Data interpretation and numerical modeling of the Mud and Suspended Sediment Experiment 1985

In 1985 a field survey was performed in the Weser estuary (northern Germany) covering a 30-km-long section to study the turbidity maximum in its complete extension. Moored instruments as well as shipborne vertical profilers were used in the survey. The measured data were employed in close connection with numerical simulations using a three-dimensional finite-difference model. The common aim of the investigations was to arrive at a better understanding of the hydrodynamics and the transient and spatial dynamics of the turbidity maximum in the Weser estuary. The overall behavior of the turbidity maximum resembles a cyclic process, wherein deposition, resuspension, and advection of resuspended sediments are the dominant processes. The conceptual model, derived from the measurements, was confirmed by the numerical simulations. This is demonstrated by the calculated distribution of suspended sediment along the estuary as well as by the calculated deposition and resuspension rates at the estuarine bed for a tidal cycle. Detailed analyses of measured data (moorings) demonstrate a close relationship between the local near-bottom velocity gradient, stratification, and turbulence on the one hand and the suspended sediment concentration on the other. A comparison between numerical results and measured data leads to an improved parametrization of the bottom shear stress, wherein the bottom shear stress is calculated from the near-bottom Reynolds stress, with consideration of the stratification. With this parametrization, it is shown that the model is able to reproduce some of the essential features reflected in the measured suspended sediment concentration.