An isopycnal numerical model for the simulation of fluid mud dynamics

The progressive extension and development of coastal waterways has led to an increase in siltation and formation of fluid mud in sections of estuarine shipping channels, ports and port approaches over the past decades. The need for a better understanding and a profound knowledge of fluid mud dynamics has increased so that it is necessary to develop new maintenance strategies and renaturation measures in estuaries as well as optimize existing ones. Numerical simulations contribute to the evaluation of such strategies. For that reason, the aim of this thesis is to enable the numerical simulation of fluid mud dynamics. Fluid mud forms by building up a structure of aggregates in regions in which there is an increasing accumulation of cohesive sediments. Although the water content of the high-concentration suspension can be very high, the flow behavior changes from Newtonian to non-Newtonian. However, most of the current established hydrodynamic numerical models solve the shallow water equations with a Newtonian assumption. A standard numerical model approach for the Reynolds-averaged Navier-Stokes equations is therefore extended in this thesis to cover the simulation of non-Newtonian behavior. The developments are based on an existing numerical model in isopycnal coordinates. A vertical resolution by isopycnal layers - layers of constant density - is pursued as the flow can be strongly stratified in systems of high-concentration suspensions. In addition, sharp density gradients characterize the transitional area of fluid mud and water body. The isopycnal discretization enables a vertically resolved simulation of the velocity and density distribution inside the fluid mud body. The isopycnal layers interact due to momentum transfer, mass transfer and interfacial shear stresses. Advective and gravitational transport, mixing and settling of cohesive sediment suspensions are realized by changes in the isopycnal layer thicknesses as each layer represents a suspension with a specific sediment concentration. The vertical transport rates are determined by parameterizations of transport subprocesses and lead to variations in the layer thicknesses. The rheology of fluid mud is dominated by the break-up and recovery of the internal structure in response to shear impact (non-Newtonian behavior). A time- and space-dependent rheological viscosity is therefore introduced into the internal stress terms for the simulation of the rheological behavior of fluid mud. Applications to schematic and realistic model domains demonstrate the abilities and performance of the extended isopycnal numerical model for the simulation of fluid mud dynamics such as simulation of fluid mud influenced by tidal currents.