The Tide Giveth And The Tide Taketh Away
On some coasts, high and low water also describes the wave size
The Tide Giveth And The Tide Taketh Away
The tide giveth and the tide taketh away.
For South Australian Mid Coast surfers this is gospel.
The tidal cycle becomes more important than the presence of swell and local winds...well, almost.
It can make or break a session and turn the swell tap on/off almost instantaneously, as yours truly was reminded of on a recent trip back to the home state.
For those unfamiliar with South Australian geography, Adelaide sits inside Gulf St Vincent, sheltered from most of what the Southern Ocean throws at the more exposed coastlines.
Just south of Adelaide there is a 30km stretch of coast that does receive surf, but only from Southern Ocean swells that are either big enough or west enough in direction to slip in between a small gap that sits north of Kangaroo Island and south of the Yorke Peninsula.
This is known as the Mid Coast and while being fickle in regards to swell direction, size, and period, it's the tide that is the biggest maker or breaker of a surf session.
Incoming tides allow the swell to push in with full force, even giving it some additional size, while on the outgoing phase, consistency, size, and energy all suffer.
A tiny 1.5ft swell can double in size to 2-3ft with the incoming push before stopping and seemingly evaporating at the turn of the tide.
Why does the Mid Coast see such significant variations in size with the changing tide compared to say the East Coast?
The answer is due to the local geography in which the Mid Coast breaks lie.
Sitting inside a gulf, ocean circulation and flow is restricted. This applies not only to water temperature and salinity but also the the tidal signal.
Across more open coasts, the tidal signal moves along the coast in an unrestricted manner, but inside a constriction such as a gulf or bay, there's nowhere for the incoming tidal signal to go but back out the way it came. This results in tidal currents flowing out either side of Kangaroo Island creating the treacherous waters of Backstairs Passage and Investigator Strait.
The tidal amplitude inside the gulf is on average 2m, reaching 3m in the upper regions and this has major implications for any swell moving through the entrance to the gulf. The large volume fluctuations can transform a tiny, unsurfable swell to consistent, fun surf with the incoming push, but just as easily suck away the size and consistency as it starts draining out. This is known as tidal modulation, and is specifically the process of breaking waves changing their height over the tidal cycle.
We see this phenomena repeated around the globe to varying degrees, with the greater the tidal movement in and out of a semi-enclosed region, or over an underwater constriction, the greater the impact on the swell height.
Waiting for that positive tidal modulation (Brokensha)
Even though regular water users are aware of the influence the tide makes to incoming swells, there have been limited real time studies on the influence on tidal currents to wind generated waves (swell).
As recently as 2008, there was a paper published “Tidal Modulation of Incident Wave Heights: Fact or Fiction?' with the end results confirming that, yes, such a phenomena exists.
But a more recent paper in 2018 captured the tidal modulation effect using wave buoys inside the Gulf of Maine (off the north-east corner of the United States).
The Gulf of Maine has the world's largest tidal range, reaching up to ~16m in the upper reaches of the Bay of Fundy. This leads to strong tidal currents over the mouth of the gulf that reach up to 1m/s.
Buoys were placed inside the gulf, recording the incoming swells and on each trace, what's clearly evident within the significant wave height observations are high-frequency oscillations (tidal modulations). To be more specific, a semi-diurnal (twice daily) oscillation of the wave height is observed within the general wave recordings. This is the brown line in the below graph.
Significant Wave Height recordings (brown) showing semi-diurnal oscillations, correlating with the tidal signal (blue). (Tidal Modulation of Surface Gravity Waves in the Gulf of Maine)
Overlaying the tidal predictions for the same period (blue line) we can see that the rising (incoming) phase brings an increase in significant wave height, while the dropping (outgoing) phase leads to a drop in significant wave height.
Keen observers will notice that the peak in swell energy occurs during the strongest push of the tide as this is when we see the strongest currents going with the swell, and vice versa, with the drop in swell being greatest on the strongest part of the outgoing phase.
Besides modulating the swell size, the currents associated with the incoming or outgoing tidal push can also refract swell into some locations while steering it away from others. Much like swell feeling a shallow bottom, slowing and bending the swell in towards the shallower region, the same occurs when swell interacts with opposing currents. The swell will bend in towards the area with the lower absolute propagation speeds, relative to the fixed bottom.
