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.

Effects of sea-level rise (SLR) on future peak water levels in tidal deltas and estuaries are largely unknown, despite these areas being densely populated and at high risk of flooding. While the rates of SLR accelerate, many channels simultaneously experience channel deepening for navigation. With globally decreasing sediment supplies, most channels are at risk of becoming deeper when the rate of SLR accelerates and sedimentation cannot keep pace with SLR. These factors potentially favor amplification of the tides and thereby increase flood risk, but the extent to which they will do so is unknown. Here, we introduce and use a validated model for an artificially deepened multi-branch delta to get a mechanistic understanding of non-linear SLR-effects on peak water levels. Results show that, when the current deepened bed level will be maintained, peak water levels do not rise on par with mean sea-level. Thus flood risk increases less than what can be expected from the predictions of the mean sea-level increase. The reason is that SLR causes a proportional reduction in convergence of channel area. This mechanism reduces tidal amplification. Nevertheless, SLR effects extend far beyond the range of present-day seasonal variations, with future low water levels being equal to present-day high water levels, while the tidal range slightly reduces. This will have consequences not only for flood risk, but also for freshwater availability, navigation and ecology.