Dipole formation by tidal flow in a channel

We discuss a laboratory model of dipole formation due to tidal flow through a channel connecting a bay to the sea. 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