Using a Holistic Modeling Approach to Simulate Mud‐Induced Periodic Stratification in Hyper‐Turbid Estuaries

Abstract

In some estuaries, deepening and dredging caused a shift to very high sediment concentrations resulting in thick layers of fluid mud. Examples for such hyper-turbid estuaries are the Ems, Loire and Yangtze river. The Ems estuary is located at the German-Dutch border, see Figure 1a, with main riverine inputs from Ems () and Leda (). Intensive dredging of the river has changed the flow profile such that the flood phase, compared to the ebb phase, is shorter, with higher velocities and acceleration. This led to an over 40 km long estuarine turbidity maximum (ETM) and the formation of a thick fluid mud layer with concentrations up to several tens to hundreds of grams per liter (Schrottke, 2006;Talke et al., 2009). Recent measurements of Becker et al. (2018a) show strongly varying concentration and velocity profiles over a tidal cycle. The processes occurring in the Ems are illustrated in Figure 1b which shows the concentration of suspended sediment over a tidal cycle in the estuary. During flood tide (position A), intense mixing occurs and the sediment concentration profile increases almost linearly from the free surface to the consolidated mud layer at the bottom. A shear layer is located near the bed, above the lower pronounced density gradient (lutocline). During slack water (position B) the sediment in the upper layer settles down and a two-layer flow forms, divided by a second lutocline. The rapid change in the concentration profile can be explained with turbulence damping due to stratification, which reduces vertical mixing and in turn leads to increased settling. The position of the largest density gradient moves upwards from the consolidated mud interface to the fluid mud-water interface. This restratification is related to damping of turbulent mixing processes.

Modelling of fluid mud flow based on a three-dimensional hydrodynamic model : incorporating a 3D rheological model

“…Knoch and Malcherek (2011) developed an isopycnal numerical model to simulate fluid mud, which divides the fluid mud layer vertically into a multilayer system, each layer with an identical density, and introduces rheological models to calculate the apparent viscosity of the fluid mud, thus reproducing the shear thinning properties of the fluid mud. This model is able to represent stratified flow conditions but lacks vertical mixing processes (Schmidt and Malcherek, 2021).…”

“…Recently, Oberrecht (2021) applied this continuous modeling approach to the Delft3D model and simulated fluid mud processes in the Ems estuary. Chmiel et al (2021) developed a vertical one-dimensional model by modifying the kw turbulence model (Wilcox, 1988) to reproduce the vertical mixing from the fluid mud layer to the water layer, and Schmidt and Malcherek (2021) applied the model to the Ems estuary and obtained satisfactory simulation results.…”