Marion Tissier

A splitting approach for the fully nonlinear and weakly dispersive Green-Naghdi model

The fully nonlinear and weakly dispersive Green-Naghdi model for shallow water waves of large amplitude is studied. The original model is first recast under a new formulation more suitable for numerical resolution. An hybrid finite volume and finite difference splitting approach is then proposed, which could be adapted to many physical models that are dispersive corrections of hyperbolic systems. The hyperbolic part of the equations is handled with a high-order finite volume scheme allowing for breaking waves and dry areas. The dispersive part is treated with a classical finite difference approach. Extensive numerical validations are then performed in one horizontal dimension, relying both on analytical solutions and experimental data. The results show that our approach gives a good account of all the processes of wave transformation in coastal areas: shoaling, wave breaking and run-up.

Nonlinear Infragravity–Wave Interactions on a Gently Sloping Laboratory Beach

A high-resolution dataset of three irregular wave conditions collected on a gently sloping laboratory beach is analyzed to study nonlinear energy transfers involving infragravity frequencies. This study uses bispectral analysis to identify the dominant, nonlinear interactions and estimate energy transfers to investigate energy flows within the spectra. Energy flows are identified by dividing transfers into four types of triad interactions, with triads including one, two, or three infragravity–frequency components, and triad interactions solely between short-wave frequencies. In the shoaling zone, the energy transfers are generally from the spectral peak to its higher harmonics and to infragravity frequencies. While receiving net energy, infragravity waves participate in interactions that spread energy of the short-wave peaks to adjacent frequencies, thereby cre- ating a broader energy spectrum. In the short-wave surf zone, infragravity–infragravity interactions develop, and close to shore, they dominate the interactions. Nonlinear energy fluxes are compared to gradients in total energy flux and are observed to balance nearly completely. Overall, energy losses at both infragravity and short-wave frequencies can largely be explained by a cascade of nonlinear energy transfers to high frequencies (say, f . 1.5 Hz) where the energy is presumably dissipated. Infragravity–infragravity interactions seem to induce higher harmonics that allow for shape transformation of the infragravity wave to symmetric. The largest decrease in infragravity wave height occurs close to the shore, where infragravity–infragravity in- teractions dominate and where the infragravity wave is asymmetric, suggesting wave breaking to be the dominant mechanism of infragravity wave dissipation.

Beach steepness effects on nonlinear infragravity-wave interactions : A numerical study

The numerical model SWASH is used to investigate nonlinear energy transfers between waves for a diverse set of beach profiles and wave conditions, with a specific focus on infragravity waves. We use bispectral analysis to study the nonlinear triad interactions, and estimate energy transfers to determine energy flows within the spectra. The energy transfers are divided into four types of triad interactions, with triads including either one, two or three infragravity-frequency components, and triad interactions solely between sea-swell wave frequencies. The SWASH model is validated with a high-resolution laboratory data set on a gently sloping beach, which shows that SWASH is capable of modeling the detailed nonlinear interactions. From the simulations, we observe that especially the beach slope affects nonlinear infragravity-wave interactions. On a low-sloping beach, infragravity-wave energy dominates the water motion close to shore. Here infragravity-infragravity interactions dominate and generate higher harmonics that lead to the steepening of the infragravity wave and eventually breaking, causing large infragravity energy dissipation. On the contrary, on a steep-sloping beach, sea-swell wave energy dominates the water motion everywhere. Here infragravity frequencies interact with the spectral peak and spread energy to a wide range of higher frequencies, with relatively less infragravity energy dissipation. Although both beach types have different nonlinear interaction patterns during infragravity-wave dissipation, the amount of infragravity-wave reflection can be estimated by a single parameter, the normalized bed slope.

Infragravity‐wave modulation of short‐wave celerity in the surf zone

The cross-shore evolution of individual wave celerity is investigated using two high-resolution laboratory experiments on bichromatic waves. Individual waves are tracked during their onshore propagation and their characteristics, including celerity, are estimated. The intrawave variability in celerity is low in the shoaling zone but increases strongly after breaking. It is maximum when the infragravity-wave height to water depth ratio is the largest, that is to say close to the shoreline. There the observed range of individual wave celerity can be as large as the mean celerity value. This variability can be largely explained by the variations in water depth and velocity induced by the infragravity waves. The differences in celerity are such that they lead to the merging of the waves in the inner surf zone for most of the wave conditions considered. Again, the location at which the first waves start merging strongly correlates with the infragravity-wave height to water depth ratio. The consequences of these findings for celerity-based depth-inversion techniques are finally discussed. Surprisingly, accounting for the infragravity-wave modulation of the velocity field in the celerity estimate does not significantly improve depth estimation in the surf zone. However, it is shown that the occurrence of bore merging decreases significantly the coherence of the wavefield in the surf zone. This loss of coherence could hamper celerity estimation from pixel intensity time series and explain, at least partly, the relatively poor performance of depth-inversion techniques in the inner surf zone.

