Breaking Waves and Coherent Structures

Vorticity and turbulence associated with breaking waves make the surf zone of special significance within near-shore dynamics and costal sediment transport. Even if been investigated since long, they are still far from being completely elucidated. Many researches showed that the turbulent flow due to wave breaking is characterized by the formation of large-scale vortex structures which swirl around each other and feature a variety of shapes, named coherent structures (CSs) (Kimmoun and Branger, 2007). They primarily control sediment movements and cross-shore morphodynamic evolution. Therefore, predicting near shore morphodynamics requires thorough knowledge of their formation and spreading throughout the water column, as well as their interactions with bottom sediments. The identification of CSs, based on vorticity or other turbulent features, is difficult along with the description of intermittent events of turbulent nature. Many methods have been used, such as quadrant analysis, wavelets, ensemble averaging to discriminate turbulent fluctuations from the ordered wave motion. Anyway, a thorough knowledge of the formation and spreading of these coherent structures in the surf zone is more expected, to better explain their interactions with bottom sediments and effects on tracers’ diffusion. With this aim, previous studies have mainly focused on spilling waves in analogy with bores and jumps, where CSs were first documented. Few experimental studies have been conducted to examine the behavior of these swirling eddies under plunging waves (Na et al., 2016), which are characterized by higher turbulence due to the presence of the overturning jet and the splash-up cycle. In the present research, we experimentally investigate both a spilling and a plunging regular wave on a fixed and impermeable slope. Measurements are carried out by means of a Laser Doppler Anemometer in some selected channel sections, located in the outer and inner surf zone. Coherent motions are identified as a function of wave phase and the bursting processes are quantified. Useful hints to turbulence spreading and transport are obtained.

References

Kimmoun O., Branger H. (2007) A particle images velocimetry investigation on laboratory surf-zone breaking waves over a sloping beach, J. Fluid Mech., 588, 353–397.

Na B., Chang KA., Huang ZC., Lim HJ. (2016) Turbulent flow field and air entrainment in laboratory plunging breaking waves. Journal of Geophysical Research: Oceans, 21 (5), 2980-3009.