Spring-neap tidal variation of fluid mud occurrence in the hyper-turbid Ems estuary

Dredging of the fairway in the Ems estuary was driven by the need to accommodate the increasing draft of ships. This modification has had negative effects on the sediment balance and ecology of the estuary. The fairway deepening results in strong alterations of the tidal dynamics, such as tidal amplitude and duration, as well as hydrodynamics such as current velocity and turbulence. This results in increased fine sediment input, which, at high suspended sediment concentrations, contributes to the formation of fluid mud—a mixture of silt, clay, and organic matter. The dynamics of fluid mud, particularly the differences between spring and neap tides, are not yet fully understood.

We investigated the formation, dispersion, and entrainment of fluid mud during the semi-diurnal tidal cycle. Therefore, the influence of flow velocity and salinity at different water depths, and the differences between spring and neap tides based on two dedicated measurement campaigns in 2023 was analyzed using high-resolution spatiotemporal monitoring data. Salinity data were used as an indicator of stable stratification. Additionally, sediment samples have been collected using a sediment corer to analyze the composition and properties of the fluid mud layer.

Our Spring-neap tide analysis showed a reduction of the flow cross-section during neap tide leading to differences in hydrodynamics between spring and neap tide driven by high sediment concentrations and fluid mud occurrence. During neap tide fluid mud was found to cover a larger fraction of the water column than during spring tide. This further highlights the strong influence of flow velocity on the dynamics of fluid mud and the need to include spring-neap considerations for future sediment management plans for the Ems.