The authors start from an old observation of a phenomenon appearing on sea harbours: under certain conditions a vortex or a group of vortices forms in bays which have a narrow opening to the sea. The authors try to answer the following questions: what leads to the formation of such vortices, why does their number vary, can such vortices form in any specific bay with known hydrology, coastal configuration and bottom topography? To answer these questions, they use a modern mathematical apparatus, namely a variational principle applied to slow motion of viscous fluids in Hamiltonian description, where the authors have made valuable contributions [Problems of hydrodynamics in the Hamiltonian description. (Problemy gidrodinamiki v gamil’tonovom opisanii.) (Russian) Moskva: Izdatel’stvo Moskovskogo Universiteta. 198 p. (1993; Zbl 0790.76003)]. The theoretical constructions are applied to a schematic two-dimensional approximation and to a complex structure, where the authors obtain numerical results followed by a useful discussion.