Estuaries are considered valuable regions, both socio-economically and ecologically. The gradients in physical characteristics like salinity present result in a high biodiversity, while the provision of many ecosystem services have attracted human settlement and activity. Human activities and estuarine biodiversity are often at odds with each others, leading to socio-ecological trade-offs in decision- and policy-making in which the ecological perspective is generally underrepresented. In this study, we implemented a hydrodynamic model to explore the socio-ecological implications of reopening the closed-off Haringvliet estuary in the Netherlands. Our socio-ecological evaluation considers the trade-off between freshwater availability and ecological diversity. In the case of the Haringvliet, we have shown that partially opening the gates enhances diversity in the system against no — or limited — loss of freshwater availability. All in all, the use of representative (non-monetary) performance indicators for the considered stakeholders allowed us to demonstrate the trade-offs in a clear fashion: the Pareto-front resulting from these performance indicators is an intuitive visualization for decision- and policy-makers as well as the communication to the public.

Luminescence dating methods are widely used to date coastal sediments, while luminescence tracing methods are a novel application to reconstruct coastal sediment pathways. Both methods rely on subaqueous resetting (bleaching) of luminescence signals by light. Differences in bleaching between grains and/or luminescence signals encode information on the light exposure history of individual grains and therefore yield information on past sediment transport. Here we assess the potential of multi-signal single-grain feldspar luminescence to inform about sediment pathways at Ameland tidal inlet in the Dutch Wadden Sea. We also tested whether nourished and native sands can be distinguished based on their luminescence signals.Single-grain infrared stimulated luminescence (IRSL50) and post-IR IRSL (pIRIR) were measured from samples collected from modern sea-floor deposits across the inlet. Equivalent dose (D_e) distributions were assessed using the Central Age Model (CAM), and bootstrapped versions of the Minimum Age Model (bMAM) and Maximum Age Model (bMAX) were applied to the IRSL50 D_e distributions. Spatial trends in CAM and bMAX-De reveal highest inherited doses at the tip of Ameland in the Borndiep channel, decreasing along transport pathways around the ebb-tidal delta. These patterns indicate erosion of Pleistocene sediments in the Borndiep channel and progressive bleaching of luminescence signals upon transport. Low D_e values in shallow areas reflect repeated reworking of Holocene sands within the active layer. Nourished and native sediments show indistinguishable luminescence characteristics for our dataset due to their shared Holocene origin.

Tracking coastal sediments can provide useful information about coastal dynamics, thereby helping coastal management. However, the highly dynamic conditions of the coasts makes analyzing the trajectories of a huge number of particles challenging. To solve this limitation, the framework of coastal sediment connectivity is designed. In this framework, recent advances in graph theory are used to quantify coastal systems as complex networks. In this context, sediment sinks/sources and pathways represent the graphical nodes and links, respectively. In this work, we take the first step to evaluate the ability of this newly-developed framework in quantifying the basic processes on a sandy beach. Firstly, we used Delft3D to obtain the velocity field and bed-level changes. Then, the Eulerian results were fed into SedTRAILS to simulate the sediment pathways. We show that the current version of the model can correctly calculate the basic metrics of the sediment-connectivity network (e.g., network link strength which is a proxy for sediment fluxes). More specifically, we show that this framework is capable of exploring the initiation of the rip channel formation.

Coastal sediment budgets are a foundational source of information for coastal management decision-making. To quantify these budgets, coastal systems are often divided into “cells” based on jurisdictional boundaries or topography. However, such divisions do not account for the pathways that water and sediment particles actually take. In this study we quantify cell boundaries that emerge from numerical simulations of sand and water pathways in a barrier island-lagoon system in the Netherlands (the Western Wadden Sea). By quantifying Lagrangian particle pathways as a network, we can derive internally well-connected but externally disconnected modules. Here we show that large (O(10 km)) coherent modules develop from flow patterns at tidal timescales (12.5 h), and are persistent through varying tide and weather conditions. Conversely, modules derived from 100 µm sand pathways are less coherent and highly spatially fragmented. The difference in patterns likely relates to the longer timescales associated with sediment transport. These emergent patterns could be used to better inform coastal and estuarine management by providing physics-based sediment cell boundaries.

