Density fronts: Sieves in the estuarine sediment transfer system?

Sharply defined water density gradients (due principally to salinity contrasts but also to temperature effects) and velocity gradients occur both longitudinally and transversely in many estuaries. These gradients are related to frontal systems (fronts) which, at the water surface, are typically manifest as lines or bands of foam, floating debris or distinct changes in the colour and transparency of the water. Fronts, which form by several mechanisms, including tidal intrusion, axial convergence, advective flow and flow separation, acts as temporary barriers, serve to inhibit exchange of water masses and to entrap fine particulate materials. It is suggested that, in terms of suspended sediment transport through and within estuaries that are characterized by fronts, such features should be considered as “sieves” in the estuarine sediment transfer system. Where characterised by many fronts, an estuary as a whole should most appropriately be considered as a complex of sieves, which collectively create a dynamic “sieve regime”.