The salt wedge is a commonly observed water system in estuaries with relatively small tidal ranges. It is formed by sea water intrusion into the river and often has a very distinctive two layered stratification.

Because of its physical, chemical, biological and economic importance, extensive investigations have been performed on the salt wedge in order to specify both over-all flow properties and mixing phenomena through its density interface. However, particularly in the reproduction of this flw system by laboratory experiments, models show significant complications and the present hydrodynamic stability theories can not interpret the interfacial mixing properties correctly.

In this study, some typical flow configurations in salt wedge flow, especially secondary flow structures, are considered. Based on previous observations, four hydrodynamic stability models are developed to investigate interface stability. Each designed to investigate the effects of the rigid boundary, boundary velocity, viscosity, and displacement of the velocity profile inflection point with respect to the density interface. The results provide qualitative explanations for the observed three-dimensional structures of interfacial phenomena and waves.

Precise experiments have been performed to produce a complete data set (i.e. information such as wedge shape, welocity and density profiles, wave length and phase velocity, interfacial displacement and the lower layer thickness) in order to verify the theoretical model predictions. Agreement between these experiments and the theoretical results is very good, although the predicted phase velocity of SVB waves is always underestimated.

This work is concluded with a discussion of the possibility of three-dimensional primary instability in salt wedge flow.