Transient scour and fill refers to the general scour and fill in a riverbed due to flood passage. It was given much attention in times of Leopold and Maddock (1953) and Colby (1964) from records of outstanding case studies. Field data from a station in a contracted, long reach of the large sand-bed Pilcomayo River –1192 newly typed in forms– are presented, with sediment concentration in the database up to 60 kg/m3, mostly wash load. Water surface rises and falls during a flood event, whereas streambed evolves in reverse (it renders the analogy to an accordion). Streambed scouring accounts for 36% of the change in flow area during floods and data reveal scouring up to 4 m from the streambed elevation when flood starts. Velocity data display large hysteresis loops inexplicable by unsteady flow only. Once streambed starts to fall, river bed scour is self-sustaining. Water depth gradient appears to be a crucial variable in a novel equation for transient scour and fill derived from Exner equation. It turns out that flow non-uniformity is relevant to explain transient scour and fill in contrast to flow unsteadiness, which is not. Quasi-steady gradually varied flow equation solved along the contracted reach is the base for a model that captures well the recorded bed evolution. The model explains most of the transient scour and fill by the downstream expansion, but the non-uniform flow and the bed material suspended load also play a role. The model fails on the days of highest sediment concentrations.

Peer Reviewed