We investigated quasi-2D sand ripple geometry (i.e., ripple height, ripple wavelength, and ripple asymmetry) on a mound subject to the influence of waves, currents, and combined wave-current flows. The results of this study quantify how ripple geometry is influenced by bed slope and combined wave-current flows. The geometry of the ripples is shown to depend on the combined wave-current flow ratio and the local bed slope. Under wave-only conditions, the wave-driven ripple length and height decreased as a function of depth and local slope. Under combined wave-current conditions, the ripples increased in height and wavelength on the stoss slope of the mound, and decreased on the lee slope of the mound. Existing ripple geometry predictors, developed for combined flows on flat sand beds, were unable to predict ripple geometry on the sloped bed accurately. We propose correction factors for ripple geometry predictors to account for slope effects and combined wave-current flow conditions. Applying the correction factors significantly improves the predictor performance for predicting ripple height, wavelength, and asymmetry on sloping beds.

Shoreface nourishments have become a popular management option to mitigate coastal retreat for sites with abundant sand supplies. With shoreface nourishments, relatively large volumes of off-site sand are placed under water in typical water depths of 4–10 m. This part of the nearshore zone has a high bed level variability and contains a myriad of (rhythmic) morphological features. As a result, understanding and forecasting shoreface nourishment morphodynamics and impacts is challenging. Significant progress on this topic is needed in due time, especially in light of emerging climate-change effects. This review paper presents an overview of field, laboratory and numerical modeling studies on shoreface nourishment morphodynamics. We have identified 4 key knowledge gaps. First, the spreading of nourished sand through the coastal zone is poorly understood, and has not been quantified. Second, it is unclear how design variables such as size, placement location and grain-size affect the lifetime, spreading and impact of shoreface nourishments. Third, the cumulative effect of repeated shoreface nourishments (scale: 1–10 km, 1–10 years) on the coastal system as a whole (100+ km, 50+ years) is largely unknown. Fourth, numerical models cannot reliably predict the complete morphological development and impact of shoreface nourishments. To tackle these knowledge gaps we propose a research agenda to ensure the generation and valuation of scientifically robust and societally relevant knowledge.

Globally, there is a growing societal need for multifunctional coastal climate adaptation of sandy shores in the coming decades. Sand nourishment strategies are increasingly regarded as promising nature-based approaches to adaptation. They may increase flood safety and mitigate coastal erosion while enhancing recreational and ecological functioning. However, their multifunctional potential has not yet been assessed under diverse climate impacts at decadal scales. This study analysed the effects of beach, shoreface and mega-nourishment strategies on the physical capacity of sandy shores to supply coastal multifunctionality, using a systems-based approach. Through a structured literature review, we identified eight indicators for recreational (2 indicators), ecological (3) and flood safety (3) functions. We integrated these indicators into a process-based sand distribution model for dissipative coastal profiles. We simulated indicator states as the coastal profile responded to the nourishment strategies under five sea-level rise scenarios and three erosion rates. Next, we calculated the extent to which the physical capacity for coastal functions and multifunctionality were supplied over six decades. Our results indicate that all three nourishment strategies can highly supply the capacity for coastal multifunctionality, although the drivers of this potential differed per strategy. These findings imply that sand nourishment strategies are viable approaches for multifunctional coastal climate adaptation in the coming decades. However, they require prioritising specific coastal features and functions. Although sand nourishment strategies remain high-impact interventions, they also allow for intentionally creating coastal landscapes. These landscapes may not only provide flood protection but also enhance the specific environmental and societal functions valued in dissipative sandy shores. Prioritising among these functions requires explicit political choices.

