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.
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.
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.