Infragravity waves are also know as "surf beat": the interference of wave trains gives a succession of high waves and low waves, and the mass and momentum transport of the waves drives an oscillation of the free surface at the scale of the wave groups that travels with the groups, this type of infragravity wave is called a "bound long wave" because it is bound to the group.
As the waves propagate in shallower and shallower water, they eventually break and release the long waves, giving rise to free infragravity waves that travel at a phase speed given by the linear wave dispersion relation. These waves are strongly refracted as they propagate offshore giving "leaky waves" that manage to escape to the deep ocean, and edge waves that propagate along the coastal wave guide.
Infragravity waves have typical periods in the range 30 to 300 s, and are thus a unique source of signals in this range of periods, longer than windseas and swell, and shorter than ocean internal waves.
Preparing for the SWOT satellite mission, researchers at LOPS and LEGOS have worked together to investigate the possible signature of infragravity waves on sea level measurements from space. This work involved the analysis of bottom pressure recorders, as well as the development of the first global infragravity wave model (GIGGLE).
1. Coastal dynamics: cliff-top storm deposits and other effects
Working at the edge of the ocean in Brest makes it easier to study all sorts of marine phenomena, and provides opportunities for looking at dynamics over cliff-lined coasts, which have been less studied because generally less prone to flooding. However, some of our cliffs are facing pretty big waves, and coastal erosion takes a very special meaning. Fichaut and Suanez (2006) have made a great detective work to pair the before and after location of big rocks (some weighing up to 6 t) that have moved during storms on the island of Banneg. These rocks start out from a cliff edge located 6 m above the highest tide, and fly / roll / bounce around during big storms. We thus went out to measure water level during storms at the foot of the cliff, and found that infragravity waves accounted for a large part of the high water levels with heights exceeding 2 meters, helping the wind-waves to reach higher elevations.
2. The global infragravity wave field
Infragravity waves are part of the surface gravity wave family: these waves have a signature on the bottom pressure provided that their length does not exceed twice the water depth. For a typical ocean depth of 5 km, this means 10 km waves. This limit cuts close to the dominant peak in the infragravity wave spectrum, so that the signature of waves with periods longer than 80 s can be observed, but shorter components are filtered out leading to a "noise notch", a gap in the pressure spectrum with very little signals in ocean bottom pressure at periods 30 to 80 s. We expect that IG waves are there on the surface but we cannot measure them.
this model was verified using the PBR data ...
3. Our Global InfraGravity wave ModeLE (GIGGLE)
Starting from empirical observations of the relationship between HIG and windsea / swell heights and periods we came up with a parameterization of the infragravity wave spectrum that could be inserted as a source all along the world shorelines, and propagated. Because we computed windsea and swell spectra with WAVEWATCH III it was natural to extend the frequency range of WAVEWATCH III to very low frequency ... even though this is very inefficient (other numerical methods would work better, we may come back to that).