Measurements and three-dimensional simulations of flow in a shallow reservoir subject to small-scale wind field inhomogeneities induced by sheltering

Spremberg Reservoir is a small and shallow water body where wind sheltering along steep and densely vegetated sections of the shoreline is expected to produce small-scale inhomogeneities of the wind field across the water surface. The potential impacts of these inhomogeneities on the quasi-steady circulation in the main subbasin of the reservoir are investigated in this study by means of a three-dimensional hydrodynamic model. ADCP measurement profiles at three locations are available for comparison. Agreement between characteristic features of the measurements and the simulation data cannot be achieved if uniform winds are applied as boundary conditions, but improves distinctly when sheltering effects are implemented. This is done qualitatively by means of analogy to the separation of flow across a backward facing step. For south-westerly winds, the modification of the flow tends from a three-dimensional to an essentially horizontal, one-cell circulation. The action of the wind sheltering on the flow is found to rely on (1) the addition of a curl to the background wind field and (2) the weakening of the topological moment along the windward shore. In Spremberg Reservoir the first mechanism appears to prevail for south-westerly winds. The study underlines the need for consideration of small-scale inhomogeneities of the wind field induced by sheltering as a potential source of vorticity in hydrodynamic simulations. Further investigations should be directed towards a physically founded parameterization of the actual wind stress distribution and towards quantifying lake dimensions at which such influences on the circulation pattern tend to be negligible.