Turbulent mixing in estuaries

Abstract

Abstract The pattern of flow in a tidal estuary is largely determined by the vertical transport of momentum and density due to turbulence. In a well-mixed estuary the turbulent mixing throughout the depth is dominated by turbulence associated with friction at the bed, although the effect of shear at mid-depth due to the density driven residual flow is not necessarily negligible. The main features of the flow resemble those in boundary layer flows, on which much information is available from other branches of fluid mechanics. Similar methods of calculation may be used, based on a mixing length approach or a turbulent energy closure model. In a slightly stratified estuary the conditions are modified without being changed radically, but as the opposite extreme of a highly stratified estuary is approached the flow is better represented by two well-mixed layers, separated by a relatively thin shear layer in which there is a steep density gradient. This type of flow has received considerable attention in theoretical studies and laboratory experiments and the results have some application to estuarine flows. Recent work on the application of relevant concepts of fluid mechanics to flows in estuaries is reviewed and some suggestions made for lines of future research. In many tidal estuaries the position is complicated in that the flow may vary between well-mixed and stratified within a tidal cycle and attention should be given to the time-varying nature of the flow. Even if only the tidally averaged result of mixing is required, a knowledge of the changing physical processes is needed in order to parameterize their effects.