Influence of river pattern and sediment input on estuary morphology

Alluvial estuaries are dynamic landscapes that are very sensitive to changes in boundary conditions such as river discharge and sediment supply. A better understanding of the influence of upstream river discharge and sediment input on the development of estuaries under various scenarios requires long-term morphodynamic models, to both predict future changes and improve geological interpretations by storing the stratigraphy. 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. Using 2D numerical models to predict the development of these systems on the scale of millennia proved to be difficult, since the modelled morphology is very sensitive to the choice in e.g. sediment transport predictor and bed slope effect. In this study, we use the knowledge of previous research that determined best parameter settings for realistic river and bar patterns to model long-term and large-scale estuary morphodynamics in Delft3D. Our objective is to quantify the effects of river discharge and sediment supply on the shape of estuaries and its deposits. Firstly, we systematically varied upstream river width and tidal amplitude to examine the relation between upstream river pattern and estuary dimensions. We quantified e.g. braiding index, bar dimensions, and tidal excursion length. Results show that flood flow velocities and tidal prism are less influenced by river discharge than suggested by 1D models, and are significantly influenced by the braiding index of the river. With relatively high tides, estuary bar patterns depend on tidal amplitude, while with lower tides estuary depth and braiding index are related to upstream river width and discharge. Next steps will include varying discharge to study the effect on the rate of adaptation of the river and estuary, and varying the grain size of the sediment input at the upstream boundary. We will input coarse sediment to explore differences between fluvial deposits and tidal currents, and fine sediment to use the model for research related to biofilm.