The integration of coastal dunes planted with vegetation and dikes combines traditional infrastructure with dynamic aeolian sediment and ecological processes to enhance coastal resilience. The functioning of such dune-dike hybrid Nature-based Solution strongly depends on aeolian sediment transport and the vertical growth rate of vegetation. We used the AeoLiS numerical model to investigate the relative importance of aeolian and vegetation dynamics in the evolution of a 120 m long and 20 m wide marram grass-planted dune field on a Belgian sandy beach backed by a seawall, constructed in 2021. AeoLiS proved to be a promising tool for predicting these systems, effectively capturing aeolian sediment deposition, vegetation growth, and profile development three years post-construction. Seasonal variations in vegetation trapping efficiency, driven by sediment burial, and seasonal plant growth emerged as important factors controlling dune growth. Profile development discrepancies were attributed to unaccounted biotic and abiotic factors, highlighting the complexity of coastal eco-geomorphological processes. Dunes planted with vegetation wider than 20 m were identified to enhance sediment trapping without an increase in dune height. These findings offer actionable insights for coastal management, promoting strategic dune design and planting approaches to reinforce shoreline resilience. Additionally, the findings underscore the necessity for advancing eco-morphodynamic models and deepening our knowledge of coastal dune dynamics.

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The formation and evolution of coastal dunes result from a complex interplay of eco-morphodynamic processes. State-of-the-art models can simulate aeolian transports and morphological dune evolution under certain conditions. However, a model combining these processes for coastal engineering applications was not yet available. This study aims to develop a predictive tool for dune development to inform coastal management decisions and interventions. The aeolian sediment transport model AeoLiS is extended with functionalities that allow for simulations of coastal landforms. The added functionalities include the effect of topographic steering on wind shear, avalanching of steep slopes and vegetation processes in the form of growth and wind shear reduction. The model is validated by simulating four distinct coastal landforms; barchan-, parabolic-, embryo dunes and blowouts. Simulations, based on real-world conditions, replicate the landform formation, migration rates and seasonal variability.

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Grain size affects the rates of aeolian sediment transport on beaches. Sediment in coastal environments typically consists of multiple grain-size fractions and exhibits spatiotemporal variations. Still, conceptual and numerical aeolian transport models are simplified and often only include a single fraction that is constant over the model domain. It is unclear to what extent this simplification is valid and if the inclusion of multi-fraction transport and spatial grain-size variations affects aeolian sediment transport simulations and predictions of coastal dune development. This study applies the numerical aeolian sediment transport model AeoLiS to compare single-fraction to multi-fraction approaches for a range of grain-size distributions and spatial grain-size scenarios. The results show that on timescales of days to years, single-fraction simulations with the median grain size, D₅₀, often give similar results to multi-fraction simulations, provided the wind is able to mobilize all fractions within that time frame. On these timescales, vertical variability in grain size has a limited effect on total transport rates, but it does influence the simulation results on minute timescales. Horizontal grain-size variability influences both the total transport rates and the downwind bed grain-size composition. The results provide new insights into the influence of beach sediment composition and spatial variability on total transport rates toward the dunes. The findings of this study can guide the implementation of grain-size variability in numerical aeolian sediment transport models.

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A model that simulates surface moisture content on sandy beaches for aeolian transport applications is developed and integrated into the aeolian transport model AeoLiS. The moisture content of a thin surface layer (≈2 mm thickness) is computed as a function of wave runup, precipitation, evaporation, percolation, and capillary rise from the groundwater table. The groundwater table is simulated using a modified Boussinesq equation accounting for the overheight due to wave runup. The surface moisture due to capillary rise is simulated with an experimentally determined soil water retention (SWR) curve of the “van Genuchten” type. Hysteresis is accounted for by differentiating between SWR curves for drying and wetting conditions. The model is tested against a data set of 221 point observations of surface moisture from Noordwijk beach in the Netherlands. The measured surface moisture within the study area displays large spatial and temporal variability. The model results display an expected cross-shore gradient of moisture content, but also a large scatter when compared to the data. The scatter may partly be explained by local variability of hydraulic properties that are not accounted for within the model. Despite the scatter, the proposed surface moisture model is a starting point to integrate the transport limiting effect of surface moisture into meso-scale aeolian transport models. To facilitate model setup and the use of this surface moisture model, the soil water retention data from 10 beaches with variable grain size characteristics are provided in this study. Future studies may focus on additional model validation against data sets with variable meteorological conditions and simultaneous moisture and aeolian transport observations.

