Physical and Numerical Modeling of the Entrainment by a High-Concentration Mud Suspension

This paper presents the results of a physical and numerical model study on the entrainment by a high-concentration, cohesive sediment suspension. This turbulent, near-bed suspension is referred to as a concentrated benthic suspension (CBS). In the current configuration, the CBS entrains low-concentrated overlying water. The results of the physical and numerical experiments are in line with the results of similar experiments, published in literature, in which, in contrast, the upper, turbulent layer entrains water and sediment from the lower layer. The entrainment rate for these experiments is almost inversely proportional to the bulk Richardson-number $E\propto {\mathrm{Ri}}_{*}^{-n}$, with $n\approx 1$, indicating a profound effect of the side wall of the experimental facility. We anticipate that the conditions studied in this paper are characteristic for the behavior of many high-concentration systems, such as the mouth of the Amazon River, and in the Loire Estuary and Ems River. A sequence of entrainment during high flow velocities, and settling/consolidation during low flow velocities, driven by tidally induced horizontal pressure gradients, keeps the sediment fluid and mobile; this process can be referred to as tidal pumping. The experimental results could be reproduced to a fair degree by a 1DV point model. This, however, does not necessarily imply that state-of-the-art numerical engineering models are readily applicable for simulations of the hydro-sedimentological conditions in complicated systems, such as the Amazon mouth and Loire Estuary. In these systems, one expects a gradual transition from a fully turbulent upper part of the CBS layer, to laminar conditions deeper down within this layer, with increasing values of viscosity and yield strength; such transition is not accounted for in state-of-the-art turbulence models. Yet, the current work provides a framework for a better understanding of these systems.