While adapting to future sea-level rise (SLR) and its hazards and impacts is a multidisciplinary challenge, the interaction of scientists across different research fields, and with practitioners, is limited. To stimulate collaboration and develop a common research agenda, a workshop held in June 2024 gathered 22 scientists and policymakers working in the Netherlands. Participants discussed the interacting uncertainties across three different research fields: sea-level projections, hazards and impacts, and adaptation. Here, we present our view on the most important uncertainties within each field and the feasibility of managing and reducing those uncertainties. We find that enhanced collaboration is urgently needed to prioritize uncertainty reductions, manage expectations and increase the relevance of science to adaptation planning. Furthermore, we argue that in the coming decades, significant uncertainties will remain or newly arise in each research field and that rapidly accelerating SLR will remain a possibility. Therefore, we recommend investigating the extent to which early warning systems can help policymakers as a tool to make timely decisions under remaining uncertainties, in both the Netherlands and other coastal areas. Crucially, this will require viewing SLR, its hazards and impacts, and adaptation as a whole.

In recent years, increased salt intrusion in surface waters has threatened freshwater availability in coastal regions worldwide. Yet, current future projections of salt intrusion are limited to local regions or changes to single forcing agents. Here, we quantify compounding contributions from changes in river discharge and relative sea level to changing future salt intrusion under a high-emission scenario (Shared Socioeconomic Pathway, SSP3-7.0) for 18 estuaries around the world. We find that the annual 90th percentile future salt intrusion is projected to increase between 1.3% and 18.2% (median 9.1%) in 89% of the studied estuaries worldwide. Our analysis also indicates that, on average, sea-level rise contributes approximately two times more to increasing future salt intrusion than reduced river discharge. We further show that the return levels of present-day 100-year salt intrusion events are projected to increase between 3.2% and 25.2% (median 10.2%) in 83% of the studied estuaries.

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.

Estuaries are among the most densely populated and heavily utilised regions in the world, where crucial functions – e.g., freshwater availability and water safety – strongly relate to the natural dynamics of the system. When developing nature-based solutions to safeguard these essential functions, a thorough understanding of estuarine dynamics is required. This study describes an elaborate sensitivity analysis on the salt intrusion length using an idealised estuary, which is parametrically designed using key estuary-scale parameters – e.g., river discharge and tidal flats – to cover a wide range of estuary classes. We were able to systematically investigate such a wide range of estuary classes due to the combination of (1) state-of-the-art hydrodynamic modelling software, (2) high performance computing, and (3) reduction and analysis techniques using machine learning. The results show that the extent of the estuarine salt intrusion length is largely determined by four estuarine features: (1) river discharge; (2) cross-sectional area (especially water depth); (3) tidal damping/amplification; and (4) tidal asymmetry. In general, the salt intrusion length shows clear correlations with (a combination of) estuary-scale parameters, which all put an upper limit on the salt intrusion length. These relations provide crucial insights for successful development of nature-based solutions to mitigate salt intrusion in estuarine environments.

The morphology of tide-dominated systems is progressively influenced by human activities and climate change. Quantitative approaches aiming at understanding or forecasting the effects of interventions and climate change are often aggregated, thereby simplifying or schematizing the investigated area. In this work, we advance on the knowledge of sediment transport processes shaping tidal systems and on methodologies translating schematized model output into physically realistic variables. In terms of improved physics, we systematically evaluate the influence of sand-mud interaction processes. Most tidal systems are shaped by a mixture of sand and mud. Morphological models typically compute transport of sand and mud independently, despite studies clearly demonstrating that their physical behavior is mutually dependent. We investigate the effects of two interaction mechanisms (erosion interaction and roughness interaction, applied with varying mud erodibility) with a schematized process-based morphodynamic model. We convert model output into metrics that describe the meso-scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. Modeled patterns and intertidal flat shape, size and composition widely vary with mud erodibility settings, but equally depend on the evaluated sand-mud interaction mechanisms (with erosion interaction having a larger effect than roughness interaction). Sand-mud interaction thus needs to be accounted for from a physical point of view, but also to improve predictions of tidal basin evolution models, particularly the (bimodally distributed) sediment composition of intertidal flats.

