Tracking Lagrangian transport in Lake Geneva: A 3D numerical modeling investigation

Lake Geneva, the largest freshwater lake in Western Europe, is subject to important environmental pressures from its densely populated shores and watershed. To maintain and improve water quality in this lake, as well as in other enclosed or semi‐enclosed basins, it is essential to understand and be able to predict how nutrients and pollutants are transported within it. A 3D numerical modeling study of Lagrangian transport in Lake Geneva is presented, showing the dispersion of water (based on tracking inert water particles) inflowing from the lake's main tributary, the Rhône River. The relation between dominant winds, circulation patterns, and transport was analyzed. The results demonstrated that transport within the lake is highly inhomogeneous in space and intermittent in time, because water mass movements are controlled by the wind‐induced formation of large‐scale gyres and their subsequent breakdown into smaller ones. Particle spreading was shown to be sensitive to the depth of the initial particle release, and to the mean depth of the particles’ trajectory. However, several preferential pathways could be identified. Some water particles rapidly (days) traveled across the entire lake, through the near‐shore region in the upper layer, while others remained trapped for months, particularly in the central region of the lake at depth. Deeper particles tended to remain longer in the lake, due to the insulating effect of stratification, bathymetry obstacles, and slower currents at greater depth.

Transport and mixing in Lac Léman

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin

Lake Geneva (Lac Léman) is the largest freshwater body in Western Europe. It is a deep peri-alpine lake whose importance stems from being not only an essential freshwater source in the region, but also a major tourist destination, a fishery and a waterway. Its dynamics have been the subject of long-term monitoring and study, and its response patterns to wind forcing (the major forcing) are relatively well understood (e.g. Lemmin et al. 2005). On the other hand, the large-scale organisation of the water circulation is less well known; the associated transport properties are even less clear. The interest in the transport of water parcels inside the lake is linked to the inflow of sediments and pollutants from the tributaries, in particular from the Rhône River, the major one in terms of water and sediment discharged into the lake. Most of the sediments entering the lake through the Rhône are believed to sink in the eastern part of the lake (Giovanoli, 1990). More recently, Halder et al. (2013) traced, using stable isotopes, water parcels from the Rhône River in the entire lake basin. This study demonstrates that the water entering the lake from its main tributary has a complex distribution inside the whole basin. How this distribution is established and evolves in time is, however, mostly unknown. The Ecological Engineering Laboratory of EPFL is trying to shed further light on this issue, by combining observational and numerical modelling tools. In particular, up to 6 Acoustic Doppler Current Profilers (ADCP’s) have been simultaneously deployed at various locations inside the lake. This data, together with available historical data, is being used to validate a hydrodynamic model of the lake, implemented using MITgcm code. Various passive-tracer release experiments were conducted using the numerical model, investigating the relative importance of wind-forcing, depth of release, stratification, and the Rhône discharge rate, for the spreading and mixing of the tracers. The preliminary numerical results confirm that the northern and southern coastal regions are preferred initial pathways for the transport of the Rhône discharge. More interestingly, the numerical simulations unmistakeably show that the transport of the Rhône River water inside the lake is highly inhomogeneous in space, and highly intermittent in time, even ignoring the discharge variability itself. This intermittency should be taken into account, in particular when interpreting point measurements, isolated in time. From a practical point of view, this is likely to have an important effect on the nutrient and oxygen availability, as well as on the concentration of pollutants. From a more fundamental point of view, this study contributes to further understanding the mixing processes in rotating, stratified flows at length scales where rotation is an important but not the only dynamic process (Rossby number small but non-zero).

