Swellnet Dispatch Onwards and Upwards: Notes from an Incredible Swell

In: Swellnet Dispatch by Craig Brokensha 18 Comments Thu 7th Jun '12

Tags: forecast , brokensha , matson , buoy , captured fetch , East Coast Low , ECL

click to enlarge

click to enlarge

click to enlarge

Wednesday's oversized East Coast swell produced one of the more memorable wave traces across the MHL network of waverider buoys.

As per the image to the right, you can see that the maximum wave heights at the Sydney buoy shot up from approximately 3 metres at 5pm, to over 12 metres by 6pm. This is an astonishing 9 metre increase in one hour, or 1.5 metres every ten minutes. A horrifying place to be if you were in a boat, that's for sure.

Incredibly, the trend replicated itself right along the eastern seaboard. The waverider buoy at Crowdy Head recorded the exact same 'J-curve' at about 1am, whilst the buoy at Byron Bay recorded a similar leap in size at 10am. Every location saw maximum wave heights increase in the order of 8-9m within the space of about an hour or so.

And this is where things get interesting. If we ballpark the travel distance between the Sydney, Crowdy Head and Byron Bay buoys (relative to the regional swell direction, of course), and estimate the swell's speed by its period, we find that the swell travelled north at approximately 32km/hr.

Also, if we ballpark the speed of the southerly change associated with this swell event (which was itself the byproduct of an East Coast Low), we also find that the change apparently tracked northwards at approximately 35km/hr.

This is enough evidence to suggest that a 'captured fetch' may have been responsible for the very large and rapid increase in wave heights. A captured fetch is a phenomena where a storm moves in the same direction and at the same speed as the swell it's generating. This significantly amplifies wave heights compared to what we'd typically expect from a storm displaying equal strength, but with a less favourable forward track.

Because the swell increase in Sydney waters occurred during the cover of darkness, we're unable to verify the size translation at the beach. However, we've received many reports from Northern NSW and SE Qld - where the swell arrived throughout the day - from surfers who were in the water as wave heights increased from 1ft to 6-8ft within an hour or two.

Despite all the amazing buoy readings, this was not a quality swell in surfers terms and only a handfull of places were rideable. The reason for that is another article in itself. //BEN MATSON - CRAIG BROKENSHA

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