A laboratory investigation into shear-generated mixing in a salt wedge estuary

The combination of strong density stratification and energetic forcing from tide and mean hows leads to complex circulation and exchange processes in estuaries. The present study was motivated by field results in the Valdivia Estuary, Chile, and others where a dramatic difference in salinity stratification was observed between low and high tides, believed to be attributable to the interacting roles of tidal forcing, topography and velocity shear effects. We describe results from a laboratory experiment which models an important aspect of the dynamics in such estuaries: the role of a changing cross-section on the stratified shear-generated internal mixing. Our experiments were conducted in a long, laboratory tank fitted with a contracting central section. Fresh water ran from one end of the tank, overflowing al an adjustable weir al the other end. A slower salt water flow was introduced at the base of the weir, and intruded upstream as a gravity current. The increased velocity shear which occurred as the salt water flow moved into the contraction caused significant overturn events at the salt/freshwater interface, so changing the density field in a manner similar to the field observations. Under certain conditions, this mixing was sufficient to arrest the wedge in the contraction, so allowing measurements of the mean density and velocity fields, as well as extensive turbulent microstructure records. This enabled us to describe the intensity and spatial variation of the turbulent mixing between the saline underflow and the fresh water overflow for a saline intrusion under the influence of topographic effects.