Analysis: The long and the short of long period swells
When East Coast surfers plan their week around the surf forecast, most simply base their decision on the swell direction, size, and local winds.
Under most scenarios this is enough information to get a grasp on what the region is doing, however when the East Coast receives long-period swell energy things get decidedly more complicated.
The East Coast usually receives swells between 5-11 seconds which offer a fairly even distribution in size across locations open to the incoming direction. For example, a 3-4ft mid-period south swell in Sydney will come in around that size across all south-facing beaches, and a 3-4ft mid-period east swell, the same across all east-facing locations.
Yet when the period ramps up to, say, 15-16 seconds, and a proper groundswell hits the coast, then that pattern breaks down and similarly exposed spots can vary considerably in size. One south facing beach may be 6ft, while the other just up the coast struggles to reach 2ft.
The reason for this is that long-period swells 'feel' the ocean floor and its various irregularities, be that canyons or sea mounts, well before the swell reaches the coast.
Long-period swells are produced by strong, prolonged storms, and their kinetic energy travels well below the ocean surface.
Swells start to feel the ocean floor at a depth equal to half their wavelength, and this wavelength can be calculated by taking the swell period, squaring it and multiplying by 1.56.
Wavelength = 1.56*period^2
So for a 10 second swell it only starts to feel the ocean bottom at a depth of 78 metres.
When looking at a 16 second swell though, it starts to feel the ocean floor at 200 metres.
And a 20 second swell will start feeling the ocean floor at an incredible depth of 312 metres.
If you've ever had a close look at the bottom topography (know as bathymetry) around the world, or even at your local surf break, you'll see that the ocean floor rises up from depths of kilometres to the continental shelf which sits at about 100-150m before then grading slower to the land.
Small features like canyons from ancient river flows lead towards certain regions and these are what start to influence long-period swells well before we see them approaching the nearshore zone.
With long-period swells 'feeling' the ocean floor at depths to 200 to 300m one can easily visualise how a canyon at this depth would focus a swell towards one part of the coast, while taking the energy away from other locations.
This is why Nazare is such a large and unique wave. North-west swells follow a deep canyon that cuts 700-1000m deep through the much shallower surrounding shelf, ending at Praia da Nazare. Besides allowing less swell dissipation due to bottom friction it also sees long-period swells bend in on themselves as the deepwater energy travels faster than that on the shelf, creating monstrous wedges.
We don't have quite as pronounced setups on the East Coast, but there are certain locations that perform very well on these long-period swells compared to other similarly oriented zones.
Now that we've got you thinking about what's happening below the ocean surface, there's one more thing to consider.
Not all long-period swells follow the same path. If a 16 scond swell feels the bottom at 200m, and a 20 second swell at 312m, they're going to be steered by different bathymetric features. The best way to visualise this is to imagine the depth contours as highways; a 20 second dead south swell will be diverted to a slightly different region than a 16 second dead south swell as it will feel the ocean floor earlier.
Throw in a tweak in direction and it complicates the matter even further.
This is why under these long-period swell events we see the coast doing funky things, and we simply don't get enough of these swells to ascertain the patterns. Without running hi-res modelling going through each iteration of swell period and direction, it's still nearly impossible to pin down where the go to zones are.
There are a few reliable locations, but as we've seen before, they can also completely miss the swell, while another location just up the coast magnifies all the swell energy. This doesn't just apply to south swells running perpendicular to the East Coast (though these are where we see the greatest effects), straight east groundswells from New Zealand are known to focus north and south of what would be perceived as a perfect bullseye right in the middle. One such east groundswell years ago was 6ft in Newcastle and Wollongong, but Sydney, smack bang in the middle, only reached 2-3ft.
Over the coming week the East Coast will receive multiple long-period southerly swell pulses, each from different directions and with different periods, so it will be great to watch and see this happening in real time, especially with tomorrow afternoon's swell, coming in with peak periods of 18 seconds.
And if there's a take home message from this aricle, it's to keep an open mind and be prepared to travel.