Employing Lagrangian particle tracking models for the study of coastal sediment transport dynamics is highly beneficial as they record the complete history of sediment transport pathways. Correctly simulating bed-particle interactions and its stochastic nature in Lagrangian models is essential to accurately estimate the direction and timescale of sediment transport. In this study we compare and assess the performance of two stochastic approaches for simulating particle erosion and deposition in Lagrangian sediment tracking models: 1) formulations proposed by Soulsby et al. (2011) that calculate probability of particles erosion and deposition from empirically-derived parameters and 2) newly-developed formulations that calculate probability of particles erosion and deposition from physical parameters. The two approaches are evaluated in the Lagrangian sediment tracking model SedTRAILS using a simulation of the dispersal of a pilot ebb-tidal delta nourishment in Ameland Inlet (Wadden Sea, Netherlands) as a case study. Our results show that the new physics-based approach represents the diffusive behavior of the nourished sediment better than the empirical approach. However, the new approach could not be fully validated yet, and the implementation of a slope term for bedload transport in the SedTRAILS transport formulations is necessary to further evaluate the new physics-based approach.

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.

Coastal regions face increasing pressure from climate change, sea-level rise, and growing coastal populations. This “coastal squeeze” threatens both the systems’ sustainability and their ecosystem services. Coastal changes depend on the distribution of sediment throughout the system, which evolves continuously through complex transport processes. While we can quantify net morphological changes, this alone provides incomplete understanding of coastal evolution as similar morphological states can result from vastly different sediment movement patterns. Coastline perturbations-deviations from straight coastlines ranging from beach cusps to headlands, deltas, and artificial nourishments-exemplify this challenge. Although their diffusive morphological evolution is well understood, we have limited knowledge of the underlying sediment movement patterns driving this change. This study reveals how coastline perturbations alter sediment transport by tracing particles from origin to destination using Lagrangian tracking at the Sand Engine mega-nourishment. Our results demonstrate that perturbations alter both sediment dispersal and accumulation. During initial stages, the longshore dispersal of sediment is strongly restricted by rapid deposition and burial on both sides of the perturbation. A backward-tracing approach reveals that sediment deposition not only originates directly from the protruding part of the coastline, but also from updrift sources. As coastline perturbations diffuse over time, sediment movement patterns gradually converge toward those of an undisturbed coast. At locations with oblique wave incidence this evolution manifests itself with predominant downdrift dispersal and updrift trapping of sediment from adjacent beaches. The successful application of our Lagrangian approach to this multi-year evolution demonstrates the potential of sediment particle tracking for understanding more complex coastal environments. Increased understanding of sediment pathways enhances our ability to predict and communicate coastal response to interventions, supporting more effective management strategies.

Sea level rise, increased storminess, and changes in sediment supply due to nourishments are all expected to drive coarsening (i.e., ‘sandification’) of muddy coastal sediments in the decades to come. Since the composition of soft-bottom benthic communities is associated with the sediment grain-size and mud content, this may result in habitats becoming less suitable for some species, leading to species shifts. Species-sediment relations can help to predict how this foreseen sandification may affect benthic fauna. We explore and quantify the sandification-sensitivity of benthic communities, with a tidal basin in the Dutch Wadden Sea as a model system. We identify the species' sediment optima and tolerance ranges using non-linear quantile regression models, summarise preference and sensitivity at the community level, and determine the difference between optimal and realised sediment habitat. We find that sediment optima are taxon-specific and that most species in this area are sediment generalists. On community level, there is a difference between the preferred and realised sediment habitat. In many areas, the actual inhabited sediment is coarser and sandier than expected based on the preferences of the resident species. Future sandification of the area would further decrease sediment habitat suitability for benthic communities in these places. This detailed knowledge of area-specific sensitivity of benthos can be used to inform coastal management decisions.

