Ebb-Dominant Mixing Increases the Seaward Sediment Flux in a Stratified Estuary

Intratidal variability in stratification, referred to as internal tidal asymmetry, affects the residual sediment flux of an estuary by altering sediment transport differently during ebb and flood. Although earlier studies suggest that flood-dominant mixing increases the residual landward sediment flux, the role of ebb-dominant mixing remains largely unknown. Based on field data, we investigate the mechanisms that cause ebb-dominant mixing and its effect on the residual sediment flux in a stratified estuarine channel. Observations based on two tidal cycles show that the pycnocline remains largely intact during flood. Vertical mixing during flood is inhibited by a strong fresh water outflow, confining landward transport of suspended sediment to the bottom layer. During ebb, the pycnocline height decreases until it interacts with the bottom boundary layer, resulting in enhanced vertical mixing and sediment transport extending further to the surface. Thus, ebb-dominant mixing increases the residual sediment flux in seaward direction. The long ebb period in combination with limited bed sediment availability further contributes to the residual ebb-flux. This is noteworthy since a long ebb duration corresponds to flood dominance, which is often associated with a landward residual sediment flux. Although our data represent average conditions and cannot readily be extrapolated to different forcing conditions, we conclude that asymmetries in vertical mixing considerably affect the residual sediment flux under average conditions.