Tony Butt’s A-Z of Surfing. I – Infragravity Waves

Tony Butt

 

An infragravity wave is a very long period wave which causes the entire shoreline to move in and out hundreds of metres, with much more force than that of the ordinary waves and at much longer timescales. As an infragravity wave comes in towards the shore, you will typically see three or four lines of whitewater stacked on top of each other. The whole water mass then turns around and draws back hundreds of metres. They have been likened to small tsunamis, although the mechanisms that cause them are completely different.

Infragravity waves were first documented in the middle of the last century. With periods in the range of one to five minutes, they were thought to be linked to the wave groups, or sets, which tend to have similar periods. Research still continues on the exact processes responsible for infragravity waves, but the results of many studies suggest that they are spawned somewhere between the breakpoint and the shore, and are somehow associated with the wave groups.

The reason they are called ‘infragravity’ waves is as follows. The ‘gravity’ part of the word means that the waves are the type whose restoring force (the force that pulls them down again after the wind has pushed them up) is gravity; in that way they are just like ordinary ocean waves. The ‘infra’ part of the word means ‘below’, which refers to the fact that their frequency is lower than that of ordinary waves. Frequency is the inverse of period, so a lower frequency means a longer period.

They exist in small surf but are much more prominent in large surf and in coastal storms, when they appear as large moving masses of water at the shoreline. Therefore, they can be dangerous for bathers and coastal walkers. They can be extremely problematical for people who own coastal properties, and they are thought to be one of the most important factors behind coastal erosion.

In fact, in some situations, when the offshore wave energy increases such as in large storms and swells, this energy reaches the shoreline almost exclusively as infragravity waves. 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.

On gently-sloping beaches, where many studies on infragravity waves have been made, the ordinary waves dissipate their energy as the lines of whitewater roll in between the breakpoint and the shore, while the infragravity waves just keep on going right to the shoreline. If the offshore wave height increases, the waves just start breaking further out, simply dissipating the energy over a greater distance. So, matter how big the offshore wave height, the ordinary waves are always pretty small when they reach the shoreline on a beach like this. Infragravity waves, on the other hand, have such long wavelengths that they never break; they keep rolling on through, maintaining all their energy right up to the shoreline. When they get to the shore, they still don’t break. Instead, they squash up horizontally and grow vertically as they slow down in shallow water, ending up as giant surging water masses at the shoreline.

The important thing to remember is that the shoreline height of the infragravity waves is bigger if the offshore wave height is bigger, whereas the shoreline height of the ordinary waves remains small no matter how big it gets offshore. As a result, in big swells on flattish beaches, the shoreline motion is completely dominated by infragravity waves.

Sometimes, if the waves become huge and out of control you have to start looking for smaller spots to surf – places facing away from the main swell direction, typically behind a headland or even a short distance up an estuary. Often these spots are more dangerous than those on the open coastline during the same wave heights. Because of the infragravity waves, these spots tend to have stronger rips and more water moving than their open-ocean counterparts, even though they pick up less swell.

If the ordinary waves are filtered down to half their size by the effect of a headland or some other feature, the infragravity waves are not. They are actually magnified 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 now only be six foot. It is the disproportionately large infragravity waves that cause all those rips and surges and that extra water moving.

If you think you’ve escaped the gigantic swells, you haven’t escaped the infragravity waves. They are still there, lurking underneath, ready to spoil your surf session or, worse, sweep you out to sea.

For a more comprehensive treatment of infragravity waves, see www.surfscience.org

infra1

infra2

On fairly flat beaches ordinary waves diminish to almost nothing at the shore, no matter how big the offshore wave height

infra3

infra4

Because they don’t break, infragravity waves always grow towards the shore. With large offshore wave heights, they are much bigger at the shore

 

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