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This study presents 62 years of hindcast wave climate data for the south coast of Sweden from 1959–2021. The 100-km-long coast consists mainly of sandy beaches and eroding bluffs interrupted by headlands and harbours alongshore, making it sensitive to variations in incoming wave direction. A SWAN wave model of the Baltic Sea, extending from the North Sea to the Åland Sea, was calibrated and validated against wave observations from 16 locations distributed within the model domain. Wave data collected from open databases were complemented with new wave buoy observations from two nearshore locations within the study area at 14 and 15 m depth. The simulated significant wave height showed good agreement with the local observations, with an average R² of 0.83. The multi-decadal hindcast data was used to analyse spatial and temporal wave climate variability. The results show that the directional distribution of incoming waves varies along the coast, with a gradually increasing wave energy exposure from the west towards the east. The wave climate is most energetic from October to March, with the highest wave heights in November, December, and January. In general, waves from westerly directions dominate the annual wave energy, but within the hindcast time series, a few years had larger wave energy from easterly directions. The interannual variability of total wave energy and wave direction is correlated to the North Atlantic Oscillation (NAO) index. In the offshore, the total annual wave energy had a statistically significant positive correlation with the NAO DJFM station-based index, with a Spearman rank correlation coefficient of 0.51. In the nearshore, the correlation was even stronger. Future studies should investigate the possibility of using the NAO index as a proxy for the wave energy direction and its effect on coastal evolution.

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In sandy beach systems, the aeolian sediment transport can be governed by the vertical structure of the sediment layers at the bed surface. Here, data collected with a newly developed sand scraper is presented to determine high-resolution vertical grain size variability and how it is affected by marine and aeolian processes. Sediment samples at up to 2 mm vertical resolution down to 50 mm depth were collected at three beaches: Waldport (Oregon, USA), Noordwijk (the Netherlands) and Duck (North Carolina, USA). The results revealed that the grain size in individual layers can differ considerably from the median grain size of the total sample. The most distinct temporal variability occurred due to marine processes that resulted in significant morphological changes in the intertidal zone. The marine processes during high water resulted both in fining and coarsening of the surface sediment. Especially near the upper limit of wave runup, the formation of a veneer of coarse sediment was observed. Although the expected coarsening of the near-surface grain size during aeolian transport events was observed at times, the opposite trend also occurred. The latter could be explained by the formation and propagation of aeolian bedforms within the intertidal zone locally resulting in sediment fining at the bed surface. The presented data lays the basis for future sediment sampling strategies and sediment transport models that investigate the feedbacks between marine and aeolian transport, and the vertical variability of the grain size distribution.

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Coastal aeolian sediment transport is influenced by supply-limiting factors caused by sediment sorting by grain size. Sorting processes can lead to coarsening of the bed surface and influence the formation of aeolian ripples. However, the influence sorting processes and bedforms might have on the magnitude of the transport is not fully understood. This study explores sorting processes and their influence on the magnitude and mode of aeolian transport by using sediment tracers. Sand was painted in different colors according to particle size and placed on a supratidal beach in Noordwijk, the Netherlands. Several experiments were conducted with varying wind speeds. Surface sampling and cameras tracked the sand color movement on the bed surface, and wind velocity was measured. The tracer experiments showed that ripples developed in moderate wind conditions. Once the ripples had formed, the supply of finer tracer grains in the downwind direction decreased over time, while the supply of coarser grains remained constant. A linear relationship between ripple migration speed and wind speed was found. For higher wind speeds, no ripples or differences in transport of grain size fractions were observed. Instead, alternating phases of erosion and deposition of the bed surface were observed, which could not be related to local variations in wind velocity. Based on these results and literature, a conceptual model was developed for an active bed surface layer with two transport regimes corresponding to moderate (I) and high (II) wind speeds. The conceptual model is intended to guide the selection of aeolian sediment transport models as a function of wind speed, bed characteristics, and upwind sediment supply. For Regime I, transport could be modeled using a linear relationship between sediment transport and wind speed and for Regime II using a third power relationship in combination with a process-based model accounting for supply limitations.

