It has been observed for a long time that under certain conditions a vortex or even a group of vortices forms in bays which have a narrow opening to the sea. What leads to the formation of such vortices confined in a quiet, almost closed bay? Why does their number vary? Can such vortices form in any specific bay with known hydrological conditions, coastal configuration and bottom topography? The answers to these questions are essential in practice because, if several vortices form in a bay, a sort of a ‘vortex cork’ is created which prevents the outflow of pollution from the bay. This pollution will be locked in the bay practically permanently. The formation of vortices can also very strongly modify the topology of the background flow and lead to the formation of structures which intensify such processes as beach drifting, silting, and coastal erosion.

This article considers the topology of the vortex regimes generated in harbour-like basins by the external potential longshore current at large Reynolds numbers. The theory discusses the issues of what solution compatible with the Prandtl–Batchelor theorem for inviscid fluids, and under what conditions, may be realized as an asymptotic state of the open hydrodynamical system. The analysis is developed based on the variational principle, the most appropriate fundamental method of modern physics in this case, modified for the open degenerated hydrodynamical system. It is shown that the steady state corresponds to the circulational regime in which the system has minimal energy and enstrophy. This state is fixed by the Reynolds number. The relation between the Reynolds number, the geometry factor and the topological number, characterizing the number of vortex cells, is found.