That is, if a current is opposing the incoming swell, it will slow the movement but also bend the swell in towards this opposing current, causing shifts in direction. The tidal currents can also slightly effect the swell period, with bunching up or lengthening out of the wavelength depending on the flow.
While each location around the world is unique, there are locations that aren't in embayments that still feel the effect of the tidal currents. This can be influenced from a number of factors such as outgoing flows from a nearby harbour/strait, steering the swell into or away from select locations.
Tidal modulation is just another piece of the puzzle and it's always worth keeping in the back of your mind when trying score the most consistency and size out of a swell.
Comments
IMO the mid coast has changed enormously over the last 5 to 8 years. The tide may be an influencing factor, but the amount of swell that comes through on a regular basis is almost non existent compared to years ago. Its almost feels like there has been a fundamental change in the ocean that allows the swell to be generated in this region or allowing it to reach the mid coast.
I don't agree. This year and last year have been terrible. The couple before that were really good.
Yep, la nina does Adelaide coasts no favours.
So funny.. I used to hang out as a grommet at Frosty's surf shop in Noarlunga in the late 70's and he was always saying about how much bigger and better the Mid used to get back in the day. Every generation of surfers has said the same thing since. Romanticising the past is a thing.
Really interesting.
Around here there are tidal influences related to water depth and hence refraction at different period signatures around near-shore reefs.
A close to 2m depth difference around certain near-shore reefs can make a big difference on certain periods and directions and influence wave focussing onto certain breaks.
Great article! It is nice to see a well articulated piece on our beloved Mid. It's amazing how in tune to the tides we become. Such a fickle place but oh, such beautiful waves when tides, wind and swell all cooperate.
Surfed in that area a lot from about 1968. No changes in swell or shape (apart from incremental seasonal ones. It was neither bigger nor more consistent in the past.
If Garry Weed is Gerry Wedd then yes I remember you surfing there back then and yes it was a impressive. I was awestruck as a grom.
Great stuff thanks Craig.
Bangko Bangko is a good example of a swell interacting with a strong opposing current.
Amazing what the change in tide can do to bangko bangko. It really is a special place. Shame about the crowds.
and west sumbawa's best spot...can be flat at hightide with a 12 foot hawaiian swell from the right direction..still not working till the tides are developing..Then hopefully the wind cooperates too. Which it often doesn't.A lot of Nusa Tengarra spots do this.Especially the more protected ones which are often offshore in the trades.
That first spot i mentioned on other days can be too big all day and still not west enough swell for some spots without any access..besides paddling your own canoe out there.Had footage of one spot pumping one 4 metre swell day- hired a canoe, paddled around the headland..shot it...but camera was stolen a week later with all footage.
Mid Coast there seems to be a big build up of sand between Seaford and Seaford Reef . Lets see what the tide and next winter does to that . Sand banks at River Mouth have built up to . All this and hasn't really changed ( to me ) Trigs, The Trough . Gulls. still the same but lots of people now.
I was always doubtful of the impact of tides on swell UNTIL I spent a week on Phillip Island (and scored the best swell of the season mind you!!! [March 2020]). The tidal impacts were phenomenal.
Yesterday afternoon I was at Balmoral so my daughter could have a swim. I had just come in from 6 foot, East Swell Manly but Balmoral was two foot! About an hour and a half from the peak of high tide, so I assume, the incoming water was amplifying the swell. Subsequently this subject matter is fresh in my mind.
Love the mid it’s like a mini hawai when it fires.
Only does it properly once in a blue moon.
Had some epic days in the 80s and 90s with a few I remember very well.
The old wind n wave surf report was called every morning and if it was a foot with the outgoing we knew it would be a couple foot at the hump on the turn, great times.
One day it was reported 1.5 to 2.5 with the outgoing when it turned it was the biggest clean day I ever witnessed there.
Hard to put a size on it but Southport looked like waits and handled the swell about the same, big closeouts with Rincon and Hump only real options Hump was very thick and juicy that day.
Thanks heaps Craig,
I was trying to look into the mechanics of this but could find scant info. Most surfers know it's a thing but I was wondering why.
Here in Wellington we get harbour tide impacts on some waves and then we have the mighty Cook Strait so we get some cross shore action with tidal flows too, makes for some interesting swell modulation. Because we have corresponding high and low tides on either side of the strait we get water flowing back and forth on the different phases.
Wow that would be super hard to get a handle on eh?
