Topology of two-dimensional flow associated with degenerate dividing streamline on a free surface is analysed from a topological point of view by considering the critical point concept. Streamline patterns and their bifurcations in the vicinity of a free surface were investigated by Brøns (Brøns, M. (1994) Topological fluid dynamics of interfacial flows. Phys. Fluids6, 2730–2736). Brøns's work is extended to the case of a stream function, including the fourth-order normal form approach. From this, a complete description of bifurcations which can occur in two-dimensional incompressible flow is obtained up to codimension three. The theory is applied to the patterns found numerically in a roll coater.
