Turbulent mixing in a sloping benthic boundary layer energized by internalwaves

A laboratory study was carried out to directly measure the turbulence prope rties in a benthic boundary layer (BBL) above a uniformly sloping bottom wh ere the BBL is energized by internal waves. The ambient fluid was continuou sly stratified and the steadily forced incoming wave field consisted of a c onfined beam, restricting the turbulent activity to a finite region along t he bottom slope. Measurements of dissipation showed some variation over the wave phase, but cycle-averaged values indicated that the dissipation was n early constant with height within the BBL. Dissipation levels were up to th ree orders of magnitude larger than background laminar values and the thick ness of the BBL could be defined in terms of the observed dissipation varia tion with height. Assuming that most of the incoming wave energy was dissip ated within the BBL, predicted levels of dissipation were in good agreement with the observations. Measurements were also made of density and two orthogonal components of the velocity fluctuations at discrete heights above the bottom. Cospectral est imates of density and velocity fluctuations showed that the major contribut ions to both the vertical density flux and the momentum flux resulted from frequencies near the wave forcing frequency, rather than super-buoyancy fre quencies, suggesting a strong nonlinear interaction between the incident an d reflected waves close to the bottom. Within the turbulent BBL, time-avera ged density fluxes were significant and negative near the wave frequencies but negligible at frequencies greater than the buoyancy frequency N. While dissipation rates were high compared to background laminar values, they wer e low compared to the value of epsilon(tr) approximate to 15 nu N-2, the tr ansition value often used to assess the capacity of a stratified flow to pr oduce mixing. Existing models relating mixing to dissipation rate rely on t he existence of a positive-definite density flux at frequencies greater tha n N as a signature of fluid mixing and therefore cannot apply to these expe riments. We therefore introduce a simple model, based on the concept of dia scalar fluxes, to interpret the mixing in the stratified fluid in the BBL a nd suggest that this may have wider application than to the particular conf iguration studied here.