The cross shore ones are usually pretty subtle except for a couple of reefs in the thick of the current.
http://graphs.gw.govt.nz/?siteName=Wellington%20Harbour%20at%20Baring%20...(Hmax)
This wave buoy near the harbour entrance shows direction shifts in phase with the tides when a south swell is running.
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Similar effect but more localised when surfing river mouths.
Agree with Kieranol, there was a period where the mid coast had size,consistency and alot more big sized days and west would pump, and south had abundant clean big days years ago. Last 4 years have been poo. (The mid has had more consistent small mid swell in this time).
After Capt Cook managed to navigate half way round the world, perhaps these tidal surges & strong currents may have put the Endeavour onto the Great Barrier Reef at 11pm 11th June 1770 ?
Location
https://goo.gl/maps/rYS7sGHWct2G34No8
Cooks Journal "Having the advantage of a fine breeze of wind, and a clear Moon light Night in standing off from 6 until near 9 o Clock, we deepned our Water from 14 to 21 fathoms, when all at once we fell into 12, 10 and 8 fathoms. At this time I had everybody at their Stations to put about and come to an Anchor; but in this I was not so fortunate, for meeting again with Deep Water, I thought there could be no danger in standing on...Before 10 o'Clock we had 20 and 21 fathoms, and Continued in that depth until a few minutes before 11, when we had 17, and before the Man at the Lead could heave another cast, the Ship Struck and stuck fast. Immediately upon this we took in all our Sails, hoisted out the Boats and Sounded round the Ship, and found that we had got upon the South-East Edge of a reef of Coral Rocks, having in some places round the Ship 3 and 4 fathoms Water...."
Reference https://www.gutenberg.org/files/8106/8106.txt
J. Banks' Account
https://jamescookjournal.blogspot.com/2013/06/d-i-s-s-t-e-r-10th-june-17...
Cairns Highest Astronomical Tide (HAT) is 3.57m above LAT
That's a very good post, thank you. Seems plausible. Were they located near a section of the reef that would amplifiy changes in depth with more water movement, I wonder.
Definitely a phenomenon I notice in areas with bigger tides.
My related query is around tidal movement and how closely it follows a Sin/Cos curve. I’ve noticed that when we get bigger high tides that the tide seems to already be very full when it’s bang on mid tide on the way in. Any actual data/papers on the issue?
So the maximum tidal movement is through that middle section of the phase, so if heading towards incoming, you'd expect it to be already quite full at mid-tide on a big incoming. That might be the perception?
You can observe the forecasts and actually observations at various sites around the country like here..
https://www.qld.gov.au/environment/coasts-waterways/beach/storm/storm-si...
Cheers Craig, not a huge amount of residual on that graph and the tides have been fairly big the last few days. I’ll compartmentalise that snippet of knowledge!
Further to that, Stu2d2, you might want to check out my comment below on the rule of twelfths. Basically, four hours after low tide, 75% of the tide has come in. With a greater volume during spring tides, it will definitely look full after the mid-tide phase, especially compared to the average (neap) high tides on your coast.
Can a big enough tide generate it's own swell? Early 90's some of the locals took me to Brims Ness, north scotland and it's 5m tide range. When we got there the tide was at the bottom and it was dead flat. The locals paddled out anyway saying the waves would come with the incoming tide. Sure enough as the tide came in it went to 2-3 foot. Can't remember what period it was but it was pretty short.
I reckon No. locals just knew it needed more water for the waves to get in there. Probably an outside shoal or something that blocks the swell at low tide
Agree with shraz, besides a tidal bore style wave, the tide can't create surfable waves.
The mid coast has good years and then bad years which are probably the result of things like La Nina influencing swell direction and wind patterns. I have been surfing it for 50 years since a grom and no overall major change in my view. Biggest negative has been the construction of the storm water outlets which damage the reef cover. Seaford being a case in point.
I agree PeterH. IMO the storm water has killed off the seagrasses we used to have at Seaford, leaving the limestone reef bottom exposed and I think the sand moves around much more for the same reason. No evidence, just my opinion. No overall change in swell for me either, we've had bad and good years for as long as I can remember. Still love the place.
johnruciak
Port Stephens bay is a good example of what you describe.Im pretty sure tidal surges cause the area to be such a dangerous place for boats..Pretty sure not just wind generated/normal swell conditions but tidal flow etc.
By the way last time i was there i did a deckhand and the first stage of skipper ticket...the very same day 50 knot westerlies were blowing and port stephens has 2 foot westerly direction waves breaking all along the west facing beaches.Unreal.could have surfed them.
