A Deeper Look Into Swell Period
Recently, Swellnet received an email asking the question: "If a wave buoy is reading 2 metres, why does the surf come in smaller than 2 metres if the period is five seconds, and why do larger period swells provide more size than that 2 metre reading”.
The answer to this and a deeper look into wave period is explained below.
A surfer's view of the ocean is usually two-dimensional. Sitting in the lineup, boardriders identify lumps of swell rising and falling in the vertical plane (up/down) while they approach in the horizontal plane (forward/backward/sidewards).
Not much thought is given to what lies beneath the surface, yet this is where the bulk of the swell energy is stored. Such is the importance of this energy, it makes or breaks a session on your local beach, reef, or point break.
We’re talking about swell period, the energy hidden below the ocean surface which only becomes apparent when the swell moves into shallower water, on approach to the coast.
Swell period is the time (measured in seconds) between two successive wave crests (peaks), and is best observed when watching an incoming set passing a stationary object such as a buoy or fixed pylon. If it takes ten seconds between wave crests then the swell period is 10 seconds. The larger the wave period, the greater the distance between two wave crests will be and this is known as wavelength.
Swellnet has previously discussed swell period and its effect on your local beach or reef. On the East Coast, lower period swells are preferred, as they generally provide peaky A-frames which are ideal for the beaches. As the swell period increases, the lines become more drawn out, longer-lined and tend to close-out beachbreaks. This favours reef and point breaks which focus the incoming lines into peeling waves.
Surfers in the southern states wouldn't get out of bed for periods under 11-12 seconds and for the reef breaks to properly fire, long-period swells of 15 seconds or more are a must.
Regardless of how peaky or straight a swell comes in, questions like the one at the start of the article arise in respect to why similar size swells out to sea can create differing wave heights when they break.
To answer it we must go back to first principles.
Tsunami aside, every wave begins its life as wind blowing across the ocean surface. The wind transfers energy into the water column which initially shows as tiny capillary waves. As the wind continues to blow, non-linear (read: non-uniform) wave-wave interactions cause the energy to be transformed into longer waves, that influence deeper into the water column. The ocean sea state will continue to grow as long as the wind input is greater than the dissipation due to wave breaking and white-capping.
Wind blowing over a stretch of ocean is known as fetch, and there comes a point where the longer waves escape the fetch and travel onwards, or the fetch dissipates allowing the organised waves to spread out and continue on their way.
The greater the wind energy that's transferred into the ocean, the longer the waves and hence the time between successive peaks and troughs. This is where swell period comes from and generally the stronger the storm, the greater the swell period it produces.
Relatively speaking, longer waves are created by strong winds transferring energy into the water column over a substantial amount of time. Just as an increasing swell period stretches out the wavelength, so too does its energy stretch down into the water column under each wave crest - this is the three-dimensional aspect of wave energy.
Therefore the larger the swell period, the greater the amount of energy that is stored below the sea surface.
To visualise this, we see the majority of the swell energy stored in the upper part of the water column, decreasing in a cone like fashion the deeper you go.
To work out how deep in the water column the energy sits in regards to period, you can use this formula:
The wavelength of a swell (gap between each crest or trough) = 1.56*(swell period)^2.
So for a 16s groundswell, the wavelength is 1.56*16^2 = 1.56*16*16 = 399.36m. Meaning there is about 400m distance between each wave crest in a 16s swell out in the open ocean.
The depth to which the energy is stored in the water column is half of the wavelength, so for a 16s swell, we see energy down to 200m below the sea surface.
This is shown in the below diagram with the orbital energy associated with each wave decreasing the deeper into the water column you go.
Having a 16s swell carry energy 200m deep into the water column is quite a distance and this is why you can see long-period swells 'feeling' the ocean floor (bathymetry) way offshore while also being steered by certain underwater features, be they canyons, sea mounts, or other features.
This steering/focussing of swell is most pronounced across the East Coast when observing long-period southerly groundswells tracking northwards. We often see these groundswells providing a wide variation in size across regions with similar exposure to the incoming southerly energy. It's not unusal for one spot to be 6-8ft while nearby another spot is only 2-3ft.
The same effect can be see at Sunset Beach, Hawaii, where waves can be twice the size of Kammieland, though it lies just the other side of the channel.
The effect of bathymetry steering or focussing is also the magic behind Nazare. When swells arrive from the north-west they refract around a deep canyon that leads all the way to the coast, in effect causing the swell to wrap back in on itself, generating super-sized wedges.
This hidden energy and the way it interacts with the ocean floor is why a 2 metre swell at 5 seconds performs much differently to a 2 metre swell at 16 seconds.
A swell of 5 seconds has a comparitively short wavelength of 39 metres, and it only feels the bottom at 19.5 metres. Compare that to the aforementioned 16 second swell which feels the ocean floor at a depth of 200 metres.
Think also of how much more energy is present in the water column. That energy will be compressed upwards as the wave approaches the shore.
With all things being equal, as the swell moves into shallower water the wavelength reduces and the amplitude (height) increases.
If both swells are 2 metres, the 5 second energy will hardly rise (perhaps even coming in smaller owing to bottom friction) compared to the 16 second swell which will rise quite significantly in size due to the extra energy in the water column being squeezed and having nowhere to go but up.
Rough estimates would be the 5 second swell coming in at 2-3 feet (waist-shoulder high) while the 16 second swell will rise right up and break in that 6 feet+ range (double overhead +).
In the next article we'll look at how to read and identify the wave period of the buoys around the country so you can better understand the expected size and conditions on your local beach or reef.