Looking for wave groups in the surf zone

This paper proposes two new parameters, temporal group steepness (S_t) and spatial group steepness (S_x), for evaluating wave grouping. The new parameters provide information on two important qualities of groups, the amplitude variation of the groups and their length, both of which have an impact on coastal problems. Computation of the parameters involves the use of the Hilbert transform to determine the wave envelope of incident waves. The Hilbert transform method is reviewed and shown to be superior to other methods involving η² or |η| and the new parameters are shown to be responsive to changes in the wave field due to shoaling and wave breaking. The new parameters are applied to field data to study the variability of wave grouping both spatially, in the cross-shore direction, and temporally, at fixed locations. Wave grouping is shown to persist through the surf zone indicating the necessity for the inclusion of nonsteady radiation stress gradients, shoreward of the mean break point, in models of long wave generation. In addition, the temporal change in group steepness is shown to correlate with the increase in long-period surf zone current modulations.