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.