Great series of articles, Craig.
A thread in this one is about the push or greater volume of water moving during the mid-tide phase. This not only affects swell, waves, currents and fish, it catches out, traps and sometimes drowns unwitting beachgoers. Alongside rips, the basic concept of exponential tide movement might be taught in schools.
As with any sine curve, the rule of twelfths applies. One twelfth moves in the first hour, then two-twelfths, and half the tide volume moves in the two hours at the middle of the six-hour cycle, 1:2:3:3:2:1, with greater volumes moving during the spring tide phases.
I publish tide calendars that graph this movement over the year. (Not sure if it's kosher to mention them, but Ocean Calendar tide charts for those interested).
Another thing you notice on a longer-term graphed tide chart is the syncopated rhythm of the tides around spring tides. High, low, high, very low, high, low, high, very low...
The rhythm of the ocean...
Thanks Afbock, love the rule of twelfths!
Thank you for that!
What’s the pick of the spots on the mid coast? Seaford?
Nice bit of a bowl at Seaford that keeps the crowd coming back but worth looking around a bit if it’s breaking everywhere else too.
Western Port Bay and Cat Bay on Phillip Island (inside WP) are very much influenced by the tide too. Very similar dynamics to what you have described here I think Craig.
Last decent swell, if I recall, boxing day 5 or 6 years ago. Which lasted a few weeks.
Commonly known a the bullshit hour.
Been surfing the mid for 51 years alot has changed especially weather patterns and the size of swell that's been hitting the coast
Most old people think this. When u are 4 a 2 foot wave looks massive, when u are 14 a 2 foot wave is pumping, when u get your licence and can travel a bit u finally realise it's just a 2 foot wave. Buuuut some people never leave the mid and still froth out on a 2 foot wave and I envy them. Ignorance is bliss
I don’t agree. I’m a Vicco surfing since the 70’s. We used to get the full force of the roaring 40s. Big cold fronts every week or so. Wind would go wsw then work its way s, se, ne, nw, humidity build, storms, then next front. Now it’s all over the place. I think weather patterns have pushed south with warming planet widening the tropical zone. Meaning smaller surf on south facing Australia. Bells would be 6’+ and westerly wind the morning after nearly every cold front. Plus generally bigger surf most of the time. Now QLD is the place for consistency with trade wind zone expanding. I hope I’m wrong tho and it’s not permanent.
Yep......I constantly chuckle to myself after years of tidal influenced sessions where everybody drives off and I paddle out when it looks flat and voila another great surf at my fave go to reef.
Lots of breaks on the Rock that kick with the tide....some actually kick on the outgoing.
A certain mysto point break that everyone thinks needs a massive swell is actually better on King tides combined with swell direction and period rather than the size of the swell.
I know a spot that benefits from overnight Nw wind swells coming down from the gulf. As soon as the S change comes in the morning it’s on. Gotta know your tides though! Couple of hours and it’s gone again.
Yeah, I lived on the Mid from '76 'til '98, many of those years doing shift work. One street back from the Esplanade behind U-Turns, and could always hear the surf. I'd be working in the garden and hear the surf get louder, grab the board & go for a surf. Often it would only last an hour or two while the tide was just right and then be crap again. It definitely had its times that only the locals surfed it, before surf cams and constant reports on the radio.
Always suprised when I hear out there..."what's the tide doing?"
It's the first thing I check.
Then there is the issue of places that only have two tides versus four per day, or little tidal variation at all.
Always trying to fine tune the effect of tide cycles at my local creek break.
Yesterday was a classic example: took my son for a surf on the inside, right on high tide, as the following run-out (sometimes) helps to stand the waves up (and allows an easy paddle out too). But the current was already quite strong from the get-go, which made it difficult to hold position. The average sand setup probably didn’t assist either (the wave itself is very fickle, hasn’t been good for a while).
However at this point there wasn’t much of a current out in the general lineup further outside. The effects of the current seemed to be restricted to the shorebreak and inner gutter, running north with assistance from the fresh southerly breeze (and I assume, regional northward current).
I then surfed out the back a couple of hours later (2+ hours after high), by which time the inside section I’d been surfing with my son was almost bare sand - but the current was only just starting to properly develop on the outside. Within an hour it was really hooking along, making it very difficult to paddle against, and also affecting wave quality a lot.
