Storm surge barriers and closure dams influence estuarine morphology. Minimizing consequential ecological impacts requires a thorough understanding of the morphological adaptation mechanisms and associated time scales. Both are unraveled using three decades of morphological measurements on the adaptation of the Eastern Scheldt estuary (The Netherlands) to a storm surge barrier and closure dams. Both the storm surge barrier (through a decrease in cross-sectional area) and closure dams (inducing a reduction in surface area of the estuary) contributed to a reduction in tidal prism. As a smaller tidal prism implies a smaller equilibrium volume of the channels, the channels demand sediment to adjust. Consequently, by providing sediment to the channels, the intertidal flats erode. Erosion rates decreased while the sediment demand of the channels attenuated. This attenuation in sediment demand resulted mainly from tidal prism gains, caused by intertidal flat erosion and sea level rise. Erosion rates of the intertidal flats decreased further while they flattened to adapt to the reduced tidal velocities. Furthermore, storms caused erosion events, after which the long-term adaptation pace of intertidal flats suddenly reduced. Despite decreasing erosion, sea level rise enhances the drowning of intertidal flats in sediment-scarce estuarine systems, thereby pressuring these estuarine ecosystems and raising the need for mitigation measures.

Sand nourishment is a widespread management strategy to protect sandy coasts against erosion. Nourishments can impact the benthic fauna directly by burial and indirectly by changing environmental conditions such as bottom shear stress and/or sediment composition (grain size distribution). The macrobenthic community in soft-bottom environments is often strongly correlated with sediment composition, however, because of the strong correlation with other environmental conditions, it is difficult to determine the direct effects of sediment composition on the macrobenthic community from field observations. Nourishments can temporarily break this correlation allowing to study the relation between sediment composition and macrobenthic community more or less independent from other environmental factors. In 2017, a shoreface nourishment was constructed at the Holland coast near Callantsoog at a water depth of about 10 m. Because the influence of waves is limited at this depth, the nourishment is not very mobile. Sediment composition and macrobenthos was monitored at the nourished site and two reference locations before (2015 and 2016), and in 2021, four years after nourishment. In 2021, the sediment composition at the nourished site was much coarser (median grain size 330 ± 59 μm) than in the years before nourishment (195 ± 34 μm), while the sediment composition at the reference locations remained comparable over the years. The average number of taxa per sample at the nourishment decreased from 14.1 before nourishment to 3.0 taxa in 2021. Also the average total density at the nourishment location decreased from 12731 ind.m⁻² before nourishment to 320 ind.m⁻² in 2021. While before nourishment, the composition of the macrobenthic community at the nourished site was comparable to the southern reference location, in 2021 the macrobenthic community was more comparable to the community at the coarser sediments of the northern reference. Clearly, four years after the nourishment, the macrobenthic community has not recovered due to the presence of coarser sediments at the nourished site. Logistic regressions show that most of the dominant species have a negative correlation with median grain size, but some species are more sensitive to changes in sediment composition than others. The sensitivity of the macrobenthos to sediment composition illustrates the importance of using sediments of similar grain size as in the placement area, especially for nourishments in the deeper parts of the shoreface.

Seagrass meadows provide valuable ecosystem services of coastal protection and chemical habitat formation that could help mitigate the impact of sea level rise and ocean acidification. However, the intensification of hydrodynamic forces caused by sea level rise, in addition to habitat degradation threaten the provision of these ecosystem services. With quantitative field measurements of the coastal protection and chemical habitat formation services of seagrass meadows, we statistically model the relationships between hydrodynamic forces, vegetation density and the provision of these ecosystem services. Utilising a high-resolution hydrodynamic model that simulates end of the century hydrodynamic conditions and three scenarios of coral reef degradation (i.e., keep up, remain or loss) we quantify how the environmental conditions within a tropical bay will change given changes to the provision of ecosystem services. Our study shows that increasing hydrodynamic forces lead to a seafloor made up of a larger grain size that is increasingly unstable and more vulnerable to erosion. The loss of a fringing reef leads to larger hydrodynamic forces entering the bay, however, the 0.87 m increase in depth due to sea-level rise reduces the bed shear stress in shallower areas, which limits the change in the ecosystem services provided by the current benthic seagrass meadow. Loss of seagrass constitutes the greatest change in a bay ecosystem, resulting in the sediment surface where seagrass existed becoming unstable and the median sediment grain size increasing by 5-7 %. The loss of seagrass also leads to the disappearance of the unique fluctuating chemical habitat, which leaves the surrounding community vulnerable to ocean acidification. A reduction or complete loss of these ecosystem services would impact the entire community assemblage while also leaving the surrounding coastline vulnerable to erosion, thus exacerbating negative effects brought about by climate change.

