Until the late 1980's, the frequency band between infragravity waves and mean flows was felt to be insignificant and was therefore unnamed. However, a result of the Superduck experiment in 1986 was the discovery of substantial levels of energy in the velocity field at wave periods roughly from 100 to 1000 seconds [ Oltman-Shay, et al., 1989], thereafter named the far infragravity band. Surprisingly, there was no accompanying sea surface elevation signal, and celerities of these longshore progressive motions were about an order of magnitude too slow to be consistent with a gravity wave explanation (Figure 2).
Using a simplified geometry, Bowen and Holman [1989] explained these motions as shear waves arising from an instability of the strong mean longshore current. Based on rigid-lid dynamics and conservation of potential vorticity, the dynamics are analogous to large scale flows but with the role of Coriolis being played by the shear of the longshore current. On barred beaches, this shear can be much stronger than would exist on the monotonic beach profiles of previous major experiments, explaining why shear waves had not been documented much earlier. Dodd et al. [1992] adapted the analysis to the measured topography and longshore current profile from the Duck experiments to show excellent agreement of the linear instability analysis to theory (Figure 2). Why linear theory works so well in cases of such strong growth rates is under investigation.
Howd et al. [1991] used a two week data set to show that shear waves contribute from 6 to 26% of the variance at the trough of an inner sand bar system, with the contribution scaling as the offshore shear of the mean current, consistent with theoretical expectations.