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. While 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 further contributes to the residual ebb-flux. This is
noteworthy since a long ebb duration, as it corresponds to flood
dominance, is often associated with a landward residual sediment flux.
Although our data represent average conditions and may not be
representative for high river discharge or storm conditions, we conclude
that asymmetries in vertical mixing considerably affect the residual
sediment flux.