This is all reasonably common though I’ve seen the opposite play out sometimes (on the outside section) when the assumed ‘poor tide’ windows have offered good surf. Throw in a reverse current under particular swell directions, and you’ve got a myriad of potential good/ bad surf scenarios that might come your way on any given day.
There are a handful of creek setups on the Tweed, and they all seem to work differently too. Kinda enjoy the challenge of working ‘em out though it does often make for a frustrating surf experience when you’ve only got a small window to get wet.
I have noticed while browsing through all the states cams most mornings to see what others are enjoying, I've noticed that SA mid coast beaches are rarely above 2ft if at all.
Spot on mate, and some people as I stated above. 4 year old think 2 foot is massive, 14 year old thinks 2 foot is pumping, 24 year old finally realises its only 2 foot, then u have 54 year olds that still thinks its pumping at 2 foot cos they never grew up or travelled! Give me a 4 foot stormy at pt willy any day! U can have your 1.5 foot clean mid with 30 on each break with kooks and angry old men that think/ compare gulls to caves its not stop pretending
Yikes.
And, scarily accurate.
Gulls was my favourite wave in the world between the ages of fourteen and sixteen and a half.
Sixteen and a half being, of course, the age when I got my Ps and could drive wherever I wanted...
Exactly Ben, and 1 other thing. U know how people say "oh the photos don't do it justice" in the mids case, like the photo in the article a photo of a mid reef looks 10 times better than it actually is. Still a beautiful place and beautiful looking waves, buuuuuttt!!!! Rant over!
It’s not been mentioned yet, but we get an earlier sea breeze in the warmer months with the change from low to incoming. Big mass of cool water filling in brings the cool air with it. Certainly has ruined many a leisurely gentleman’s hour surf..
Oh yes, very interesting, though very localised and depending on the current situation/placement of the EAC, eddies etc.
heard that one before, about how it's changed so much, and used to pump for weeks offshore at 6ft
tide changing swell direction or bending direction looking spot on with today’s cape de coudic readings. this site is a great way to learn alot more about our great countries oceans. thankyou everyone and keep up the good work. and yeah, i’ve got photos that of my local that makes any surfer froth but we would always say that it prob only happens 4 times a year and alltime once every 15yrs ,d’oh! lol
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Wonder if the incoming tidal surges could combine with or amplify the infra-gravity waves when a groundswell is flowing in the same direction? A super-surge.
Great question!
It would allow it to penetrate further inland and also inside lagoons with the increased water depth on the incoming, but modulation, looks like there's no real trend as far as I can see.
Here are observations from Patonga, in the Hawkesbury from the most recent swell event..
More zoomed in..
Looks to be thanks to the dynamics driving infragravity waves and swell waves being different.
Cruisy canoeing up Patonga Creek when there's a big incoming tide.....
The tide gauge is on the Patonga wharf; very sheltered from most swells.
Hawkesbury River mouth would bend swell to the south with an incoming tide....
Sheltered from swells but not infragravity waves as you can see.
I grew up surfing the Mid in the late 70s & early 80s, and strangely, the reefs were always bigger coming up to full tide but The Hump got bigger on the low tide. Counterintuitive. Local bottom contours at play.
What a wonderful article, many thanks, Craig.
I felt very fortunate to surf the mid through the mid-'80s until 2002, and rarely since then. It is (The Mid) a locale that requires the surfer to learn (dare I suggest) more oceanography requirements than many other spots in Australia (noticeably the East Coast). Though once learned, it allows the surfer to forecast with a little more confidence.
I cannot say I nailed the knowledge required (as did a young Ben and even younger Craig as demonstrated here), but it did allow for a little comfort in forecasting the ideal 'sixty minutes'. After some 40 yrs of surfing, I will still suggest that the Mid, on her day, is a place of sheer joy.....just don't forget manners and etiquettein the process.
I’m late to this thread, just came back from Bali …… and would have to say that the majority of Bali and specifically Keramas must fall into this category to some degree. Tide was such a factor to surf quality
Thanks for the write up Craig it go me thinking and digging a bit.
I surf the mid coast waves of 2 types and I was thinking about the tidal effect on the fluid around us/the atmosphere.
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/atmosp....
Here's link that does the tech talk.
At the end it touches on the interplay between the land and the ocean and the atmosphere and how they interact differently.
Just making it all fit in place a bit better for myself anyhow.)