Benthic boundary mixing in a stratified lake

The hydraulic characteristics in a turbulent benthic boundary layer (TBBL), induced by the breaking of the internal waves in a linearly stratified fluid were investigated through laboratory measurements. Experiments were performed at subcritical, critical, and supercritical wave conditions and at 3 different slopes of 6, 12, and 18 degrees with the horizontal. Microscale fast response conductivity and temperature probes in conjunction with laser-Doppler velocimetry were used to measure the time series of the salinity, temperature, and velocity in a TBBL. The boundary layer thickness was measured by the rainbow schlieren method. From the measured velocity data it was found that the phase-averaged velocity follows the law-of-the-wall for 25% of the wave period. The diffusion coefficient for momentum increases with the increase of the angle of the sloping boundary. The time-averaged volume flux for a 6 degree slope is significantly smaller than the time-averaged volume fluxes at the slopes of 12 and 18 degree. Internal wave or flow characteristics at the sloping boundary can be used to estimate the time-averaged boundary layer thickness. The energy dissipation rate can be estimated from the energy spectrum of velocity data in the space domain. Conversion of the measured temporal velocity data to the spatial data requires an appropriate definition for the advective velocity. An integral time scale was used for averaging the absolute value of the measured velocities to estimate the advective velocity in the TBBL. The integral time scale is related to the average frequency of the spectral energy density of the flow velocity. The estimated energy dissipation rates were comparable with the values obtained by curve-fitting a theoretical Batchelor spectrum for the temperature gradient spectra. At 12 and 18 degrees slopes the mixing efficiency is higher than the mixing efficiency at a 6 degree slope. Stratified flow can sustain turbulent mixing if normalized a energy dissipation rate is larger or equal to one. The experimental measurements of this investigation are in agreement with the theoretical model of Ivey et al. (2000)