Infragravity Waves: Part 1

First published in The Surfers Path

© Tony Butt 2010 - Please be decent enough to contact me before plagiarizing my stuff

Along the coast where I live, you can normally find at least one or two rideable spots even in the biggest, stormiest conditions.  That’s what I thought during the first few years I lived here anyway.  Recently, however, I’ve realized that simply heading for the most out-of-the-way sheltered cove doesn’t always guarantee rideable surf.  If the waves are so huge that you end up at some spot half way up an estuary or behind three or four breakwaters, something can still make the waves unrideable even if they are down to a reasonable size. 

The problem, in crude terms, is that there is ‘a lot of water moving’ – something I’m sure you are familiar with.  One thing you might not be so familiar with is just where all that moving water comes from, and why it always seems to be there at sheltered spots in big storms. 

In this, the first part of a three-part article, I’m going to introduce the concept of infragravity waves – extra long period waves that manage to find their way up estuaries, around headlands and behind breakwaters, actually getting bigger while the ordinary waves are getting smaller.

If you are looking for rideable surf during a huge storm, the further you get from the main exposures, the more you start to notice large surges of water at the shoreline.  Even if the ordinary waves are only a few feet high, the shoreline itself moves in and out hundreds of metres, quite often flooding over dunes and car parks and into shops and cafes, and then turning around and drawing back out again, sucking all sorts of debris with it.  The line-up, if there is one, is typically a wash of swirling currents as all that water moves beneath it, making surfing uncomfortable and dangerous.

During the last couple of winters, I’ve noticed a lot of this sort of thing going on along the coast where I live.  On the occasions that I’ve gone down to check the damage the day after the storm, I could see evidence of infragravity waves everywhere.  At the most affected spots there were whole areas of squashed grassland, with piles of seaweed and other debris carried a long way inland by the infragravity waves.  In some cases they had reached over a kilometre inland, often well out of sight of the sea.

If you didn’t know what these waves were, you might think that they were small tsunamis.  After all, they do have the same characteristics and behave in pretty much the same way.  However, they come at regular intervals of five or ten minutes rather than just once, and they are not associated with any particular geological event, which means that they are not tsunamis.  The fact that they only appear during large stormy surf suggests that they are probably somehow associated with the wind-generated waves.

Infragravity waves are a keen subject for coastal researchers, because they are thought to be an important contributor to coastal erosion, as well as being dangerous for bathers, coastal road users and waterfront hotel owners.  They were first documented in the late 1940s and early 1950s by Walter Munk and other legendary oceanographers.  They were originally referred to as ‘surf beat’ and were acknowledged to have something to do with the wave groups, or sets.  While the period of the waves we normally see breaking on beaches rarely gets above about 20 seconds, the period of infragravity waves is more akin to the period of the wave groups, ranging from about 30 to 300 seconds. 

Why are they called infragravity waves?  The term ‘infragravity’ is actually slightly confusing.  The prefix ‘infra’ comes from the Latin for ‘below’; and the ‘gravity’ part comes from the fact that most ocean waves are categorized as ‘gravity waves’ because the restoring force (that which pulls them down again after the wind has pushed them up) is gravity.  Therefore, infra-gravity waves are gravity waves, but ‘lower’ than normal ones.  But ‘lower’ what?  Well, wave physicists like to talk in terms of frequency, the inverse of period.  Infragravity waves have a lower frequency than ordinary waves, which means that they have a longer period.  Confused?  Well, if you want, you can refer to them as ‘long waves’ which is really saying the same thing.  But infragravity waves are not just waves that are longer than normal ones; they are a bit more special than that.

With harbours, estuaries and headlands, the ordinary waves gradually lose energy as they propagate in and around all the corners, whereas the infragravity waves just plough on right through to the most far-reaching pocket beaches and boat ramps.  But infragravity waves have mostly been studied on gently-sloping beaches, where it is easy to compare their behaviour with that of the ordinary waves.  Here, the ordinary waves dissipate their energy through breaking, while the infragravity waves just keep on going right to the shoreline. 

One important feature of infragravity waves is that they actually increase in size as they approach the shore.  Because of their exceptionally long wavelength, they never get steep enough to break, not even on the most gently-sloping beaches.  But as they come into shallow water they do slow down, which causing them to ‘jack up’ in the same way as ordinary waves do when they hit shallow water.  Because the front of the wave slows down before the back, the wave is squashed up horizontally and pushed up vertically.  In fact, because infragravity waves never break, they keep on growing like this all the way to the shoreline and reach their maximum size at the shoreline itself.  This process is greatly enhanced during large stormy conditions (see Figure 1).

In contrast, the ordinary waves, which have a much shorter wavelength, generally become steep enough to break before they reach the shoreline.  Once they break and become lines of rolling whitewater, they start to dissipate their energy.  On gently-sloping beaches, they tend to break a long way out, and can lose practically all their energy before they get to the shore.  This is why the lines of whitewater arriving at the shoreline are usually quite weak and dribbly, even if the waves breaking out the back are really big.  In this case, the ordinary waves are said to be saturated.  In theory, completely saturated waves diminish to nothing at the shoreline. 

Now, if the offshore wave height changes, the ordinary waves at the shoreline stay the same size (very small), but the infragravity waves change in response to the offshore wave height.  If the offshore wave height gets bigger, the ordinary waves simply start breaking further out in deeper water, dissipating that extra energy over a greater distance.  No matter how big the offshore wave height gets, the ordinary waves never get any bigger at the shoreline (Figure 2).   In summary, any changes in offshore wave height are only manifest at the shoreline as changes in the size of the infragravity wave motions.

As a result, the shoreline becomes more and more ‘infragravity-dominated’ as the offshore wave height increases.  This concept, which is very important for coastal oceanography, was proven in the early 1980s in a series of classic field experiments, which I am going to talk about in Part 2 of this article.

Figure 1: Infragravity waves are so long that they never break, therefore they squash up and get bigger as they slow down in shallow water, reaching their maximum height at the shoreline itself

Figure 2: In truly saturated conditions on gently-sloping beaches it doesn’t matter how big the offshore wave height gets, the ordinary waves still diminish to virtually nothing at the shoreline

A good video of infragravity waves hitting Cornwall, England, on 10 March 2008, can be seen HERE