An edge wave is, as the name suggests, a wave that propagates along the shore – along the edge, if you like. Edge waves were first discovered in theory in 1846 by Sir George Stokes (1819-1903). However, at this time they were considered “a mathematical oddity, having no consequence to the real world”. They remained this way until 1973, when they were first measured in real life by David Huntley and colleagues from the University of Plymouth, using electronic instruments at a beach in Devon, England.
An edge wave is a type of infragravity wave – a very long wave that co-exists underneath the ordinary incoming waves. The period and wavelength of an infragravity wave are typically about ten times as long as that of an ordinary wave. You can see them in big swells and storms, as the entire shoreline moves in and out sometimes hundreds of metres, often taking more than a minute to do so. Infragravity waves are thought to be generated somewhere near the breakpoint of the ordinary waves, although there are still several schools of thought as to their exact process of generation.
To understand an edge wave, we need to first visualize an infragravity wave propagating towards the shore. As the infragravity wave hits shallow water it squashes up and increases in height in the same way as an ordinary wave, but its wavelength is so long that it never actually breaks, no matter how steep the shoreline.
To end up as an edge wave, the infragravity wave must start off by approaching the shore at an oblique angle. It reaches the shore without having lost any energy, so it tends to bounce back off the shoreline. The angle at which it bounces back (the reflected angle) is opposing to the angle from which it approaches (the incident angle) – a principle of basic physics, which can be applied to a ray of light shining into a mirror or a football being kicked against a wall.
Now, as the reflected wave tries to make its way out to sea again, it will be refracted back in towards the shore. Refraction is the bending of a wave front as it begins to propagate over varying depths of shallow water. In this case, one end of the wave front – the end nearer the shore – is in shallower water than the other end. Therefore, the end nearer the shore will slow down more than the other end. Due to refraction, the wave swings around and eventually ends up facing the beach again.
On a long stretch of beach with no headlands, this process of reflection and refraction continues, with the edge wave bounding along the coast like a bouncing ball in a world laid down on its side. If there are headlands at the ends of the beach, the wave can become ‘trapped’, bounding back and forth between the headlands. Of course, infragravity waves and edge waves are so long that you can rarely see this happening, but it can easily be measured with scientific instruments.
So how can edge waves affect our waves for surfing? Well, they could help to explain how peaks with right- and left-peeling waves are sometimes formed on a beach with no sandbars – in other words on a beach that would normally produce nothing but close-outs. One mechanism for doing this is the interference caused by an offshore shoal, but if there are no sandbars and no offshore shoal and you still get good peaks, edge waves might just be the reason.
On a beach with sandbars, the water is shallower over the bumps and deeper over the dips. The basic idea is that the waves are focused onto the bumps and defocused away from the dips, which makes them form into peaks. But how about if the water surface itself, instead of the sea floor, had bumps and dips in it? This would also produce areas of deep and shallow water, which would cause the waves to focus and defocus in just the same way.
One thing that can produce this water surface variation is an edge wave. The edge wave would be like a moving bump creeping its way across the water surface. It would not have to be very high to affect the bathymetry enough for the incoming waves to bend around it. Also, the movement of an edge wave is much slower than that of the ordinary waves, which gives the normal waves plenty of time to respond to the variations in water depth, just as if they were coming in over a set of fixed sandbars.