Human-induced land subsidence causes many coastal areas to sink centimetres per year, exacerbating relative sea level rise (RSLR). While cities combat this problem through investment in coastal infrastructure, rural areas are highly dependent on the persistence of protective coastal ecosystems, such as mangroves and marshes. To shed light on the future of low-lying rural areas in the face of RSLR, we here studied a 20-km-long rural coastline neighbouring a sinking city in Indonesia, reportedly sinking with 8–20 cm per year. By measuring water levels in mangroves and quantifying floor raisings of village houses, we show that, while villages experienced rapidly rising water levels, their protective mangroves experience less rapid changes in RSLR. Individual trees were able to cope with RSLR rates of 4.3 (95% confidence interval 2.3–6.3) cm per year through various root adaptations when sediment was available locally. However, lateral retreat of the forest proved inevitable, with RSLR rates up to four times higher than foreshore accretion, forcing people from coastal communities to migrate as the shoreline retreated. Whereas local RSLR may be effectively reduced by better management of groundwater resources, the effects of RSLR described here predict a gloomy prospect for rural communities that are facing globally induced sea level rise beyond the control of local or regional government.

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.

In recent years, coastal management has been facing new challenges: socio-economic growth and consequent climate change impose new boundary conditions pushing coastal systems towards unseen states. For adaptation and mitigation strategies as well as risk management, the resilience of systems to these projected changes must be tested and quantified using predictive tools, given the scarcity of observations. Process-based models, which limit the number of assumptions, are the preferred tools. However, these models are computationally expensive and therefore unattractive for global sensitivity and uncertainty analyses. Input and model reduction techniques, as well as behavioural empirical models, have been widely used to overcome these computational difficulties. In this paper, we propose a process-based hybrid workflow—that combines statistical and machine learning with a process-based numerical model—to provide sensitivity analyses on complex systems. As an example we explore salt intrusion in estuaries. The novelty of the method presented is the implementation of an adaptive sampling technique of numerical experiments with a process-based hydrodynamic model, and the training of a neural network to augment the set of numerical runs executed. The first uses predictive uncertainty to automatically explore the response of the complex system to varying environmental boundary conditions and geomorphological configurations. The second is trained to provide system responses around the sampled points. This exploration is closed by simulating the extremes in the output space as found by a genetic algorithm. This scheme is shown to be highly efficient in non-linear, heteroscedastic, and highly non-stationary systems.

Nature-based strategies, such as wave attenuation by tidal marshes, are increasingly proposed as a complement to mitigate the risks of failure of engineered flood defense structures such as levees. However, recent analysis of historic coastal storms revealed smaller dike breach dimensions if there were natural, high tidal marshes in front of the dikes. Since tidal marshes naturally only experience weak flow velocities (~0-0.3 ms^-1 during normal spring tides), we lack direct observations on the stability of tidal marsh sediments and vegetation under extreme flow velocities (order of several ms^-1) as may occur when a dike behind a marsh breaches. As a first approximation, the stability of a tidal marsh sediment bed and winter-state vegetation under high flow velocities were tested in a flume. Marsh monoliths were excavated from Phragmites australis marshes in front of a dike along the Scheldt estuary (Dutch-Belgian border area) and installed in a 10 m long flume test section. Both sediment bed and vegetation responses were quantified over 6 experimental runs under high flow velocities up to 1.75 ms^-1 and water depth up to 0.35 m for 2 hours. These tests showed that even after a cumulative 12 hours exposure to high flow velocities, erosion was limited to as little as a few millimeters. Manual removal of the aboveground vegetation did not enhance the erosion either. Present findings may be related to the strongly consolidated, clay- and silt-rich sediment and P. australis root system in this experiment. During the flow exposure, the P. australis stems were strongly bent by the water flow, but the majority of all shoots recovered rapidly when the flow had stopped. Although present results may not be blindly extrapolated to all other marsh types, they do provide a strong first indication that marshes can remain stable under high flow conditions, and confirm the potential of well-developed tidal marshes as a valuable extra natural barrier reducing flood discharges towards the hinterland, following a dike breach. These outcomes promote the consideration to implement tidal marshes as part of the overall flood defense and to rethink dike strengthening in the future.

