/ Vidy Bay hydrodynamics under different meteorological conditions EDCE 2011 Auteur(e)s A. M. Razmi1 Encadrement Prof. D.A. Barry1 / Prof. U. Lemmin 2 1 Ecological Engineering Laboratory (ECOL) / 2 Environmental Hydraulics Laboratory (LHE) Introduction & Objectives Results & Discussion Numerical validation. Simulations are agreed well with modelling (Figure 5) Results demonstrates the high temporal and spatial variability of the currents in Vidy Bay. This variability is observed in the drifter experiments (Figure 6) Vidy Bay is located on the northern shore of Lake Geneva. There is a sewage outfall in the bay and a drinking water intake at St. Sulpice, to the west. As has been found in many places, wastewater entering the lake contains low levels of pharmaceutical residues. The wastewater plume is highly diluted in the bay by dispersion/diffusion, but its motion under different meteorological conditions is uncertain. Ultimately, water-current patterns and mixing processes, especially due to wind forcing, determine the mixing dynamics in the water column. The objective of this work is to quantify the bay hydrodynamics under the highly variable wind conditions found on Lake Geneva Materials & Methods Experiments with Five Instrumented Drifters Lagrangian campaign: Trace the flow features such as vortices in a wide range of spatial and temporal scales Instruments are equipped with on-board GPS and different sensors (air and water temperature, humidity, wind speed) a) b) Figure 5. Lagrangian campaign results (19 January 2011) The drifters started above WTP outfall (X). Drifter tracks over 7h in Vidy Bay Numerical current field in the near-surface Simulated drogue pattern from the WTP outfall c) Figure 1. Lagrangian drifter study Numerical Modelling A commercial 3D finite-difference hydrodynamic model (Delft3D-FLOW) was used. It solves the Reynolds-Averaged Navier-Stokes equations with a k-ε turbulence-closure model. The simulations covered all of Lake Geneva, and produces non-steady flow and transport phenomena Local stream and River Rhone inflows, detailed, spatially distributed lake-wide wind, temperature and humidity data were used as model boundary conditions a) b) Figure 6. Panels (a), (b) are the simulated current vectors for 10:00-16:00, 14 January 2011 Panel (c) shows drifter tracks, for drogues that were released above the outfall at two different times. The results show that the current directions change on the time scale of several hours c) Typical current response (Figure 7, numerical simulations) of the Vidy Bay currents to the two strong wind events. (a) and (b): from Southwest (Vent) and (c) and (d) Northeast (Bise). a) b) Figure 2. a) Overall grid and bathymetry-Delft3D and b) Nested numerical weather prediction model of the COSMO-2 system (MeteoSuisse) Data analysis Previous ADCP velocity profiles over the entire the water column at the location of the Vidy Bay outfall (data collected by Prof. U. Lemmin, 2005) were analysed. Effects of strong winds (from Northeast and Southwest) and stratification are noticeable in Figure 3 and Figure 4, respectively a) b) Figure 3. Comparison with currents (b) and wind events (a) above the Vidy Bay outfall (location; X in b) in November 2005. In plot b), the X, Y axes are the East and North directions and numbers are days in November. a) Wind vectors from St. Prex, b) Progressive vectors for 5-m depth, giving an indication of water movement from the outfall c) d) Conclusions Numerical and experimental studies as well as data analysis were carried out to investigate currents in Vidy Bay. Different patterns were found depending on the wind regimes and lake thermal structure The results demonstrate that Vidy Bay displays high temporal and spatial variability due to the variable forcing and relatively rapid hydrodynamic response to changing conditions Water motion in upper layer and wind direction follows Ekman Spiral (due to wind stress, friction and Coriolis force) Thermocline separates two body of water mass in the stratified seasons Maximum displacements of water mass take place in the strong events of “Bise” or “Vent” in Vidy Bay a) b) Figure 4. Water movement under stratified (b) and unstratified (a) conditions at different depths (3-19 m) above the Vidy Bay outfall. (a) December 2005 (late autumn, unstratified) (b) September 2005 (early autumn, stratified). Two different directions of water movement are evident