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Influid dynamics andnautical terminology, abreaking wave orbreaker is awave with enough energy to "break" at its peak, reaching a critical level at whichlinear energy transforms intowave turbulence energy with a distinct forward curve. At this point, simple physical models that describe wave dynamics often become invalid, particularly those that assume linear behaviour.
The most generally familiar sort of breaking wave is the breaking ofwater surface waves on a coastline. Wave breaking generally occurs where the amplitude reaches the point that the crest of the wave actually overturns. Certain other effects in fluid dynamics have also been termed "breaking waves", partly by analogy with water surface waves. Inmeteorology, atmosphericgravity waves are said to break when the wave produces regions where thepotential temperature decreases with height, leading to energy dissipation throughconvective instability; likewise,Rossby waves are said to break[1] when thepotential vorticity gradient is overturned. Wave breaking also occurs inplasmas,[2] when the particle velocities exceed the wave'sphase speed. Another application in plasma physics isplasma expansion into a vacuum, in which the process of wave breaking and the subsequent development of a fast ion peak is described by theSack-Schamel equation.
Areef or spot of shallow water such as ashoal against whichwaves break may also be known as a breaker.[citation needed]
Breaking of water surface waves may occur anywhere that the amplitude is sufficient, including in mid-ocean. However, it is particularly common on beaches, because wave heights are amplified in the region of shallower water because the group velocity is lower there(see also:Waves and shallow water).
There are four basic types of breaking water waves. They are spilling, plunging, collapsing, and surging.[3]
When the ocean floor has a gradual slope, the wave will steepen until the crest becomes unstable, resulting in turbulent whitewater spilling down the face of the wave. This continues as the wave approaches the shore, and the wave's energy is slowly dissipated in the whitewater. Because of this, spilling waves break for a longer time than other waves, and create a relatively gentle wave. Onshore wind conditions make spillers more likely.
A plunging wave occurs when the ocean floor is steep or has sudden depth changes, such as from a reef or sandbar. The crest of the wave becomes much steeper than a spilling wave, becomes vertical, then curls over and drops onto the trough of the wave, releasing most of its energy at once in a relatively violent impact. A plunging wave breaks with more energy than a significantly larger spilling wave. The wave can trap and compress the air under the lip, which creates the "crashing" sound associated with waves. With large waves, this crash can be felt by beachgoers on land. Offshore wind conditions can make plungers more likely.
If a plunging wave is not parallel to the beach (or the ocean floor), the section of the wave which reaches shallow water will break first, and the breaking section (or curl) will move laterally across the face of the wave as the wave continues. This is the "tube" that is so highly sought after by surfers (also called a "barrel", a "pit", and "the greenroom", among other terms). The surfer tries to stay near or under the crashing lip, often trying to stay as "deep" in the tube as possible while still being able to shoot forward and exit the barrel before it closes. A plunging wave that is parallel to the beach can break along its whole length at once, rendering it unrideable and dangerous. Surfers refer to these waves as "closed out".
Collapsing waves are a cross between plunging and surging, in which the crest never fully breaks, yet the bottom face of the wave gets steeper and collapses, resulting in foam.
Surging breakers originate from long period, low steepness waves and/or steep beach profiles. The outcome is the rapid movement of the base of the wave up the swash slope and the disappearance of the wave crest. The front face and crest of the wave remain relatively smooth with little foam or bubbles, resulting in a very narrowsurf zone, or no breaking waves at all. The short, sharp burst of wave energy means that the swash/backwash cycle completes before the arrival of the next wave, leading to a low value of Kemp's phase difference (< 0.5). Surging waves are typical of reflective beach states. On steeper beaches, the energy of the wave can be reflected by the bottom back into the ocean, causingstanding waves.
As ocean surface waves enter shallow water, the water particle velocities begin to move faster relative to the speed of the waveform. As a result, the waveform becomes unstable and the crest of the wave overturns, which is known as the wave breaking process.[4] During breaking, a deformation (usually a bulge) forms at the wave crest, either leading side of which is known as the "toe".[clarification needed] Parasitic capillary waves are formed, with short wavelengths. Those above the "toe" tend to have much longer wavelengths. This theory is anything but perfect,[clarification needed] however, as it is linear. There have been a couple non-linear theories of motion (regarding waves). One put forth uses aperturbation method to expand the description all the way to the third order, and better solutions have been found since then. As for wave deformation, methods much like theboundary integral method and theBoussinesq model have been created.
It has been found that high-frequency detail present in a breaking wave plays a part in crest deformation and destabilization. The same theory expands on this, stating that the valleys of the capillary waves create a source forvorticity. It is said thatsurface tension (andviscosity) are significant for waves up to about 7 cm (3 in) in wavelength.[5]
These models are flawed, however, as they can't take into account what happens to the water after the wave breaks. Post-break eddy forms and the turbulence created via the breaking is mostly un-researched. Understandably, it might be difficult to glean predictable results from the ocean.[citation needed]
After the tip of the wave overturns and the jet collapses, it creates a very coherent and defined horizontalvortex. The plunging breakers create secondaryeddies down the face of the wave. Small horizontal random eddies that form on the sides of the wave suggest that, perhaps, prior to breaking, the water's velocity is more or less two dimensional. This becomes three dimensional upon breaking.[citation needed]
The main vortex along the front of the wave diffuses rapidly into the interior of the wave after breaking, as the eddies on the surface become more viscous.Advection andmolecular diffusion play a part instretching the vortex and redistributing the vorticity, as well as the formation turbulence cascades. The energy of the large vortices are, by this method, transferred to much smaller isotropic vortices.
Experiments have been conducted to deduce the evolution of turbulence after break, both in deep water and on a beach.
A theoretical limit on the steepness of non-breaking waves in finite depth is given by theMiche criterion, derived by French engineer Robert Miche in 1944.[6][7][8]
