Anatomy of turbulence in a narrow and weakly stratified estuary

A weakly stratified straight channel in the Swan River estuary was examined using a CTD probe and a microstructure profiler capable of measuring two components of velocity, temperature and conductivity. In addition, a pair of acoustic Doppler current profilers were deployed at the bottom to record profiles of the horizontal velocity. The results indicated that the mean flow was barotropic with little shear. The turbulent overturns were mostly active and viscous forces were negligible in their energetics. Although the stratification was too weak to influence the mean motion, it was found that the turbulence was influenced by buoyancy, and a collapse of the vertical anisotropy was documented with decreasing turbulent Froude number, Fr-t. Using the results from laboratory and numerical studies, we infer that the mixing efficiencies associated with these turbulent events ranged between 0 and 0.3, with a mean value of 0.07.