Mechanics of Backwater-Mediated Avulsions on River Deltas

Abstract:

River avulsions – abrupt relocation of a river channel – are a primary mechanism by which deltas grow and field evidence suggests that backwater hydrodynamics play an important role in setting the location of avulsions on deltas; however, we still lack a mechanistic understanding of backwater-mediated avulsions. Here we use physical experiments to unravel the mechanics of river avulsions caused by backwater hydrodynamics where low water discharges cause flow deceleration and sediment deposition and high water discharges result in flow acceleration and erosion in the lowest reaches of alluvial river near its shoreline, i.e., the backwater zone. We performed two experiments in the river-ocean facility at Caltech, where a 7 cm wide, 7 m long alluvial river feeds into a 6 m by 3 m “ocean” basin, building its own delta under subcritical flow and constant sea level conditions. The first experiment was conducted under constant sediment and water discharge, where backwater hydrodynamics are muted, while in the second experiment, we alternated between a low flow and high flow such that the erosion caused by the high flow is not balanced by the deposition caused by the low flow in the backwater zone, thus, allowing for the persistence of backwater hydrodynamics. In the first experiment, avulsions occur at the imposed change in flow confinement and are driven by a loss in sediment transport capacity, which are akin to avulsions near a canyon-fan transition on alluvial fans. In the second experiment, however, avulsions occur further downstream of the imposed change in flow confinement, and the avulsion length (distance of the avulsion location from the shoreline) scales with the backwater length. Thus, our results demonstrate, for the first time, that backwater hydrodynamics exert a primary control on avulsion location thereby setting the delta-lobe size. Further, we document key differences in development of shoreline rugosity, channel migration and avulsion frequency between the two experimental deltas.