Fate of fluid mixed at the boundaries of a lake

Three field studies were conducted in Ada Hayden Lake in Ames, Iowa to study generation of turbulence on the sloping boundary and to investigate boundary-interior communication in a lake with dye tracking experiments and measurements of meteorological conditions, internal wave response, and turbulence. The objectives of these studies were to (1) predict the occurrence and strength of turbulent mixing in terms of meteorological forcing and stratification by investigating the dependence of internal waves and turbulence on the slope on the Lake number, (2) investigate the fate of mixed fluid by tracking an intrusion generated at the boundary; and (3) evaluate offshore transport by basin scale seiches. To predict the Lake number conditions under which turbulence will be generated at the slopes (objective 1), the rate of dissipation of turbulent kinetic energy was determine from near-bottom velocity measurements using the structure function method, and histograms of ε/νN2 were analyzed for all the data and for five different Lake number regimes. Although a quantitative relationship between the Lake number and the turbulence intensity could not be determined, some relationships between the Lake number and ε/νN2 for different Lake number regimes could be observed. For example, for high Lake number, most of the values of ε/νN2 were low enough to suggest that transport was mainly caused by molecular diffusion, while for low Lake number, turbulence was energetic. For moderate Lake numbers, the value of ε/νN2 at the peak in the histogram increased as the Lake number decreased from 30 to 1. To investigate intrusion generation and propagation (objective 2), temperature microstructure measurements on the slope and horizontal and vertical dye mapping were used. Profiles of temperature microstructure measured soon after the injection both at the injection site and offshore showed large eddy diffusivity near the boundary. The propagation characteristics of the intrusion were predicted most closely by a formulation for an axisymmetric intrusion governed by a balance between buoyancy and inertia. To evaluate offshore transport by basin scale seiches (objective 3), the horizontal variation in internal wave shear and strain, which can increase the lateral dispersion between the boundary and the interior, was analyzed. The strain can spread the mixed fluid far enough from the boundary that vertical shear becomes an important dispersion process. These findings improve the understanding of the pathway from energy input from the wind to offshore transport