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Cascading Density Currents from the Petit Lac to the Grand Lac of Lake Geneva

Titel Englisch Cascading Density Currents from the Petit Lac to the Grand Lac of Lake Geneva
Gesuchsteller/in Barry David Andrew
Nummer 159422
Förderungsinstrument Projektförderung (Abt. I-III)
Forschungseinrichtung Laboratoire de technologie écologique EPFL - ENAC - IIE - ECOL
Hochschule EPF Lausanne - EPFL
Hauptdisziplin Hydrologie, Limnologie, Glaziologie
Beginn/Ende 01.07.2016 - 30.06.2019
Bewilligter Betrag 305'620.00
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Alle Disziplinen (2)

Disziplin
Hydrologie, Limnologie, Glaziologie
Fluiddynamik

Keywords (7)

Gravity currents; Delft3D; Grand Lac; Petit Lac; Density currents; COSMO-2; Winter cooling

Lay Summary (Französisch)

Lead
De quelle manière le refroidissement hivernal influence-t-il de manière durable l’équilibre énergétique des grands lacs ?
Lay summary

En hiver, le refroidissement de la surface conduit à une augmentation de la densité de l’eau à la surface d’un lac. Loin des rives, cette situation conduit à un échange d’eau vertical, i.e. la surface d’eau dense refroidie uniformément glisse de manière descendante étant ainsi remplacée par une eau moins dense provenant des couches du lac plus profondes. Dans les parties peu profondes, plus proches des rives, la circulation de l’eau dense étant descendante, l’eau est contrainte de couler le long des pentes vers le fond du lac. Diverses campagnes de terrain ont mis en évidence que l’eau dense semble se déplacer sur des distances substantielles jusque dans les parties profondes d’un lac. 

Ce projet vise à investiguer la production d’eau froide des courants denses et son mouvement subséquent des rives aux fonds du lac Léman. Le lac Léman est un excellent site pour cette étude puisqu’il est représentatif des grands lacs d’eau douce. Les campagnes de terrain auront pour objectif d’explorer ce phénomène à la confluence des Petit et Grand Lacs du lac Léman, grâce à une palette de méthodes de mesures. Les données acquises par ces campagnes seront complémentées par des numérisations hydrodynamiques en 3D. Ce mécanisme de transport d’eau froide est un des éléments actuellement les moins compris dans la balance énergétique du lac. Ce projet fournira donc une meilleure compréhension de l’influence à long terme des courants d’eau froide sur la température du lac.

Direktlink auf Lay Summary Letzte Aktualisierung: 12.10.2015

Verantw. Gesuchsteller/in und weitere Gesuchstellende

Mitarbeitende

Name Institut

Verbundene Projekte

Nummer Titel Start Förderungsinstrument
123034 Water Dynamics and Contaminant Transfer Processes from the Shallow to the Deep Waters of a Mid-sized Lake 01.02.2009 ProDoc
146652 Oxygen depletion in a deep perialpine lake 01.07.2013 Projektförderung (Abt. I-III)
178866 Variability of Mesoscale Surface Energy Flux in a Large Lake 01.06.2018 Projektförderung (Abt. I-III)

Abstract

During cold weather, the surface water of a lake becomes denser than the underlying water, leading to vertical mixing. Dense water in the nearshore can reach the lake bottom, then travel downslope to deeper parts of the lake, until it reaches the neutral buoyancy depth. At the larger scale, a shallow shelf area, if rapidly cooled to a vertically uniform temperature, becomes denser than deeper parts of the lake, creating a horizontal temperature gradient driving offshore flow (this is a version of the “lock-exchange” problem). The literature on these issues is well established, both from theoretical and experimental perspectives. However, the fate of this dense water away from the shoreline is much less investigated. This project will address this question using a combined field measurement and numerical modelling approach, with Lake Geneva as the study site. Lake Geneva is particularly interesting in this context as it consists of two basins, the relatively shallow Petit Lac in the west, and the deep Grand Lac in the east. The Rhône River passes east to west through Lake Geneva, whereas the Petit Lac lakebed slopes in the opposite direction. Existing field data show the presence of a cold layer at the base of the Petit and Grand Lacs in cold periods, which thus moves under gravity against the mean flow in the lake. The project is designed to test the hypothesis that cold-water density currents propagate from the shallow littoral zones of the Petit Lac, travel downgradient (subject to deviation by Coriolis forcing) along the lakebed and enter the Grand Lac, possibly reaching the deepest parts of Lake Geneva (310 m).The existing field data consists of numerous CTD profiles taken during 1986-1992, and in 2005. The data are a valuable resource since they show dense water near the lakebed, and provide an excellent numerical modelling validation target (Delft3D will be the hydrodynamic model used). Flows in mid-sized lakes like Lake Geneva are driven by Coriolis and meteorological conditions, especially wind. Spatial and temporal variations in meteorological forcing for the whole of Lake Geneva have been available since 2007 (COSMO-2 model output from MeteoSuisse). Prior to that, shore-based meteorological data are available (again from MeteoSuisse), although this makes model validation a more difficult task. To provide direct field evidence of the project hypothesis, and improve the model, two types of field measurements will be undertaken. Over two winters, within-lake measurements will be taken at four locations in the vicinity of the Petit Lac-Grand Lac confluence. These measurements will be designed to capture near lakebed temperatures and currents, and will therefore identify occurrences of density currents. We will also deploy our boat and autonomous measurement platform on several occasions during these two winters. CTD profiles, surface temperatures and ADCP profiles will be measured. The CTD profiles will show the presence of any near-bed dense layer. ADCP measurements will provide current vectors in the water column. Data from these (synoptic) campaigns will be also be used for model validation.Besides providing evidence on the behaviour of cold-water density currents in a mid-sized lake, with the numerical model we can estimate the overall transport volumes to deeper parts of the lake, and ascertain the effect on the lake energy balance.Two collaborations are planned. Prof. em Ulrich Lemmin (EPFL) will bring significant field experience in data collection and analysis. Dr. Rob Uittenbogaard (Deltares, the “home” of Delft3D) brings expertise in environmental fluid mechanics, and the application of Delft3D to such problems.
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