Infragravity waves II

Seminal work of Stokes, based on linear wave theory, suggest that there is a net forward drift in the fluid beneath a propagating surface wave. However, a recent work supported by field measurements suggest that in many cases, particularly in waves propagating over a shear flow, either a net forward or backward Eulerian flow may develop according to the local average water depth. In this work we further investigate what has been observed in the field by means of laboratory experiments in a wave flume, where monochromatic short waves and bichromatic (short + long) waves are run. Fluid particles are tracked and their velocity evaluated through Particle Tracking Velocimetry (PTV) and processed to extract vertical profiles of longitudinal velocity. Fluid trajectories are to be compared with theoretical predictions.

Infragravity (IG) waves are low-frequency waves (0.0033 Hz < f < 0.05 Hz) that become important or even dominant compared to the sea-swell waves at the toe of a dike on a very or extremely shallow foreshore, therefore becoming important to the wave overtopping process. This paper presents the field setup for observations of IG waves offshore and nearshore at Living Lab Raversijde (LLR) with the Research Dike Raversijde (RDR) (presented by Gruwez et al. (2024), also at Coastlab24) for measuring the wave-structure interactions (overtopping, impact,…), and presents the results of IG wave influence on wave overtopping, including free IG waves (never done before). Finally, a first evaluation of existing empirical formulas with the field measurements is done. At the conference, the field setup and results will be presented in more detail, based on storms measured during winters '22-'23 and '23-'24.

Traditionally, the evaluation of overtopping on coastal structures is carried out by the combination of meteo-oceanographic information and the analytical evaluation of the variable, through semi-empirical formulae.The meteo-oceanographic variables that allow the analytical calculation of overtopping are, in general, only associated with waves and sea level, with no considerations given to wind and infragravity waves effects. Both variables, wind and infragravity waves, can dramatically modify run-up and ovetopping, specially in reflected beaches. This contribution takes advantage of the waves forecast system SO3, which combines wave and infragravity waves to couple them to the XBeach model, to determine more realistically the run-up and its derived effects, e.g. overtopping. A segment of Caldera beach, located on the Pacific coast of Costa Rica, is used as study place. This site frequently experiences overtopping throughout the year.

Infragravity waves are surface waves with relatively longer periods in comparison to periods of the spectrum-dominant gravity waves. They are characterized by oscillations between 20 and 300 seconds (0.0033 Hz < f < 0.05 Hz), amplitudes that range from a few millimeters to tens of centimeters, and wavelengths of kilometers. Implementing optimal sampling strategies for observing and characterizing infragravity waves might be a challenging. This study explores multi-sensor (ADCP, Quartz Pressure, Wave Buoy) in-situ deployments conducted at four selected sites off the Belgian coast. Results focus on detection of infragravity waves in both calm as storm periods. Conclusions reveal new insight on the occurrence of infragravity waves along the Belgian coast, their magnitude, generation, propagation, and relationship with sea swell waves and bathymetric features along the Belgian Coast.