A Regime Shift From Low to High Sediment Concentrations in a Tide-Dominated Estuary

Many estuaries are strongly deepened to improve navigation, with sometimes large and poorly understood consequences to suspended sediment dynamics. To improve understanding of such large changes, we study the Ems River Estuary, where a regime shift from low to high sediment concentrations was observed after deepening. The aim of this study is to improve understanding of the development of the sediment concentration regime over time and estimate the associated time scale. Using the idealized width-averaged iFlow model, we identify the coexistence of two distinct stable equilibrium regimes representing low and high sediment concentrations, qualitatively matching the regimes observed in the Ems. Depending on the river discharge, a critical depth profile is identified at which the regime shifts. By combining the model results and long-term observations of the tidal range, first indications of the regime shift are observed around 1989, taking approximately 6-7 years to develop. Plain Language Summary Many estuaries have been extensively deepened to accommodate large ships. In the Ems River Estuary, such deepening has resulted in a large increase of the amount of fine sediment suspended in the water, referred to as a regime shift. However, as historical observations of sediment concentrations in the Ems are scarce, it is unclear when the regime shift occurred and how long it took to develop. Using an idealized mathematical model, we investigate the depth and flow conditions that allow for this regime shift to occur. Depending on the discharge of the river, a critical depth is found at which a regime shift from low to high sediment concentrations occurs. Combining the model results and observations of the tidal range, it is estimated that the regime shift took approximately 6-7 years to develop, between approximately 1989 and 1995.