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In recent years, dune-in-front-of-dike projects have been carried out at several locations, e.g. at Raversijde and Oosteroever in Belgium and the Hondsbossche dunes in the Netherlands. In the near future, many coastal defence systems require reinforcement to adapt to rising sea levels, and often, natural values along the coasts may also be enhanced. Therefore, it is anticipated that this type of hybrid coastal protection – a mix of grey and green solutions – will become more common in the future. Contrary to grey defence structures, such as earth dikes and rock or concrete structures, dunes are dynamic features. Their level of flood protection depends on their morphological evolution due to aeolian and marine transport processes, vegetation dynamics, and anthropogenic impact. Numerical models are commonly used tools to assess the safety level of dunes and predict their future evolution. In addition to event timescales (storms), the decadal timescale is typically of interest from a coastal management perspective, especially when considering sea level rise. On this timescale, dune build-up through aeolian transport depends on the wind's transport capacity, and the availability of sediment of the appropriate size exposed to the wind is an important process. Sediment availability for aeolian transport is controlled by other sediment transport processes, such as dune erosion and longshore sediment transport, nourishments, and limiting factors, such as surface moisture and armour layers. Simulation of dune evolution at the decadal timescale requires an integrated model approach that accounts for the non-linear interactions between marine and aeolian transport processes in the longshore and cross-shore direction. Reduced complexity approaches are required when these models are applied to large temporal (decades) and spatial scales (kilometres). This study aims to predict medium to long-term dune evolution by developing a new coupled long- term beach and dune evolution model, coDaC (coupled Dunes and Coasts ). The new model combines a semi-empirical cross-shore transport model, the CS-model (Hallin et al. 2019a), with a longshore transport and coastline evolution model, Unibest CL+ (Figure 1). The coupled model is applied to simulate 22 years of morphological dune evolution along an 8 km-long coastal stretch at the Kennemer Dunes in the Netherlands. @en

From November 12th to 13th in 1872, an extreme coastal flood event occurred in the south Baltic Sea. An unusual combination of winds created a storm surge reaching up to 3.5 m above mean sea level, which is more than a meter higher than all other observations over the past 200 years. On the Danish, German, and Swedish coasts, about 300 people lost their lives. The consequences of the storm in Denmark and Germany were more severe than in Sweden, with significantly larger destruction and higher numbers of casualties. In Denmark and Germany, the 1872 storm has been more extensively documented and remembered and still influences local and regional risk awareness. A comparative study indicates that the collective memory of the 1872 storm is related to the background knowledge about floods, the damage extent, and the response to the storm. Flood marks and dikes help to remember the events. In general, coastal flood defence is to the largest degree implemented in the affected areas in Germany, followed by Denmark, and is almost absent in Sweden, corresponding to the extent of the collective memory of the 1872 storm. Within the affected countries, there is local variability of flood risk awareness associated with the collective memory of the storm. Also, the economic dependency on flood-prone areas and conflicting interests with the tourism industry have influence on flood protection decisions. The processes of climate change adaptation and implementation of the EU Floods Directive are slowly removing these differences in flood risk management approaches.

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Sea level is rising due to climate change and is expected to influence the development and dynamics of coastal dunes. However, the anticipated changes to coastal dunes have not yet been demonstrated using field data. Here, we provide evidence of dune translation that is characterized by a linear increase of the dune toe elevation on the order of 13–15 mm/year during recent decades along the Dutch coast. This rate of increase is a remarkable 7–8 times greater than the measured sea level rise. The observed vertical dune toe translation coincides with seaward movement of the dune toe (i.e., progradation), which shows similarities to prograding coasts in the Holocene both along the Dutch coast and elsewhere. Thus, we suspect that other locations besides the Dutch coast might also show such large ratios between sea level rise and dune toe elevation increase. This phenomenon might significantly influence the expected impact of sea level rise and climate change adaptation measures.

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