Swellnet Dispatch Waves Misbehavin': Why the size varied during the East Coast swell

In: Swellnet Dispatch by Stu Nettle 24 Comments Mon 20th Sep '10

Tags: period , refraction , radial spread , big waves , central coast , shellharbour , Craig Brokensha , Cronulla

The weekends incredible swell event on the East Coast was a striking example of how swell period effects wave size.

Most surfers already know how the period - the time between consecutive wave crests - effects the power and size of a swell. In short: the greater the period, the more energy the swell train carries and the more powerful the swell is. The visible manifestation is long, well-defined swell lines that increase greatly in size when they 'feel' the shoreline. They also have the ability to refract (bending: as in around a headland or point) a greater amount when they feel the bottom then lesser period swells.

This is due to the fact that long-period swells carry much of their energy deep below the ocean surface and, as they approach shallow water, they feel the ocean bottom earlier than short-period swells.

The swell that hit the East Coast had its genesis in a storm near the Antarctic ice shelf. By the time it reached the East Coast it had travelled over 3000 kilometres and the period had drawn out over 15 seconds. Periods of this magnitude are very rarely seen on the East Coast and thus the confounding nature of the swell caught some people by surprise.

On Friday afternoon reports of twelve foot plus waves came in from certain Central Coast, Cronulla and Shellharbour-region waves. See todays Wave of the Day (and stay tuned for tomorrows!) for proof of this. Yet, during the same period beaches that often get swell, and faced the same direction as those listed above, barely got over six feet. What was happening?

Well, here's an analogy: If you've ever been to, or seen photos of, Sunset Beach in Hawaii you may have noticed that the peak at Sunset can be ten feet while the reef at Kammie's - not 200 metres away - can be half that size, and the channel seperating them can have no waves except a chest-high shorebreak. The size difference in a short stretch of coast with equal access to swell is startling.

What is happening is that the long-period swells hitting the North Shore of Hawaii feel the offshore reefs beyond the wave zones and the swell train refracts and focuses its energy toward different parts of the coast. The result is a magnification of swell energy in some areas and a paucity of it in others. In this case, Sunset Beach receives an abundance of energy while Kammie's doesn't. It all depends on the shape and depth of the reefs offshore.

Meanwhile, back home, we had a swell of Hawaiian-style period hitting the coast and behaving in a way we were unfamiliar with. The waves were feeling the ocean floor further out than is usual. For those people wanting to make use of large waves on the East Coast there were valuable lessons to be learnt, and it would serve those people well to remember how their local big wave spot behaved during this swell.

Yet there were also lessons to be learnt for the average surfer. Since the weekend there have been a few comments appearing on websites and Facebook pages that the swell was a hoax. One can only assume that the people making those claims visited beaches that didn't receive the full extent of the swell. It's important to understand that the forecast had to cover the whole coast, yet, as I've stated above, certain regions behaved very differently.

Obviously this swell wasn't a hoax and Swellnet forecaster, Craig Brokensha, got the numbers and timing correct.

Also, there was much media hype over the 18.4 metre wave that passed the Cape Sorell wave buoy on Thursday. Because that wave and the East Coast swell had a common source there was an expectation that we would see a far larger swell. It's important to note the waves that passed Cape Sorell were in a very different stage of evolution when that wave was recorded. Cape Sorell was very close to the source and completely exposed to the full extent of the storm. On the other hand, the swell that hit us had two days to calm down, draw out the period, and depreciate in size. And, very importantly, the swell was heading north-easterly up the Tasman Sea toward Fiji. The waves we saw had spread out radially toward the East Coast and lost some of their energy in the process.

If anyone has any questions about the swell or the characteristics that were on display (refraction, period, radial spread etc.) please type them below and our East Coast forecaster, Craig Brokensha, will answer them for you.//STUART NETTLE

Frontpage image for this story taken by Matt Burgess

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