Shear-induced convective mixing in bottom boundary layers on slopes

Field data from a stratified lake demonstrate that buoyancy-driven convective mixing can be an important mech-anism for the formation of well-mixed bottom boundary layers (BBLs) on slopes. Convective turbulence is caused by differential transport of stratified water masses along the slope of a basin. Because the current velocity usually decreases toward the sediment, water at some distance from the bottom is transported faster than is water below. During upslope flow, this process leads to transport of heavier water on top of lighter water and hence to unstable stratification within the BBLs. Analogously, strong BBL stratification occurs during downslope flow. High-frequency acoustic Doppler current profiler (ADCP) and temperature measurements in the BBLs of a lake revealed the cyclic occurrence of convective turbulence driven by periodic across-slope currents of internal seiching. The estimated maximum buoyancy flux within the BBLs was in good agreement with the highest observed dissipation rates of turbulent kinetic energy. This suggests that, in our specific case, classical bottom shear production and the mentioned buoyancy-driven (convective) production contributed by similar amounts to the turbulent kinetic energy. It is a frequently observed feature of many stratified water bodies that a well-mixed bottom boundary layer (BBL) ex-ists directly above the sediment surface. The height of such well-mixed layers varies between a few meters in lakes and reservoirs (Gloor et al. 2000; Hondzo and Haider 2004; Lemckert et al. 2004) to several tens of meters in ocean