Saturday, 18 September 2010

Swell

The worlds bravest, or silliest fisherman, take your pick...
There was a prediction yesterday for a very strong pulse of swell to come through Sydney after lunch, and considering the wave buoy off Tasmania's west coast had recorded an all time high of 18 metres from the same system, there was reason to think things were going to get a bit bigger than usual.
Rob & I decided to take the afternoon off & head out around the Sydney cliffs to see what rolled through. Although not a 'seat-of-the-pants' experience, it was an afternoon when on more than one occasion, I felt very small. The swell buoy off Manly showed wave heights between 4-6m, with wave periods from 12-16 seconds, and that was pretty much what we figured on the water. Most sets were coming through at about 3m, with the occasionally very big, very long wavelength. There was no wind to speak of & virtually no sea, so we got our kicks riding the 2m rebound that was thrown up off the cliffs between North Head & Blue Fish Pt. As the swell was so southerly, even that effect was negated somewhat, and it did feel at times like we were paddling among apartment blocks moving at 35 knots. 
The photo above shows a couple of dudes in an old timber clipper out trolling through any whitewater they could find. More than once as we watched them crawling up the cliffline, the place they has just been copped a high sig. set which obliterated everything. If they'd been there a minute earlier it would have a very entertaining spectacle. We came to the conclusion that these guys either had no idea, or they new exactly what they were doing, or they were pissed.... 
Days like this remind you of the sheer power of the ocean. As a kayaker these sorts of swell & wavelength conditions make virtually our entire local coast unlandable. They turn dormant shallows or bomboras into boat killers, and heighten your awareness of just how thin the line can be in the sea. Something often overlooked is the fact that waves as big as this can break according to the length of the wave period, so sometimes (rarely thankfully) even the deep water offers no sanctuary. They need to be a bit bigger than this around the deep water off Sydney for this to happen, but it doesn't make it any less intimidating when a wave of this size starts to look a bit steeper....You know there's no way it will break, but it's hard not to think about what would happen if it did.
Turning & heading back into the swell was comforting, as Rob said, at least running this way you can say you saw the one that got you...
There are a couple of snaps of the day below.
Rob behind a yacht that has disappeared behind a crest

Mercer's head blighting the skyline

On the way back in we found a solid lump of tree trunk floating around the ferry lanes, & figured we should move it to shore. A harbour gin palace would have come a mighty cropper if it had collided with this floating detritus. Then again, maybe one less gin palace wouldn't be such a bad thing. Here's Rob with the offending article under tow. He's not that fast when he's towing a big log....

2 comments:

  1. That's interesting about the wave period affecting the shape of the wave Mark. A quote from another Oceanography website "At a depth of half its wave length, the rounded waves start to rise and their crests become shorter while their troughs lengthen." So the waves off Tassie's West Coast had a 17 second period and peaked at 18.4 metres in 100 metres of water with a wavelength of about 470 metres. And they apparently would be travelling at close to 100 kph!! I bet there weren't any fishing boats out there.......

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  2. Boy, that Cape Sorrell reading was off the chart, its hard to imagine what a sea like that would look like. Yesterday was big, but apart from the odd big set producing rebound quite serene. Nothing like west coast Tassie......

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