LOW-FREQUENCY SWASH MOTION-INDUCED BY WAVE GROUPING

This paper concerns the low frequency motion of swash directly induced by wave grouping on a steep beach. A new experimental investigation i s presented which considers the hydrodynamics of the inner surf zone a nd swash zone using vertical wave gauges and a run-up wire. Results fo r regular waves, wave groups and random waves are discussed, with part icular reference to low frequency motions. The inner surf zone and swa sh zone are found to be unsaturated at incident short wave frequencies and, as a result, significant wave grouping is apparent at the shorel ine. The low frequency motion in the surf zone is found to be in phase with the incident wave grouping and may therefore be regarded as a ti me varying set-up (Watson and Peregrine, 1992). The low frequency moti on of the swash is shown to be an order of magnitude greater than that in the inner surf zone, inconsistent with cross-shore standing long w aves, for which no evidence is found. We demonstrate that the low freq uency motion of the shoreline provides an excellent approximation to t he run-up of individual bores and therefore describes the run-up envel ope. Spectral analysis shows that the low frequency motion of the swas h is directly linked to the modulations in offshore wave height, i.e. the low frequency energy in the incident wave envelope, In addition, t he random wave run-up spectra show an f(-4) high frequency roll-off, a s found by Huntley et al. (1977). The accumulated data show that, unle ss the surf zone is totally saturated, a significant proportion of the low frequency swash motion may be directly due to incident wave group ing and rot standing long waves. (C) 1997 Elsevier Science B.V.