Tom Shand

ON THE INFLUENCE OF WAVE GROUPS ON SHOALING AND BREAKING WAVES

1Determining the largest wave height H which can occur in water of depth d without breaking is a fundamental reference quantity for the design of coastal structures. The ratio of breaking height (Hb) to depth (Hb/d), is known as the breaker index and has been the subject of much research over the past 150 years. Current design guidelines are based on investigations which, predominantly used monochromatic waves, thereby neglecting group effects. Groupiness or height modulation in wave trains is inherent to the free propagation of waves in deep water. Whilst our understanding of the formation and propagation of wave groups remains limited, significant progress has recently been made in the prediction of the onset and strength of breaking of wave groups in deep water. Recent two-dimensional laboratory studies have also shown that wave group effects have a marked affect on wave shoaling and breaking, influencing both the position and form of the breaking waves. These studies have used idealised wave spectra to generate modulating wave groups which are repeatable on a characteristic wave group time scale. Some scenarios yielded breaker indices up to 35% systematically above current design guideline values. Due to limitations of conventional wave probes when used near and within the breaking zone, a laser induced fluorescence (LIF) technique has recently been developed at the Water Research Laboratory. The technique yields high spatial and temporal resolution measurements of propagating wave forms from deep water, through the breaking region and inner surf zone. This technique has been verified using conventional capacitance wave probes and high resolution digital video images with resolution of better than 0.05mm obtained giving errors less than ±0.1 mm. Extensive investigations using repeatable wave groups and incorporating the LIF technique have been completed for a uniform bed slope of 10:1 in which the location of a shoaling bed was incrementally adjusted along the wave tank. In this manner, the shoaling and breaking properties of individual waves within the group could be carefully examined. It was observed that, as a wave group encounters a sloping bed, the behavior of the shoaling wave group is critically affected by the spatial phasing of the group relative to the bed. A critical depth between L0/5 > d > L0/8, where L0 is the deepwater wavelength, has emerged as the primary factor delineating two types of group shoaling. When a wave crest is coincident with the group energy maximum at this critical location, wave breaking tends to occur earlier, in deeper water and in a more gentle manner. However, where there is coincidence of a wave trough with the group energy maximum at this depth, delayed shoaling is observed. Notably, delayed shoaling stabilises the waves within the group, delaying the initiation of breaking and can yield Hb/d ratios up to 38% greater than present design guidelines. Our results show that present design guidelines based primarily on monochromatic waves are not conservative when wave group effects are considered.