The establishment of young organisms in harsh environments often requires a window of opportunity (WoO). That is, a short time window in which environmental conditions drop long enough below the hostile average level, giving the organism time to develop tolerance and transition into stable existence. It has been suggested that this kind of establishment dynamics is a noise-induced transition between two alternate states. Understanding how temporal variability (i.e. noise) in environmental conditions affects establishment of organisms is therefore key, yet not well understood or included explicitly in the WoO framework. In this paper, we develop a coherent theoretical framework for understanding when the WoO open or close based on simple dichotomous environmental variation. We reveal that understanding of the intrinsic timescales of both the developing organism and the environment is fundamental to predict if organisms can or cannot establish. These insights have allowed us to develop statistical laws for predicting establishment probabilities based on the period and variance of the fluctuations in naturally variable environments. Based on this framework, we now get a clear understanding of how changes in the timing and magnitude of climate variability or management can mediate establishment chances.

Biological traits of benthic macroinvertebrates from a large area of the North Sea soft sediments were used to explore habitat occupancy within seascapes of contrasting hydrodynamics. The area, the Dutch sector of the North Sea, is mainly composed of 2 habitats: shallow dynamic bottoms of heterogeneous geomorphologies and deep homogeneous muddy bottoms. Higher within-habitat heterogeneity was hypothesized to more specifically select benthic life strategies according to environmental filtering, i.e. through the action of abiotic forces. Functional community patterns were explored through the RLQ method, which relates habitat and trait variables, at different spatial scales of specific seascape heterogeneity, and functional diversity indices were used to shed light on community assembly mechanisms. Locally, 3 associations between habitat characteristics and biological traits were shown to correspond with predictions of life history theories, whereas only 2 emerged when considering all types of seascapes. This spatial scaledependence was explained by abiotic alternations masked over the larger scale at which all the existing strategies could not be properly disentangled. The relative composition in strategies obeyed specific assembly rules as identified by functional diversity indices. Seascape geomorphology was locally discriminant of functional patterns, and could account for biodiversification, much beyond basic taxonomic counts. Whereas habitats of higher physical stability hosted the taxonomically richest communities, stress or disturbance frequency increased functional variations within communities due to different strategist habitat occupancies. This study proposes a generic mechanism of benthic community structuring in soft sediment shelves.

As a response to climate change and sea-level rise, new nourishment strategies for low-lying sandy coasts are developed. These interventions affect the habitat quality of coastal ecosystems for benthic communities. Unraveling the relationship between benthic fauna and their environment facilitates the design of sustainable management strategies for the coastal ecosystem. At the ebb-tidal delta of Ameland, The Netherlands, a unique dataset of 166 benthic and sediment samples is collected and allowed for an investigation of the macrobenthic fauna distribution at the spatial scale of morphological features. The benthic community at the ebb tidal delta is composed of species capable of withstanding the dynamic nature of these sandy coastal ecosystems. Despite the dynamic environment, the geomorphology of the ebb-tidal delta is reflected in the benthic species distribution. Distinct species assemblages were identified, covering a gradient of physical stress from extremely exposed to waves or currents, to relatively low energetic environments such as found on the delta plane seaward of the ebb-tidal delta terminal lobe. This gradient is reflected in the median grain size, organic matter content, and oxygenation of the sediment. A second gradient distinguishes well-sorted, mainly wave-exposed sediments from less well-sorted, mainly current-exposed sites. The functional characteristics of the benthic fauna show a clear contrast between the three most exposed, and the three most sheltered assemblages. Small, short-lived, surface deposit-feeding, highly mobile, burrowing organisms dominate in the most exposed sites, whereas with increasing shelter also larger, long-lived, filter-feeding and sessile organisms become more dominant. The functional characteristics suggest that the fauna of the most exposed sites will likely show a fast recovery of disturbance by sand nourishments. A much longer-lasting effect can be expected on sheltered parts of the ebb-tidal delta.

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.

