The swash zone is an important region for the coastal morphodynamics. Often, model studies of the swash zone use depth-averaged models. These models typically assume a vertically uniform velocity and sand concentration for calculating the sand transport flux. However, this assumption is not always accurate in the swash zone. In order to investigate the vertical distribution of velocity and sand, we use a depth-resolving model that is able to capture these vertical variations. We simulate the flow and suspended sediment transport induced by bichromatic waves using a 2DV depth-resolving RANS model. Our verification of the model shows that special care needs to be taken to deal with bubbles in 2DV simulations. Furthermore, we show that turning off the (Wilcox, 2006, 2008) limiter for turbulence, increases the modelled turbulent kinetic energy that is induced by wave-breaking, resulting in improved predictions of sediment concentrations. Using the depth-resolving model, we show that the vertical distribution of velocity and sand is far from uniform in the swash zone. The results show that if one assumes vertically uniform depth-averaged velocities and concentrations, one can overpredict the sediment flux by 50%.@en

Sand waves are found on shallow, sandy seabeds throughout the world and their dynamics may pose an imminent threat to offshore construction. Therefore, there is a pressing need to understand bed level dynamics in sand wave areas. These bed level dynamics lead to variations in sand wave shape and migration rate over time. However, these variations cannot be explained with the present-day process-based sand wave models, which all include a purely periodic tidal forcing. To explain these fluctuations a more intricate description of the hydrodynamics is necessary. The aim of this study is to explore the importance of time-varying, non-tidal currents for sand wave dynamics in the North Sea. We adopted the three-dimensional Delft3D-Flexible Mesh model, and were able to reconstruct time-varying, non-tidal currents on top of the periodic tidal forcing, while significantly reducing computation times. The simulated currents and water levels showed a good agreement with in-situ measurements. Compared to the situation with only tidal forcing, the simulated sedimentation and erosion rates were amplified up to 15 times due to time-varying, non-tidal currents. Additionally, periods of net erosion were found at locations in the sand wave transect where tidally forced models only showed net-sedimentation. It is therefore important to consider time-varying, non-tidal currents when predicting future sand wave dynamics in the field.

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Reconstructions of the most severe historic flood events contribute to improved quantification of design discharges corresponding to large return periods. Reducing the uncertainty of design discharges has a great significance in constructing proper flood defences to protect the hinterland from future flooding. However, reconstructions of the peak discharges of such historic flood events are generally associated with large uncertainties, which arise from the accuracy of the historic topography, hydraulic roughness of the river channels and floodplains, and the historic hydrograph shape. This study sets up a one dimensional-two dimensional (1D-2D) coupled hydraulic model, stretching from the upstream of Bonn at Remagen to downstream of Düsseldorf, Germany, with the length of 113 km to reconstruct the maximum discharge of the 1374 flood event (Qmax,1374), which is considered to be the largest flood of the last millennium in the Lower Rhine catchment. An uncertainty analysis was performed by adopting different river bed levels and roughness values in order to estimate the influence of these uncertainties on the reconstructed peak discharge. The upstream discharge wave was varied corresponding to a wide range of peak discharges from 12,000 to 24,000 m3/s. The resulting Qmax,1374 was determined of between 14,400 and 18,500 m3/s, were then used in a flood frequency analysis to determine the design discharges corresponding to different return periods. Compared to the design discharge computed with previous estimations of the 1374 peak discharge, we found a significant reduction of 2,000 m3/s in the design discharge corresponding to a 100,000 year return period, which is the maximum safety standard adopted in the Dutch water policy for some downstream dike sections.@en

Transdisciplinary research requires improved knowledge exchange between science and practice. Such improvements include diversifying and scaling up knowledge accessing and sharing through online platforms. We conducted twenty interviews informed by behavioral science methods to clarify the aim, components, and participants' perspectives on the usefulness of the proposed components for an envisioned platform. Participants were members of a Dutch community of practice for river studies and a research programme into integrated and collaborative management. The proposed concept included storylines, data repositories, user profiles, interactive visualisations, and collaborative sessions. Interview results include drivers and barriers from prospective users that we translated into requirements to increase the potential adoption and effective use of online platforms with similar components. From the experiences with implementing these requirements, we provide recommendations for enabling primary drivers: (i) Combining online and offline interactions to provide various options for knowledge exchange between disciplines and organisations. (ii) Sharing the content and application of the research with a non-scientific audience. (iii) Reusing existing online platforms as much as possible without restricting any to improve the reuse of research methods and results. We further provide recommendations to overcome the main barriers: (i) Partnering with various communities to extend knowledge exchange. (ii) Following a participatory approach to improve the design and content while considering the time and resources that such a process entails. (iii) Providing flexible options to contribute and tailor overviews of available knowledge in different ways according to prospective users' roles in practice. (iv) Purposefully facilitating online interactions according to the transdisciplinary process-intended attributes.@en

