi-permanent gyres in the estuary and the coastal zone, the origin of which is still unclear. A sensitivity analysis suggests that these gyres, whose size is about half the width of the estuary (i.e. a few kilometers), could be due to the peculiar bathymetry of the navigation channel that has been made a few decades ago

The AMANDES tidal model for the Amazon estuary and shelf

International audienc

Setting up and validating a 2D model of the Congo River estuary and the adjacent coastal zone

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Preliminary results of an unstructured mesh model of the Congo River, estuary and ROFI

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2D and 3D modelling of the Congo River and Region Of Freshwater Influence (ROFI) with SLIM

With the second largest river outflow in the world and one of the widest hydrological basins in the world, the Congo River is of major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other great rivers such as Amazon, Nile, Yangtze or Mississippi rivers. e Congo River estuary contains important natural resources, like oil and gas, whose extraction could benefit from ocean currents simulations. As it is also the location of several vulnerable ecosystems, such as the Mangroves National Park, these simulations can also be a useful tool to ensure these areas will not be impacted by the exploitation of the up-cited resources. To model this area, we are using the hydrodynamical model SLIM ( http://www.climate.be/slim/ ), a finite elements model using unstructured grid. ere are many challenges to accurately model such a complex area. ese include the river bathymetry that alternates very shallow (less than 5 m) and very deep (more than 500 m) areas. e river fan is the only river fan of this importance still active nowadays. ese constraints justify an approach combining 2D and 3D high-resolution modelling. Achieving this implied to take great care in creating a numerical code which is flexible and modular as well as efficient in term of computation time. While 2D unstructured mesh ocean models are now quite mature, unstructured-mesh 3D models are still in their infancy. Many challenges remain, such as finding a trade-off between solution accuracy, robustness and computation time, or σ -layer versus z-layer modelling. Moreover, the Congo river being located in a very remote region,datasets are sparse and setting boundary conditions or validating the model is thus particularly challenging. We have developed several tools to help us to deal with these data. In this talk, we will present the technical issues, either dealing with data or numerical issues, we had to tackle with to set both the 2D and the 3D models. Solutions are validated with respect to tidal gauges and satellite altimetry data. We also simulate the water age evolution under various hydrodynamical regimes. All these developments show the relevancy of this approach and open the way for modelling including more phenomenon