Sorry if this sounds confusing but,
At what stage does the size of a swells height or the period start overtaking one another.
Eg. If a swell has a reading of 8ft @ 18seconds, when will that start being bigger than a swell with a bigger height but smaller period, eg. 10ft @ 15 seconds.
Or vice versa.
Sorry if it's confusing but it's hard to get across what I'm actually trying to say!
Think I know what your asking . But you might be trying to ask something different than what im thinking. Drop another example to whittle it some more please
"When will it start being bigger" do u mean open ocean or breaking wave ?
Ah sorry should of clarified that. I meant the size of the breaking wave. I know a longer period swell will be "thicker" when it breaks, but I'm talking more height.
So using bells as an example, if I go there one day and the forecast is 8ft @ 18 secs and then the next day it's 10ft @ 15 secs, which day will have a bigger breaking wave height.
Or does it also depend on the bathymetry of each spot as well as other factors?
Just wondering if there's a general ratio of height and period where one will overtake the other
Bathymetry . Learn about its effects on deepwater reefs . Read "ghost wave" book for fascinating story about sean collins discoveries . Bells will have to be explained by someone familiar with the mechanics but then they shouldn't publish it on here
Yeah cool I'll have a look for it.
Bells was just an example because it was the first wave that came to mind, just pretend I said Waimea.
There is a correlation between swell height and period, but it also depends on the surf location.
It depends on bathymetry as Camel said, with tropical islands offering more size from the same open ocean swell than locations with a continental shelf.
As you said the larger the period, the large the surf expected from a similar sized swell.
For places like Indo etc I'd say very roughly any swell at 13/14s = it's height. If the period is higher, it will be bigger, and if smaller, then smaller.
Ie 2m @ 13/14s = 6ft surf.
But 2m @ 18s = 8ft surf.
If you had these open ocean swells moving in towards the Mornington Peninsula from outside Bass Strait you could expect less size due to swell attenuation from the continental shelf.
But if the size directly off the coast was as in the examples above, similar sized surf would be seen across magnets (hope that makes sense).
also depends on the travel distance from swell source to surf spot. Bigger period less decay.
but locally generated swells on the east coast like cyclones and ECL's suffer very little swell decay.
If your referencing the same swell , from one day to the next . Then the second day will most likely be bigger . Down here there will often be two swells in the water , its just the bathymetry and refraction will often make it look like one .
We'll pretend the first day was the fore runners with higher period and less height . As a general rule i would say that the peak swell height of initial fore runners will be alot smaller than the peak height/power of the swell at roughly 20-25 % less period as the swell fills in . But then as the storm producing front or even the left over energy that might re invigorate closer to the coast advances east then the swell angle may shift more south and open up the coast to more energy . Each spot will also have significant local effects one of which in Bass Strait is tidal flow . The more you look into it the less enjoyment .;-)
You're confusing things though Southey.
If the swell is pure and not mixed in with any other swell, the peak will be the peak of the height and greatest period.
You're getting into when the wave models mix the new long-period forerunners with the existing swell, so it looks like it's overforecasting the size, when the most size is due the next day.
Our model does fairly well at splitting the different swell trains and showing the true peak in size compared to sites like Buoyweather and windguru.
But I don't look at yours and others models that much . Infact only in the last year or two had i bothered . When i do though i often see how accurate they are . ;-)
Your right , but it doesn't address his question , unless he's talking about the same swell comparitive over two consecutive days .?
I think he's trying to compare different peaks in different swells . At which each has totally different directions , longevity , residual .etc etc .
To add to the confusion, I will add my understanding of goofyfoot's question.
I think goofyfoot is asking if there is a simple shoaling coefficient that applies to wave period.
The shoaling coefficient being the difference between the deep water wave height and the shallow water breaking wave height.
For example, that an 18 second period swell will have a standard shoaling coefficient of 1.5, therefore a 2 metre deep water wave will break as a 3 metre wave.
And a 13 second swell will have a standard shoaling coefficient of 1, therefore a 2 metre deep water wave will break as a 2 metre wave.
Therefore, goofyfoot could say, as long as the 13 second deep water wave's height is more than 1.5 times the 18 second deep water wave's height, then the 13 sec swell will have a taller breaking wave than the 18 sec swell.
The answer I guess is: no goofyfoot, there is no standard shoaling coefficient. It would vary according to the local bathymetry, swell direction etc.
After all that I think Wally has come closest to what I was getting at!
It was a confusing question with a lot of variables.
Thanks for the replies y'all!
3m @ 20 sec will be a 20ft wave at breaks like jaws , todos , mavs etc .
In oz in a place where there is not any special bathymetry you might have a 7m @ 12 sec and barely see a 20ft wave .
Look at " wave attenuation due to bottom friction" to understand how big periods 16-25s are not special in shallow coastal bathymetry . The waves might be way better & bigger when the periods 12sec . Its really easy to understand although 98% of surfers dont have this knowledge it seems .
The only time a big period swell will make big waves is in deepwater breaks with large reef domes . If the coast you surf has a major shelf like the mp & sa then its low period swells that will give you taller waves
Yeah Wally is getting at what I was trying to explain it looks.
Sorry but wallys attempt seems earnest but it means nothing, to me cos you have bathymetry that is the real factor . There are so many varieties of surf break bathymetry that is very complex . Gf you really have to talk in metres in Australia because our buoys measure metres and thats the easiest way to cross check each and every swell event . The forecast gives an indication of what to expect but hindcasting the swell buoy is how you get accuracy to work out the real period & size . You have 3 buoys close by to measure from , sa , tas & pnp , then u just study it yourself year by year u will know more about how big each break is due to the swell signature
Thanks caml. It's such a complex thing. (For me anyway)
I'll start studying the reading of each decent swell a bit more and see if I can get my head around it a bit more. I suppose I'm like a lot of people who just use the luxury of the forecasts on here and not really do too much of my own observations. This winter I'll take note!