SURF SCIENCE: What is Swell Dispersion?

Tony Butt

by on

Updated 212d ago

Last week’s big swell reminded me of one of the most fundamental principles of ocean wave science: radial dispersion.

When the swell first reached spots in northern Spain, Portugal and Morocco, the buoys were showing periods of around 18-to-20secs.

The period then dropped to about 15 or 16secs for a while, and then continued to drop until the swell lost its punch at around 11secs. The swell on the charts at the moment is not as solid as last week’s, but it shows another really good example of dispersion.

The reason why the long periods tend to arrive first, particularly if you are a long way from the storm centre, is because ocean waves are dispersive. This means that, unlike other types of waves such as sound waves or electromagnetic waves, they don’t all travel at the same speed. Their speed is governed by their period. The longer the period, the faster they go.

Pressure for the beginning of the swell generated a couple of days ago.

Pressure for the beginning of the swell generated a couple of days ago.

It follows that, as waves of different periods propagate away from the generating area, they disperse, or spread out radially. When a swell is first generated, a whole range of different period waves are produced at the same time, all bunched up together.

As the swell begins to propagate away, the longer, faster waves start to race out in front, and the shorter, slower ones lag behind. Once the swell has travelled some distance, there will be a clear separation between the waves of different periods. Instead of being bunched up, the swell will be stretched out in the direction of propagation.

If you find this difficult to understand, think of a group of marathon runners, whose speed depends on their height (not always true, I know, but useful just to illustrate the point). They all start off together, all bunched up. But then, after a while, the taller ones pull out in front of the pack, leaving the shorter ones behind. At the finishing line, the first ones to arrive will be the tallest, with the height gradually decreasing over time until the last ones finally arrive.

The first waves of a new swell are usually the longest, but they are not always the biggest. Typically, the peak in wave height arrives several hours after the first waves of the swell. However, if the spot has intense focusing whose effect increases with period, the peak in the local wave height could occur earlier than the peak in the offshore wave height. This is a really important topic, which I will dig deeper into in future articles.

Waves of different periods propagating away from a storm separate themselves out like runners in a marathon.

Waves of different periods propagating away from a storm separate themselves out like runners in a marathon.

Period charts at three-hour intervals showing the propagation of the swell away from the storm centre. The size of the arrows gives an idea of the size of the swell. You can see how the long periods make their way to the front of the pack, then arrive at the coast first - see above.

Cover shot: Puerto Escondido by Edwin Morales