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Cross-shore sediment transport in the swash-zone : large-scale laboratory experiments
High temporal and spatial coverage measurements of swash hydrodynamics and sediment concentrations are needed to be able to quantify and understand sediment fluxes near the shoreline. Additionally, this relation between intra-swash processes and swash averaged sand transport rates are essential to further development of numerical and empirical models for sand transport in the swash zone. The present research focuses on the hydrodynamics, sand transport processes and net sediment transport in the swash zone through a series of large-scale wave flume experiments. This research aims to improve the under-standing of swash zone sand transport processes, in particular the role of cross-shore sand advection and wave-swash interactions, and bring new detailed insights into the relation between intra-swash processes and net sand transport rates.
RANS modelling of cross-shore sediment transport and morphodynamics in the swash-zone
Most numerical studies of sediment transport in the swash zone use depth-averaged models. However, such models still have difficulty predicting transport rates and morphodynamics. Depth-resolving models could give detailed insight in swash processes but have mostly been limited to hydrodynamic predictions. We present a depth-resolving numerical model, based on the Reynolds Averaged Navier-Stokes (RANS) equations, capable of modelling sediment transport and morphodynamics in the swash zone.
Vegetated hydrodynamic system : parameterization and stochastic dependence modelling
Vegetation as a nature-based solution, along with conventional solutions like dikes, has convincingly shown potential for flood hazard (wave load) reduction. However, be it numerical or experimental, only a few isolated case evidences have been presented. This study introduces stochastic dependence modelling using non-parametric Bayesian networks (NPBN) for vegetated coastal systems. The system has been parametrized using continuous distributions, and likely (conditional) correlations among variables. NPBNs can cater continuous marginal distributions and use Gaussian copulas. The model represents a consistent joint probability distribution and hence can be used to generate conditions in physically realistic windows. It adds value to numerical modelling by reducing the number of simulations required to get meaningful generalized results.
Field measurements of very oblique wave run-up and overtopping with laser scanners
An alternative flexible solution te measure wave overtopping was developed, using two terrestrial laser scanners (Oosterlo et al., 2019). The goal of this paper is to further validate this innovative system for measuring wave run-up heights, depths and front velocities, and for determining the wave overtopping, (peak) wave period and angle of incidence during an actual severe winter storm with very oblique wave attack. To this end, the paper will describe the analysis of the data obtained during storm Ciara (10-12 February 2020). Furthermore, the laser scanner results will be validated with data from the overtopping tanks and video recordings. Finally, the data gathered during storm Ciara will be compared to the current knowledge on wave overtopping, to possibly gain new insights in the influence of very oblique wave attack on wave overtopping.
Advanced modelling of wave penetration in ports
This study evaluates to what degree SWASH models correctly simulate wave penetration per wave process, separately in simplified models and in combination in the full harbour layout, to identify their role in the model accuracy. The study shows SWASH’s capability to reproduce qualitatively the most important reflection and diffraction trends. To a large extend, diffraction is the main process determining the wave height inside the harbour; reflection at the harbour end comes second. Outside the harbour, reflection off a quay wall is the dominant process, while reflection off a gravel slope is noteworthy. All in all, it is concluded that for non-breaking, relatively low waves, SWASH accuracy in modelling wave penetration is sufficient for engineering purposes in a harbour environment.
Modelling transitions in grass covers to quantify wave overtopping erosion
Transitions in vegetated dike covers, such as geometry changes or roughness differences, are identified as weak spots in dikes for grass cover erosion by wave overtopping. Although several erosion models exist to model grass cover erosion on dikes, it is unclear how the effect of transitions on grass cover erosion must be included in these models. Therefore, we have developed a model approach to analyze the effects of transitions on grass cover erosion using field experimental data and to derive representative influence factors for one transition type. The model approach has been applied to the transition at the landward toe where the slope changes to a horizontal plane. The model approach is general applicable and can be transferred easily to other transitions. The derived factors can be used to improve predictions of dike cover erosion near transitions.
The broad application of a depth inversion algorithm based on the dynamic mode decomposition
Mapping coastal bathymetries from remote sensing is an attractive alternative to in situ measurements due to the large spatial coverage and relatively low costs. The technological boost of drones and satellites now offers new and more flexible platforms for video-based depth inversion algorithms (DIAs); it thereby signals a desire to generate depth estimates on-the-fly, requiring more flexibility and high computational speeds of the DIAs. For this purpose, a novel algorithm was developed, which is fast enough to be used for on-the-fly depth and surface current estimation at a broad range of application areas.
Analysis of the performance of different sediment transport formulations in non-hydrostatic XBeach
Process-based, wave-resolving models are essential tools to resolve the complex hydro-morphodynamics in the swash zone. The open-source Non-Hydrostatic XBeach model can solve the depth-averaged wave-by-wave flow in the nearshore region up to the shoreline and the intra-wave bed changes during time-varying storms. However, validation and testing of its morphological response are still limited in the context of sandy beaches. This work aims to assess the performance of the wave-resolving sediment dynamics modelling within Non-Hydrostatic XBeach for different sediment transport formulations. The sediment transport modelling approaches considered in this study were tested and compared to laboratory experiments involving wave trains over an intermediate beach. Numerical results show a better performance in the prediction of the intra-swash sediment dynamics when the newly implemented wave-resolving transport equation is applied compared to the existing approach within the model.
Reduced complexity modeling of shoreline response behind offshore breakwaters
Prediction of the shoreline response behind offshore breakwaters is essential for coastal protection projects. Due to the complexity of the processes behind the breakwaters (e.g., wave diffraction, currents, longshore transport), detailed modelling needs high computational efforts. Therefore, simplifying the process effect in a simpler coastline model could be efficient. In this study, the coastline evolution model ShorelineS is used. A new routine was implemented in the model to adjust the wave heights and angles behind the offshore breakwaters. Two approaches from the literature and a newly introduced one were tested in this study. The model free grid system was used to simply track the breaker line; such an advantage also helped to form tombolo, which is not common for these types of models. The tests showed promising results for single and multi breakwaters systems; however, the newly introduced approach still needs further testing and refinement for better performance and less computational cost.
Implications of coral reef mining pits on 2D hydrodynamics
Reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures on Small Island Developing States (SIDS). There are strong concerns about this practice, because partial removal of the protective reef could increase wave impacts on the islands. Here we present the potential 2D effects of reef flat excavation pits on nearshore hydrodynamics. More specifically, we aim to assess the impact of excavation pits on nearshore circulation patterns, which could be an indicator of coastal morphological changes.