Cohesive sediments drive the transition between river- and tide-dominated estuaries through floodplain infilling

Estuaries are dynamic landscapes where river and tidal currents interact and form complex channel-bar patterns and intertidal floodplains. Mud has cohesive forces and settles mainly in calm or protected areas, such as on top of intertidal flats and along estuary fringes. In general, an increase in river discharge dampens the tidal currents and increases the supply of sand and mud from the hinterland, which results in a transition from tide-dominated estuaries to aggrading river-dominated estuaries. However, it is unknown how increasing river discharge, relative to the tidal prism, and a resulting increase in mud supply controls bar patterns, sediment transport rates and estuary width and length. Our objective is to systematically quantify these characteristics and the large-scale estuary equilibrium as a function of river discharge, tidal prism and mud concentration using a large-scale and long-term Delft3D-2DH idealized estuary model with freely migrating bars and channels. Results show that mud reduces the intertidal area and significantly changes bar dimensions by infilling, stabilizing floodplains and preventing bar splitting. Infilling of the intertidal area focuses the flow in the channels, resulting in two contrasting trends: 1) the tidal currents are amplified by channel deepening or 2) tidal currents are dampened by the river discharge when channel deepening is prevented, which results in a quicker transition from tide-dominated to river-dominated estuaries. This difference in response depends on mud supply, estuary size and river discharge relative to tidal prism. Furthermore, sediment transport in the intertidal area becomes more ebb-dominant due to the reduced water depths and flood flow velocities, resulting in increasing sediment export and stronger ebb-dominance compared to sandy estuaries.