Infragravity Waves: Part 3

First published in The Surfers Path

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

In the last two articles I’ve been talking about infragravity waves, what they are, where they come from and how they can sometimes completely dominate the water motions at the shoreline. In this, the final part, I’m going to talk about why infragravity waves are perhaps the most important link between oceanic storms and coastal erosion; plus I’m going to say a bit more about what I mentioned right at the very beginning – how infragravity waves can penetrate right through to the most far-reaching pocket-beach halfway up some estuary or behind several breakwaters, even when the ordinary waves have been whittled down to a fraction of their open-ocean size.

For many years, people have known that beaches tend to erode in large storms. The classic pattern is a long, slow phase of onshore sediment movement during periods of small waves, followed by a short, powerful burst of offshore sediment movement during an episodic storm event. But how this pattern actually takes place is a bit more of a mystery. What is the process that links big storms with coastline erosion? We still don’t know the complete answer, but infragravity waves go some way towards explaining it.

The standard theory linking storms with coastal erosion through infragravity waves is as follows. Under the crest of a wave, including that of an infragravity wave, the water motions are in the same direction as the wave itself. Under the trough, the water motions are in the opposite direction. Therefore, under the trough of a shoreward-travelling infragravity wave, the water motion is offshore (Figure 1).

Figure 1: The water motions beneath a passing wave

The ordinary waves have underwater motions associated with them too. These water motions go shoreward and seaward much faster than those of the infragravity waves, and stir up sediment on the bed. But the amount of sediment stirred up depends on where you are in the wave group, or set (waves travel in sets in deep water, way before they reach the breakpoint – sometimes you can see them from the top of a cliff). Assuming that the waves get bigger towards the middle of the group and taper off towards the beginning and end, we can say that the large waves in the middle of the group produce the most powerful water motions. Therefore, the most sediment is stirred up under the middle of the group. As a result, you have areas of high sediment suspension under the middle of the group, and areas of little or no sediment suspension under the beginning and end of the group.

Now, according to the most accepted theory of infragravity-wave formation, which I briefly explained in the last article, the trough of the infragravity wave coincides with the middle of the wave group. So the trough of the infragravity wave, under which there is an offshore flow of water, coincides with the large ordinary waves in the middle of the group, under which there are areas of high suspended sediment concentration. This, logically, will transport large amounts of sediment offshore. In contrast, the peak of the infragravity wave, where the water motion is onshore, coincides with much smaller ordinary waves in between groups, under which there are areas of little or no sediment suspension. Hence there will be only small amounts of sediment transported onshore.  The whole story is shown in Figure 2.

In summary, if you add up the onshore and offshore sediment transport in stormy conditions when you have large infragravity waves, you will find a net offshore transport.  This contributes to coastal erosion.

Figure 2: Standard theory for sediment transport under infragravity waves

Of course, the theory above will only work if at least some of the infragravity waves are still locked to the groups. But if they are no longer locked to the group after the breakpoint, surely they can only produce coastal erosion beyond the breakpoint, right? If that were the case, infragravity waves wouldn’t be a very convincing explanation of coastal erosion.  After all, much more coastal erosion takes place shoreward of the breakpoint than seaward of it.

In fact, what happens in practice is that the grouping structure is not entirely lost at the breakpoint; some of the infragravity waves are still at least partly attached to the groups. So we can say that the above process can explain some of the erosion that takes place shoreward of the breakpoint.

Right on the shoreline, there is also another process that contributes to sediment erosion. It is in the swash zone where the majority of coastal erosion happens during storms. The swash zone is the part of the beach right at the shoreline where the water surges in and out. And the particular nature of the infragravity motions in the swash zone is one of the most important factors. It is thought that, when the infragravity waves get really big, they take on an ‘asymmetric’ form, where the shoreward surge is long and slow, and the seaward return is short and fast. If you watch carefully in really stormy conditions you can see the infragravity wave gradually pumping shoreward, with the ordinary waves ‘stacked up’ on the back of it, before the whole thing slows down, turns around, gathers itself up and gets sucked out to sea again. Because the onshore phase is slow, relatively little sediment is churned up.  So, even though the flow is longer, there is less sediment available to be transported shoreward. The offshore phase, on the other hand, is much faster, so colossal amounts of sediment are scooped up from the sea bed, which then get transported rapidly seaward and dumped offshore somewhere.

If you’ve managed to follow me up to this point, you’re probably beginning to slide down the slippery slope of idealized models and theoretical thinking, which means you’ll probably think both of those ideas fairly reasonable. In fact, like most things related to infragravity waves, they are both really just hypotheses, and a subject of ongoing research.

Lastly, some of live in places where the waves sometimes become so huge and out of control that you have to start looking for smaller spots to surf. Usually these are places facing away from the main swell direction, behind a headland or even a short distance up an estuary. In my experience, these spots seem to be much more dangerous than those on the open coastline, even though they pick up a lot less swell. They nearly always have stronger rips, more water moving and give you a lot more grief getting in and out of the water than their open-ocean counterparts. But why? 

It is because of the infragravity waves. Even if the ordinary waves are filtered down to half their size by the effect of a headland, an estuary or some other feature, the infragravity waves just keep ploughing on through, actually getting bigger as they pour into the nooks and crannies of tucked-away pocket beaches. The infragravity waves corresponding to a 30-foot swell will still reach that round-the-corner spot even though the ordinary waves might have be filtered down to a manageable six foot. It is the disproportionately large infragravity waves that cause all that extra water moving, and all those rips and surges. Even if you think you’ve escaped the gigantic swells, the infragravity waves are still there, lurking underneath, ready to spoil your surf session or, worse, sweep you out to sea.