Infragravity waves I

This study generalizes SWAN model capabilities for free infragravity (IG) waves based on a new collection of measurements over the southern part of the North Sea. As a first step, existing SWAN model capabilities for free IG are generalized to unstructured grids. In contrast to the common nesting approach, unstructured grids reduce modelling complexity through the use of a single computational grid that allows local refinement nearshore and high flexibility to generate grids along coastlines. The new collection of measurements provides a unique opportunity to validate this modelling approach over the spectral domain and over the full IG frequency band. Moreover, these new measurements enable model validation for both deeper water offshore-sites and shallower nearshore-sites and allow to optimize modelling performance for North Sea forecasting.

Wave overtopping is not easy to measure in real field situations. A 12-years long program started in 2018 to measure wind, wave and water level conditions in a complex estuary, together with wave overtopping measurements at dikes. Wave and water level conditions are measured directly in front of the dike. Overtopping is measured with two overtopping boxes that were placed in the dike at different levels. In recent years, a system was invented that measures wave run-up and wave overtopping with two parallel lasers. The laser scanner system was placed next to the overtopping boxes to enable a proper validation. This paper describes the storms that have been measured up to now, including new measurement devices, improvements to measurements and new analysis methods. This all brings new insights, but more storms and improvements of measurements are needed to come to a full understanding of this complex estuary.

This study investigates the spectral distribution of  the wave energy in a coral reef-lagoon system, focusing on very low frequency waves (VLF) and among them resonant waves. The data analysis is constructed on a dedicated methodology for identifying evanescent resonant waves, mainly based on the elaboration of two criteria: the energy persistence and the phase shift. The methodology is used on field data from an in-situ campaign in Maupiti Island (French Polynesia). Wave energy spectra are calculated at various positions from pressure time series; the method developed then results in persistence spectrograms that demonstrate how VLF (and possibly resonant waves) evolve in time and space in the Maupiti lagoon and across the Maupiti barrier.

Reef barriers play a major role many coral islands, by sheltering the lagoon from the ocean wave energy and then creating a unique habitat for many species. This filtering action becomes increasingly crucial for ecosystems health and shoreline protection in the context of climate change and related sea level rise, degradation of coral systems and modification of wave conditions. A strong research effort has therefore been engaged by the coastal oceanographers community for the last two decades to improve our knowledge and prediction skills of wave dynamics over coral reef systems. A widely reported observation is the importance of infragravity waves (IG) over wave-driven reef systems, whether fringing or barrier reefs. IG are primarily forced by groups in the incoming short-wave (SW) field, either by the release of bound waves or the breakpoint oscillations (Bertin et al. 2018). IG period typically ranges between 30 and 200s, which makes them prone to excite or interact with natural seiching modes in reef-lagoon systems often ranging in the Very Low Frequency (VLF) band. Further research efforts are now necessary to better understand the interaction between long IG/VLF oscillations and SW field. In particular, long waves are expected to play a dynamic depth-filtering role on SW energy, acting as long carrier wave able to promote the propagation of larger SW groups by IG/VLF crests. More generally, the spectral energy transfers over the reef crest-flat system and their relative importance w.r.t. frictional and breaking dissipation are not fully understood over the complete range of surface waves. 
The aim of the present study is to analyse and to discuss a series of field observations performed on the barrier reef of Maupiti Island, French Polynesia. A particular focus is placed on the interaction between SW and IG wave fields across the reef crest-flat system.