Tide Driven Dynamics of Subaqueous Fluid Mud Layers in Turbidity Maximum Zones of German Estuaries

Abstract

Fluid mud is a highly concentrated suspension of fine-grained cohesive sediment, which often occurs in estuarine turbidity maximum zones (ETMZs) throughout the world. As an interface between the sea and inland, the estuarine zone is considered to be an ideal location for trade and thus ports are constructed in the estuarine zone. The occurrence of fluid mud is problematic as it might restrain navigability. Besides, cohesive sediments like mud particles are known to be the main carrier of pollutants as they specifically absorb contaminants from the water. Thus the management of estuarine and harbour fluid mud is a matter of considerable importance in the practice of hydraulic and environmental engineering. Current fluid mud models which are being used as a tool for making predictions require substantial site-specific validation to observations, and often give disappointing results due to an insufficient knowledge of underlying biogeochemical and physical mechanisms governing fluid mud dynamics. Detailed in situ measurements of fluid mud with high temporal and spatial resolution remain lacking due to laborious and costly measuring campaigns. In this study fluid mud dynamics in ETMZs were investigated by application of a numerical model and based on data of two field measurement campaigns. The numerical modelling study had been based on data from an early field survey in the Weser estuary. The entrainment of fluid mud deposited in dune troughs was studied in a series of scenario simulations and the suitability of entrainment formulae implemented in the model has been examined. The numerical model has been shown to simulate the entrainment of fluid mud layer from the dune troughs at peak tidal currents and reformation at slack waters, which were observed in early field surveys. It was concluded that a further development of the numerical fluid mud model cannot be independent of field measurements. In order to gain more insights into associated processes of fluid mud, two field campaigns have been conducted. Based on data of the first measurement campaign which was conducted in a stable ETMZ of the German Ems estuary, a new method to rapidly examine position, strength and dynamics of the fluid mud lutocline is introduced. With high temporal and spatial resolution the intratidal variability of the fluid mud lutocline characteristics and its dependency on forcing hydrodynamics are demonstrated. The lutocline appeared to undergo a cyclical evolution governed by several processes like entrainment, hindered settling and dewatering under the tidal forcing. In the second campaign which was conducted in an oscillatory ETMZ being recovering in the German Ems estuary, quantitative measures of a mobile fluid mud layer and its lutocline stability were explored based on observed calibrated gradient Richardson numbers. The calibrated gradient Richardson numbers have shown a stabilization and formation of the lutocline which is existent for several hours over high slack water. A new method to derive backscatter gradient Richardson numbers has been introduced, which does not require any calibration of backscatter gradients to suspended sediment concentration (SSC) gradients. The backscatter gradient Richardson numbers have been found to be also able to measure the fluid mud lutocline stability. A snapshot of reformation of fluid mud layer was captured in the ETMZ being recovering. These two field campaigns have both shown a strong tidal asymmetry with significant flood dominance and much longer high slack water than low slack water, which can be considered as a major cause of the formation of ETMZ in the Ems estuary. The hindered settling which played a crucial role in the increase of strength of lutocline as the field measurements revealed, could be implemented in future model developments. The dataset collected in the field campaigns would definitely provide strong support for the validation of models in future research.