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.

The sediment composition of the seabed governs its mobility, hence determining sediment transport and morphological evolution of estuaries and tidal basins. Bed sediments often consist of mixtures of sand and mud, with spatial gradients in the sand/mud content. This study aims at increasing the understanding of processes driving the sediment composition in tidal basins, focusing on depositional processes. We show that bed sediments in the Wadden Sea tend to be either mud-dominated or sand-dominated, resulting in a bimodal distribution of the mud content where the two modes represent equilibrium conditions. The equilibria depend primarily on the sediment deposition fluxes, with bimodality originating from the dependence of suspended sand/mud concentrations on the local bed composition. Our analysis shows that bimodality is a phenomenon that is not only specific for the Wadden Sea; it can be expected for a wide range of suspended sediment concentrations and thus also in other systems worldwide.

The sediment exchange between the Dutch Wadden Sea and the North Sea coastal zone is of key importance to Dutch coastal management. Net sediment import from the coastal zone to the Wadden Sea results in coastal erosion which needs to be compensated through nourishments. At the same time net sediment import is the source of sediment for the intertidal flats in the Wadden Sea to adapt to sea level rise (SLR). Understanding the current and future sediment exchange is therefore essential for sustainable coastal management. Insights in the sediment exchange directly influence the coastal nourishment strategies applied to the Dutch coasts. Projections of the future sediment exchange between the Dutch Wadden Sea and the North Sea are established using the aggregated morphodynamic model ASMITA for five sea level rise scenarios, viz. the present rate of 2 mm/yr and accelerated rates of 4, 6, 8 and 17 mm/yr in 2100. The differences in the projected import rates between the five sea level rise scenarios until 2100 are not as large as the differences in sea level rise rates may suggest. For the Eastern part of the Dutch Wadden Sea, where the morphology is near its dynamic equilibrium, the projected import rate in 2100 varies with a factor 3 (300%), for sea level rise rates from 2 to 17 mm/yr (factor 8.5, 850%). In the western part of the Dutch Wadden Sea, where the morphology is still far from equilibrium due to the closure of the Zuiderzee, the projected import rate in 2100 varies a factor 1.45 (145%) for these sea level rise rates. For the total Dutch Wadden Sea this is a factor 1.7 (170%). The projected increase of the import rate until 2100 with respect to the present situation (2020) is up to a factor 1.45 (145%) for the highest sea level rise scenario, which is significant but not substantial.

The Wadden Sea is a unique intertidal wetland area, forming an important hub for migratory water birds. A feared effect of accelerated sea-level rise (SLR) is the gradual loss or even disappearance of the ecologically valuable intertidal flats. To date, the effect of SLR on the time-evolution of the intertidal areas in the Dutch Wadden Sea has not been studied. To explore the sensitivity of the intertidal flats to SLR and the spatial differentiation of the response, simulations are carried out with the reduced-complexity model ASMITA for four sea level rise scenarios: one with a stable rate of 2 mm/yr (current rate), and three with accelerated sea level rise rates to respectively 4, 6 and 8 mm/yr. In addition, a scenario with a linearly increasing rate to 17 mm/yr in 2100 has been added to get an impression of what may happen under more extreme SLR-rates. The results show that the intertidal flats in the larger basins are most vulnerable to drowning. Due to differences in tidal flat geometry, the intertidal flats in the smaller basins mainly reduce in average height, while the intertidal flats in the larger basins mainly reduce in surface area. Within the basins, largest losses are expected to occur just off the land reclamation works and along the western part of each tidal watershed. The intertidal flats are sensitive to the rate of SLR. With doubling the rate of SLR, losses nearly double as well. Complete drowning is not predicted for any of the considered scenarios, but for the larger basins volume losses of nearly 50% by 2100 are predicted for the highest considered scenario. This will transform these basins into more lagoon-like basins, which is expected to have major consequences for the ecology.

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.