Influence of wind direction, wind waves, and density stratification upon sediment transport in shelf edge regions: The Iberian shelf

Abstract

A prognostic baroclinic model including a suspended sediment transport module is used in cross‐sectional form to examine processes, namely, internal tide, wind, and wind waves, which lead to cross‐shelf sediment movement. Although the main focus is a process study to quantify the role of various mechanisms giving rise to ocean‐shelf sediment exchange, the calculations are set in the context of the Iberian shelf. Eddy viscosity and diffusivity are computed from a turbulence energy submodel. Calculations with upwelling and downwelling favorable winds showed that friction velocity and turbulent mixing were a maximum in near‐coastal regions. In these areas, suspended sediment concentration was a maximum. For upwelling favorable winds in homogeneous water, near‐bed sediment was moved toward the coast where upwelling occurred. In this area, sediment was transported into the near‐surface layer and advected offshore by the wind‐driven flow. For the parameters considered here, advection in the near‐surface layer exceeded vertical settling, and maximum sediment concentration occurred at the surface. For a downwelling favorable wind the strong surface currents are onshore, and hence any sediment in the surface layer is advected toward the coast. Although the bottom current is off‐shelf, it is weak, and hence the off‐shelf export of sediment in this layer is less than that found in the surface layer with an upwelling favorable wind. In stratified conditions, with an upwelling favorable wind the intensity of the stratification at the shelf edge increases. Associated with this, there is an increase in the vertical sediment concentration gradient. With downwelling favorable winds, rapid mixing occurs on the shelf, and a shelf front is formed. Sediment concentration on the shelf and at the shelf edge shows a similar distribution to that found under homogeneous conditions. When tidal forcing is included an internal tide is produced that interacts with the wind‐driven flow in a nonlinear manner. Increased bed stress due to the internal tide and vertical circulation cells lead to enhanced sediment erosion above that found with wind forcing only. For an upwelling wind this leads to enhanced off‐shelf sediment movement in the surface layer. For a downwelling wind a circulation develops that tends to inhibit off‐shelf sediment transport. The effect of wind waves is to increase significantly sediment suspension on the shelf. Although the concentration of the sediment is influenced by the wind waves, its spatial distribution is determined by the wind and tidal flows. A detailed analysis of the terms in the sediment transport equation is used to quantify the effects of each process upon the sediment distribution in the water column.