What is Wave Refraction?

Tony Butt

by on

Updated 35d ago

In this series of articles so far we’ve looked at where the waves we surf come from, how they are formed and what happens to them as they propagate over the ocean surface. Now we’ll have a look at what happens to them once they start to approach the coast. You can view all those and more articles explaining surf science, HERE.

In the deep ocean, the sea floor is so far away from the surface that its influence on the waves that travel across that surface is negligible. But at some point near the coast, the water becomes shallow enough for the presence of the sea floor to affect the behaviour of the waves. The configuration of the sea floor (the bathymetry), either offshore of a surf spot or below the breakpoint itself, is a vitally important factor in determining the characteristics of the waves when they break.

Check the cam: Nazare

Refraction can magnify swells, such as here at Jaws.

The way in which the bathymetry affects a wave approaching the coast is by making it bend, through the process of refraction. Refraction can turn, twist and mould the waves into a thousand different shapes and sizes, all depending on the bathymetry. It can pre-condition the waves to come in at different angles and form spectacular A-frames like those at Nazaré, or groom the swells into long lines before they arrive at spots like Jeffrey’s Bay. Or it can focus and magnify swells to enormous proportions, right there just before they break, at places like Peahi or Sunset Reef.

Refraction is the bending of a wave-front as it travels at different speeds over water of different depths. When different parts of the same wave-front travel at different speeds, the wave bends towards the slower part. The shallower the water, the slower the wave; therefore the wave bends towards the shallower water.

In the deep ocean, the orbital motions of the particles beneath the waves diminish to nothing way before they reach the sea bed. But once the water starts to get shallower, these orbital motions begin to reach all the way down to the bed, and so, now, the sea bed does have an influence on the waves on the surface. It slows them down.

You might be forgiven for thinking that the reason they slow down is because of friction. You can imagine those particles ‘rubbing’ on the bed and causing the waves to lose energy. Well, almost, but not quite.

What really happens is a bit more complicated, but I’ll try to explain it as simply as possible. The sea bed is far from flat, especially from the point of view of the water molecules. It has irregularities in it which the molecules bump into as they try to move back and forth along the bed. This slows them down in the same way as driving along a really bumpy road will slow your car down. Some of the energy in the horizontal motion is converted to vertical motion as the car bumps up and down, but not much energy is actually lost.

A simplified way of thinking about it is that the change in motion on the sea bed is ‘communicated’ upwards through the water column by each molecule ‘touching’ its neighbour, rather like billiard balls. As a result, the slowing down of the molecules on the bed produces a slowing down of the molecules on the surface, which causes a slowing down of the waves themselves.

At what depth does this start to happen? The depth at which the waves start to ‘feel’ the bottom depends on the wavelength of the waves. Longer waves have larger orbital motions. Therefore, the orbital motions of longer waves reach down further below the surface, and longer waves ‘feel’ the bottom before shorter ones.

The depth at which the presence of the sea bed starts to affect the wave speed is about half the deep-water wavelength, or about 0.8 times the period squared, with the period measured in seconds and the depth in metres. I prefer to talk in terms of period rather than wavelength, firstly because the period is something we see quoted much more than wavelength and, most importantly, the period doesn’t change as the waves travel into shallow water.

Now, using this simple rule, you can find out how far from the shore waves of a particular period start to bend, or refract. All you need is a bathymetric chart and a knowledge of the period of the waves. This can be extremely useful if you are scouring the charts for another potential Nazaré or Peahi, or perhaps just want to know how a spot will behave in different swells of different periods.