Fluid Mud Occurrence in the Hyper-Turbid Ems Estuary: Insights into Tidal Cycle Influence

The Ems estuary (NW Germany) is characterized by a hyper-turbid state with sediment concentrations ranging from < 1 g/L up to several hundred grams per liter. In summer, when upstream discharge decreases and biomass production increases, substantial fluid mud layers (sediment concentration > 10 g/L) accumulate, particularly during low tides. The thickness of the fluid mud layer reaches a significant fraction of the water column (up to 60%), thus affecting cross-sectional hydrodynamics as well as navigability. Extensive monitoring with high temporal and vertical resolution is required to understand the processes controlling the formation and compaction of the fluid mud layers, given their high temporal and spatial variability.

Between August 2019 and September 2019, two vertical probe arrays were installed in the Lower Ems estuary (km 11.775) equipped with multi-parameter probes (pressure, salinity, oxygen, and current velocity), optical backscatter turbidity sensors, and additional oxygen loggers. The probes/loggers are deployed at three different depths (approx. 1.15m above bed, 1.95m above bed, and attached to a buoy at the surface) measuring continuously every 5 min. Thus, a high-resolution time series with decent vertical resolution over a period of six weeks has been recorded.

Six weeks of monitoring revealed two superimposed cycles of fluid mud occurrence. On a shorter timescale, fluid mud occurrence is separated by distinct stages of formation, entrainment, and stratification throughout a tidal cycle. However, on a longer timescale, fluid mud occurrence is affected by the neap-spring tide. Here, we identify four main stages: buildup, stationary state, breakup, and no presence of fluid mud, each stretching over multiple tidal cycles. Each stage is characterized by distinct differences in, e.g., turbidity, salinity, and oxygen gradients/dynamics. Our approach successfully enables monitoring of fluid mud in the hyper-turbid Ems estuary for a better understanding of the processes of fluid mud formation and breakup. Future research aims to develop predictive models for the occurrence of hyper-turbid conditions and different fluid mud stages in the Ems estuary.