Modelling flows in shallow (fluvial) lakes with prevailing circulations in the horizontal plane: limits of 2D compared to 3D models

Abstract

The numerical modelling of circulations in shallow lakes is a relevant tool for all environmental applications in which flow advection processes are of interest, e.g. for studies on nutrients, microorganisms, pollutants and sediment dynamics. While three-dimensional (3D) models are needed to properly describe the flow fields of basins with the main circulations in the vertical plane, two-dimensional (2D) models are commonly deemed to yield adequate results for lakes with prevailing horizontal circulations. However, the depth-averaged approximation is more limiting for wind-driven flows than for gravity-driven ones, such as rivers, as the driving force is a surface rather than a volume one, distributed along the depth through turbulence. In this work, the effects of such inaccuracy on the reproduction of circulation layouts are evaluated through compared simulations between a 2D Shallow Water solver and a 3D Reynolds-Averaged Navier-Stokes one. The models are first applied to a simple enclosed elliptical test basin and then to the real case of the Superior Lake of Mantua, a shallow fluvial lake in Northern Italy, thereby also investigating the influences of the interaction of wind with a riverine current and of a complex bathymetry on the compared results.