Circulation patterns in open, wide and deep lacustrine embayments with different aspect ratios

David Andrew Barry, Ulrich Lemmin, Amir Mehdi Razmi
2013
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Résumé

Numerical simulations were carried out to examine the effect of different horizontal aspect ratios on wind-induced circulation within open, wide and deep lacustrine embayments. Two adjacent embayments in Lake Geneva, Switzerland, located between Morges and Lausanne on the north shore of the lake, were compared. These embayments were selected because of their similar exposure with respect to wind forcing. However, their aspect ratios are different, making them ideal case studies to evaluate the impact of the aspect ratio on the resulting circulation pattern. A previously validated 3D hydrodynamic model (Delft3D-FLOW) was employed to simulate currents in the lake for the year 2010. Detailed over-lake maps of wind, temperature and humidity were used as input to drive the model. The embayments were compared in terms of circulation patterns and conditions leading to gyre formation. Furthermore, we carried out a systematic comparison, focusing on the embayment aspect ratio by constructing different “synthetic” embayments with aspect ratios in the range 1 - 4. Subsequently, the flow field within these embayments was computed for typical meteorological regimes (dominant wind regimes and seasonality). In particular, the generation of gyres was compared for different cases. The results revealed that, even for large aspect ratios (~ 3), wind-induced circulation can still occur in open, wide, and deep lacustrine embayments. The results showed that, as expected, gyres are formed less frequently as the embayment aspect ratio increases. For this reason, gyres are much more probable in the Morges embayment than in Vidy Bay.

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https://infoscience.epfl.ch/record/185943?ln=fr

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David Andrew Barry, Ulrich Lemmin, Amir Mehdi Razmi
2013
Discussion par affiche

Résumé

Official source

https://infoscience.epfl.ch/record/185943?ln=fr

À propos de ce résultat

Concepts associés (6)

Concepts associés

Chargement

Publications associées (5)

Publications associées

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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

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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

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Numerical simulations were carried out to examine gyre formation within open, wide lacustrine embayments. The present study was motivated by observed differences in gyre formation within two open and wide embayments (located at Vidy and Morges in Lake Geneva, Switzerland). These two embayments are located within about 3 km of each other on the northern shore of Lake Geneva, and are subjected to similar pelagic currents. Vidy is deeper and has a greater aspect ratio than Morges. The flow field in the embayments was modeled using a previously validated 3D hydrodynamic model (Delft3D-FLOW). The model solved the Reynolds-Averaged Navier-Stokes equations, combined with a k-ε turbulence closure in σ (lakebed-following) coordinates. Our study focused on the influence of the embayment geometry on the (uniform) longshore (pelagic) current, specifically the occurrence and magnitude of circulation within the embayment. We built a set of numerical experiments using synthetic embayments, and systematically examined embayment geometry, thereby capturing the differences between the Vidy and Morges embayments. The numerical experiments considered single rectilinear embayments with different aspect ratios (i.e., 1-6), depth, shore-parallel flow rates, and embayment corner angle between 0°-50°. The circulation magnitude changes abruptly for an angle of about 40°. Embayments with angles greater than 40° have much greater circulation then those with lesser angles, other factors remaining the same. Of the factors considered (i.e., aspect ratio, offshore current velocity, corner angle, bottom slope, and viscosity), bottom slope and the viscosity have almost no impact on embayment circulation. For uniform offshore current patterns, gyres form in embayments with large aspect ratios (up to ~3). For the Vidy and Morges embayments, the results showed that gyre formation is more likely in Morges due to its smaller aspect ratio, a finding that is supported by field data gathered in drifter studies. For example, simultaneous drifter releases in 2011 showed parallel-to-shore currents in the Vidy embayment and a gyre in Morges.

2013

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2013