Enhanced mixing in narrows: A case study at the Mainau sill (Lake Constance)

Abstract:

Previous work has identified bottom currents as a significant source of turbulence in stratified lakes. Sills may therefore be a major factor determining overall turbulent diapycnal (vertical) exchange in lakes with multi-basin hypolimnia.¶In order to investigate the contribution of the Mainau sill (separating Upper Lake Constance from Lake Überlingen) to the overall diapycnal mixing in Lake Constance, a series of temperature microstructure profiles was taken at Mainau Island in October 1993. From these profiles, using Batchelor's method, the rate of dissipation of turbulent kinetic energy was determined and related in an energy balance to the turbulent kinetic energy input from the wind, the energy content of internal seiches and the energy dissipation of bottom currents at the sill. Further, the vertical diffusivities were calculated using the dissipation method.¶The analysis shows that ≈ 5.5% of the wind energy flux was found in to the water column (below 2 m depth) and that energy dissipation was 8 times higher in the shear zone of the thermocline (≈ 8.4 ˙ 10^-3 W m^-2) than in the bottom boundary layer (≈ 1.1 ˙ 10^-3 W m^-2). Dissipation above the Mainau sill (≈ 9.5 ˙ 10^-3 W m^-2) exceeded the basin-wide average dissipation of internal seiche energy (≈ 0.3 ˙ 10^-3 W m^-2) by a factor of 34. However, since the areal extent of the sill is small (≈ 1% of the lake area), the sill contributes only about 40% to the basin-wide dissipation. Also vertical diffusivities within the thermocline were consistently enhanced by approximately the same amount over the sill. The synthesis of the observations implies that the sill plays a disproportionately large, but not dominant, role for small-scale diapycnal mixing in the hypolimnion of Lake Constance.