Dynamics of horizontal turbulent mixing in a nearshore zone of Lake Geneva

Time series of currents measured during the fall/winter period in a nearshore zone (near a headland and in an adjacent bay) of Lake Geneva, Switzerland, were used to determine horizontal turbulent mixing coefficients. The data were filtered to obtain components in the time range 0.5–12 h, which are suitable for mixing studies because spectra showed no energy source or system response there. From the filtered data, eddy diffusion coefficients were calculated by the integral time‐scale method and eddy viscosity coefficients by the Ertel mixing length method. Viscosity coefficients were systematically higher than diffusion coefficients by a factor of at least two. Longterm horizontal turbulent mixing coefficients calculated over periods of several months were of order 10 3 cm 2 s 1 , a factor of 10 lower than coefficients obtained for coastal zones of larger lakes. Short‐term (3 d) mixing coefficients varied between 10 2 cm 2 s −1 for quiet background situations and 10 4 cm 2 s −1 for periods of wind‐induced, large‐scale advection. This variability was traced to the history of wind events, indicating that energy input is not sufficiently homogeneous in time to maintain mixing at a constant level. Sheltering by local topography caused the sensitivity of the mixing coefficients to wind direction. Variability in space, with larger values at the headland and smaller ones inside the bay, underlined the modifying influence of shoreline geometry. Inside the bay, diffusion diagrams show that near the surface mixing is generated by shear, while near the bottom inertial subrange diffusion is found. The importance of shear is also evidenced by a correlation between wind forcing and diffusion coefficients for events of long wind fetch. Near the headland none of these simple concepts hold because local topography influences mixing.