A model of tidal flushing of an estuary by dipole formation

We discuss a laboratory model of the flushing and mixing between a bay and a sea connected by a narrow channel. Tidal changes in water elevation lead to periodic flow in and out of the bay through the narrow channel. Due to flow separation there is an asymmetry between the flow entering and leaving the channel on different phases of the tide. A consequence of this asymmetry is that every tidal cycle a fraction of fluid is not returned through the channel, so that a passive tracer in the bay is successively flushed away. We consider what happens when the creation of vorticity in the channel leads to the formation of a dipole that can propagate away from the returning flow of the later phase of the tide. When the dimensionless ratio of maximum channel velocity U, width of channel W and tidal period T is such that W/UT<0.13, we find that dipoles can propagate away from the source region without being drawn back into the sink. This process leads to a larger exchange of water between the estuary and sea, than the classical model of tidal flushing by the jet-sink asymmetry of Stommel, H., Farmer, H. G., 1952. On the nature of estuarine circulation WHOI Tech. Rep. 52–88, 131].