Salt marshes can contribute to coastal protection, but the magnitude of the protection depends on the width of the marsh. The cross-shore width of the marsh is to a large extent determined by the delicate balance between seaward expansion and landward retreat. The influence of the magnitude of daily occurring mild weather conditions and sediment availability on the variability of salt marsh width has not been systematically assessed. This paper investigates how the magnitude of homogeneous hydrodynamic forcing, combined with sediment availability, affects the biophysical development, and more specifically retreat and expansion of salt marshes. The dynamic extent of the salt marsh is assessed by modeling online-coupled hydrodynamics, morphodynamics and vegetation growth using the numerical Delft3D-Flexible Mesh model, and a vegetation growth module. Simulated patterns around the salt marsh edge resembled field observations, as well as the simulated temporal variability of the lateral position of the salt marsh edge. In the model, the salt marsh extended seaward at low wave forcing (0.00 m; 0.05 m), and retreated landward at higher wave forcing (0.10 m; 0.15 m). With increasing physical stress, the salt marsh edge was found at lower elevations, indicating an unhealthy system with a retreating marsh edge due to vegetation mortality, whereas decreasing physical stresses result in a higher salt marsh edge, enabling expansion. This balance suggests the importance of response time of vegetation to physical stress. Yet, the salt marsh forced with higher waves was able to switch from a retreating extent retrogradational to an expansional behavior as sediment supply increased.

Epifauna and infauna are often sampled by different types of sampling devices including grabs, and dredges, depending on various factors such as local environmental conditions, research questions and available budget. Because grabs and dredges have a different species-specific selectivity, both methods could be complementary but for some research questions they could give comparable results. In this study, a comparison is made between macrobenthos (epifauna and infauna) sampled simultaneously by a Van Veen grab and a quantitative benthic dredge, based on a large dataset that was collected from 2010 to 2019 as part of an extensive monitoring program to study the impact of the Sand Motor, a mega beach nourishment that was implemented in 2011 on the sandy North Sea coast in the Netherlands. Because of the larger mesh size of the sieve in the benthic dredge (5 mm) compared to the Van Veen (1 mm), only larger mollusks, echinoderms and crustaceans are identified in the dredge samples. This resulted in a lower number of taxa and average densities in the 676 benthic dredge samples (42 taxa and 98.7 ind. m⁻², respectively) compared to the 636 Van Veen samples (228 taxa and 1380.9 ind. m⁻², respectively). Due to the larger sampling area of the benthic dredge (10–15 m²) compared to the Van Veen (0.1 m²) the chance to encounter a species is higher in a benthic dredge than in a Van Veen grab sample. Although the benthic dredge only samples a subset of the benthic community that is sampled by the Van Veen grab, multivariate (nMDS) analysis of the data showed remarkable similarities between Van Veen and benthic dredge samples with major gradients correlated to water depth and temporal changes in environmental conditions due to the presence of the Sand Motor. This suggests that both techniques are indicative for the spatial variation and development of the macrobenthic community.

Although tidal flats appear homogeneous from a distance, morphological variations are found on various spatial scales. These are driven by physical and/or biological processes. In this paper, we consider the creeks that are present on fringing tidal flats and which are orientated approximately perpendicular to the main channel. To explain why these creeks occur, we analysed high-resolution aerial pictures and yearly measured LiDAR data of the Ems-Dollard and Western Scheldt estuaries, located in the Netherlands. We selected nine bare fringing tidal flats, with and without creeks in both estuaries. Subsequently, we related the flat shape to the creek occurrence by evaluating cross-sections of tidal flats from the two estuaries. Finally, we studied how the flat shape affects the cross-shore flow velocity with a 1D numerical model to link creek occurrence to tidal flow. The results show highest ebb velocities, the largest velocity gradients and the largest erosion potential at the transition area between the lower and the upper flat. The milder the slope of the upper flat and the shorter the transition zone, the larger the flow velocities. Based on the data analysis and numerical model outcomes, we conclude that the conditions are favourable for creeks on convex-up intertidal flats with a sharp transition between the upper part and lower part of the flat and that they are predominantly found in this transition zone. We finally argue that these tidal creeks are not only a consequence of the tidal flat profile, but also affect the (equilibrium) profile of the tidal flat.

The increasing pressure on Earth's ecosystems due to climate change is becoming more and more evident and the impacts of climate change are especially visible on coral reefs. Understanding how climate change interacts with the physical environment of reefs to impact coral growth and reef development is critically important to predicting the persistence of reefs into the future. In this study, a biophysical model was developed including four environmental factors in a feedback loop with the coral's biology: (1) light; (2) hydrodynamics; (3) temperature; and (4) pH. The submodels are online coupled, i.e. regularly exchanging information and feedbacks while the model runs. This ensures computational efficiency despite the widely-ranged timescales. The composed biophysical model provides a significant step forward in understanding the processes that modulate the evolution of coral reefs, as it is the first construction of a model in which the hydrodynamics are included in the feedback loop.

