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.

River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake. In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works.

Deltas and estuaries worldwide face the challenge of capturing sufficient sediment to keep up with relative sea level rise. Knowledge about sediment pathways and fluxes are crucial to combat adverse effects on channel morphology, e.g. erosion which enhances risk of bank collapse and increasing tidal penetration. We constructed sediment budgets which quantify annual changes for the urbanized delta of the Netherlands affected by fluvial and coastal fluxes of sediment, engineering works and dredging and dumping activities. The Rhine-Meuse delta shows a negative sediment budget in recent decades due to anthropogenic intervention. Following a large offshore port expansion, dredging in ports and harbours in the region has doubled in the past five years, likely due to the induced change in net sediment fluxes. In addition, the deeper navigation channels, ports and harbours are trapping siltier sediment than before, changing sediment composition in the mouth. The removal of sediment from the system through dredging is adverse to the necessity for sediment in heavily eroding branches. To allow for sustainable sediment management in the future and to cope with sea level rise, further measurements are required to properly quantify the amount of incoming sediment from the rivers and the seaward boundary and the mechanisms of transport which are key to solving the sediment issues in the delta. The varied response of the branches has important consequences for navigation, ecology and flood safety and management of the sediment in the system will be of pivotal importance in coming decades and for other deltas worldwide.