Where rivers meet the sea, deltas form: branched, dynamic coastal ecosystems that have historically been attractive places for human settlement. Within these deltas, fresh riverine water and saline water meet in transitional zones, estuaries, and at the point where the river flows out into the sea, a river plume may be formed. To provide humans with services such as food, drinking water, and shelter, many deltas have been heavily modified. At the same time, these deltaic systems are typically highly vulnerable to such environmental change, so that the combination of human pressures and climate change can significantly amplify risks related to freshwater resources, ecosystem biodiversity, and coastal safety.

While climate-change-intensified flood risk has been extensively studied, risks to freshwater availability, such as salt intrusion, have received less attention. Salt intrusion is controlled by a complex interplay of river discharge, tides, wind, stratification, and human interventions. The combined effect of these processes on salt intrusion remains poorly understood, particularly in complex, multi-branch deltas or estuarine networks, such as the Rhine-Meuse Delta.

The overarching objective of this thesis is to gain more insight into salt intrusion dynamics in the coast-delta system of the Rhine-Meuse Delta. Understanding these dynamics is critical for both scientific and management purposes. This dissertation takes a modelling approach to addressing this objective. However, currently available realistic hydrodynamic models of the region often do not resolve all relevant scales, and analysis tools are unable to extract the underlying processes from the complex datasets they generate. Thus, this work is structured into two complementary parts: the first focuses on developing a suitable model and analysis tools, and the second applies these tools to investigate salt intrusion dynamics.

First, the scientific basis of this dissertation is established by reviewing the physical mechanisms governing salt intrusion in estuaries. The multi-scale nature of these processes is highlighted, from small-scale mixing to estuary-scale exchange flows. For salt intrusion in the Rhine-Meuse Delta, the most important processes are shown to occur on the scale of meters to the entire coast-delta system and on the intratidal to fortnightly timescale. Identifying these scales helps guide the choice of analysis methods. To extract the underlying processes from complex, realistic models spanning all these scales, this thesis analyses terms in the salinity variance analysis.

Identifying relevant scales for salt intrusion also informs hydrodynamic model selection. For this thesis, the realistic, three-dimensional, unstructured Delft3D Flexible Mesh Rhine-Meuse Delta model is chosen. The model is further developed to increase its predictive capability for an average-discharge year by adding a heat flux model and improving some of the boundary conditions. This new version of the RMD model is then validated for an average-discharge year using an extensive set of measurements in the region. This validation shows excellent reproduction of water levels, temperature, and good reproduction of salinity throughout the estuarine domain, thus ensuring that this model provides a reliable tool for investigating the mechanisms that control salt intrusion in the Rhine-Meuse Delta.

Next, the challenge of applying the chosen analysis methods to the RMD model is considered. Models with staggered unstructured grids, like the RMD model, are made up of polygons that can have any shape, as long as they adhere to certain orthogonality constraints. This, combined with the way datasets of models with staggered unstructured grids are structured, results in large, complex model output datasets. To circumvent computationally expensive calculations on these types of grids, output is often interpolated onto structured grids. However, the salinity variance analysis requires volume conservation and can thus not be applied to interpolated data. It is found that three practical implementation strategies are key to enabling complex computations on larger-than-memory unstructured datasets: unchunking alignment dimensions, ensuring perfect chunk alignment for element-wise operations, and strategically circumventing automatic alignment operations. A scientific software Python package, pySVA, is developed to apply the novel practical implementation strategy to model output datasets generated by the RMD model.

Using the tools developed in the first part of this thesis, the feedback between wind direction, river plume distribution, estuarine exchange flow, and salt intrusion is explored. Although river plumes are formed at the outflow of estuaries and the two are thus logically connected, it is unclear whether there is a dynamic two-way coupling between the estuary and the river plume — i.e., whether the river plume also influences the estuary. This is investigated by considering the influence of wind on the Rhine-Meuse Delta through the modification of the Rhine river plume. It is found that wind (direction and speed) modifies the plume's extent, stratification, and the strength and location of vertical mixing. Upwelling winds stretch and thin the plume, strengthen stratification, and influence the estuary by enhancing exchange flow through a shoreward bottom current. Downwelling winds thicken and narrow the plume, oppose estuarine circulation via altered cross-shore residual flow, and reduce exchange flow strength. Furthermore, downwelling winds can completely suppress cross-shore straining by prolonging the alongshore flood. Onshore winds attach the plume to the coast and suppress cross-shore straining, but through a different mechanism than under downwelling winds: by directly altering the cross-shore flow. Onshore winds decrease the cross-shore residual flow but increase the depth-averaged onshore current, resulting in relatively large exchange flows but low salt exchange efficiency. Thus, plume changes propagate into the estuary, altering the stratification, residual velocity profiles, and, ultimately, the strength and composition of the estuarine exchange flow. The net effect is that upwelling winds generally reduce salt intrusion, while downwelling and onshore winds increase it.

