Tidal intrusion and mixing in the highly stratified Palmiet Estuary are discussed as an example of a typical small bar-built estuary. The flux of salt through the estuary is dominated by advection owing to the strong gravitational circulation. This vertical circulation pattern is characterized by four processes. The dense saline water enters the estuary as a hydraulically controlled flow through a constricted entrance channel. Beyond an internal hydraulic jump, landward of the flood tidal delta, the saline intrusion propagates as a non-mixing bottom density current and eventually replenishes deep basal reservoirs of saline water in the upper estuary. On the ebb tide this lower saline layer is prevented from flowing seaward by the presence of the sill at the mouth (upstream blocking)-upward entrainment of halocline water occurs via subcritical shear and the sub-halocline water is not removed. The entrained salt is exported to sea by way of the outflowing buoyant surface layer This pattern of circulation is expected to be present in countless small bar-built estuaries around the world. While these systems are generally characterized by micro-tidal and high wave-energy coasts, they are observed in a variety of other situations. In the Palmiet Estuary this pattern occurs during both the drier summer and the wetter winter, exhibiting a certain robustness to seasonal changes. These estuaries are generally classified as highly stratified, although the actual stratification varies as a function of season, tide and position in the estuary. Of consequence is the long residence times expected, and observed, for the deep saline water found in the scour holes of the upper estuary. With decreasing river flow and more constricted connections with the sea, tidal mixing decreases and this deep basal water can be resident long enough to become anoxic.

The nature of estuarine circulation under highly stratified conditions is reviewed in the context of southern African estuaries that are small and have a constricted connection to the ocean. While the focus is on the generic hydrodynamic processes and features, attention is given to field observations from a particular estuary as a way of illustrating these processes and features. Notable features are the tidal intrusion front, the bottom density current and the long-residence deep water in the upper estuary. The important hydrodynamic processes, which account for these features, are internal hydraulic control of exchange through the mouth, buoyancy-driven landward intrusion of sea water, upward shear-driven entrainment and ebb tidal outflow. The context in which these processes and features are expected to occur is addressed through a discussion of seasonal variations in the prototype and through a discussion of these features observed in other southern African estuaries. A distinction is made between those systems that are permanently highly stratified and those that display highly stratified phases. The ecological and management implications of strong stratification in an estuary are discussed briefly.

Following the closure of the mouth of the Palmiet estuary in February 1988, the thermohaline structure was recorded at approximate weekly intervals. The evolution of the two-layer structure was studied and the mixing processes causing the changes were determined. Vertical diffusivities were computed and the qualitative understanding of the slow mixing rates in the deep pools was confirmed. The study indicates a probable summer condition of the estuary following upstream impoundment.