At a global scale, deltas are vital economic hubs, in part due to the combination of their access to inland regions via river systems with their proximity to sea. However, with the sea in close vicinity also comes the threat of freshwater contamination by saline seawater, especially during droughts. This study explores the potential of a mitigation measure to estuarine salt intrusion, namely the construction of a (temporary) earthen sill—a measure implemented in the Lower Mississippi River near New Orleans (LA, USA). This study suggests design guidelines on how a sill can be used to mitigate estuarine salt intrusion: the design should focus on the longitudinal placement and the height of the sill, and the mitigating efficiency of the sill reduces with increasing tidal range. Overall, a (temporary) sill has great potential to reduce salt intrusion in salt wedge estuaries if there is sufficient water depth available.

Worldwide, estuaries are increasingly constrained by human interventions, such as wetland reclamations. Intertidal area has an important influence on the extent of estuarine salt intrusion. Previous research has shown conflicting effects of intertidal area on the salt intrusion. Therefore, this study explores this interaction for three estuary classes: (a) salt wedge, (b) partially mixed, and (c) well-mixed. Our findings show that the effect of intertidal area on the salt intrusion depends on the estuary class: enlarging the intertidal area reduces the salt intrusion for salt wedge and partially mixed estuaries, but vice versa for well-mixed estuaries. These opposing responses are explained by the balance between salt fluxes driven by the estuarine circulation versus by the tidal oscillation. In general, enlarging intertidal area results in the suppression of the estuarine circulation. Such system understanding is especially relevant in an era of increasing coastal urbanization.

Optical turbidity and acoustic sensors have been widely used in laboratory experiments and field studies to investigate suspended particulate matter concentration over the last four decades. Both methods face a serious challenge as laboratory and in-situ calibrations are usually required. Furthermore, in coastal and estuarine environments, the coexistence of mud and sand often results in multimodal particle size distributions, amplifying erroneous measurements. This paper proposes a new approach of combining a pair of optical turbidity-acoustic sensors to estimate the total concentration and sediment composition of a mud/sand mixture in an efficient way without an extensive calibration. More specifically, we first carried out a set of 54 bimodal size regime experiments to derive empirical functions of optical-acoustic signals, concentrations, and mud/sand fractions. The functionalities of these relationships were then tested and validated using more complex multimodal size regime experiments over 30 optical-acoustic pairs of 5 wavelengths (420, 532, 620, 700, 852 nm) and six frequencies (0.5, 1, 2, 4, 6, 8 MHz). In the range of our data, without prior knowledge of particle size distribution, combinations between optical wavelengths 620–700 nm and acoustic frequencies 4–6 MHz predict mud/sand fraction and total concentration with the variation <10% for the former and <15% for the later. The results also suggest that acoustic-acoustic signals could be combined to produce meaningful information regarding concentration and mud/sand fraction, while no useful knowledge could be extracted from a combination of optical-optical pairs. This approach therefore enables the robust estimation of suspended sediment concentration and composition, which is particularly practical in cases where calibration data is insufficient.

