Topographically induced internal waves and enhanced vertical mixing in an estuary
Stratified tidal flow over topographic features has often been thought to give rise to enhanced vertical mixing in estuaries. So far no estimates have been made of the transfer of energy from the barotropic tide to internal waves, and subsequently to increased potential energy of the water column. Observations have been made over a topographic depression at Cargreen in the Tamar Estuary. This work shows clearly the formation and evolution mechanisms for internal waves on the thermocline, on a neap and a spring tide. On a neap tide an internal wave formed as a nonlinear response to the tidal flow. It subsequently dissipated its energy to mixing through turbulence due to enhanced shear across the thermocline. On the spring tide the internal wave broke in situ in the form of a hydraulic jump. Estimates of the energy transfers are in excellent agreement with studies on the continental shelf edge, and in laboratory experiments of wave breaking, and are the first of their kind for estuaries. A nonlinear, hydrostatic, two-layer numerical model of the flow has been applied to the Cargreen topography, for the neap and spring tide observations. The model predicts the maximum wave heights and energies well, but fails to reproduce the more subtle details of the thermocline response. Other observations of the flow at Cargreen are put into a conceptual framework based on an internal Froude number, Fi, and it is indicated that Fi = 2 to 3 is appropriate for internal wave/hydraulic jump transitions at Cargreen.