Theoretical modelling of deep ocean sediment transport

Abstract

Predicting the fate of suspended material requires knowledge of both sediment concentration and velocity fields. These are governed by the advection and diffusion of mass and momentum respectively and are coupled by the turbulent diffusion process crucial to both. The link between the two is the boundary shear stress which determines force exerted on the bottom by the flow and vice versa. The response of sediment to fluid forcing is complicated by its cohesiveness both from physical and biological processes. This is important not only in the bed but also in suspension where flocculation can occur. During eroding events the friction velocity is much greater than the mean settling velocity so the sediment essentially behaves as a passive contaminant; however, in the low-velocity periods the suspension is a collection of different settling velocity classes to be treated separately but coupled by the aggregation process. The deposition process can be complex as is evidenced by the formation of longitudinal ripples. These features may be formed due to helical circulation arising from non-uniform turbulence field. The cause of the variation in turbulence is postulated to be inhomogeneities in the concentration field that damps the turbulence through the effect of stratification.