2016

Wintertime Coastal Upwelling in Lake Geneva: An Efficient Transport Process for Deepwater Renewal in a Large, Deep Lake

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

Combining field measurements, 3‐D numerical modeling, and Lagrangian particle tracking, we investigated wind‐driven, Ekman‐type coastal upwelling during the weakly stratified winter period 2017/2018 in Lake Geneva, Western Europe's largest lake (max. depth 309 m). Strong alongshore wind stress, persistent for more than 7 days, led to tilting and surfacing of the thermocline (initial depth 75–100 m). Observed nearshore temperatures dropped by 1°C and remained low for 10 days, with the lowest temperatures corresponding to those of hypolimnetic waters originating from 200 m depth. Nearshore current measurements at 30 m depth revealed dominant alongshore currents in the entire water column (maximum current speed 25 cm s−1) with episodic upslope transport of cold hypolimnetic waters in the lowest 10 m mainly during the first 3 days. The observed upwelling dynamics were well reproduced by a 3‐D hydrodynamic model (RMSE 0.2°C), whose results indicated that upwelled waters spread over approximately 10% of the lake's main basin surface area. Model‐based Lagrangian particle tracking confirmed that upwelled waters originated from far below the thermocline, that is, >150 m depth, and descended back to around 150–200 m depth over a wide area after wind stress ceased. Observational and particle tracking results suggest that wintertime coastal upwelling, which can occur several times during winter, is an overlooked transport process that is less sensitive to the effects of global warming than convective cooling. It can provide an effective but complex 3‐D pathway for deepwater renewal in Lake Geneva, and other large, deep lakes with a sufficiently long wind fetch.

2020

Wind-driven interbasin exchange and hypolimnetic upwelling during wintertime in a large, deep lake (Lake Geneva)

David Andrew Barry, Ulrich Lemmin, Rafael Sebastian Reiss

Distinct sub-basins and large embayments are a ubiquitous feature of many lakes. Horizontal gradients in water quality between basins can result from a number of processes. For example, different seasonal mixing regimes between basins with different maximum depths can produce biochemical gradients between their hypolimnia. Consequently, interbasin exchange can be an important process with significant ecological consequences. Combining field observations, 3D hydrodynamic modeling, and model-based Lagrangian particle tracking, we investigated wind-driven interbasin exchange between the shallow Petit Lac (max. depth 75 m) and deep Grand Lac (max. depth 309 m) basins of Lake Geneva, Western Europe’s largest lake, during early winter. In addition to CTD casts conducted in the Petit Lac, several ADCP and thermistor chain moorings were deployed at the confluence between the two basins during the winter 2018/2019. Following a strong northeast-bound, along-axis wind event lasting from 7 to 10 December 2018, a two-layer flow pattern established at the confluence: epilimnetic water from the Petit Lac was pushed by the wind into the Grand Lac and was compensated for by a bottom inflow of deep hypolimnetic waters from the Grand Lac into the Petit Lac. Consequently, temperatures in the lower part of the water column gradually decreased at all moorings, with the lowest temperatures corresponding to values found at 180 m depth, as indicated by full-depth temperature profiles taken in November and December 2018. For approximately 3.5 days, deep Grand Lac water was continuously transported into the Petit Lac, with observed inflowing current velocities near the bottom exceeding 27 cm/s. Approximately 1.5 d after the wind subsided, the current patterns at the confluence reversed and the previously upwelled Grand Lac water was drained again from the Petit Lac in a bottom-hugging current with measured velocities reaching 19 cm/s. The current and temperature patterns at the confluence were well represented by a 3D hydrodynamic model (MITgcm). Model-based particle tracking confirmed the deep origin of the upwelled Grand Lac waters. Furthermore, it revealed that the interbasin upwelling event effectively formed a current loop, during which, over the course of more than one week, hypolimnetic water from below 150 m depth first upwelled into the Petit Lac, intruding approximately 10 km into the shallow basin, and subsequently descended back into the Grand Lac hypolimnion. Moreover, low model-based gradient Richardson numbers and temperature inversions observed in the CTD profiles indicate turbulent mixing between the deep, upwelled Grand Lac waters and the “fresher,” i.e., better quality Petit Lac waters. Our field observations and modeling results show that enhanced wind-driven interbasin exchange and deep hypolimnetic upwelling between the shallow Petit Lac and deep Grand Lac basins of Lake Geneva frequently occur during early winter. Furthermore, our results suggest that these hypolimnetic interbasin upwelling events may present a potentially important mechanism for hypolimnetic-epilimnetic exchange and deep-water renewal in Lake Geneva and possibly in other deep multi-basin lakes under similar wind conditions; especially, when considering the expected weakening of the classical deep convective cooling during wintertime due to climate change effects.