Observations and numerical modelling of a non-buoyant front in the Tay Estuary, Scotland

Acoustic Doppler current profiler (ADCP) and density data have been collected for a section of front which consistently occurs on the flood tide along a break in bathymetry in the Tay Estuary. Lateral velocity shear in a vertical profile through the front was measured to be 0.52 s −1. An estuarine cross-sectional numerical model was developed with buoyancy-driven flow. Results from the numerical model showed that shears of such magnitude cannot be produced by buoyancy alone. Instead, a hypothesis was devised for the generation of the bathymetry-aligned front, and tested using the numerical model. The flooding current flows over sandbanks at the southern bank of the estuary and is then directed over (rather than along) the bathymetry break due to a sudden topographic restriction at the Tayport Narrows. Due to tidal phase effects, this overbank flow has a lower density than the ambient main channel water, hence behaving as a buoyant plume. The plume entrains higher density bottom water and a recirculation cell is set up in the lee of the bathymetry break. A surface convergent front occurs because a corresponding towards-bank flow (confirmed by field data) occurs in the centre of the channel. The numerical model was applied to this configuration using suitable initial and boundary conditions based on field observations. Lateral velocity profiles and the strength of shear show good agreement with the field data. It is suggested that the presence of a density gradient is required to generate the front but is not the main driving force.