Period and refraction
Good question.
No shorter period swells don't have as much energy stored below the ocean surface and hence don't refract well at all.
Longer period swells have more energy stored in the water column and hence feel the ocean floor to deeper depths and refract in much better than shorter period swells. The larger the swell period the greater the refraction.
There's also other steering affects by under water canyons and ridges which may focus a swell towards a certain area before it then refracts in to shore.
So say for a short period S'ly swell pushing up the NSW coast it'll struggle to get into any location that doesn't face south (ie east facing beaches and southern corners). A longer period swell will wrap in a lot more and provide much more size across these more protected spots.
Thanks Craig, that is what I had always believed as well but after living in Singapore for a while and watching super short period swells in Malaysia and Vietnam refract quite dramatically it made me start thinking about some other places back in West Oz that I used to surf a fair bit.
I started thinking maybe the larger period swells have so much grip on the bottom that they cant bend as quickly? Whereas the shorter period swells are more flexible.
W.A. is not short on for long period ground swells and I have spent endless annoyed days watching bulk swell file straight past without even so much as a hint of refraction into where I wanted to surf but then on junkier smaller swell the energy seems to get in a lot better even from the same direction.
Everything Craig said .
But i'll add .
Generally speaking longer period swells will refract more , and depending on how deep the near shore waters are then they will always refract more in degrees than short period .
But like a large ship their turning circle is longer/ larger .
Hence most the world class waves are the second , third or just some distance from a deviation in coastal angle / regional point or cape .
With shallow offshore bathymetry , strong crosshore winds with obligatory short period swell .
Sometimes protected nooks ( the first point ) can work better , and often short waves will work there .
The tighter refracting radius of such swells will only see wave s concentrated on that first nook in the lee of the exposed first point or cape .
None of that is what i would call learned or able to be backed up by science , just purely observational from searching new areas .
That is interesting Reecen, but this may be because the longer period swell is being steered or focussed away at those particular numbers, while the shorter-period swell just gets straight in.
A few locations seem to do this, along with the South Australia Mid Coast, 10-14s swells seem to penetrate into the gulf much easier than a longer-period groundswell and this may be due to the longer-period swell feeling the bottom a lot earlier and losing energy due to bottom friction when the lower period swell passes straight on in.
Definitely an area for further study, but each location has to be taken on its own merits with no blanket rule applied.
Yes as Southey pointed out as well.
How about if you have a location where there's a wide bay and refraction draws the incoming swells to either side of that bay (lets pretend they are pumping Left and Right points). the beach in the centre of that bay might make more of a short period swell than the points do (lets say the beach gets 4ft when the points are 2ft in a short period swell), whereas when a groundswell comes into that bay, the groundswell would bend into a u-shape due to bathymetry and relative to a short period swell, you might not get the same surf size ratio (ie: the points get 3ft and the beachie gets 3ft). Is that the theory?
Yeah yocal, that's about right.
But you'd need more size in the windswell to get surf up to the size of the groundswell even with the spreading of energy. Ie two same sized swells with different periods, the larger period swell would still offer the same if not more size in the middle than the lower period swell. All case by case scenarios though.
Many times they said its a lazy swell just becos the wind was light .then they said the swells moving fast when were surfing a short period slab
Could it also be about direction?
Could it be that short period swells are more likely to have a mix of swells from slightly different directions? while a longer period swell is more likely to be from one constant direction?
Short period swells may give the appearance they are refracting more, when it could just be the mixed swells hitting things at different angles?
Just an idea.
Yeah reecen if you take east coast aus this weekend, primary direction of groundswells is usually some degree W of S. Whereas ya windswells will usually have an E component as the the lows/fronts are not in the westerly wind belt.
Yeah?
ID ,
You've touched on a very important point ..... Onshore winds can and do hide some vital long period swells that could even be up to 45 degrees at variance from the short interval storm associated with the those near shore winds . Anything on the southern Australian shores , tends to see multiple swell trains form up into one , and so less of an impact where there would be a huge variance between any miserly local wind swell and the dominant swell that's usually a much longer interval .
craig, just wondering how you guys determine period for your surf forecasts? I know you calibrate stuff to coasts but I would have thought stuff like period would be fed straight in from the models you use. I've noticed for today you have period going up to 16 and 17 seconds for uluwatu and sumatra but other sites have it going up to 19 and 20 seconds, maybe splitting hairs but seems to make a big difference. observations show other sites to be more accurate today
Indo dreaming your idea is actually a fact as far as I learned . Re More variety of directions in short periods
caml wrote:Indo dreaming your idea is actually a fact as far as I learned . Re More variety of directions in short periods
Spot on. Here in NSW that's verified on the spectral buoy data where lower period swells can be spread across a quadrant - and also, it should be said a range or periods, i.e a mix of 5 to 9 second waves - while higher period swells are more specific in their direction and narrower in the period range. Uniform in direction, uniform in period.
What is the relationship with period on refraction? Will a short period swell refract quicker then a long period swell?
I don't know if it is observation or imagination but it seems that larger period swells have a higher tendency to march straight past the books and crannys and shorter period swells tend to wrap tighter from the same direction.