Rising sea levels and anthropogenic activities are intensifying pressure on coastal zones. Process-based coastal morphodynamic models are increasingly used to forecast natural and anthropogenic beach morphology changes at various spatio-temporal scales. Such predictions are crucial for the sustainable management of coasts. However, process-based morphodynamic models contain numerous free model parameters, introducing uncertainty in predictions. Systematically exploring the parameter space has remained a challenge due to the high computational demands of these morphodynamic models. Here, for the first time we quantify parameter uncertainty of a state-of-the-art morphodynamic (2DH) coastal area model (Delft3D) by systematically varying key model parameters, utilizing the Dutch national supercomputer: SurfSara. We simulate the initial (14-month) response of the Sand Engine, an innovative mega-nourishment placed along the Holland coast with 1024 strategically chosen parameter sets. The resulting simulations are analysed using Generalised Likelihood Uncertainty Estimation (GLUE) to attain probability distributions of morphological evolution and its sensitivity to parameter settings. The model simulations all show an alongshore redistribution of sediment resembling what is observed. However, even simulations with similar skill reveal substantial differences in predicted morphologies (same order of magnitude as the predictions’ 90% confidence interval). Our findings suggest that identifying a single optimal parameter set for coastal numerical models might be unrealistic, even for well-defined cases like large-scale coastal interventions, and that an ensemble modeling approach that quantifies parameter uncertainty is likely better suited for studies relying on morphodynamic predictions. Furthermore, we find that the magnitude of the uncertainty induced by the free model parameters is comparable to that resulting from year-to-year variations in wave climate, underscoring the importance of including both sources in uncertainty assessments.

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.

Wave nonlinearity plays an important role in cross-shore beach morphodynamics and is often parameterized in engineering-type morphodynamic models through a nonlinear relationship with the Ursell number. It is not evident that the relationship established in previous studies also holds for sheltered sites with fetch-limited seas as they are more prone to effects of local winds and currents, the waves are generally steeper, and the beaches are typically reflective. This study investigates near-bed orbital velocity nonlinearity from wave records collected at two sheltered beaches in The Netherlands and contrasts them to earlier observations made along the exposed, wave-dominated North Sea coast. Our observations at sheltered beaches show that the Ursell number has comparable skill in predicting wave nonlinearity as it has on previously studied exposed coasts. However, the orbital velocities at sheltered coasts are more asymmetric for the same Ursell number than on exposed coasts. When exposed coast data were examined for moments with comparable high-steepness waves, a similar effect on asymmetry was observed. In addition, following and opposing winds were found to have a clear relationship with total nonlinearity, while they did not affect the phase between skewness and asymmetry at the sheltered beaches. Refitting the free parameters of an Ursell-based predictor improved the bias for the asymmetry parameterization. Whether this has implications for modeling of the magnitude of wave-nonlinearity-driven sediment transport using engineering type models is strongly dependent on the sediment transport formulation used, as these formulations depend on additional calibration coefficients too.

Increased climate impacts threaten coastal functions globally, highlighting the need for multifunctional coastal climate adaptation. Sand nourishment can adapt sandy coasts to sea level rise, mitigate erosion, increase flood safety, enhance ecological habitats and expand recreational space. Therefore, sand nourishment is increasingly regarded as a promising nature-based strategy for coastal climate adaptation. However, despite this growing recognition, the assessment of how sand nourishment design impacts multifunctional adaptation remains limited. In this perspective article, we argue for three key lessons for researchers to optimise assessing multifunctional coastal climate adaptation by sand nourishment. We conducted stakeholder workshops to scope and inform our perspective, performed semi-structured literature reviews to concretise and validate this for international applications, built a qualitative model to visualise our interdisciplinary overview of how nourishments impact coastal multifunctionality, reflected on this in expert workshops, and identified implications for researchers. In this manner, we assessed the effects of nourishment design on coastal morphology, ecology, socio-economics and ecosystem services in realising the key policy goals of flood safety, nature and recreation. We found that sand nourishment design can result in conflicts between policy goals, generate ambiguous outcomes and lead to system-wide feedback effects. As such, we identified three key lessons: (1) conflicts between policy goals require informing political decision-making on prioritisation between coastal functions, (2) concreteness is needed on otherwise ambiguous functions, and (3) ongoing, multidisciplinary system-wide monitoring is essential. We thus call for a holistic approach to sand nourishment design and encourage researchers from diverse expertise and localities to expand on and adapt our findings to optimise informing sand nourishment design for delivering multifunctional coastal climate adaptation worldwide.