The Sand Motor is a very large (20 million m³) nourishment constructed along the coast in The Netherlands. The huge volume of sand is redistributed along the coast by natural forces stemming from tidal currents and waves. For environmental evaluation of this large construction, the benthic subtidal fauna has been sampled prior to the construction of the Sand Motor, and at 1, 2, 4, and 6 years after construction. Although some significant differences between years were detected, overall the total density, total biomass and average number of species per sample were surprisingly constant over this time period. However, large differences were found in the species accumulation curves over samples, and in the rank-biomass and rank-abundance plots. These were related to two important trends in the communities. First, the invasive mollusk Ensis leei, the biomass dominant in the years before construction of the Sand Motor, dwindled in importance in later years. Recruitment of the species failed, but it is unclear whether, and how, this is related to the construction of the Sand Motor. Second, the correlation structure between depth, grain size, bottom shear stress due to waves and currents, which is very tight along a linear coast, was disrupted by the Sand Motor. The community composition was shown to depend strongly on these physical factors. The nature of the dependencies did not change, but the range of different combinations of factors after construction of the Sand Motor was widely larger than before. Although samples had similar number of species per sample before and after construction, the average difference between samples after construction was much larger than before. The Sand Motor is a very large construction, leading to loss of a substantial area (order 100 ha) of submarine area, which recovers at a long time scale. Total disturbance of benthos by burial, expressed as area^∗(time before full recovery) was shown to be similar for the Sand Motor and for other coastal nourishment schemes when expressed per unit volume of sediment applied. However, in contrast to beach and shoreface nourishments, the Sand Motor led to a habitat diversification in the coastal zone.

The intensity of major storm events generated within the Atlantic Basin is projected to rise with the warming of the oceans, which is likely to exacerbate coastal erosion. Nature-based flood defence has been proposed as a sustainable and effective solution to protect coastlines. However, the ability of natural ecosystems to withstand major storms like tropical hurricanes has yet to be thoroughly tested. Seagrass meadows both stabilise sediment and attenuate waves, providing effective coastal protection services for sandy beaches. To examine the tolerance of Caribbean seagrass meadows to extreme storm events, and to investigate the extent of protection they deliver to beaches, we employed a combination of field surveys, biomechanical measurements and wave modelling simulations. Field surveys of seagrass meadows before and after a direct hit by the category 5 Hurricane Irma documented that established seagrass meadows of Thalassia testudinum remained unaltered after the extreme storm event. The flexible leaves and thalli of seagrass and calcifying macroalgae inhabiting the meadows were shown to sustain the wave forces that they are likely to experience during hurricanes. In addition, the seagrass canopy and the complex biogeomorphic landscape built by the seagrass meadows combine to significantly dissipate extreme wave forces, ensuring that erosion is minimised within sandy beach foreshores. The persistence of the Caribbean seagrass meadows and their coastal protection services during extreme storm events ensures that a stable coastal ecosystem and beach foreshore is maintained in tropical regions.

Despite the well-recognized importance of plant traits for biogeomorphic development of landscapes, our understanding remains limited of how species-specific plant traits respond to and serve as drivers for the sedimentary dynamics within a biogeomorphic landscape. By manipulating a series of laboratory experiments, using mesocosms and a flume, we examined how species-specific differences in expansion strategy, i.e., clonal step-length of laterally expanding tillers, both respond to sediment type and drive cliff formation and persistence. We compared three marsh species, with contrasting clonal expansion traits, that are dominant in many estuaries worldwide: Spartina anglica, Scirpus maritimus, and Phragmites australis. Our results revealed that S. anglica tussocks tend to have high shoot density due to a short clonal expansion step-length, whereas S. maritimus tussocks were much more diffuse and tend to have a longer clonal expansion step-length. P. australis showed intermediate traits. Clonal expansion step-length did show within-species variation in response to sediment texture, but species-specific differences remained the most important. Species with smaller clonal step-lengths, such as S. anglica in this study, are more likely to induce cliffs at the marsh edge by driving formation of larger cliff heights and also having lower capacity to grow out from cliffs. Our findings thus illustrate how dynamic landscape features like cliffs at marsh edges depend on the clonal expansion traits of the dominant species. This enhances current understanding of the formation and development of marsh edges, and is instructive for understanding the role of species-specific traits in driving distinct biogeomorphic landscape dynamics.