The formation of a tidal intrusion front at the mouth of a macrotidal, partially mixed estuary: a field study of the Dart estuary, UK

High-resolution measurements of density and velocity were made before, during and after the formation of a transient tidal intrusion front at the mouth of a narrow, partially mixed estuary during spring tides. Detailed Acoustic Doppler Current Profiler (ADCP) and density measurements throughout 1 km longitudinal transects at hourly intervals during spring tides indicated that the front formed when the barotropic inflow was sufficient to arrest the buoyant outflow from the estuary. This has been shown to be driven by a constriction of the channel at the mouth and a depression in the seabed just landward of the mouth, with these changes in topography shown to interact with the flow in a similar manner to theoretical two-layer flow over the lee side of a sill. A detailed analysis of the flow and stratification across the narrow frontal interface found a high degree of shear and mixing just prior to the plunge point, which together with a hydraulic jump at the seabed depression, eventually led to the destruction of the density interface and the total intrusion of seawater as the flooding tide strengthened. There was no surface manifestation of this front during neap tides, and further measurements have confirmed that the relatively weak barotropic flow at this time was not sufficient to overcome the stratification in a two layer regime. An evaluation of the key frontal discriminators has confirmed the validity of their use in such an estuary.