The present work presents physical laboratory measurements of surface elevation and pore water pressures in a fine sand bed under bichromatic waves in a large-scale laboratory experiment. This was done at three cross-shore locations in the swash zone, with pressures being measured at different depths in the bed. The measurements show that the pore pressure signal decays and shifts with increased depth. These measurements are used to validate a practical model, based on the theory of Yamamoto et al. (1978, https://doi.org/10.1017/S0022112078003006) and Guest and Hay (2017, https://doi.org/10.1002/2016JC012257). The model corresponds well with the measurements (nRMSE < 0.2 and R² > 0.95 for most probes) and shows that a frequency-based model can reproduce the pressures in the bed, despite the bed being exposed during dry periods. Furthermore, the model provides the opportunity to calculate pressure gradients, both throughout the bed and at the bed surface. These modeled pressure gradients at the bed surface show that the vertical pressure gradient can have an important impact on the Shields parameter, thereby influencing sediment transport.

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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.

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Buildings affect aeolian sediment transport and bedform development in sandy environments. Cellular automaton (CA) models have, however, only been used to simulate natural bedform dynamics. This study extends a well-known aeolian CA model to include sediment dynamics around buildings, and uses this model to explore the interaction of building-induced deposition and erosion with natural bedform dynamics. New CA rules are introduced to represent acceleration, deceleration and sideward transport of sediment around obstacles. The simulated deposition and erosion patterns show good agreement with field experiments. The model reproduces the shape and location of the morphological pattern around a single building, and effects of building spacing on this pattern for building groups. Model results further demonstrate that building-induced effects interact with local bedform dynamics and can alter the shape, growth and migration of sand dunes.

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We present a fully coupled 2DV morphodynamic model, implemented in OpenFOAM^® that is capable of simulating swash-zone morphodynamics of sandy beaches. The hydrodynamics are described by the Reynolds-averaged Navier–Stokes (RANS) equations with a (Formula presented.) turbulence model and the Volume of Fluid (VoF) approach for discriminating between air and water. Sediment transport is described in terms of bedload and suspended load transport. We show that the default divergence scheme in OpenFOAM can become numerically unstable and lead to negative sediment concentrations, and propose a solution to avoid this problem. The model performance is assessed in terms of surface elevation, flow velocities, runup, suspended sediment concentrations, bed profile evolution and sediment transport volumes by comparing with measurements of field-scale (wave height of 0.6 m) solitary waves. The model shows reasonable agreement in terms of hydrodynamics and predicts the correct sediment transport volumes, although the deposition is predicted more onshore compared to the measurements. This is partially attributed to an overprediction of the runup. The model shows that the suspended sediment concentration displays a strong vertical dependence. These results show the potential of depth-resolving models in providing more insight into morphodynamic processes in the swash zone, particularly with respect to vertical structures in the flow and suspended sediment transport.

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In this study we tested to what extent grain count data from a laser particle counter, when enriched with granulometric data, can lead to accurate measurements of aeolian sediment fluxes in the field. Field experiments were conducted at Koksijde beach (Belgium) with a vertical array of five Wenglor fork sensors and co-located vertically stacked mesh sand traps. Sand collected in the traps was used to both obtain the reference values for sediment flux as well as to obtain granulometric data at the five Wenglor sensor elevations. Grain counts were transformed to sediment fluxes by combining the granulometric data with the grain size dependent, effective detection width concept. It was found that the limitation of the Wenglor sensor to have a minimum detectable grain size, well within the diameter range of sand grains in transport, could easily be corrected for through a linear relation of Wenglor detectable sediment flux with total sediment flux. However, we found that the Wenglor derived fluxes deviated from the sand trap derived fluxes in an inconsistent manner, both in the vertical and over time, which made us conclude that there is no uniform calibration possible to match the Wenglor data with the trap data. This suggests that further studies using optical aeolian transport sensors should focus on analysing the raw photoelectric signal rather than on internally processed count data.

