Control of river discharge on large-scale estuary morphology

Abstract:

Estuaries are dynamic landscapes that are very sensitive to changes in boundary conditions such as river discharge, sediment supply and sea-level rise. A better understanding of the influence of upstream river discharge and sediment input on the development of estuaries under various scenarios requires long-term and large-scale morphodynamic models, to both predict future changes and improve geological interpretations. Past 1D model studies have shown that upstream river discharge has a significant effect on the equilibrium bed profile of estuaries, but these studies ignore the effect of 2D bar and channel formation. Our objective is, firstly, to study the effect of bar and channel formation on the interaction between river discharge and tidal currents, and secondly to examine the control of river discharge on the sediment transport and the large-scale morphology of estuaries. To this end, Delft3D has been used to model a self-formed river and estuary system with dynamic bars and channels, in which tidal amplitude, river discharge and river width are systematically changed. We analyzed e.g. tidal prism and excursion length, channel dimensions, braiding index, and net sediment transport throughout the estuary for the different scenarios. Results show that the presence of bars and channels causes larger tidal flow velocities and tidal prism, and a smaller dampening effect of river discharge than suggested by 1D models. Furthermore, bar formation creates intertidal areas where net sediment transport in the flood direction occurs. As a result, the system can be in a dynamic equilibrium, even though river discharge is relatively large. With relatively high tides, tidal amplitude controls estuary dimensions and bar patterns. However, with lower tidal amplitude and/or increasing river discharge, flood flow velocities are dampened and ebb velocities are enhanced by the discharge, resulting in a decrease in flood dominant transport on the bars and river discharge controlling bar pattern and channel depth. Therefore, with increasing discharge, the system shifts from an estuary in equilibrium to a prograding delta.