Flow convergence and stability at a tidal estuarine front: Acoustic Doppler current observations

Characteristics of the flow field in an estuarine frontal zone have been investigated in a field study in the lower James River estuary. Underway sampling with an acoustic Doppler current profiler (ADCP) on repeated transects across the front provided information on the structure of the flow field near the front and its evolution in time. As this tidal intrusion front advanced up the estuary during the flooding tide, prominent and consistent features in the velocity field included a localized zone of convergent flow beneath the visible surface line and a stratified shear layer just upriver of the front. Within the shear layer between the buoyant surface water and the faster, higher-salinity undercurrent, gradient Richardson number estimates suggest that the flow was at or near the threshold for sheer instability. Another shear-type gradient in the flow field, the across-front variation of the along-front velocity component, strengthened over a sequence of transects, with intensity increasing toward the surface. Tracking of the front was then interrupted when the identifying line of foam and accumulated material on the surface, previously sharp and well defined, broke up and dispersed to such an extent that the visible signature of the front was lost temporarily. A visible frontal expression later reappeared, and propagation upriver continued. Lower bound estimates of downwelling flow in the frontal zone were determined by continuity considerations.