Finally, the effects of human intervention into the Rhine-Meuse Delta system are explored by considering the scenario of adding a second sea connection to the Rhine-Meuse Delta through permanently opening the Haringvliet floodgates. To assess this, two simulations with the RMD model are conducted: a reference scenario representing the average-discharge year and a scenario with permanently open floodgates and the same forcing as in the reference. It is found that the intervention changes the discharge distribution, alters tidal wave propagation and phasing, and reduces stratification in the northern branches of the estuary and the river plume. Placing the branches into the estuarine parameter space (Geyer & MacCready, 2014) shows that, as a result, almost all branches in the estuarine network undergo regime changes, and the salt intrusion length increases in all branches — an effect that cannot be explained by the commonly used steady-state salt budget. Additionally, comparing the two scenarios provides insight into the processes affecting salt intrusion in the Rhine-Meuse Delta in more general terms. Counterintuitively, analysis of the salinity variance analysis reveals that positive straining is negatively correlated to salt intrusion length, which challenges our current understanding of salt intrusion in salt wedge estuaries. The key to understanding this is to realise the importance of horizontal dissipation. Horizontal dissipation is found to be strongly correlated with straining, which is hypothesised to be related to baroclinic convergence and frontogenesis. Additionally, horizontal dissipation and the salt intrusion length are strongly negatively correlated, suggesting that horizontal dissipation is an important factor affecting salt intrusion in the Rhine-Meuse Delta.

Overall, this thesis demonstrates how coupled estuary–plume dynamics, advanced hydrodynamic modelling, and process-based analysis methods can be integrated to quantify and understand the physical drivers of salt intrusion, offering new insights into estuarine behaviour and informing sustainable management of complex deltaic systems. ... Read more

The Rhine-Meuse Delta is a low-lying delta in the Netherlands that is subject to both salt intrusion events and storm surges. Typically, storm surges only temporarily cause increased salt intrusion and do not cause severe problems for freshwater availability. However, during the storm surge of December 2013, salt reached the closed southern branch of the delta and higher salinities were observed for weeks after the storm surge. The purpose of this study is to examine the mechanisms controlling salt intrusion in the Rhine-Meuse Delta during and after a severe storm surge event. A three-dimensional hydrodynamic model (Delft3D-FM) of the Rhine-Meuse Delta was developed that successfully reproduces salt intrusion for both normal and storm surge conditions. During the storm, high water levels in the northern branch caused a salt flux toward the southern branch. The southern branch of the Rhine-Meuse Delta is closed off by an estuarine dam, consequently salt was retained landward of the dam. Local stratification in the southern branch caused salt to remain in the deeper parts, limiting the effectiveness of flushing after the storm surge. In the post-storm period, salt was gradually released from the southern branch, raising salinity levels in an adjacent channel. The river discharge was only just below the yearly average, showing prolonged salt intrusion can also occur outside of dry periods. ... Read more

Deltas are home to billions of people and are often highly developed and engineered systems. Extreme weather events such as droughts are a threat to deltas worldwide. During droughts salt can intrude far inland and threaten the drinking, agricultural and industrial water supply of many people. Under climate change the frequency of extreme events is expected to increase and the threat of salt intrusion may intensify. Here we use data and models to explore salt intrusion in the Rhine-Meuse Delta (RMD) during the severe European drought in the summer of 2022. The RMD is one of the most highly managed deltas in the world, with numerous interconnected waterways and an open connection to the sea at the mouth of the Rotterdam Waterway. The outflow of the Rhine River through the Rotterdam Waterway generates the strongly stratified Rhine River plume. Under normal conditions a salt wedge intrudes about 16-18 km inland on every tide. In contrast, under drought conditions in summer 2022, observations show salt intruding over 42 km inland and the Rhine River plume diminished in size. We explore the changes in estuarine dynamics during the drought using velocity, salinity and temperature data from various field campaigns near the mouth of the Rotterdam Waterway and within the delta, together with numerical models. We also compare drought condition observations with data from prior field campaigns during normal discharge conditions. Shifts in the relative strength of the dominant mechanisms of landward salt flux throughout the drought are explored and linked to the changes in estuarine response. ... Read more

Under the influence of climate change, estuaries around the world are increasingly exposed to more extreme weather conditions. In recent years droughts specifically have been occurring more frequently and for prolonged periods. During a period of drought, salt intrusion is exacerbated, impacting the availability and quality of water resources and the estuarine ecosystem. As the impacts of droughts can be severe, assessment of droughts and their influence on the estuarine system is of great importance.

