A model study of tidally induced suspended sediment transport in the Iberian shelf edge region

Across shelf suspended sediment transport due to tides is investigated using a three-dimensional baroclinic hydrodynamic model, including a sediment transport capability, and parameterizing sub-grid scale diffusion using a turbulence energy model. The model in cross shelf form, with topography characteristic of the shelf edge off the west coast of Spain, is used to examine sediment movement due to both barotropic and baroclinic tides and an along-shelf flow. Calculations showed that the bed stress due to the barotropic tide and an along-shelf flow exceeded the critical bed stress for part of the tidal cycle, leading to sediment suspension. The across shelf residual flow (due to tidal rectification and the bottom Ekman transport associated with the along-shelf flow) was on shelf in the surface layer and off shelf near the bed and produced a net off shelf sediment transport in the bottom boundary layer.Whena fixed stratification representing the thermocline was imposed, this inhibited the upward diffusion of sediment, thereby increasing its concentration in the bottom boundary layer. Comparison with and without stratification allowed its influence upon sediment concentration to be quantified. Calculations in which the stratification evolved with the flow field produced internal tides with an associated increase in current amplitude at a number of locations, leading to greater spatial variability in the tidal residual, and an additional residual due to the across shelf tidally induced thermal front. Increased diffusion and vertical velocity associated with the internal tide produced a thickening of the sediment bottom boundary layer. A detailed examination of terms in the sediment transport equation enabled the importance of each in different regions to be quantified. It showed that in the near bed layer, the main balance was between vertical diffusion and settling. However, sediment concentration in the upper part of the water column was increased by the presence of the internal tide due to enhanced vertical advection and diffusion into the region. Changes in vertical stratification, were found to influence both the horizontal and vertical distributions of sediment, due to its influence upon the internal tide.