Does anyone posses any info about radial spread of swell? Such things like rates of spread, how far after leaving the fetch does spread start to happen? Does spread change with swell period? How much energy does spread rob from swell coming from the source direction?

This question wouldn't possibly have anything to do with the likely ECL in the Tasman this weekend, would it?

yup.

Just trying to work out the mechanics of it. I know theres quite a decernable difference in size say Caloundra compared to Sunshine. Just dont know how to calculate it. (maybe not the best example as Caloundra is semi blocked by Morton)

More so for lows that hover off the west coast of NZ. Positioning a little further north or south of say New Plymouth, can make a lot of difference.

Also for trade wind swell. Ive noticed it points quite ESE and we only seem to get part of the radial spread and not the main swell.

99% sure it has to do with the period of the swell . As this will influence the speed of said swell. And i wouldn't believe you would see much of this in Periods below 13 secs . Obviously fetch will have to influence the period more so than longevity . At a rough guess i would say that radial spread would've come into any noticable effect untill periods approached 12 sec's and the distance from the swell source to differing ( low winds ) were atleast 500 but more likely 800-1000 KM's . And i think we've been down this path before , but bathymetry really does have a massive effect on this especially in this case as you are looking at , as the swell thats travelling pretty much parallel to the coast .
Again Radials will most likely propagate in open ocean , and only through and also helped by an opposing trade or existing sea state , that has been overpowered by this new swell . And i mean an abating wind . I'm not 100 % sure on local bathymetry there , but I would at a guess say that the submerged River beds ( out where they used run when the oceans where lower ) entering into the 100-200 m drop offs will probably have more of an influence than actual direction block of an island or point .
You'll find that most literature on this will only give an indication of open/ deep water ( >500m ) .
Best ways is to observe and take notes on differing swell directions/ periods effecting such spots . Don't forget to note tides , seasonal currents , and even recent rainfall .
You tend to take these things for granted ( growing up and knowing a coast ) , untill you decide to spend some time elsewhere or even relocate permanently . Unfortunately all those idiosyncracies of your original stomping grounds , are taken for granted . I kow one thing is for sure . The more fickle or variable your original place of learning the ropes , the easier it is to be open to possible influences when travelling or setting up elsewhere .

Talk to fisho's that surf or used to surf that area , always golden . That and the cagey old bastards of your knew local , but generally this involves getting them drunk and pumping up their self esteem a bit .
Having a fridge in you car stocked with ice cold cans next time its epic , and then offering a few around post surf is always another way . This particularly tends to work in even the heaviest of localised regions ( SA / WA ) , these two more than not coz its usually far from a drive thru . lol

thanks SO. Yeah, any literiture would be useful.

I guess the other thing to consider is lateral spread of energy as energy travels the path of least resistance. Easier to travel sideways than head on. This will obviously be after it has left the confines of the source.

I always thought bathometery played a bigger factor in refraction and it was more friction (which is directly related to swell size and energy/period) of the outer ends of the swell train that relates to radial and lateral spread. i.e. less energy at the outer ends, but with the same friction as source direction, this makes the swell bend and stretch laterally like a rubber band.

All I can say is that the Sunny Coast is not the place to be when there's a near shore ECL in the lower Tasman Sea!!!

Looking at the latest GFS model, I'd be thinking the Sunny Coast won't see much of the swell from this ECL until the low moves well offshore (even considering radial spreading). Monday/Tuesday look to be your best bet for any real surfable waves from this girl.

And of course, head to your swell magnets which accentuate the S'ly groundswells up there.

But in the meantime.....tomorrow through to Friday morning look very fun up your way for some waves!!!

yeah, I image 1-3ft at the most(depending on location).

Finally got a new camera after 2 1/2 months without one. Tomorrow through Friday is looking excellent especially Friday. Just hope the rain stays away.

found this useful info... with gibberish equations

http://books.google.com.au/books?id=hupZ...epage&q&f=false

ok and this is very useful. Turns out deep water long period swells have very little "angular spread"

http://chl.erdc.usace.army.mil/library/publications/chetn/pdf/cetn-i-28.pdf

TC P ,

Take note of the section in the first one that talks of Great circle path . Only swell trains of significant strength will have this influence .

Very little ( as pointed out in the second and first link ) indepth analysis has been carried out on open ocean swells . Ie the shear scale is too hard to replicate accurately . Most analysis's in these /all literature is gathered from experiments within lakes . And this as such , which i believe was touched on in the first piece has little or no experience of currents on swell . And this along with large scale continental refraction has alot to do with NSW's great circle path swells that tend be written up under the pretense of Radial spread influences .

Where in fact the North /South flowing East Coast Current has alot to do with this along with Bathymetrics in the 500 -1000M line around the South East Cape of Tassie . Another influence would have to be , that the lower Tasman would tend to be in a Swell shadow or basicly ( at rest ) and this has anything significant with any sort of W in its direction radiate up into relatively tranquil Tasman , but only with the current running down and intermixing on the shallower Bathymetry of the Sth East Tasman Shelf .

Again the sea state in the path of this Radial spread swell needs to be little to nothing and perhaps with a decaying Sth East to East Sth east component , or the eastern flank of a High Pressure system leaving the continent into the Tasman . Unfortunately for your scenario A: the swell will not get to the swell energy heights that it needs to have underlying smaller frequency waves that have the energy to make a beyond 90 degree swell radial swing . Infact I'm reasonably confident that anything originating in the Tasman is incapable of Projecting around beyou 20-30 degrees .
So your position ( anything North of D'Bah/ the islands ) is probably along way off seeing any sort of action from this direction , what is more probable is something radiating around the Nth Isle of New Zealand to your selves although alot of this would be lost in any residual E - ENE swell that is running at the time . But to be sure it does exist , you'll see on certain days ( rarely ) rises of swell from these directions ( for there will be an unexpected increase for little observation on the charts ) . And this is where i expect that your finger being on the pulse and all this reading may come in handy .

yeah, anything out of the south really just goes straight past.

I see what you mean about west swell trains refracting up into settled Tasman waters. Ive noticed recently a good run of swell at Newcastle consistently being 3ft when the rest of the southern nsw coast is flat.

I came across this site http://www.meted.ucar.edu/marine/mod3_wlc_propdis/print.htm#32 (you need to join to view) but this is bar none the most indepth publication I have been able to find on the mechanics of swell. what your saying is true though. Most stuff I could find previously was mostly based on fetch's across great lakes.

you mention north/south current on the east coast. Do the eddies that spin off the eastern side of the current help with refraction or am I missing the point?

Im now on the hunt for info about currents and its effects on swell trains, something I know very little about. Just used to rely on the old rule of thumb "incoming tide push" but then again like you said above, different from my old stomping ground so I need to keep my eyes and ears peeled and a few cold ones in the car.

Newcastle is also one the most exposed bits of the NSW coast as far as southerly aspects go. Seal rocks is the other south swell magnet. Its great in summer because Birubi (north end of stockton) is offshore in a N to NE wind as it faces dead south.

I have bodyboarded 6ft days there, while pretty much everywhere else in the state is flat.