Fine Sediment Dynamics Affected by Large-Scale Interventions: A 3D Modelling Study of An Engineered Estuary

Estuaries around the world are extensively modified to support dense human populations. They trap both fluvial and marine sediments, creating regions with high suspended sediment concentrations (SSCs), called estuarine turbidity maxima (ETM), which affect port navigability and ecological values. The impact of human-induced narrowing and deepening on sediment trapping and flushing is still subject to debate, particularly in strongly stratified estuaries. This study evaluates the effects of channel deepening and intertidal wetland reclamation on sediment trapping and flushing in a highly engineered and stratified estuary. A schematized 3D numerical model, inspired by the Rotterdam Waterway, was developed and validated under present-day conditions. Subsequently, human interventions were simulated in combination with high discharge events which occur every 1, 10 and 100 years. Results indicate that the amount of sediment flushed during high discharges is almost entirely dependent on river discharge and channel depth. Net sediment flushing occurred at Freshwater Froude numbers of 0.2 or higher. Sediment trapping decreased with shallower channels and larger intertidal area: on average, a 1.5m shallower channel reduced trapping by 13%, while a 0.5km wider intertidal area reduced it by 35%. Namely, while both interventions reduced trapping due to tidal flow, they enhanced trapping due to estuarine circulation flow, the dominant trapping mechanism in the system. Finally, a conceptual model is presented, explaining the impact of intertidal wetlands and channel depth on the key processes relevant for fine sediment dynamics in stratified estuaries.