Development of supercritical motion and internal jumps within lock-release radial currents and draining flows

The unsteady radial flow of relatively dense fluid released from an axisymmetric lock is analysedwhen it freely drains over an edge and when it generates a gravity current propagating over ahorizontal surface. In both situations when modelled using the shallow water equations, despiteinitiation from rest within a lock, the flow thins, accelerates and becomes supercritical in a regionclose to the symmetry axis. For free-drainage, this alters the outflow, while for radial gravity cur-rents, it leads to the formation of an internal jump connecting rapidly moving fluid to more tranquilflow at the front. Both scenarios share the same supercritical flow structure, which is related totheir initiation from lock-release conditions and is a function of axisymmetry; the phenomena isnot found in analogous two-dimensional flows. Through analysis and numerical integration of thegoverning equations, the common onset and consequences of supercriticality are revealed in bothflows, and in particular, the progression of the fluid motions to self-similar states that develop atlate times.