Caractérisation des composantes hydrodynamiques d'un système côtier semi-fermé : Le Bassin de Marennes-Oléron

Introduction and objectives of the study: Understanding the operating modes of the coastline systems requires the study of the hydrodynamics responsible for transporting the whole of the geobiological compartments of which they are composed. The idea of maintaining a close connexion between the physics, biology, geology and chemistry of the oceans is not new, and was for that matter the subject of a note in the first report of the International Council for the Study of the Oceans (ICSS) in 1902 (Sverdrup et al., 1954): ... it was seen from the beginning that the study of the physical conditions, of the chemical nature of the ocean waters, of the currents, etc., was of the greatest importance for the investigation of the problems connected with life, that on the other hand, the study of the floating organisms had particular worth for the solution of hydrographic problems, and consequently that sharp line should never be drawn between these two main divisions ... Since then, computing capabilities have continued to increase and it is currently possible to simulate the hydrodynamics of the coastal zones on a spatiotemporal scale compatible with biogeochemical processes. Nonetheless, refinement of modern models creates, and particularly for lagoon or estuarine coastal systems subject to the rocking of the tides, a huge complexity for analysing that, very often, makes it difficult to see the system in its entirety. Furthermore, the great structural diversity of the models developed does not facilitate inter-system comparisons, the latter being possible only by using common synoptic magnitudes. These are probably some of the reasons for which, for some thirty years now, new tools and concepts have appeared that make it possible to understand in a more summary way the hydrodynamics of coastal systems by calculating Lagrangian1 or Eulerian2 magnitudes, derived from hydrodynamic models, such as: Renewal time, transit time, residence time, age of water masses, etc. The following, for example, are cited: Zimmerman (1976), Takeoka (1984), Oliviera & Baptista (1997), Brooks et al. (1999), Delhez et al. (1999), Deleersnijder et al. (2001), Delhez & Deleersnijder (2002), Monsen et al. (2002), Delhez et al. (2004), Shen & Haas (2004). The final objective of this study is to describe the hydrodynamic components of two mesotidal lagoons of the Atlantic coastline, the Marennes-Oléron and the Arcachon basins, by calculating characteristic magnitudes such as residual fluxes, tidal prism, renewal time and age of the water masses, considered as measurements of the remanence of the water masses inside the defined spatial limits. The same calculation code, MARS-2D, developed on the two sites, was used to characterise the hydrodynamics and the above-mentioned magnitude calculation. The same computing methods were used so as to be able to compare the two sites and attempt to respond to the following questions: What type of circulation do the water masses have, instantaneous or residual? What are the water's renewal times? How do the tide and the wind act on these variables? What is the influence of the rivers and how do they disperse the plumes that come from them?