The influence of density fronts on sediment dynamics within river-to-sea estuarine transitional waters

The interaction of river-borne suspended and bedload sediments with marine-derived sediments is complicated by the sharply defined lateral and transverse water density gradients of frontal systems that characterise many estuaries worldwide. These features, often recognised at the water surface as linear bands of foam or flotsam, develop principally due to tidal intrusion, axial convergence, longitudinal shearing and flow separation. However, knowledge of the relationships of such features with bathymetry is still poorly developed. Not only do frontal systems impact upon intra-water column fine particulate transport by entrapment and compartmentalisation, they also exert a control on the distribution of bedforms and bottom sediment grain size distributions, thereby delimiting often closely juxtaposed, but differing bottom current velocity fields on the ebb and flood tidal phases. In consequence, fronts are now considered as "sieves" within the transitional river-to-sea, estuarine sediment transfer system; this perception has been reinforced by numerical study of lateral grain size sorting.