Particle Image Velocimetry (PIV) measurements of the flow induced by breaking waves over a smooth steep slope of 1/15 were performed. Five cases of incoming regular waves were tested. Upstream of wave breaking, the induced oscillatory flow was not sinusoidal in time as the wave crests became steeper and the troughs wider. The profiles of the period-mean velocity showed the existence of a return current. Outside the surf zone, the current magnitude decreases linearly with increasing distance from the free surface, but it is influenced by the parabolic nature of the surf-zone undertow just upstream of the breaking position.
Wave breaking is a physical process of great importance in coastal dynamics. As sea waves propagate towards the shoreline, they are influenced by the sea bottom, their height is amplified due to shoaling until their crest overturns and causes large amounts of wave energy to be transformed into turbulent kinetic energy. The above mechanism initiates other physical processes that influence the flow characteristics in the surf zone and hence affect the cross- and -alongshore sediment transport, as well as shoreline and bed evolution.
Several studies exist of waves propagating and breaking over sloping beaches that have significantly improved our understanding of wave breaking hydrodynamics in the surf zone. Hansen and Svendsen (1984) performed small-scale experiments using a bi-directional micropropeller, in order to study the generating mechanisms of the undertow in a sloping beach of 1/34.26. Svendsen (1987) analyzed literature data of turbulent kinetic energy k generated by breaking waves on mild smooth slopes. He found that the variation of k over depth in the surf zone is remarkably weak, a finding that suggests the existence of strong vertical mixing attributed to large-scale turbulent vortices, and he also suggested a simple empirical formula for the variation of k over depth. Furthermore, Svendsen (1987) concluded that turbulence kinetic energy due to breakers on a mild slope resembled the one in a plane wake, and its distribution among the corresponding velocity fluctuations in the streamwise, transverse and vertical directions is 43%, 32% and 25%, respectively. Ting and Kirby (1994, 1995, 1996) studied experimentally the undertow current and the turbulence statistics in the surf zone of spilling and plunging breakers (regular waves) on a smooth slope of 1/35. Their results showed that under a plunging breaker the turbulence levels are much higher and the vertical variations of the undertow current and the turbulent kinetic energy are more uniform in comparison to the spilling breaker. They also showed that the turbulent kinetic energy is transported seawards under a spilling and landwards under a plunging breaker, while it is dissipated much faster under a plunging than under a spilling breaker. Cox et al. (1996) performed experiments of spilling breakers (regular waves) on a smooth slope of 1/35. They found that a logarithmic layer exist in the bottom boundary layer, for most of the phases over a wave period, seawards of the breaking point and in the surf zone. Cox and Kobayashi (2000) analyzed laboratory measurements for spilling breakers (regular waves) on a smooth slope of 1/35 and they showed that in the surf zone, from the trough level into the wave boundary layer, intense, intermittent turbulent events exist that did not occur with the passing of each wave, while the instantaneous velocity fluctuations were of the same magnitude as the phase-averaged horizontal velocity. Huang et al. (2009) performed experiments of spilling breakers (regular waves) on a smooth slope of 1/20. They found that significant turbulent dissipation occurs in the roller region of the breaker, while the turbulence dissipation rate follows an exponential decay from the crest to the bed. Ting and Nelson (2011) studied experimentally the spilling breakers (regular waves) on a smooth slope of 0.03. They showed the existence of coherent downburst events induced by wave breaking in the wave boundary layer, and that their spatial distribution was not entirely random, but correlated to the point of incipient breaking and the longitudinal spacing of the descending eddies.
