A new approach for studying small scale turbulence in the thermocline region of Lake of Geneva

The aim of this PhD thesis is to contribute to a better understanding of turbulent mixing in lakes. This research is focused on the response dynamics of the stratified Lake of Geneva to strong wind forcing. We especially investigated the dynamics of small scale turbulence in the thermocline layer during such events. A preliminary study of the large scale dynamics in the stratified Lake of Geneva was first carried out. Therefore, some characteristics of internal Kelvin waves are described in detail, and strong shear across the thermocline induced by the passage of these waves is highlighted. Furthermore, this large scale study significantly helped to improve the design of a new platform called "Diver" for taking turbulence measurements developed at the Environmental Hydraulic Laboratory (EPFL-LHE). The Diver is displaced in the vertical by a bottom-resting winch and is connected to the shore by a long cable which provides real time access to the data. Probably the greatest advantage of our measuring system was its ability to follow the time evolution of a two-meter thick layer with a resolution of turbulence scales. To succeed in taking accurate high resolution measurements with this innovative collection of sensors was an exciting challenge and various results are presented throughout this dissertation. Our inexpensive moored platform provides an interesting alternative device for the scientific community working on turbulence in the coastal zone. Various questions have been addressed in this dissertation. Each time, our goal was to link small scale processes to large scale dynamics. Therefore, we tried to explain in detail the following points: What are the typical profiles of dissipation of turbulent kinetic energy and of vertical eddy diffusivity for the stratified Lake of Geneva? Since this question has not been addressed before, these results may be useful for the broad scientific community (biology, physical limnology and oceanography). What is the level of stability of the thermocline under strong external forcing conditions? The occurrence of instabilities away from boundaries has rarely been documented in lakes. Can we investigate turbulent patches? The occurrence of Kelvin-Helmholtz instabilities was investigated with this measurement platform by following the time evolution of a turbulent event, thereby allowing to estimate the efficiency of mixing by turbulent overturns. Time evolution profiles of dissipation, diffusivity and mixing efficiency were also carried out through a length scale analysis. In so doing, we tried to provide simple criteria to investigate turbulent events.