Infragravity Waves Reach Full-Force

Craig Brokensha picture
Craig Brokensha (Craig)
Swellnet Analysis

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.

Comments

mikehunt207's picture
mikehunt207's picture
mikehunt207 Wednesday, 13 Apr 2022 at 10:45am

Incredible footage , looked like ks wave pool for a minute.
Does this happen there very often? I imagine not looking at the manicured lawns on the foreshore.
Just lucky the swell doesnt get that big in Manly very often? Never been there but talk about extreme beach front properties, always blown away by how built right to the sea it is on the east coast.
Shame that wasnt local boy Tony Abbot getting washed into the park , Climate change ? No way says Tony!

Ronson's picture
Ronson's picture
Ronson Wednesday, 13 Apr 2022 at 12:16pm

No it doesn't happen very often, if ever? Brings new meaning to the term "novelty wave" because its totally novel.

stunet's picture
stunet's picture
stunet Wednesday, 13 Apr 2022 at 12:46pm

Reckon it happens a little more than people realise, however they're really only noticeable to the eye when the surge goes up an estuary, which as Craig explains is now possible cos of rainfall.

When amongst breaking waves they're largely indistinguishable; perhaps a surge attributed to a rogue wave or whatnot.

Craig's picture
Craig's picture
Craig Wednesday, 13 Apr 2022 at 4:27pm

It happens noticeably every large swell event but is way more evident due to these estuaries and inlets being flushed open by the flooding.

thermalben's picture
thermalben's picture
thermalben Wednesday, 13 Apr 2022 at 11:04am

Far out, that video is insane. How's old mate getting outrun in the park!

seeds's picture
seeds's picture
seeds Wednesday, 13 Apr 2022 at 11:09am

Amazing footage. Still remember as young kids me and my brother playing in calm water on surf mats way inside Currumbin Ck(I think) and out of nowhere a surge wave came and picked us up and up the creek we went for what seemed ages.
Probably what it was. Was the only one.

donweather's picture
donweather's picture
donweather Wednesday, 13 Apr 2022 at 12:39pm

Insane footage. You’d swear looking at that footage that it was a tsunami.

Roystein's picture
Roystein's picture
Roystein Wednesday, 13 Apr 2022 at 12:59pm

I found your original article very interesting Craig and certainly kept it in the back of my mind following. The footage makes a lot of sense when combined with your explanation. Thank you for the increase in knowledge and awareness of the wonders of the ocean. Saw one in action at a out of the way spot on Monday, cleared the lineup and washed a gigantic plume of sand into the deeper water.

Craig's picture
Craig's picture
Craig Wednesday, 13 Apr 2022 at 4:28pm

Thanks Roystein.

Bungan33's picture
Bungan33's picture
Bungan33 Wednesday, 13 Apr 2022 at 1:10pm

That video is incredible. Having surfed all my life and done white water river guiding - it blows me away how people underestimate the power of moving water. That little kid was about 30 seconds away from drowning I reckon. Intense.

mjh's picture
mjh's picture
mjh Wednesday, 13 Apr 2022 at 1:13pm

This whole area of land beside the lagoon that was swamped is all reclaimed so in my limited knowledge is this not just water filling in areas where it would naturally have been ebbing and flowing for millenia?

mike.logan's picture
mike.logan's picture
mike.logan Wednesday, 13 Apr 2022 at 1:37pm

A few years ago near Bundaberg Qld I noticed how on a set of waves coming into a small creek inlet how the whole water level rose (about 30cm) in the creek. This only on a small wave size of about 3-4' face waves 250m away. Then due to a long time between sets, a big out flow of water back to the ocean. Can't say I have noticed it any other time over 40 years.

t-diddy's picture
t-diddy's picture
t-diddy Wednesday, 13 Apr 2022 at 3:16pm

very interesting!

thermalben's picture
thermalben's picture
thermalben Wednesday, 13 Apr 2022 at 3:45pm

Gravity waves pushing into Cudgera Creek (Hastings Point, Far Northern NSW) in January this year.

Background info here: https://www.swellnet.com/comment/787794#comment-787794

Distracted's picture
Distracted's picture
Distracted Wednesday, 13 Apr 2022 at 4:12pm

Awesome, had been hoping you would be doing an article Craig!
Similar footage from Avoca, really lucky no one was hurt.
https://m.

Craig's picture
Craig's picture
Craig Wednesday, 13 Apr 2022 at 4:23pm

Thanks Distracted, and wow! It's incredible eh!

thermalben's picture
thermalben's picture
thermalben Wednesday, 13 Apr 2022 at 4:26pm

Did the Pet Shop Boys assist with the soundtrack? Expecting some bloke to jump in front of the camera and launch into 'It's a Sin'.

Nae07's picture
Nae07's picture
Nae07 Saturday, 23 Apr 2022 at 9:11am

That is definitely not Avoca Beach NSW. The Shark tower at Avoca is to the right of the lagoon and carpark. Whoever posted the video doesn't even know where it is if you read the comments they are asking other people what beach it is and this video is taken from a balcony. If you look on Google Earth you can see there are no homes in Avoca in that particular spot that could have taken this kind of footage.

