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Institut für Wasser- und Umweltsystemmodellierung - IWS



"Characterisation of tracer distribution in a lake using spatial moments "

In this study tracer transport and mixing in Lake Überlingen will be examined, using spatial moments. The tracer’s concentration was simulated using the three-dimensional numerical model ELCOM. Flow was driven by a uniform steady wind field. In most simulations, the wind velocity was 3 m/s and came from the NE, but a number of different simulations had different wind directions and velocities. Tracers were released in an initially stratified lake at different locations near the surface and in deeper waters. The concentration distributions were described qualitatively, but also quantitatively using spatial moments to characterise the motion of the centre of mass and the size of the tracer cloud over time. The centres of mass followed the local flow pattern induced by the wind. Mixing and transport depended on the tracer release locations as well as on wind conditions. There were two main regions of mixing in Lake Überlingen, separated by the transition zone betw! een Lake Überlingen and the main part of Lake Constance. Mixing in the western part of the lake remained limited to the western part of the lake, with little transport across the transition zone to the main Lake Constance. Mixing in the transition zone resulted in tracer transport towards the main lake, and corresponding larger tracer cloud sizes. Mixing over the sill of Mainau resulted in separate concentration regions, depending on the wind direction.