Vidy Bay hydrodynamics under typical meteorological conditions

Vidy Bay (Lake Geneva, Switzerland) is the site of a sewage outfall, located 700 meters offshore at a depth of 30 meters. Estimates of water circulation in this area are required to assess the transport and fate of water quality components. Numerical simulations and field measurements were carried out to examine current patterns in this area. A commercial 3D finite-difference hydrodynamic model (Delft3D-FLOW) was used. The simulations, which incorporated a non-uniform grid system, sigma coordinates and high-resolution bathymetry, covered all of Lake Geneva. In addition to local stream and River Rhone inflows, detailed, spatially distributed lake-wide wind, temperature and humidity data were used as model boundary conditions. Several scenarios based on typical meteorological conditions were simulated to determine the water currents in the lake and, particularly, in Vidy Bay. Different patterns were found depending on the wind regimes and lake thermal structure. A Lagran-gian drifter experiment was conducted in the bay to capture current patterns and to measure local air and water temperature, as well as wind velocity. The modeling results and field data compared well. The results demonstrate that Vidy Bay displays high temporal and spatial variability due to the vari-able forcing and relatively rapid hydrodynamic response to changing conditions.

Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold water density currents on the lateral slopes caused by differential cooling between the shallow literal zone and the deep off-shore region, and upwelling of deep water masses over the lateral slopes which results from wind forcing and may be influenced by the Coriolis force. Previous field experiments in Lake Geneva, the largest freshwater lake in Western Europe (max. depth 310 m), have shown that cold-water density currents are a common phenomenon during wintertime, frequently reaching the thermocline at O(100m) depth. As part of a project investigating winter mixing dynamics in Lake Geneva, we examine wind-driven, Coriolis-influenced upwelling in order to better understand its role in the deep-water exchange during the weakly stratified period (December-March). The study is based on field observations taken on the northern shore of the lake, 20 km west of Lausanne in the winters of 2017-19. Near-bottom water temperatures are inferred from a fiber-optic based Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1m) and O(0.1°C), respectively. Temperature and current velocities in the water column are recorded by several ADCP and thermistor chain moorings. The data set is completed by regular CTD campaigns following wind events favoring upwelling. These field measurements were combined with a validated 3D hydrodynamic model (MITgcm) in order to analyze and quantify the exchange between the littoral zones and deeper parts of the lake. It was observed that following medium to strong northeast-bound wind events (i.e. winds parallel to the main axis of the central lake) lasting for at least one day, near-shore temperatures at all instruments suddenly dropped to the same temperatures as those in the deep open waters and remained low for several days. This indicates that, triggered by persistent wind-stress and under the influence of the Coriolis force, hypolimnetic water reached the surface in the near shore zone, where it is subject to heat and gas exchange with the atmosphere, before returning to the deep layers. Numerical modeling of these events showed comparable results and allowed to trace the origin of this cold water to the deep layers. Our field observations and the numerical results suggest that in addition to the previously observed cold-water density currents, wind-driven upwelling is a common phenomenon in Lake Geneva during the weakly stratified winter period and potentially a significant mechanism for deep-water renewal.

2019

Measurement and modeling of Vidy Bay (Lake Geneva) hydrodynamics

David Andrew Barry, Ulrich Lemmin, Amir Mehdi Razmi

Vidy Bay is located near Lausanne on the northern shore of Lake Geneva in an area where sewage outfalls and drinking water intakes are positioned. Local circulation patterns may control water quality aspects in this part of lake. Currents are mainly influenced by variable wind regimes and the lake’s thermal structure. Field experiments and numerical simulations were carried out to quantify the circulation patterns in this part of the lake. A set of Lagrangian drifters was used to capture current patterns and to measure air and water temperature and wind velocity in Vidy Bay. Current measurements were compared with simulations from a 3D hydrodynamic model (Delft3D-FLOW), covering all of Lake Geneva. Higher resolutions of bathymetry and grid mesh size were used for the Vidy Bay. Meteorological data for the model’s surface boundary conditions were provided by the high-resolution (2.2 km gridded) MeteoSwiss COSMO-2 model. Modeled currents compared well with the field data. The validated model was used to identify typical current patterns in the bay. Consistent with previous observations of the bay’s currents, the modeling results showed that Vidy currents are highly variable both spatially and temporally, although current patterns were found to be follow the forcing by the different wind regimes across the whole lake.

2011

Current variability in a wide and open lacustrine embayment in Lake Geneva (Switzerland)

Roham Bakhtyar, David Andrew Barry, Nicolas Le Dantec, Ulrich Lemmin, Amir Mehdi Razmi, Alfred Johny Wüest

Field measurements and numerical simulations were used to determine the effects of dominant meteorological conditions on the hydrodynamics of a wide (aspect ratio ~2), relatively deep (seasonally stratified) and open lake embayment (Vidy Bay, Lake Geneva). A three-dimensional hydrodynamic model (Delft3D-FLOW) was employed to simulate flow in the lake. High-resolution maps of wind, temperature and humidity (over the lake) were applied as input to drive the model. Because wind was the main force driving flow in the lake, currents in the embayment were investigated systematically for different wind conditions and seasonal stratification. Satisfactory model validation was achieved using drifter and moored measurements within the embayment. Markedly different circulation patterns were measured within the embayment, with the transition from one pattern to another occurring abruptly for small changes in wind direction. These distinct patterns resulted from relatively small changes in the large gyre of Lake Geneva’s main basin, especially the angle between the current in front of the embayment and the embayment shoreline. The boundary between the embayment and the pelagic zone was defined by the largest gyre within the embayment. This study shows that, (i) in a large lake, complex current patterns can occur even within a minor embayment, and (ii) that these patterns can transition rapidly over a small range of wind directions. Near-shore gyre can occur for lengthy periods, which has implications for flushing of discharges within the embayment.

Elsevier Sci Ltd2013

Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

2019