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Wave breaker types

As the waves approach the beaches, they undergo heavy transformations, which occur in four distinctive zones:

  • 1. the shoaling zone, where the waves slow down and their height begins to increase. The waves lose their symmetrical appearance, and the front of the wave begins to steepen.
  • 2. the breaking zone, where the waves overturn and break. When the waves start to decelerate and the front face becomes almost vertical, the water particles at the crest of the waves will have a higher velocity than the local wave celerity, thus leading to irreversible breaking (Peregrine, 1983).
  • 3. the surf zone, where we observe the transformation of the waves into a strongly aerated, highly unsteady and chaotic flow. The turbulence is generated at the surface and advected to bottom, potentially causing the suspension of the elements constituting the bottom (e.g., sediment) (Battjes, 1988; Svendsen and Putrevu, 1996).
  • 4. the swash zone, where the broken waves finally dissipate a significant amount of energy until they reach the beach, and finally the breaking bores are observed to run-up and run-down the sloping beach (Могу et ah, 2011).

Therefore, the surf process can be seen as an irreversible transition from a well-organized, two-dimensional wave motion, into a rotational, highly turbulent and three-dimensional motion.

When arriving on beaches under oblique attack, the waves produce a current parallel to the coastline (coastal drift current). In addition, water masses brought in by breaking waves on beaches then return to the open sea, sometimes generating strong cross-shore currents: rip-currents and undertow currents, channeled by bathymetry channels. However, since the wave breaking area is strongly related to the water depth, the tide also modulates the effects of the swell by varying the average level of the water body, between high and low tide. The trigger point of the surge, therefore, varies as the day progresses.

There are several types of breaking waves. The terminology associated with breaking waves dates back to the late 1940!s, when the terms “plunging” and “spilling” can be found in a U.S. Navy Hydrographic Office military manual (Bigelow and Edmondson, 1947). The collapsing and surging breaker will be identified in later works in which definitions will be given to distinguish them (Galvin, 1968). This following terminology is conventionally adopted:

  • spilling breaker, the front face of the wave turns into foam, and large rollers are observed made of a mixture of air and turbulent water propagating from the breaking point to the beach, the wave gradually losing its height and speed, as it dissipates its energy ;
  • plunging breaker, which is the most spectacular form, where a very impressive overturning motion of the wave can be seen, with the formation of a jet ejected from the crest of the wave, which encloses a large tube of air before violently impacting the water to form large splashes of foam ;
  • collapsing breaker, observed when the waves are propagating to the beach and deforming, but without breaking, to finally form an almost vertical wall of water that will fall in its lower part, violently throwing a mixture of water, air and sand up the beach ;
  • surging breaker, is a form of breaking wave that is observed when the waves propagate to the beach, with very narrow surf zone, deform, and break through their base where a tongue of water is propelled upward from the beach, with almost no foam. This type of breaker is usually encountered in steep slopes or in the vicinity of man-made structures.

By observation, the Iribarren number or Iribarren parameter (also known as the surf similarity parameter or breaker parameter) is a dimensionless number used to describe the occurrence and the type of wave breaking on sloping beaches (Battjes, 1974). It has been established that there is a continuous spectrum ranging from spilling to surging breaking, varying according to three parameters: the slope of the beach (slope inclination from mild to strong), the steepness of the wave (the ratio between the height of the wave and its wavelength, which is an indicator of the degree of non-linearity of the wave) and the dispersion parameter (the ratio of the depth and wavelength). This last parameter, which reflects the influence of the bathymetry variations on the propagation of the wave, classifies if the water depth is shallow or deep. For an equivalent incoming wave condition, the breaking will be “softer” or “milder” for low beach slope, while the breaking will be more violent on steep slope (breaking plunging to frontal). Surging breaking is the limit of the spectrum, since we do not observe a large amount of foam nor a violent impact. The triggering of the breaking event occurs later when the beach goes from a gentle to a steep slope, when waves break closer and closer to the top of the beach. So, this confirms the intuition of the members the United States Army Corps of Engineers, who wanted to get information about the sea bed by observing breaking waves: when a wave breaks, we are often in the presence of a high point of bathymetry.

Moreover, according to the type of breaking, beaches will not undergo the same fate (Ting and Kirby, 1994). Surprisingly, beaches subjected to spilling waves are those that will erode the most (Ting and Kirby, 1995): indeed, the foam rollers propagating up the beaches will generate stronger return currents (“undertow”) that will generate in the lower part of the water column, oriented from the beach to the open sea, transporting away the suspended sediment. On the contrary, the more violent breaking events, while occurring closer to the beach, will have a strong impact and put the sediment in motion mainly towards the beaches (Ting and Kirby, 1996).

 
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