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Flow structure

During the last few decades, important contributions have focused on breaking waves. The mechanisms involved during the breaking process have been detailed (Peregrine, 1983), while the dynamics of the surf zone have been intensely studied (Battjes, 1988; Svendsen and Putrevu, 1996). Spilling breaking waves have been extensively studied, allowing a better understanding of the dynamics of surface tension effects, which can dramatically alter the deformation of the crest shape as the wave approaches breaking (Duncan et al., 1999; Duncan, 2001). The latest advances in numerical modeling and experimental measurement techniques used for the surf zone investigation have been discussed (Christensen et al., 2002). More recently, the most significant results have been presented and the scientific challenges and obstacles that remain to be addressed have been identified (Kiger and Duncan, 2012).

Experimental observations made over the last 30 years allowed significant progress to the understanding of flow physics and its consequences on the littoral zone, in terms of of induced currents, sediment suspension, and subsequent erosion or accretion phenomena. In the 1970’s and 1980’s, the first experimental works, complemented by the first numerical simulations, allowed scientists to detail the large structures of turbulent flow generated by the waves breaking on beaches. The wave breaking processes induce vortices that play an important role in the pick-up and suspension of sediments in the surf zone.

When a wave breaks, the wave that propagates to the shore loses its symmetrical shape and steepens. When the front of the wave becomes almost vertical, a jet of water is ejected from the crest (the lip of the wave, as surfers call it). This jet then free-falls down and will impact the front of the wave, enveloping more or less air (sometimes forming a tube, a cavity prized by surfers). The impacting jet is often followed by large series of splash-ups, projected upward and forward. According to the plunge point position on the front face of the wave, different types of breaking waves can be observed. If the plunge point is located near the crest of the wave, the subsequent splash-up will be directed downwards leading to spilling breaking. If the jet impact is further down the face of the wave, away from the crest down the front face of the wave, then a plunging wave is observed.

Among the most important works, experiments have been presented to discuss the internal structure of the flow under breaking waves (Miller, 1976), highlighting what is now coined the “breaker vortices”, whose size and intensity depend on the type of breaking experienced. The process of successive splash-ups has been shown for the first time Miller (1976): once the impacting jet creates a first splash-up, a second smaller splash-up is then generated, and so on until the energy of the wave is dissipated. Pictures show the formation of large quantities of air bubbles during these splash-up cycles. Vortex structures generated by plunging breaking waves were proved to affect significantly the bottom (Miller et ah). Sand bars were suggested to be formed when the structures created by the plunging wave were present throughout the flow, while these bars were suppressed by spilling breaking when the structures were confined in the upper part of the flow, near the free surface.

This difference between spilling and breaking has also been confirmed by a series of experiments conducted to identify the internal structure of the velocity field in the surf zone (Nadaoka and Kondoh, 1982). Measurements clearly indicate that the velocity field is divided into two regions, the upper part of the water column, near the free surface, and the lower part, near the bottom. The high rates of turbulence associated with air entrainment were quantified and proved to be responsible for the attenuation of wave energy in the surf zone (Nadaoka and Kondoh, 1982). Air bubbles are mainly contained in large vortical structures and transported to the bottom by them. The upper part of the flow is characterized by the presence of large structures and a high aeration rate, while the bottom zone is where smaller structures coexist, coming from the upper part of the flow and those generated by friction at the bottom.

During all the stages of a breaking event, different vortices will appear (Zhang and Sunamura, 1994). Two major categories have been identified: structures with a horizontal axis of rotation and those with an oblique rotation (pointing from the surface towards the bottom). The first category, mixing со- and counter-rotating vortices (Sakai et al., 1986; Bonmarin, 1989; Kimmoun and Branger, 2007) due to successive splash-ups generated during plunging breaking, generates high levels of turbulence and consists of a large amount air. They are found in the large rollers of foam in the surf zone. Oblique structures are those that are most likely to reach the bottom and put sediment in suspension (Nadaoka et al., 1989). These structures appear more typically under spilling breakers, confirming observations of greater beach erosion for this type of breaking. It can be seen that the large horizontal structures are present in the bore front propagating towards the shore, while behind the crest of the breaking wave, the structure of the flow changes rapidly into a three-dimensional structure stretching along an oblique axis from the free surface towards the bottom. These structures are very intense and carry a great amount of turbulence from the surface, thus agitating the bottom. The schematic structure of the flow is classified and synthesized in (Zhang and Sunamura, 1990; Zhang and Sunamura, 1994).

More recently, (Kubo and Sunamura, 2001) identified a new type of large-scale turbulence, named “downbursts”, which can be present in the breaker zone along with the previously observed oblique vortex. (Ting, 2006) also identified these downbursts of turbulence descending from the free-surface. A downburst is characterized by a rapidly descending water mass without marked rotational characteristics, impinging the bed and causing the sediment particles to move more vigorously than oblique vortices. Very few numerical works have been dedicated to the simulation of these three-dimensional structures observed under the breaking waves (Watanabe and Saeki, 1999; Christensen and Deigaard, 2001; Lubin, 2004; Watanabe et al., 2005; Christensen, 2006; Lubin et al., 2006; Iafrati, 2009; Lakehal and Liovic, 2011). In addition, a very limited number of these works have been devoted to smaller processes, such as the striation of free-falling jets or their dislocation into drops before impact (Longuet-Higgins, 1995; Narayanaswamy and Dalrymple, 2002; Watanabe et al., 2005; Saruwatari et al., 2009).

 
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