Flood/ebb tidal asymmetry in a shallow sandy estuary and the impact on net sand transport

Flood/ebb tidal dominance plays a pivotal role in estuarine sediment transport. The Dyfi Estuary, mid-Wales, UK, is used as a case study, together with an idealized trumpet-shaped estuary morphology, to illustrate key processes giving rise to flood/ebb dominance. Observations made in the main entrance channel to the Dyfi indicate that the system is presently ebb-dominant with regard to both the relatively long duration of the ebb tide and also the faster ebb velocities. Predictions made across the mouth of the estuary using the Telemac Modelling System suggest further that the net sand transport presently occurs out of the estuary. In contrast, due to the varying distribution of channels and flats within the estuary, the tidal flow in the upper estuary is flood-asymmetric and here the net transport is up-estuary. Flood versus ebb dominance is shown to be attributable to the relative extent of the channels and sandflats in relation to the mean water level. An attempt is made to develop a simple metric to determine present-day transport pathways within an estuary in response to its cross-channel flow asymmetries.