Infragravity: The Waves You've Never Heard Of
It came as a surge, pushing inshore and at a rate to what I'd expect a small scale tsunami to move. Paddling against it was impossible, the rock wall only 15 metres away being a great reference point - I was losing ground despite my efforts. In the end I had to jump off the board and anchor myself, wait for the surge to stop, and then resume paddling as it retreated back out to sea about thirty seconds later.
While surfing South Steyne during the peak of last week's south swell an interesting phenomena was taking place. One that I've experienced before but not enough for me to look further into. The surge in the sea was due to the presence of 'infragravity waves', something different to the normal swell waves we usually surf. In scientific circles swell waves are called gravity waves, so infragravity refers to waves that occur within gravity waves.
We know that wind creates waves, initially tiny capillary waves, growing bigger and becoming more organised into windswell and then groundswell with increasing strength, length and longevity.
As these swell waves travel away from their source they sort themselves into wave groups - what you and I call sets - that contain individual waves of similar size and period.
If we imagine these sets travelling through the deep ocean you'll have an area of increased wave amplitude (wave height), maximised in the middle of the set and then immediately ahead and behind this a low point in amplitude.
Because the larger waves carry more momentum with them, there's a slight decrease in the water level under the larger waves and an increase under the small waves.
This in turn creates a long wave spanning the peaks and troughs between wave groups [see image below], with periods upwards of twenty five to thirty seconds though they can reach up to five minutes. These are known as infragravity waves, and they're bound (attached) to the wave groups they are travelling with.
In the open ocean, the height of infragravity waves are only in the order of centimetres but as they approach shallower water they increase in height much like swell waves increase in size just before breaking.
This effect - which is also known as 'surf beat' - is most apparent on slowly sloping beaches, and can reach up to 1 metre in height under big swells.
However, instead of breaking like swell waves, infragravity waves surge up the beach, and this is what I experienced multiple times during last Tuesday's swell event. During large swell events, storm-surges and coastal inundation beyond the normal high tide line are usually the result of infragravity waves.
My personal experience last Tuesday was the most noticeable in my surfing life and I'd put that down to the very dynamic and incredible swell event that was created directly off the coast, with wave heights rising to 7-14 metres offshore. With such a large, energetic sea state and South Steyne having the gradual sloping sea floor, the infragravity waves would have been in the order of 1 metre once inshore, surging in and out towards the beach at a rate you can't fight against, even on a board.
While infragravity waves were discovered by the late Walter Munk in the 1950's the way they seperate away from swell waves once these themselves break inshore is still not fully understood. Sometimes they reflect back out to sea and at other times on steeper shores they dissipate much quicker.
The implications of infragravity waves regarding the transportation of sand has been researched more, and observations show that they influence the temporal behaviour of rip currents, fostering the detaching of eddies which than transport sand alongshore.
Next time you're struggling to push out across that sand bar that seems to be constantly surging you inshore, have a think back to this article and maybe wait a minute or so and try and push out as the infragravity wave recedes.