A submerged, low-relief nearshore berm was constructed in the Pacific Ocean near the mouth of the Columbia River, USA, using 216,000 m³ of sediment dredged from the adjacent navigation channel. The material dredged from the navigation channel was placed on the northern flank of the ebb-tidal delta in water depths between 12 and 15 m and created a distinct feature that could be tracked over time. Field measurements and numerical modeling were used to evaluate the transport pathways, time scales, and physical processes responsible for dispersal of the berm and evaluate the suitability of the location for operational placement of dredged material to enhance the sediment supply to eroding beaches onshore of the placement site. Repeated multibeam bathymetric surveys characterized the initial berm morphology and dispersion of the berm between September 22, 2020, and March 10, 2021. During this time, the volume of sediment within the berm decreased by about 40%to 127,000 m³, the maximum height decreased by almost 60%, and the center of the deposit shifted onshore over 200 m. Observations of berm morphology were compared with predictions from a three-dimensional hydrodynamic and sediment transport model application to refine poorly constrained model input parameters including sediment transport coefficients, bed schematization, and grain size. The calibrated sediment transport model was used to predict the amount, timing, and direction of transport outside of the observed survey area. Model simulations predicted that tidal currents were weak in the vicinity of the berm and wave processes including enhanced bottom stresses and asymmetric bottom orbital velocities resulted in dominant onshore movement of sediment from the berm toward the coastline. Roughly 50% of the berm volume was predicted to disperse away from the initial placement site during the 169 day hindcast. Between 9 and 17% of the initial volume of the berm was predicted to accumulate along the shoreface of a shoreline reach experiencing chronic erosion directly onshore of the placement site. Scenarios exploring alternate placement locations suggested that the berm was relatively effective in enhancing the sediment supply along the eroding coastline north of the inlet. The transferable monitoring and modeling framework developed in this study can be used to inform implementation of strategic nearshore placements and regional sediment management in complex, high-energy coastal environments elsewhere.

Coral reefs represent an efficient natural mechanical coastal defense against ocean waves. The focus of this study is La Saline fringing coral reef, located in the microtidal West of La Réunion Island in the Indian Ocean, frequently exposed to Southern Ocean swell and cyclonic events. The aim is to provide a better understanding of the reef's coastal defense characteristics for several Southern Ocean swell events. Pressure sensors were placed across the reef to measure water level fluctuations and to study wave transformation. A numerical model (XBeach surfbeat), validated using field observations, was used to deepen understanding of wave transformation, wave setup and runup. Field measurements and model outputs show that as gravity waves dissipate over the reef, and frequency-dependent dissipation of infragravity waves by bottom-friction occurs, the reef acts as a low-pass filter. Wave-induced setup is found to be the dominant hydrodynamic component. Setup and runup are each 98% and 79% driven by the offshore significant wave height, and 2% and 21% driven by the tide. The modulation of the water level by setup is the main contributor to runup in the fringing reef. At semidiurnal timescales, setup and runup are in antiphase with tidal variations as lower water levels result in higher gravity wave energy dissipation, setup and runup. Simple-to-use transfer functions relating incident wave characteristics to these hydrodynamic components are proposed. The effects of bottom friction and water level on the defensive capacity of the coral reef highlight future implications of structural damage and sea level rise.

Tropical Cyclones (TCs) are singular storms causing intense wind, large waves, extreme water levels, and heavy rainfall. TCs prove every year to be one of the most destructive natural phenomena worldwide. The quantitative assessment of the hazards resulting from TCs (i.e., flooding and extreme winds) is challenging since satellite data are only available for recent decades, whereas older historical observations are incomplete and less accurate. In addition, long-term prediction through numerical weather forecasting is still limited. This often results in large uncertainties in the definition of TC hazards associated with events with longer return periods or in areas infrequently impacted by TCs. Even when this information is available, for example through statistical sampling of synthetic TC tracks, the numerical modelling of the associated hazards for all the different TC conditions can lead to computational costs which are often infeasible. Several methodologies that overcome the issues of accuracy and computational efficiency currently exist, but these are not generically applicable, and they tend to focus on specific areas only, for example where TCs typically make landfall. The main contribution of this paper is a novel methodology for the estimation and analysis of TC hydro-meteorological conditions and induced hazards. The method is generically applicable and maximizes accuracy while accounting for computational efficiency. Our approach identifies a smaller but representative set of TC tracks (RTCs) that preserves the information about extremes and the frequency of events of the larger population. The method is successfully applied and validated in a case study in the Bay of Bengal, using a set of synthetic TC tracks representing 1000 years of TC climate. For the best-performing configuration, the required number of scenarios and associated computational costs were reduced by 90% while maintaining accuracy in the simulated offshore storm surges, significant wave height, and windspeeds typically within 10% of the prediction based on the original full set of scenarios. This method is globally applicable and greatly improves the efficiency of TC-related hazard estimation, making it particularly valuable for areas with limited historical data.