Accelerated sea level rise prompts the upscaling of nourishment strategies, either through larger individual nourishment volumes or increased frequency of implementation. In such strategies, the nourished sand may lack time to effectively redistribute in the designated timeframe, leading to significant deformation of the profile over multiple nourishment cycles. This study quantifies subsequent effects, focusing on profile steepening, nourishment lifetimes, and the feasibility of operational objectives. We simulated two common nourishment strategies at a Dutch case study location using the cross-shore morphological model Crocodile over a 50-year timespan under sea level rise rates of 2–32 mm/year. The choice of strategy led to a variation of up to 75% in the total amount of sand used. Our results show increasing profile deformation with nourishment volume applied and duration of the nourishment strategy, with sand accumulating in the nourished section and little dissipation to the lower shoreface. The consequent profile steepening leads to reduced nourishment lifetimes by up to 30%. Additionally, under high sea level rise rates, more erosive coasts experience a reduction in nourishment lifetimes to annual intervals, while less erosive areas require up to four times more sand than currently needed. These findings illustrate key dilemmas in the formulation of future nourishment strategies and highlight the importance of optimizing these strategies to account for sea level rise.

Quantitative predictions of marine and aeolian sediment transport in the nearshore–beach–dune system are important for designing Nature-Based Solutions (NBS) in coastal environments. To quantify the impact of the marine-aeolian interactions on shaping NBS, we present a framework coupling three existing process-based models: Delft3D Flexible Mesh, SWAN and AeoLiS. This framework facilitates the continuous exchange of bed levels, water levels and wave properties between numerical models focussing on the aeolian and marine domain. The coupled model is used to simulate the morphodynamic evolution of the Sand Engine mega-nourishment. Results display good agreement with the observed aeolian and marine volumetric developments, showing similar marine-driven erosion from the main peninsula and aeolian-driven infilling of the dune lake. To estimate the magnitude of the interactions between aeolian and marine processes, a comparison between the simulated morphological development by the coupled and stand-alone models was made. This comparison shows that aeolian sediment transport to the foredune, i.e. 214,000 m³ over 5 years, extracts sediment from the marine domain. As a result, the alongshore redistribution of sediment from the main peninsula by marine-driven processes decreased by 70,000 m³, representing 1.7% of the total marine-driven dispersion. From the aeolian perspective, marine-driven deposition and erosion reshape the cross-shore profile, controlling the supply-limited aeolian sediment transport and the magnitude of sediment deposition in the foredunes. In the region with persistent accretion along the Sand Engine's southern flank, a higher than average foredune deposition was predicted due to morphological development of the region where sediment is picked up by aeolian transport. Including these marine processes in the coupled model resulted in an increase of 1.3% in foredune growth in year 1 and up to 6.7% in year 5 along this accretive section. At the northern flank, where the developing lagoon and tidal channel provided increased shelter to the supratidal beach, predicted foredune deposition reduced up to −11.5% over the evaluation period. Our findings show that both aeolian and marine transports impact reshaping the nourished sand, where developments in one domain affect the other. The study findings echo that the interplay between aeolian- and marine-driven morphodynamics could play a relevant role when predicting sandy NBS.

There is a relative lack of research, targeted models and tools to manage beaches in estuaries and bays (BEBs). Many estuaries and bays have been highly modified and urbanised, for example port developments and coastal revetments. This paper outlines the complications and opportunities for conserving and managing BEBs in modified estuaries. To do this, we focus on eight diverse case studies from North and South America, Asia, Europe, Africa and Australia combined with the broader global literature. Our key findings are as follows: (1) BEBs are diverse and exist under a great variety of tide and wave conditions that differentiate them from open-coast beaches; (2) BEBs often lack statutory protection and many have already been sacrificed to development; (3) BEBs lack specific management tools and are often managed using tools developed for open-coast beaches; and (4) BEBs have the potential to become important in nature-based management solutions. We set the future research agenda for BEBs, which should include broadening research to include greater diversity of BEBs than in the past, standardising monitoring techniques, including the development of global databases using citizen science and developing specific management tools for BEBs. We must recognise BEBs as unique coastal features and develop the required fundamental knowledge and tools to effectively manage them, so they can continue providing their unique ecosystem services.

