Estuarine water circulation

Estuarine water circulation is controlled by the inflow of rivers, the tides, rainfall and evaporation, the wind, and other oceanic events such as an upwelling, an eddy, and storms.

However, the circulation near the mouth of the estuary is complex due to the tidal mixing processes that occur between the estuarine and ocean waters.

If the coast is rugged with headlands, a mosaic of complex flow fields consisting of eddies, jets, and stagnation zones will occur, further complicating the circulation patterns outside of the estuary.

[1] The exposure time τ' is estimated by: Vestuary is defined as the mean estuarine volume and Ttide is the tidal period.

Vertical mixing determines how much the salinity and temperature will change from the top to the bottom, profoundly affecting water circulation.

As a velocity difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater.

[citation needed] As tidal forcing increases, the control of river flow on the pattern of circulation in the estuary becomes less dominating.

Turbulent eddies mix the water column, creating a mass transfer of freshwater and seawater in both directions across the density boundary.

[citation needed] In these estuaries, tidal flow is greater relative to river discharge, resulting in a well mixed water column and the disappearance of the vertical salinity gradient.

If tidal currents at the mouth of an estuary are strong enough to create turbulent mixing, vertically homogeneous conditions often develop.

[6] Fjords are examples of highly stratified estuaries; they are basins with sills and have freshwater inflow that greatly exceeds evaporation.

A slow import of seawater may flow over the sill and sink to the bottom of the fjord (deep layer), where the water remains stagnant until flushed by an occasional storm.