Structure thermique et courantologie du Léman (Thermal structure and circulation patterns of Lake Geneva)

Hydrodynamic processes in Lake Geneva (Switzerland and France) have been studied since the pioneering work of F.A. FOREL who initiated the science of limnology as a branch of oceanography. Lake Geneva is characterized by a thermal stratification of the water column from spring to autumn and a homogenization from the end of winter to the beginning of spring. Lake Geneva is a « monomictic » lake according to theclassification of Hutchison. To obtain a better understanding of the hydrodynamics of Lake Geneva, a three-dimensional finite element model with the ability to simulate the temperature structure and water movement was applied. The finite element grid superimposed on the bathymetric data has a resolution of 8149 triangular elements different size ranging from 10 to 100 meter each side and 4553 nodes, covering the two lake basins : Petit-Lac and Grand-Lac. The model was run for a period of one year (2005) with continuously updated weather data. The model solves the equations for the conservation of mass transport, dynamics and temperature, which can simulate the water temperature, current velocity and direction. The annual thermal cycle begins with isothermal conditions in winter, followed by thorough mixing in late winter, and by stratification from May to October. The annual cycle of temperature distribution in the Grand-Lac shows strong seasonal trends. Depending on the difference in volume and morphology of the basin, cooling and warming during the transition periods (spring and fall) are slower in the Grand-Lac than in the Petit-Lac. Over much of the year, the waters of Grand-Lac circulate in a gyre turning counterclockwise. The size and lifetime of the vortex varies depending on weather conditions. Appendices of the main gyre are found in major bays of the northern shore, in Morges and Vidy (Lausanne). These are less stable than the main gyre and their direction may switch depending on wind direction. In the transitional area between the Grand-Lac and the Petit-Lac a gyre oriented clockwise is established. The western end of the Petit-Lac is characterized by a counterclockwise circulation system. In the Petit-Lac, flow towards the Rhone outlet mainly occurs in surface waters and along the borders of the basin, while the return currents to the Grand-Lac are deeper-water currents and found in the central part of the basin. In the Grand-Lac, the downstream flow from East to West follows the northern edge of the basin, while the return current, from the western to the eastern part of the basin, predominantlyfollows the southern border. Results of modelling are generally in good agreement with field data ; however systematic current surveys in the different parts of the lake basin are still missing for the model validation