Modelling of high concentration fluid mud water injection dredging density currents

Abstract

Siltation of fine (cohesive) sediment in navigation channels and harbour basins may cause serious problems for the shipping industry. Intensive maintenance dredging is required to ensure safe shipping traffic. Conventional dredging methods are highly expensive, and therefore port authorities seek for more efficient solutions to reduce costs. One of these solutions is water injection dredging (WID), which in general, proves to be cheaper than hopper dredging by leaving the sediment in place, thus, eliminating substantial costs for relocation of the dredged sediment.

WID is generally applied on soils, which show high concentrations of fines in their particle-size distribution. These kinds of soils, known as fluid mud, exhibit non-Newtonian behaviour. An extensive rheological analysis of fluid mud from the Caland Canal (Port of Rotterdam, The Netherlands), confirmed that fluid mud does indeed show non-Newtonian behaviour. Flow curves obtained by a rotational rheometer provided evidence that fluid mud can be classified as a visco-plastic fluid and furthermore exhibits thixotropical behaviour. The focus of this study is on gaining more insight into the influence of this non-Newtonian behaviour on fluid mud density currents created by WID using detailed numerical simulations.

The numerical simulations were carried out by a variable density 3D computational fluid dynamics (CFD) model. Visco-plastic behaviour was added through the Bingham-Papanastasiou model, in addition, two rheological models, relating the Bingham model parameters to the volumetric concentration of solids were used to capture the rheology of fluid mud. Numerical results were validated by new experimental measurements performed in the water-soil flume at Deltares. Comparison was made between a regular simulation, without rheology, and the two rheological models for different magnitudes of viscosity regularisation. The main difference between the two is the formation of a new bed layer due to the influence of yield stress. This is caused by the relatively high (apparent) viscosity, which is approximately two orders of magnitude larger, decelerating the density current. Apart from this, a sharper density gradient and a lower density current are developed as a consequence of the non-Newtonian rheology.