Evidence of transient scour and fill

Sergi Capapé, Juan Pedro Martín Vide

Tuesday 30 june 2015

15:05 - 15:20h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 6B. Sediment - River

Transient scour and fill refers to the general scour and fill in a riverbed due to the passage of a flood. It was given much attention in the times of Leopold and Maddock (1953) from records of outstanding case studies. Data from the large sand-bed Pilcomayo River are presented. The maximum sediment concentration in the database is greater than 60 kg/m3, mostly wash load. The water surface rises and falls during a flood event, whereas the streambed evolves in reverse, in a way that the streambed scour accounts for 36% of the change in flow area and renders the analogy to an accordion. The database reveals scouring up to 4 m from the initial streambed elevation. The analysis follows in the footsteps of Leopold and Maddock (1953), who stressed the role of the sediment load in the transient scour and fill. Mass conservation equations for water and sediment and momentum equation of the diffusive wave are used to derive a change in streambed elevation. The convective terms prove to be crucial for the prediction of transient scour and fill. Especially the velocity displays large hysteresis loops inexplicable by the unsteady flow over a fixed bed. Instead, it is suggested that these distinct loops stem from the high sediment load (bed material and probably also wash load) that enhances the transient scour and fill. Although spatial variation data would improve the results, the prediction equation captures the amount and sign of the transient scour and fill.