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$X_{{bp}}$, 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$\approx 2.25$m shoaling solitons with small height variations propagating at$C=6.7\ \mathrm {m}\ \mathrm {s}^{-1}$. Observed non-dimensional cross-wave wind$U_w$was onshore and offshore, varying realistically ($-1.2 < U_{w}/C < 0.7$). Georectified images, a wave staff, and lidar are used to estimate$X_{{bp}}$,$H/h$, overturn area$A$and aspect ratio for 22 waves. The non-dimensionalized$X_{{bp}}$was inversely related to$U_{w}/C$. The non-dimensional overturn area and aspect ratio also were inversely related to$U_{w}/C$, 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.