Processes influencing suspended sediment movement on the Malin–Hebrides shelf

Abstract

A three-dimensional baroclinic model, including suspended sediment transport, is used in cross-sectional form to examine processes (tidal, along-shelf current, wind-waves, and wind) influencing suspended sediment transport off the west coast of Scotland. Sub-grid scale processes are parameterized using a turbulence energy model. Calculations using appropriate sediment types and tidal forcing for the region, show that the barotropic tide alone essentially cannot suspend bottom sediment in the area. However, when an along-shelf flow is added some sediment is suspended. An off-shelf net transport of sediment occurs in the bottom layer, with a weaker on-shelf transport in the surface region. This is consistent with recent measurements in the area which show the lack of tidal resuspension, and the importance of an along-shelf flow in sediment suspension. Observations reveal a weak off-shelf flow in the bottom boundary layer. The magnitude of this transport is increased by the presence of wind-waves, or by a reduction in the sediment settling velocity. Observations during a storm clearly show the importance of wind-waves in causing sediment resuspension particularly close to the shore. When density stratification was included, internal tides were generated with associated regions of enhanced bottom friction velocity u * and hence increased sediment resuspension. Although the dominant feature of the flow is similar to that found with the barotropic tide, this is modified by smaller scale currents due to the presence of the internal tide. Summer observations indicate that strong internal waves can enhance sediment suspension. However from the limited number of observations and the high spatial variability of the internal tide found in the model, the importance of the internal tide is difficult to assess from the measurements. When an upwelling favourable wind is added, the vertical density gradient in the near-bed region increases, which changes the internal tide and hence its effect upon sediment suspension and transport. The presence of an along-shelf flow induced by the wind significantly increases the bed stress leading to enhanced sediment resuspension. Sediment transport is dominated by the wind-driven flow, with coastal upwelling moving sediment from the bottom boundary layer to the surface layer where it is advected off-shelf. Observations made during upwelling favourable winds, confirm this off-shelf advection of sediment in the upper part of the water column. Conversely with a downwelling favourable wind, the thickness of the bottom boundary layer is increased, and in the shelf edge region sediment is suspended to a greater height in the water column. In the near-coastal area, under downwelling conditions, the sediment is confined to the bottom boundary layer and is advected away from the shore in the near-bed region.