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In sandy environments, like the beach-dune system, buildings not only affect the airflow, but also the aeolian sediment transport in their surroundings. In this study, we determine how the horizontal size of sediment deposition patterns around buildings depends on the building's dimensions. Four one-day experiments were conducted at the beach using box-shaped scale models. We tested 32 building geometries, where scale model height, width and length ranged between 0.3 and 2.0 m. The deposition patterns were substantial in size: the total length and width of the deposition area were up to an order of magnitude larger than the horizontal building dimensions. It was found that the size of upwind and downwind deposition patterns depended more on the building width perpendicular to the wind direction (w), than on the building height (h). Building length had little influence. Especially the combined effect of w and h correlated well with horizontal deposition size. This is expressed in a new scaling length B for deposition around buildings, with B=w^2/3·h^1/3. As a first validation, the spatial dimensions of the initial deposition patterns observed around a scale model of 2.5 × 12 × 2.5 m, placed at the beach for five weeks, showed good agreement with those predicted based on B.

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Accurate calculation of wave overtopping is important for determining the required crest height and geometry of a dike. Berms and roughness elements are widely used to reduce the average overtopping discharge at dikes while the reductive effects of berm and roughness are still not fully understood. Several empirical formulae are available to predict the overtopping rate at coastal structures. However, the extrapolation of these empirical formulae is not always applicable for complex structures (e.g. a dike that has a berm and/or roughness elements on the waterside slopes) or wave conditions that are outside the applicability of the empirical predictors. A 2D numerical model based on OpenFOAM® is set up in this study for predicting wave overtopping at dikes that have complex configurations with berms and roughness elements. The validation results show that this OpenFOAM® model is capable of reproducing the incident waves accurately and predicting the wave overtopping discharge with good accuracy. Subsequently, the numerical model is applied to study the reductive influence of a berm and protruding blocks on the mean overtopping discharge at dikes. The roughness of protruding blocks is incorporated by explicitly modelling the protrusions using refined mesh. The model shows reasonable behaviour of the reduction of wave overtopping influenced by a berm and roughness. This indicates the capabilities of the numerical model in the design and safety assessment of dikes.

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The long-term physical existence of sandy shores critically depends on a balanced sediment budget. From the principles of Building with Nature it follows that a sustainable protection of sandy shores should employ some form of shore nourishment. In the spatial design process of urbanized sandy shores, where multiple functions must be integrated, the knowledge and the prediction of sediment dynamics and beach-dune morphology thus play an essential role. This expertise typically resides with coastal scientists who have condensed their knowledge in various types of morphological models that serve different purposes and rely on different assumptions, thus have their specific strengths and limitations. This paper identifies morphological information needs for the integrated spatial design of urbanized sandy shores using BwN principles, outlines capabilities of different types of morphological models to support this and identifies current gaps between the two. A clear mismatch arises from the absence of buildings and accompanying human activities in current numerical models simulating morphological developments in beach-dune environments.@en

Observations of barrier coasts around the world suggest that some systems do not conform to the O'Brien-Jarret law. Here we explain this by investigating how resonance and bottom friction affect the response of tidal inlets to variations in basin geometry. Therefore, we develop a morphodynamic barrier coast model that is based on the stability concept of Escoffier for the morphological evolution of the inlets, coupled with an idealized hydrodynamic model that describes the water motion in the outer sea, inlets, and arbitrarily shaped back-barrier basin. We find that the total tidal prism through all inlets is predominantly determined by the (cross-shore) width of the basin and identify three regimes for this. First, a linear regime for narrow basins (i.e., basin width (Formula presented.) tidal wavelength) where a larger basin leads to a linear increase in total tidal prism. Second, a resonant regime for basins with a width around the resonant condition in which the total tidal prism reaches a peak. This resonance condition is a quarter tidal wavelength for basins without friction, which shifts to narrower basins as friction becomes stronger, down to 0.15 tidal wavelength. Third, a dissipative regime for wide basins (i.e., the cross-shore basin dimension or basin width (Formula presented.) resonant condition) with sufficiently strong bottom friction in which the total tidal prism does not change for wider basins, because the tidal wave completely dissipates in the basin.

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The average overtopping discharge is an important parameter for the design of flood defences. Several empirical formulas are available for predicting the overtopping discharge at dikes. However, these empirical formulas often have their specific applicable conditions. To complement with the empirical methods, a numerical model has been developed using the open source CFD package OpenFOAM to model the wave overtopping at dikes. Systematic calibration and validation of the numerical model are performed. The influences of the mesh, solver, turbulence model and roughness height on the modelled results of the average overtopping discharge have been investigated during the model calibration. The simulations show that the turbulence model increases the accuracy of the numerical model for predicting the average overtopping discharge under wave breaking conditions. The calibrated model is then validated by comparing the modelled average overtopping discharges with the measured ones from the physical model tests. Results show that the OpenFOAM model is capable of predicting the average overtopping discharge accurately at dikes that have a smooth straight waterside slope.