Cross‐shore sand transport by infragravity waves as a function of beach steepness

Two field data sets of near-bed velocity, pressure, and sediment concentration are analyzed to study the influence of infragravity waves on sand suspension and cross-shore transport. On the moderately sloping Sand Motor beach (≈1:35), the local ratio of infragravity wave height to sea-swell wave height is relatively small (HIG/HSW 0.4), most sand is suspended during negative infragravity velocities, and qIG is offshore directed. The infragravity contribution to the total sand flux is considerably larger and reaches up to ≈60% during energetic conditions. On the whole, HIG/HSW is a good indicator for the infragravity-related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance.

Testing numerical hydrodynamic and morphodynamic models against BARDEX II Experiment data sets

ABSTRACT Castelle, B., Dubarbier, B., Tissier, M., Bonneton, P., Conley, D.C., Ruessink, B.G. and Masselink, G., 2013. Testing numerical hydrodynamic and morphodynamic models against BARDEX II Experiment data sets. The hydrodynamics on barred beaches and mechanics of sediment transport related to sandbar migration, berm formation/destruction, barrier crest dynamics and washover deposition are extremely complex. At this time, process-based models encompassing all these processes are non-existent. Among other shortcomings, the lack of existing intensive high-frequency full-scale data limits the range improvement and validation of nearshore process-based models. In June 2012, the large-scale Barrier Dynamics Experiment (BARDEX II) was performed in the Delta Flume, providing new datasets for rigorous testing of existing hydrodynamic, groundwater and morphodynamic models and further assisting their development. Three types of models are expected to be applied and further improved: (1) the short-wave averaged surfzone beach profile evolution models 1DBeach and UNIBEST-TC, (2) the short-wave averaged and infragravity-wave resolving XBeach model that addresses morphological changes of the nearshore area, beaches, dunes and backbarrier during storms including cross-barrier groundwater fluxes and (3) the short-wave resolving hydrodynamic models SURF-GN and SWASH. In this contribution, we present the application of three of the five models. 1DBeach is applied to a morphological sequence characterised by onshore and subsequent rapid offshore sandbar migration for time-invariant wave forcing and falling tide. XBeach model is applied to a rising tide sequence characterized by a rapid shoreline retreated and overtopping and overwash processes. SURF-GN is applied to a high tide run with occasional overtoppings. All these model applications are described and model skills are qualitatively assessed. Guidelines for future model improvements and validation on BARDEX II dataset are further discussed.

A Bayesian‐Based System to Assess Wave‐Driven Flooding Hazards on Coral Reef‐Lined Coasts

Many low-elevation, coral reef-lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave-induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave-induced flooding a challenge. A process-based wave-resolving hydrodynamic model (XBeach Non-Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef-lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef-lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave-induced flooding on coral reef-lined coasts may change due to climate change.

Rip current system over strong alongshore non-uniformities: on the use of HADCP for model validation

Modeling and understanding topographically-controlled rip currents remains a challenging task. One of the reasons is the lack of intensive, high-spatial resolution, flow field measurements in the rip channel vicinity. During the ECORS (DGA-SHOM) intensive field measurements, an intertidal inner-bar rip channel was instrumented with fixed eulerian current meters. In addition, for the first time in such a system, a Horizontal ADCP (HADCP) was implemented in the vicinity of the rip current, on the sandbar edge, for horizontally profiling wave induced-currents. Results show that the HADCP provides unique information on the shear in the vicinity of the rip neck, which is particularly useful for model calibration. The HADCP data was compared with local flow measurements for various tide and wave conditions, showing a very good agreement at a 5 m range. Restrictions and recommendations for HADCP implementation in the field are pointed out. The use of HADCP for horizontally profiling rip current circulations would benefit from being deployed outside of the breakers to measure the cross section of the rip head where sediment plumes and bubbles are essentially surface dominated. In this rip current system area, which would suffer from acoustic opacity only during high energy conditions, the rip current jet is strongly unstable owing to the current shear. HADCP would provide unique information on the rip current instabilities and vortex shedding in this poorly understood area of the rip current system.

Field Observations of Wave-Induced Set-Up on the French Aquitanian Coast

We report and analyze extreme wave-induced set-up heights obtained during a large international field experiment on the Atlantic coast of France. The field experiment associated with a large storm with the maximum offshore wave height of more than 12 m, enabled us to record extreme set-up heights up to 2 m. Such extreme data which are necessary for developing further numerical and analytical studies in this field, were lacking in the literature. Our data agrees reasonably well with existing set-up data reported from other coasts in the world. A good correlation was observed between set-up and offshore wave height. Similar to other coasts, the setup-offshore wave height relationship was linear up to a value of about 1 m. Nonlinear behaviour was observed for higher setup values. This study will help to further improve and validate the existing analytical and numerical solutions.Copyright