How waves form (and how surfers handle them)

If you choose the sea (which dances along the clear cliffs) as your vacation destination, with its changing reflections, privileged witnesses of the whims of the sky, will undoubtedly find you find yourself waiting for a wave to take you away, before it happens. not coming to be pushed over the edge. We have all experienced it: there is nothing more complicated than predicting how a wave will come to carry us away or how to get out of the water once in strong waves.

At the moment of appreciating the size of a wave, within a few seconds and in an improvised way, one becomes a physicist who models a very complex phenomenon to predict its effects.

Waves are a natural phenomenon as rare as they are dangerous. The height of the waves varies from a few centimeters to 32.3 meters for the highest recorded by an instrument, or even more than 34 meters for the highest wave seen visually.

Origin and nature of waves

A wave is a deformation of the surface of a body of water usually caused by another fluid: the air. At the interface, the wind creates waves in oceans, seas and lakes.

The Ninth Wave, painting by Hovhannes Aivazovsky (1850) – Hovhannes Aivazovsky / Wikimedia CC0

Other events, less frequent, are also a source of waves: some earthquakes, volcanic eruptions or meteorite falls that create waves called tsunamis or tidal waves. Tides are also the source of more specific waves, called tidal bores, sudden increases in water in a river or an estuary, due to the wave of rising water at the mouth and downstream of some rivers during high tide. Finally, ships are also a source of waves.

Waves are waves, that is to say events that move periodically in time (we are talking about time) and in space (we are talking about wavelength). They can spread for miles before making landfall, and gain strength depending on the slope of the ocean floor. To understand this phenomenon, it is necessary to find a physical model of the wave.

A brief history of waves

The astronomer and mathematician George Biddell Airy gave the simplest theory for regular (periodic) waves. An Airy wave has a free surface, the water surface, which is sinusoidal in shape. This is a very simplified vision of reality, valid for waves of low “camber”, which is the ratio of the length of the wavelength.

If we look closely at ocean waves, we see that most of them are not sinusoidal: the peaks are sharper, the troughs are flatter.

But what interests us now in Airy’s theory is that the fluid particles describe almost closed ellipses, the size of which decreases with depth. We often think wrongly that a wave carries us: in fact, if we swim in the depth during the expansion of a wave, we realize that there is a repetitive movement, not necessarily intuitive from the point of view view of everyday experience but well emphasized. by Airy’s theory.

A wave develops and the particles carried by the waves have elliptical trajectories
A wave development and the particles carried by the waves have an elliptical trajectory – Kraaiennest / Wikimedia CC BY-SA 4.0

In deep water, that is, for depths greater than half a wavelength, these ellipses are circles. The fact that the ellipses are less closed is a manifestation of “Stokes drift”. Near the free surface, the velocity of a water particle is greater under a crest than the opposite velocity when passing through the next trough. This results in drifting in the direction of wave propagation which can be reversed with depth.

What freedom!, painting by Ilya Repin (1903)
What freedom!, painting by Ilya Repin (1903) – Ilya Repin / Wikimedia CC0

Wave breaking approaching the beach

Let’s go back to what worries us this summer: facing the waves … with dignity! In fact, “owning a wave” means understanding its surge.

In fact, some waves are too steep to be strong: they break.

Approaching a coast, the depth decreases, the shape of the waves changes, first in an almost symmetrical way then generally with a steeper front face when the wave height is the same order of depth. When a wave breaks, most of its energy is lost in eddies and air bubbles. The accompanying impulse contributes to the acceleration of the current in the direction of the rupture.

The shape of a wave around a beach depends on the slope of the bottom. Going in the direction of the rise of the slopes, we often distinguish three types of destruction. Progressive or slippery erosion usually occurs on beaches with very low slopes. The waves begin to break away from the shore with a foamy looking crest that recedes as it advances leaving a layer of foam.

The sinking wave is especially unique in its rolls that are appreciated by surfers. The wave surrounds a pocket of air and then collapses creating a noticeable splash. This usually occurs on a steep slope or a sudden change in depth (a trap), and that’s why there are “spots” that are popular with surfers.

Draw energy from the waves

There is more lost energy than reflected energy at the beach. The energy lost on the surface of the broken wave is useful to the surfer who can use it as a source of kinetic energy: the wave gives him the necessary dynamics to move, and make the figures he wants on his board moving thanks to the wave.

A frontal or swelling surf is formed like a plunging surf but the wave moves up the coast before the crest curls. The breaking zone is very narrow and most of the energy is reflected to a greater depth. If the crest goes faster than the wave itself, a thick layer of crest foam forms on top. We are then talking about a slippery wave, usually on a gently sloping beach. This type of breaking completely destroys the surfer because the energy is given to the depth. The surfer’s talent as a “casual physicist” is to identify types of surf for this purpose.

Types of waves
Types of waves – SL Douglas and J. Krolak / FHWA / Wikimedia CC BY-SA 4.0

There is also an intermediate case between plunging and frontal breaking. Instead of forming a roll, the wave presents a vertical surface before collapsing.

Surfers are often nicknamed the “tamers” of waves. Good wave conditions allow for good surfing practice (with a lot of technique though!). This means good wind or swell conditions, which can vary by location.

Hollow waves, with a break, are the favorite of “shortboarders”, who use short boards. Hollow waves are the strongest waves, but not necessarily the largest. In fact, some waves that can be described as hollow are more powerful than a soft wave of the same size. The principle of the shortboard (short board) is to allow a more “dynamic” surfing practice with more changes in the direction of the wave to achieve different figures. This is made possible by a wave whose breaking is better controlled.

Finally, a surfer primarily applies research to the “good waves” discipline.
All in all, whether you play with them, meditate on them or face them, enjoy these moments of freedom that Nature offers. And let’s remember that there is only one planet Earth that spends beautiful summers!

This analysis was written by Waleed Mouhali, teacher-researcher in Physics at ECE Paris.
The original article was published on the site of The Conversation.

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