Roystein's picture
Roystein's picture
Roystein Wednesday, 13 Apr 2022 at 5:03pm

This is fascinating - I remember reading about a historical rescue/drowning event and I would hypothesise that it is likely the circumstances and outcome could be explained by these infragravity waves. I am not sure if this particular tragedy has ever been explained away as this?
Article of event here
https://www.surfsportsforum.com/black-sunday-on-bondi.html

matt.evans's picture
matt.evans's picture
matt.evans Friday, 15 Apr 2022 at 8:31am

Wow, @Roystein, that's fascinating history! What an amazing event and story!

endru's picture
endru's picture
endru Saturday, 23 Apr 2022 at 7:31pm

Thanks for sharing this link. I've shared it with everyone who might be interested. Don't know how many times I've heard that myth about a "collapsing" sandbar (but I'm guessing there was more than enough going on to explain the tragedy, without including infragravity waves).

Tooold2bakook's picture
Tooold2bakook's picture
Tooold2bakook Wednesday, 13 Apr 2022 at 5:38pm

Amazing

Max Wax's picture
Max Wax's picture
Max Wax Wednesday, 13 Apr 2022 at 6:36pm

Saw this happen at shelly beach in manly that same morning, the water surged the whole way up the beach onto the grass and flooded the whole area, nearly flooded the boathouse had it risen a few cm higher! On top of that shelly beach faces WNW which means that the waves wrapped 180 degrees to get in there

andy-mac's picture
andy-mac's picture
andy-mac Wednesday, 13 Apr 2022 at 6:57pm

Wow, well you do learn something new everyday. Thanks Craig.

Sprout's picture
Sprout's picture
Sprout Wednesday, 13 Apr 2022 at 8:54pm

Great article thanks Craig.

Jamyardy's picture
Jamyardy's picture
Jamyardy Wednesday, 13 Apr 2022 at 8:59pm

Cool stuff.
If Manly lagoon is not always open to the ocean, how do they measure water level (in this case tides) 800m upstream ?

Craig's picture
Craig's picture
Craig Thursday, 14 Apr 2022 at 5:25am

It usually just measures the height in regards to flooding, but with it open, lucky for us it also recorded the infragravity waves.

wayneoz's picture
wayneoz's picture
wayneoz Wednesday, 13 Apr 2022 at 10:12pm

Cool article Craig. We witnessed one at Easties in the Gong. It was insane.

Yendor's picture
Yendor's picture
Yendor Thursday, 14 Apr 2022 at 6:17am

Thanks Craig,

That's super interesting to know about.

Do you think that the infragravity wave from a large set out the back of a big bay could intensify a smaller set breaking on the inside of the bay?

I feel like I might have observed this. The scale would probably be about 1km from the outside to the inside of the bay.

Finnbob the terror's picture
Finnbob the terror's picture
Finnbob the terror Thursday, 14 Apr 2022 at 6:20am

Great info,
Craig do you think ifragravity waves could make their way through the bay as far around as Sorrento and Blairgowrie? I have witnessed a few weird surges, but a friend had a strange experience last year at Sorrento boat ramp, a few waves surged in and flooded the boat ramp, submerging the Jetty and flooding 3 or 4 meters up the ramp.

Craig's picture
Craig's picture
Craig Thursday, 14 Apr 2022 at 6:42am

Yes totally! As Max said above, the Boathouse inside Shelly was getting smoked and the waves were surging 100m or so inside the bay there and this is 180 degrees around from the primary swell direction.

Yippee's picture
Yippee's picture
Yippee Thursday, 14 Apr 2022 at 9:51am

Only been in, on or looking at the ocean for about 60 years, and never knew (the science) of these types of waves. I guess we just called it "surge" .
So thanks Craig!

dawnperiscope's picture
dawnperiscope's picture
dawnperiscope Thursday, 14 Apr 2022 at 1:27pm

Interesting stuff! I feel like these little buggers might also be responsible for damage to old man shoulders at certain north coast sweepy headlands.

matt.evans's picture
matt.evans's picture
matt.evans Friday, 15 Apr 2022 at 4:42am

Really great and interesting stuff Craig! Very well done.
And, thanks everyone for those fascinating videos above. I just love this stuff.

You mention these waves building in the open oceans, then being released from the sets when the sets hit the coast.

Is it building that energy the whole distance that the swell travels? Or, is this a more regional effect that's influenced by regional weather/swell/tide conditions? I've experienced some surge-y activity from SoPac swells in CA, but if it were just distance that influenced it I'd expect it to happen strongly every south swell.

Craig's picture
Craig's picture
Craig Friday, 15 Apr 2022 at 6:29am

Thanks Matt!

The size of the infragravity waves are a function of size of the sea state. Ie on normal swells it's hardly noticeable unless you're a very keen observer watching intertidal pools slightly rising and receding.

So more localised, energised sea states produce the most noticeable and largest infragravity waves.

dawnperiscope's picture
dawnperiscope's picture
dawnperiscope Tuesday, 3 May 2022 at 8:03am

Looking back over the years I reckon Dad's who've dug a pool on the beach for their toddlers would also be unsuspecting keen observers! The amount of times I watched a set fall short, but in between sets it gets flooded unexpectedly. Great to have an explanation.

johnruciak's picture
johnruciak's picture
johnruciak Tuesday, 3 May 2022 at 6:35am

Great article Craig, had no idea. Now do meteotsunamis :)