Infragravity Waves in a Complex Macro-tidal Environment: High Frequency Hydrodynamic Measurements and Modelling

Nearshore high frequency (> 1 Hz) hydrodynamic measurements over month to year scale face many issues, especially in inter-tidal areas. We present measurements done in the intertidal area of a macro-tidal site (Gavres, France), and on a time span of 2 years. The measurements are done with a WiSens NKE pressure sensor (burst mode of 17 minutes). Infragravity (IG) waves are present in the signal with IG wave heights reaching 0.5 m. The hydro-meteorological conditions (tide, surge, offshore short wave, wind) are characterized for each burst, showing for instance a linear increase of IG wave height with the offshore wave height (Hs) as long as Hs remains smaller than 4 m. To further investigate the temporal and spatial variability of IG waves, the analysis is completed using an unsteady phase resolving model (SWASH). We show how the IG wave height depend on the offshore wave conditions, the still water level, local wind conditions, and the local configuration, and provide hypothesis on the IG wave mechanisms generation on Gavres.