Meandering streams in a shallow fluid layer

Decaying turbulence in a shallow flow is shown to be characterized by the emergence of long-lived meandering currents, which are closely related to pronounced vertical flows inside the shallow layer. These vertical flows are concd. in regions that are dominated either by vorticity or by strain of the flow field. Upwelling of fluid is obsd. in patch-like domains near elliptic points. Downward flow takes place close to hyperbolic points where the hyperbolic nature of the streamlines leads to thin, elongated regions of intense downwelling. The latter results in long, contracting regions in the free-surface flow. Particles that float on the liq. surface will congregate in these strain-dominated regions, thus lining out the large-scale meandering streams.