Surfing Explained: Ep15 How Waves Travel the Oceans

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In this week's episode of Surfing Explained we continue with ocean waves and go through the interesting & sometimes unusual ways waves travel through the ocean.

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Script:
Continuing from our last episode, today we’re going to explore what happens to a wave once it leaves the fetch area, and begins it journey outwards from the wind source.

As the waves leave the fetch area that created them, they radiate outwards. As the circumference of the expanding area increases, so must the width of the wave, and in doing so the energy and size of the wave decreases.

As the distance from the original storm doubles, the energy decreases by around 15-20%. This means that waves will initially loose a lot of their energy in the first 24 hours after they leave the storm area, but after that they can actually travel huge distances while still maintaining a lot of their energy. This is known as Circumferential Dispersion or Swell Decay - and it means that if we want really big waves, we’re going to need to be fairly close to the storm.

We’ve mentioned before that as waves travel their energy moves in an orbital path within the water, and that it is only the energy that really travels as its transferred from molecule to molecule, with each molecule being picked up by the orbital path, and then deposited more or less where it started.

The bigger the orbital path, the more distance the energy covers with each revolution. This means that, given enough time, the waves with more energy and a higher wavelengths and periods, end up overtaking and passing lesser energy waves with a lower wavelengths and periods. This is called Radial Dispersion and it means that new ground swell can be identified on a forecast chart by sudden jumps in wave period as those bigger, faster waves arrive at the beach first.

It also means that If your surf beach is too close to the storm the waves will probably be disorganized, with higher quality longer period waves mixed with lesser quality shorter period waves.

This creates a Goldilocks zone between the effects of Circumferential Dispersion (i.e how long the waves take to lose energy), and Radial Dispersion (whether the waves have had enough time to become organized) - which is obviously different for each swell, but it’s where we’ll find swell that is both powerful and clean.

A group of waves traveling at the same speed together is known as a wave train And they do some unusual things when traveling through open ocean. As wave trains of different wavelengths travel across the ocean at different speeds, their wavelengths overlap, with two differing effects:

- Constructive Interference: Where the 2 wavelengths combine together to create a bigger wave.

And,

- Destructive Interference: Where the 2 wavelengths cancel each other out. Similar in principle to to how noise cancelling headphones work.

This process results in the formation of groups of waves on the ocean surface that we know as ‘Sets”, and the rhythm of these sets is governed by the overlapping wavelengths.

The more similar the different wavelengths, the more drawn out the overlap becomes, which creates more waves in a set, and longer the lulls between them. The more varied the overlapping wavelengths are the fewer waves to the sets. This rhythm is not fixed throughout the wave trains lifespan, and will fluctuate as the swell travels away from the storm, and as radial dispersion separates the different wavelengths. 

Within a group or set, each visible surface wave has a limited lifespan, and as it decays, a new surface wave is produced at the back of the group or set.

This means the speed of these Wave Groups or sets through the ocean is slightly different to the speed of the individual waves. To calculate their speed in m/s we use the average period in seconds multiplied by 0.78. This is what we use to forecast the approximate arrival of a swell: (on screen examples) 10sec = 28kph, 15sec = 42kph, 20sec = 56kph.

It’s easy to picture the rolling motion of wave energy, but waves are much more complicated than this, and in reality each wave travels as a stack of decreasingly sized cylinders of energy from ocean surface down, sometimes hundreds of meters. In the deep ocean, the energy in the cylinders dissipates to nothing without touching the ocean floor - all that changes though once a swell reaches a shallower coastline, and the energy reaches the ocean floor.

In next week’s episode, let’s explore what happens when these open ocean waves reach shallower waters near the coast. Thanks for watching, and we’ll see you next week.