Modeling the Transition to High Sediment Concentrations as a Response to Channel Deepening in the Ems River Estuary

Many estuaries are strongly modified by human interventions, including substantive channel deepening. In the Ems River Estuary (Germany and Netherlands), channel deepening between the 1960s and early 2000s coincided with an increase in the maximum near-bed suspended sediment concentration from moderate (approximate to 1kg/m(3)) to high (>10kg/m(3)). In this study the observed transition in the suspended sediment concentration in the Ems is qualitatively reproduced by using an idealized width-averaged iFlow model. The model is used to reproduce observations from 1965 and 2005 by only changing the channel depth between the years. Model results show an increase in sediment concentrations from approximately 1-2kg/m(3) to 20-30kg/m(3) near the bed between 1965 and 2005 if the river discharge is below 70m(3)/s, which holds approximately 60% of the time. Thereby, this study for the first time provides strong evidence for earlier published hypotheses that channel deepening was the main driver of the increased sediment concentrations in the Ems. The results are explained using two aspects: sediment transport (longitudinal processes) and local resuspension (vertical processes). The magnitude of the sediment import increased, because a combination of channel deepening and sediment-induced damping of turbulence increased the M-2-M-4 tidal asymmetry. This effect is particularly strong, because the M-4 tide evolved to a state close to resonance. All imported sediment is kept in suspension when it is assumed that resuspension is sufficiently efficient, which depends on the value of the erosion parameter used and inclusion of hindered settling in the model. Plain Language Summary Between the 1960s and early 2000s, the sediment concentration observed in the Ems River Estuary (Germany and Netherlands) has increased strongly. This increased sediment concentration is relevant as it is associated with a severe deterioration of the ecosystem. During the same period, the estuary underwent multiple human interventions including substantive channel deepening to support large ships. This study demonstrates that the observed increase in sediment concentrations can be reproduced qualitatively using a computational model when only accounting for the change of depth between the 1960s and 2000s. Thereby, this study provides strong evidence for earlier published hypotheses that channel deepening is the main driver resulting in increased sediment concentrations. The used model is an idealized width-averaged model called iFlow. The model is idealized in the sense that it specializes in modeling the large-scale trends and uses simplified descriptions of some physical processes, focused on capturing the qualitative behavior. The model simplifications allow for a thorough and systematic analysis of the underlying physical processes and uncertainties. Using this analysis, a deeper understanding of the model results and related uncertainties is obtained.