Cohesive sediment transport on an estuarine intertidal zone

A field study of cohesive sediment movements on a muddy intertidal zone (ITZ) at Portishead, Severn Estuary is presented. Five deployments of 1-3 days have been made to improve the understanding of temporal and spatial variations in suspended and bed sediments at a site which is found to be wave dominated. Pressure transducers, electro-magnetic current meters, optical turbidity meters and ultra-sonic bed level transducers are used to monitor the hydro/sediment dynamics in the bottom metre of the water column at two points on the ITZ. A one-dimensional (vertical) k-ϵ turbulence model has been modified to include bed sediment exchange and density stratification to enable further interpretation of field data. Modelling results show that the weak current on the ITZ would simply provide a mechanism for diffusing suspended sediment into the upper part of the flow, and does not enhance the wave boundary layer by its presence and cannot actively support suspended sediment. Hence near bed (wave generated) turbulence controls the vertical SSC distribution. A bed model based on bed sediment exchange via critical erosional and depositional shear stresses only predicts high near bed concentrations for some of the hydrodynamic conditions reflected in field data, suggesting hindered settling is an important factor in their formation. The introduction of density stratification reduces near surface SSC creating steeper SSC gradients in the upper half of the flow, for a weak current with waves