They've been incorrectly called tidal waves or mini-tsunamis, yet infragravity waves are their own particular kind of wave.

The mistake can largely be attributed to the infrequency of infragravity waves, however, during the recent clustering of East Coast swell events, they've become more regular. In fact, the last two years have been a boon for observing infragravity waves.

So what is this infragravity wave we speak of?

Much like light, oceanic waves fall along a spectrum ranging from, at one end, tiny, high-frequency capillary waves, then less frequent but more drawn out groundswells, next are tsunami waves, and then there's the waves with the longest wavelength (yet lowest frequency), which are the tides.

Sitting in between wind-generated groundswell waves (periods of 10-25 seconds) and tsunamis (periods of 10-60 minutes) sit infragravity waves.

They have periods of minutes and wavelengths of kilometres, linking sets across the open ocean.

They form as a result of the ocean surface trying to balance itself out as sets lift and depress the sea level.

Imagine a large set of multiple waves travelling through the deep ocean. Because the set displaces the ocean above its normal level, a counter-force - in this instance called the conservation of momentum - causes a depression of the sea surface spanning the set. 

Now, with all things being equal, due to the slight depression of the ocean surface under the sets we see a slight rise in the ocean surface between the sets.

This slight rise and fall of the ocean surface is known as an infragravity wave (normal ocean waves are correctly called gravity waves, while infragravity waves exist within them)

The size, strength, and intensity of infragravity waves in the open ocean is directly proportional to how large and active the sea state is. That being, the larger and more consistent the swell, the larger the displacement of the ocean surface and hence, the infragravity waves.

These waves are invisible in deep water, but once the swell and sets start pushing into shallower water they are released from the sets, ebbing and pulsing into the near-shore zone.

As their frequency is in the order of minutes and the wavelengths span kilometres, they surge into the coast much like a small tsunami, amplified by shallow bays and inlets, again much like tsunamis do. However, while they act similarly, it bears repeating that they aren't tsunamis of any kind.

While only being a few centimetres high in the deep ocean, they become larger when they reach the shallow water of the coast, and during last weekend's large south-east groundswell sea level gauges observed infragravity wave action between 10-20 centimetres.

While this doesn't sound like much, when added on top of a localised storm surge they become very destructive, with torrents of water surging and slamming into the coastal zone over a much longer period of time compared to wind-generated waves.

For surfers, these infragravity waves are most noticeable along point and reef breaks, with sporadic surges of water and sweep pushing you out of position, with even the strongest of paddlers unable to make any ground against them. They come and go and if you push through for a few minutes you'll feel the surge release and even reverse as the infragravity wave retreats back out to sea.

During last weekend's swell we had a unique setup where a lot of the coastal lagoons and inlets were open due to intense, sustained rainfalls. Not only that, they're scoured deep from running floodwater.

This allowed the infragravity waves to penetrate much further inland than seen before, causing significant erosion while also catching many people unaware.

Below is footage of an infragravity waves surging 500m up Manly Lagoon, gathering speed while pushing into shallower water. So much so it couldn't be outrun, with it engulfing one unlucky punter who saw it approaching. Similar scenes occurred at Avoca on the Central Coast, with reports from the Illawarra, Shoalhaven, and far South Coast.

Below is water level data from the Manly Lagoon, measured 800m inland and what it clearly shows is the localised storm surge on top of the forecast tides and also infragravity oscillations through the peak of the swell event.

The purple line shows the forecast tidal height, with the green showing what was observed. We can see that observed heights were higher than forecast thanks to the storm surge and lower than normal local pressure (inverse barometer effect), but what's also visible through the peak of the swell event on the 1st and 2nd of April is the 10-20cm oscillations on this above normal sea level. Much like oscillations on a seismograph register earthquakes.

Manly Lagoon water level observations thanks to Manly Hydraulics Laboratory

These are the infragravity waves being recorded with them being most active and visible during the peak of the storm, easing back noticeably through Sunday the 3rd as the swell dropped rapidly. Following this the heights returned to near forecast with much smaller infragravity waves of a few centimetres recorded.

The destructive combination of elevated sea level along with 20cm infragravity wave action saw the sea level pushing 40cm above the forecast tide of 1.6m to around 2m. The implications regarding future sea level rise and stronger more intense storm events is obvious.

Ulladulla Harbour water level observations thanks to Manly Hydraulics Laboratory

Observations from inside Ulladulla Harbour show a similar signal with infragravity waves between 10-20cm peaking through the swell event on the 1st and 2nd of April.

Special thanks to Manly Hydraulics Laboratory on behalf of the NSW Department of Planning and Environment Biodiversity and Conservation Division.