Modelling Cohesive Sediment Dynamics in the Marine Environment

Abstract

One of the most challenging tasks in modelling marine processes is the description of fine sediment dynamics. Cohesive sediments enter the marine environment from physical or anthropogenic sources and affect physical, biochemical and biological processes. Their movement and fate is the result of various physical processes and forces that interact with one another. These processes extend from the scale of the microfloc and the effects of the small-scale forces, like cohesion, to the macroscale of the hydrodynamic field and the effects the local hydrodynamics. The present chapter aims at covering the most significant processes that control the motion and fate of cohesive sediment in microtidal areas, focusing both on the physical and mathematical description of the phenomena.More specifically, the physical processes that take place in the water column and near the seabed and the corresponding mass-exchanges between them are presented and analyzed. The motion of sediment in suspension is determined by the seawater velocities and local shear, the stratification of the water column and the characteristics of the cohesive flocs that control the processes of advection, dispersion flocculation-deflocculation and settling. Near-bed processes are highly dependant on the prevailing shear stress conditions and the characteristics of the sedimentary matter and control the mass-exchanges between the water column and the seabed; these processes include deposition, self-weight consolidation, resuspension and erosion of the bed. Modelling results from a three-dimensional sediment transport model, formulated based on the aforementioned considerations, are also presented and discussed; the applications were performed in the domain of Thermaikos, a microtidal gulf in the NW Aegean Sea (Eastern Mediterranean), for different (point or distributed) sources of fine particulate matter, including river-borne matter, physically and mechanically (trawling-induced) eroded sediments and aeolian transported dust.