Sediment Bypassing of River Mouths: Mechanisms and Effects on Delta Evolution

Abstract:

Wave-influenced deltas are shaped by wave-driven transport of river-borne sediments. Near the river mouth, combined jet and wave dynamics, along with morphodynamic feedbacks, control the fraction of sediment transported alongshore by littoral currents that can bypass the river channel. Here we study how different bypassing rates influence large-scale delta evolution and examine the effect of waves and the river mouth jet on alongshore sediment bypassing.

First, we use a modified version of the Coastline Evolution Model (CEM) to look at the effects of wave climate, fluvial sediment supply, and alongshore sediment bypassing rates on channel orientation. This modified version of CEM progrades the channel in a direction perpendicular to the local shoreline orientation at the river mouth, allowing feedbacks between alongshore sediment transport and fluvial sediment delivery to steer the river channel. Additionally, we allow a prescribed fraction of littoral sediment to bypass the river mouth. We find that deltas that have a large fluvial sediment flux can orient themselves into the direction of dominant wave approach. Lower fluvial inputs result in channels that are deflected downdrift, with increasing deflection as bypassing is reduced. In contrast, channels do not deflect downdrift (but can reorient themselves updrift for large fluvial fluxes) when full bypassing is allowed. These results demonstrate the importance of river mouth sediment bypassing on delta growth patterns, but, as we explore arbitrary bypassing laws, the simulations cannot help us constrain natural bypassing fluxes.

To further investigate the natural extent and mechanisms of bypassing, we use the coupled hydrodynamic and morphodynamic model Delft3D-SWAN. With a simplified shoreface and river channel, the model is able to construct river mouth morphology from the combined action of alongshore transport and a river mouth jet. Exploring river mouth morphology and sediment bypassing under various wave and fluvial conditions, we find that bypassing rates can be highly variable, and strongly depend on jet stability, fluvial sediment properties and inlet geometry. Waves increase jet spreading and promote jet deflection, and can help create an efficient bypassing regime through morphodynamic feedbacks that involve bar creation and elongation.