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A reliable estimation of average wave overtopping discharge is important for dike design and safety assessment. Berms and roughness elements are widely applied to reduce the overtopping discharge over dikes. In this study, the effects of a berm and roughness on the wave overtopping discharge are investigated by means of physical model tests. New empirical formulae are derived from the analysis of the experimental data in order to provide more accurate estimations of reductive influence of berms and roughness on the wave overtopping discharge. Additionally, a new formula is developed to estimate the reductive influence of varying roughness along the waterside slopes with a berm. The new equations show a significantly better performance within the tested range when compared with existing formulae for the average overtopping discharge.

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The composition of benthic species communities in the nearshore zone is closely related to the hydrodynamic and morphodynamic conditions. Sustainable management of the coastal ecosystem requires knowledge about the natural dynamics as well as human-induced changes on the ecosystem. To improve our knowledge of the benthic species distribution along a dissipative sandy shore with multiple breaker bars, an extensive dataset was collected in the nearshore zone of the barrier islands Ameland and Schiermonnikoog in the Dutch North Sea. From 2010 to 2014, every year, approximately 180 grab samples along 18 cross-shore transects were collected and analyzed for sediment characteristics and macrobenthic species composition. Mixed-effect-models and partial redundancy analysis were used to analyze the importance of morphological features (i.e., slopes, bar crests, and troughs) as an explanatory variable for the benthic species distribution. The results indicate that the morphological features in themselves explain three times more variation than the environmental parameters used. This demonstrates the importance of morphological features as a factor in explaining the distribution of benthic species communities in the nearshore. Detailed information on morphological features is easy to obtain from bathymetry maps or visual inspection. Incorporating morphological features in species distribution models will therefore help to improve sustainable management of our valuable sandy coastal systems.

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The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping discharge still need to be developed and/or validated. Therefore, this study aims to develop empirical equations to quantify the reductive influence of rock armour on wave overtopping at dikes. Empirical equations for estimating the effects of rock berms and roughness are derived based on the analysis of experimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of waterside slopes is estimated by introducing the location weighting coefficients. Results show that the newly derived equations to predict the average overtopping discharge at dikes lead to a significantly better performance within the tested ranges compared to existing empirical equations.

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Foreshores consisting of both bare tidal flats and vegetated salt marshes are found worldwide and they are well studied for their wave attenuating capacity. However, most studies only focus on the small scale: just some isolated locations in space and only up to several years in time. In order to stimulate the implementation of foreshores serving as reliable coastal defense on a large scale, we need to quantify the decadal wave attenuating capacity of the foreshore on the scale of an estuary. To study this, a unique bathymetrical dataset is analyzed, covering the geometry of the Westerschelde estuary (The Netherlands) over a time-span of 65 years. From this dataset, six study sites were extracted (both sheltered sites and exposed sites to the prevailing wind direction) and divided into transects. This resulted in 36 transects covering the entire foreshore (composed of the bare tidal flat and the vegetated salt marsh). The wave attenuation of all transects under daily conditions (with and without vegetation) and design conditions (i.e. events statistically occurring once every 10,000 years) was modelled. Overall, the spatial variability of the geometry of a single foreshore was observed to be much larger than the temporal variability. Temporal changes in salt marsh width did not follow the variability of the entire foreshore. Both under daily and design conditions, vegetation contributes to decreasing wave energy and decreases the variability of incoming wave energy, thereby decreasing the wave load on the dike. The southern foreshores, sheltered from the prevailing wind direction, were more efficient in wave attenuation than the exposed northern foreshores. A linear relation between marsh width and wave attenuation over a period of 65 years was observed at all marshes. The present study provides insights needed to calculate the length of a salt marsh to obtain a desired minimum wave attenuating capacity.

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Side channels are commonly constructed to reduce the flood risk or to increase the ecological value of a river. Such artificial side channels generally aggrade. We categorize the development of side channels based on the sediment that is deposited in these channels. Based on this categorization, we determine the main mechanisms that affect their development, and we propose an initial framework on how to predict the long-term development of side channels. The results can be used to design, operate, and maintain side channel systems.@en