We aim to improve modelling of sediment transport in stratified channels such as the New Waterway, so we can better predict how dredging requirements respond to climate change or human interference.
Figure 1: Measurement campaign to understand sediment dynamics in the New Waterway near Maassluis. Photo by Judith Poelman (8/3/2022).
Motivation and Practical Challenge
The Rhine-Meuse Estuary (RME) is the region where the Rhine and Meuse rivers flow into the North Sea. Since the construction of storm flood protections and large dredging works in the port area, some channels in the RME become shallower while others are eroding. This is already a problem for shipping and threatens stability of dikes and bridges in the RME. Furthermore, it is uncertain how the bed levels will respond to future changes such as sea level rise or human interferences such as further deepening of the entrance channel.
Existing models fail to simulate these long-term bed level changes, because the interaction between salt and fresh water and the effects on sediment transport are not properly included. This study aims to improve our understanding of sediment transport in the New Waterway and similar channels.
Figure 2: Overview of the Rhine-Meuse Estuary (looking eastward), with the New Waterway on the left side of the picture. Source: NASA World Wind web application
The future bed level development of the channels in the Rhine-Meuse Estuary (Figure 2) is uncertain due to uncertainty in the sediment import and export, and due to lacking understanding of local sediment transport. We aim to better understand sediment transport in the New Waterway through field campaigns (Figure 1) and modelling.
An extensive field campaign in the New Waterway was carried out to better understand to transport of sediment (ranging from fine silt to sand) in this channel. Since the New Waterway is characterized by a strong salt-wedge type of flow (Figure 3), the results of the data analysis will contribute to our understanding of sediment transport in heavily stratified channels.
In the New Waterway, and many other estuarine channels, a mixture of sand, silt and clay is present. The interaction between these different types of sediment is not well understood. We will carry out laboratory experiments in the Kraijenhoff van de Leur Laboratory for Water and Sediment Dynamics at Wageningen University, studying the transport processes of sediment mixtures.
The resulting knowledge of sediment transport in stratified flow and of sediment transport in mixtures, will be incorporated in a numerical model. This allows to better predict bed level changes in estuaries as a response to climate change or human measures.
Figure 3: Visual explaining landward transport of sediment as a result of saline water with a large density flowing underneath an upper fresh water layer.
Figure 4: Transport of uniform sediment (left) depends on flow strength and resulting bed shear stress. The transport of sediment mixtures such as present in the New Waterway (right) is more complex due to hiding-and-exposure and possible cohesiveness.
Relevant for whom and where?
The results will be incorporated in existing modelling tools, which can be applied by river managers to study the effect of measures such as further deepening of port entrance channels.
Findings and practical application
The measuring campaign reveals the along-channel residual flow velocity and shows a sharp transition between landward flow and seaward flow at the transition depth between salt and fresh water. Combined with the measured sediment concentrations, we can calculate the landward and seaward sediment fluxes during a spring tidal cycle and a neap tidal cycle. The next step is to distinguish between different sediment fractions: this is important, since fine sediment (mud) typically settles in harbour basins, while coarse sediment (sand) settles both in harbour basins and channels. The sediment size thus determines where dredging is required. Moreover, information on grain sizes are relevant for modelling applications. With these models, we try to predict how the required dredging volumes and locations will change under changing conditions such as sea level rise, increased river flow or channel deepening.
Status for day to day practise
In the future, these findings will (hopefully) be incorporated in existing modelling tools, so that the residual sediment transport can be predicted in various deltas around the world.
The field data analysis and laboratory experiments are ongoing. In the end, we’d like to incorporate these findings in existing models such as Delft3D. One component that we still need more information on, is the sediment size distribution on the bed of the New Waterway. Sampling in this channel is not straight-forward, due to its depth and large amount of shipping.
Last modified: 05/10/2022
Residual sediment transport in a stratified estuarine channel
Based on field measurements and sediment transport modelling, we aim to unravel the mechanisms controlling residual sediment fluxes in highly stratified estuarine channels, by focusing on the New Waterway.
13/04/2022 by Iris Niesten