Projections of high rates of sea level rise have stimulated proposals for adaptation strategies with increasingly high nourishment volumes along sandy beaches. An underlying assumption is that coastal profiles respond rapidly to nourishments by redistributing sediments towards a (new) equilibrium shape. However, this perception may not be valid when high volumes of nourishment are applied, as the profile shape may then undergo significant deformation. Current state-of-the-art modelling techniques often concentrate on a single spatio-temporal scale, either lacking the necessary temporal horizon or failing to provide the required level of cross-shore detail. This article introduces Crocodile, a diffusion based cross-shore model designed to bridge the gap between short- and long-term nourishment modelling. The model simulates the effects of nourishment strategies on coastal volume, coastline position and beach width over a decadal timeframe. It incorporates different elements which compute cross-shore diffusion, sediment exchange with the dune and longshore sediment losses. To test the model performance, a series of idealized nourishment scenarios are examined, along with three case studies along the Dutch coast with different nourishment strategies over the past few decades. The modelled coastal volume, shoreline position and beach width strongly resemble the observations with only a 12% overestimation in profile volume and 13% underestimation in beach width. Averaged over selected periods of nourishment, trends and trend reversals between different strategies are well replicated with slight overestimation for coastal volume trends by 1.5m³/m/yr(10%), while beach width trends are underestimated by 0.2m/yr (15%). Given that the added nourishment volumes are typically in the order of 100m³/m, these model errors are considered sufficiently low to conclude that Crocodile effectively simulates variations in coastal volume, coastline position and beach width over a decadal timeframe in response to different nourishment strategies. Therefore, Crocodile can facilitate the evaluation of future nourishment strategies.

We investigate pathways of sediment diffusion for a Gaussian-shaped sand mound subjected to monochromatic waves. Our unique results nearly close the sediment budget by quantifying each of the sediment transport processes responsible for mound diffusion associated with sediment flux due to slope driven transport and ripple migration. Downslope ripple progression was observed as ripples formed at the mound top advanced down the side slopes in a direction perpendicular to wave propagation. Once ripples formed on the sides of the mounds the ripples became pathways for sediment flux from the top to the bottom of the mound, persisting even after ripples reached the base of the mound as sediment avalanching due to gravity and mound slope. Lateral ripple migration caused ripples to migrate along the sides of the sand mound in a direction parallel to wave propagation. Once ripples reached the base of the mound, lateral migration of ripples caused spreading of sand around the sides of the mound. Lateral ripple migration was largely driven by ripple splitting caused by a large downslope sediment flux from the center of the mound that generated ripples with longer wavelengths than wave orbital hydrodynamics could support. To restore equilibrium between sediment and flow conditions, ripples with longer wavelengths continuously split and migrated laterally around the mound. Our results reflect the importance of slope driven transport, bed fluidization, and ripple dynamics on the larger scale diffusivity and suggest that slope driven and ripple driven sediment fluxes should be more explicitly included in sediment transport formulations.

Field observations of small scale seabed morphology were obtained over 4 weeks at two locations separated 66 m along a cross-shore transect during the 2014 MEGAPEX Experiment conducted as part of the longer term Sand Engine mega-nourishment project along the North Sea Coast of The Netherlands. The seafloor was continuously covered by dynamic bedforms with amplitudes ranging 0.02–0.40 m and wavelengths ranging 0.20–2.5 m. Ripple migration rates were up to 3.6 m/h, dependent on the energy of the waves and currents. Under the assumption of bedload dominant transport, cross-shore and alongshore sediment volume flux by ripples was estimated from observations at the spatially separated imaging locations. The average and maximum ripple sediment volume flux was found to be 0.22 and 1.7 m³/m/day, respectively, with larger fluxes during spring flood tides and storm wave conditions. The daily averaged fluxes were usually oriented about 30° north of shore-normal, moving in the same direction as a nearby transverse sandbar migration direction. Estimated gradients in the sediment flux within the surfzone were computed from bed level change measurements of the inner surfzone including a larger scale transverse sandbar measured from subsequent jetski surveys. We find that the estimated gradients in surfzone sediment flux are conceivably driven by small variations in the sediment flux driven by sand ripple migration, supported by our observations of ripple driven sediment flux at the two ripple imaging stations. A simple conceptual model is presented that shows how small scale bedforms may contribute to the growth and decay of larger scale bathymetric features, such as sandbars. Results suggest that sediment flux by small scale sand ripples and megaripples could significantly contribute to larger scale morphologic development in the surfzone.