Phase relationships between fine sediment suspensions and tidal currents in coastal seas

In shelf and coastal waters, point measurements of fine suspended sediment concentrations commonly display phase and frequency differences with respect to local current forcing. Phase shifts are often attributed to tidal advection and slow-settling velocities of cohesive sediments. The phase relationship between tidal current speeds and near-bed mud concentrations obtained at a site off the east coast of England is investigated. A simple depth-averaged advection-diffusion model is introduced and used to explore the influence of relative changes in settling, advection, and local erosion on the temporal evolution of suspension events. The model includes two source terms: local resuspension in phase with current speed and advection of a horizontal gradient in phase with tidal displacement. Incorporating a settling (or sink) term introduces a negative or positive phase shift in the advection or resuspension term, respectively. The settling term accounts for the vertical structure of suspension and becomes important at slack water when the near-bed concentration, and thus deposition, rapidly increases. Model results are compared with suspended mud concentration data to help explain the observed phase relationships between concentration and current speed.