Using a high-resolution 3D coupled ocean-delta model we investigate the influence of the record-breaking European drought of the summer of 2022 on the Rhine-Meuse Delta and compare this to the estuarine response under average discharge conditions, putting the drought’s influence into perspective. Spatial patterns of stratification, mixing, and straining and their evolution throughout the drought period are studied by a salinity variance analysis. The progression of the salt wedge and retreat of the tidal plume fronts are examined and related to the changing strength of the individual estuarine processes influencing stratification. We show that as the tidal plume fronts retreat during the drought, we see a corresponding change in the structure of the salt wedge, demonstrating the importance of the coupling between the tidal plume fronts and the estuarine dynamics. ... Read more

Tidal river plumes dominate many shelf seas, transporting freshwater, sediment, nutrients, pollutants and larvae downstream. The Rhine River Plume is one of the largest in Europe, under typical discharge conditions it is dominated by tidal plume fronts in the near to mid-field plume and by tidal straining in the mid- to far field plume. Moreover, in agreement with other tidal river plumes discharging onto the shelf, internal waves generated ahead of tidal plume fronts are an important source of mixing in the river plume. We compare field data collected downstream of the mouth of the Rhine River in 2013 and 2014 under typical discharge conditions, with data collected in the near field plume during 2022 during a major drought. Together with numerical models we explore how extreme variations in freshwater discharge impact both tidal straining and the formation and strength of tidal plume fronts. Furthermore we explore how in turn, this influences the structure and mixing of the near to far-field Rhine River Plume. We use a 3D hydrostatic model of the Rhine River Plume and a potential energy anomaly analysis to explore changes in the mixing. We explore how the river plume adjusts to extremely low discharge conditions and discuss the possible impact on the transport of freshwater, tracers, larvae and fine sediment. ... Read more

Strongly forced salt wedge estuaries are known to demonstrate significant variability in stratification, currents, and mixing within a tidal cycle. Such estuaries also show strongly spatially varying patterns of stratification and mixing, and the processes causing this spatial and temporal variability are known to interact. As a consequence, the dynamics of these estuaries are distinctly different from well-mixed and partially mixed estuaries, and intratidal processes may play a more central role in the evolution of stratification.

The Rhine-Meuse estuary is an example of a strongly forced salt wedge estuary. It is very dynamic, mesotidal, and stably stratified. In this study, we investigate how individual intratidal estuarine processes contribute to the evolution of stratification in the Rhine-Meuse estuary. Data from recent shipboard measurements are used to assess their relative influence and highlight the potential importance of interactions between these processes.

From measurements in the Rhine-Meuse estuary we find that the availability of salt is determined by tidal advection of the salt wedge. Additionally, exchange flows transfer salt from high-density to low-density regions such as harbour basins and side branches while the salt wedge is advected through the estuary. The combination of the barotropic tidal asymmetry imposed at the river mouth and turbulence damping at the pycnocline results in strong shear and subsequent formation of mid-depth jets at the onset of flood. These mid-depth jets contribute to the transfer of salt by transporting salt from regions of higher momentum to regions of lower momentum. Furthermore, the measurements suggest that several bathymetric transitions locally generate internal wave activity, although the resulting turbulent mixing is not strong enough to erode the persistent salt wedge structure.

These findings underline the importance of interactions between intratidal processes on different spatial scales and their effect on the evolution of stratification in the Rhine-Meuse estuary. As an extension to our findings, measurements in the Rhine region of influence (ROFI) are used to further examine the role of the seaside forcing on the individual physical processes and the resulting intratidal variability of stratification in the estuary. ... Read more

Plastic debris has become an abundant pollutant in marine, coastal and riverine environments, posing a large threat to aquatic life. Effective measures to mitigate and prevent marine plastic pollution require a thorough understanding of its origin and eventual fate. Several models have estimated that land-based sources are the main source of marine plastic pollution, although field data to substantiate these estimates remain limited. Current methodologies to measure riverine plastic transport require the availability of infrastructure and accessible riverbanks, but, to obtain measurements on a higher spatial and temporal scale, new monitoring methods are required. This paper presents a new methodology for quantifying riverine plastic debris using Unmanned Aerial Vehicles (UAVs), including a first application on Klang River, Malaysia. Additional plastic measurements were done in parallel with the UAV-based approach to make comparisons between the two methods. The spatiotemporal distribution of the plastics obtained with both methods show similar patterns and variations. With this, we show that UAV-based monitoring methods are a promising alternative for currently available approaches for monitoring riverine plastic transport, especially in remote and inaccessible areas. ... Read more