Cross-shore wind-induced changes to field-scale overturning wave shape

Abstract

The shape of depth-limited breaking-wave overturns is important for turbulence injection, bubble entrainment and sediment suspension. Overturning wave shape depends on a nonlinearity parameter H/h, where H is the wave height, and h is the water depth. Cross-shore wind direction (offshore/onshore) and magnitude affect laboratory shoaling wave shape and breakpoint location Xbp, but wind effects on overturning wave shape are largely unstudied. We perform field-scale experiments at the Surf Ranch wave basin with fixed bathymetry and ≈ 2.25 m shoaling solitons with small height variations propagating at C = 6.7 m s−1. Observed non-dimensional cross-wave wind Uw was onshore and offshore, varying realistically (−1.2 < 0.7). Georectified images, a wave staff, and lidar are used to estimate Xbp, H/h, overturn area A and aspect ratio for 22 waves. The non-dimensionalized Xbp was inversely related to Uw/C. The non-dimensional overturn area and aspect ratio also were inversely related to Uw/C, with smaller and narrower overturns for increasing onshore wind. No overturning shape dependence on the weakly varying H/h was seen. The overturning shape variation was as large as prior laboratory experiments with strong H/h variations without wind. An idealized potential air flow simulation on steep shoaling soliton shape has strong surface pressure variations, potentially inducing overturning shape changes. Through wave-overturning impacts on turbulence and sediment suspension, coastal wind variations could be relevant for near-shore morphology.