Do strong local offshore winds affect the size of the surf?

thermalben's picture
thermalben started the topic in Sunday, 6 Aug 2017 at 7:22am

*note: the first dozen or so comments here are copied from a discussion in a recent Victorian Forecaster Notes thread which, for posterity, I have transferred to a seperate forum thread - as it's a great discussion. Any comments before 7:30am Sunday 6/8 have a later timestamp than when initially posted (12-18 hours prior) but are in the correct sequence. 

Vic Local's picture
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Vic Local commented Sunday, 6 Aug 2017 at 7:23am

I wouldn't be putting a size on it and neither should you guys considering how unpredictable the weather pattern is. BOM is calling for gale force offshores. That alone makes height predictions dubious at best. This looks like a really fragile swell that's not going to handle high tides, strong offshores, and slight change in directions.

"angry online, smiley in the brine"

thermalben's picture
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thermalben commented Sunday, 6 Aug 2017 at 7:24am

Haha!

"We have decided not to estimate the surf size this weekend, given how unpredictable the weather pattern is".

Yeah, that'll fly. Would love to see the BOM roll that one out too.

BTW, the strength of the local offshore wind has nothing to do with the size of the surf.

Vic Local's picture
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Vic Local commented Sunday, 6 Aug 2017 at 7:24am

"BTW, the strength of the local offshore wind has nothing to do with the size of the surf." Really Ben. Do you seriously believe that?

"angry online, smiley in the brine"

thermalben's picture
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thermalben commented Sunday, 6 Aug 2017 at 7:24am

Yes, I believe the science.

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thermalben commented Sunday, 6 Aug 2017 at 7:25am

"The effect of opposing winds or wave energy on propagating swell is surprisingly small, especially if swell steepness is small. If a swell moving north encounters a north wind and waves moving south, only negligible changes in swell height and period will occur. The wave interactions between the swell and opposing waves are so small that errors associated with neglecting them are much smaller than errors that result from poorly defined wind speeds. These effects are, for the most part, ignored in wave/swell computations."

Source: MetEd department of NCAR, "a federally funded research and development center sponsored by the National Science Foundation and devoted to service, research and education in the atmospheric and related sciences. NCAR is managed by the nonprofit University Corporation for Atmospheric Research, a consortium of more than 100 universities granting degrees in atmospheric and related science."

Nick Bone's picture
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Nick Bone commented Sunday, 6 Aug 2017 at 7:25am

Just out of curiosity, why is that? Simply because swell has more energy in it than a wind?

Could a 40knot offshore squash a short period swell coming in from the opposite direction (i.e Home. Strong North with short period SW swell?)

I am the bone

Vic Local's picture
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Vic Local commented Sunday, 6 Aug 2017 at 7:25am

So last weekend when the swell came in a day later than you predicted had nothing to do with the howling offshores? And howling winds wrapping around a low don't effect swell direction at all?
I may not go in for all that book learning, but I'm just smart enough to know that swells can be kept out to sea on the Surf Coast by 70-90km northerlies. But hey 20 + years of watching the waves down here counts for nothing eh?

"angry online, smiley in the brine"

pigdog's picture
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pigdog commented Sunday, 6 Aug 2017 at 7:26am

was "vic local" smart enough to invest in the property market 20 years ago down there….

Vic Local's picture
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Vic Local commented Sunday, 6 Aug 2017 at 7:26am

Yes. pigdog. So technically I'm still a blow in.

"angry online, smiley in the brine"

carpetman's picture
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carpetman commented Sunday, 6 Aug 2017 at 7:26am

Hey Ben, what are the contributing factors to swell decay over any given distance? From my understanding decay is quite complex and so a constant is applied as there are many, but only minor, impacts. But that is in the open ocean. As a wave stands up, and as you've highlighted above, steepness is likely to impact this. I'm not arguing with the science just trying to get a better understanding of nearshore.

carpetman's picture
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carpetman commented Sunday, 6 Aug 2017 at 7:26am

Also, why does the opposite apply when a storm moves over an already agitated sea state?

crustt's picture
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crustt commented Sunday, 6 Aug 2017 at 7:27am

I'll side with Ben, using the logic that wind doesn't travel under water, can't see how it could affect a good groundswell.

carpetman's picture
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carpetman commented Sunday, 6 Aug 2017 at 7:27am

Also, my observations from the surfcoast on a howling NW make me think there are significant impacts near shore. Bass straight is shallow and waves will begin steepen up a long way out from shore. They tend to feel much weaker in this scenario. But maybe that's just because of the wind pushing strongly up the face?

indo-dreaming's picture
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indo-dreaming commented Sunday, 6 Aug 2017 at 7:28am

I know the theory and science says swells can't be affected by strong offshore's, or even blown in by an onshore.

But in my experience i swear it happens.

And at some spots in a strong offshore waves often feel like they have lost energy compared to the same swell period with a light offshore or light onshore or no wind, IMO the effect is worse the lower the swell period.

goofyfoot's picture
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goofyfoot commented Sunday, 6 Aug 2017 at 7:28am

You don't think it's the low period swell making it feel like the waves don't have much energy?

Wharfjunkie's picture
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Wharfjunkie commented Sunday, 6 Aug 2017 at 7:28am

Agree with ID a exceptionally strong offshore tends to feel as if the power of the wave has been diminished. Wonder if the research ThermalBen referred to was researched well offshore.

Vic Local's picture
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Vic Local commented Sunday, 6 Aug 2017 at 7:28am

I think there's pretty good anecdotal evidence that a very strong offshore will keep a swell out or significantly delay the arrival of a swell. How often do you see a swell build from flat in gale force northerlies or NWers?

"angry online, smiley in the brine"

thermalben's picture
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thermalben commented Sunday, 6 Aug 2017 at 7:29am

I'm happy to discuss and debate the specifics, but you have to be very careful argueing anecdotal evidence against emperical data collection. 

There are complex, valid reasons why swell events appear late, or end up smaller (or bigger) than expected. The main reason is because weather forecasting as a general service is very hard. Secondly, because marine forecasting has almost no observational data to work with, and thirdly because surf forecasting (a third generational forecast product) can see large variations in size between small stretches of coast, related to incremental changes in period and direction. So a swell event across a small stretch of coast can be spot on at one location and completely wrong at a neighbouring break.  

And lastly, surf forecasting is also unique in that we are forecasting "good" or "bad" surf days - not neccesarily a measure of wave heights, but more related to whether someone has a good session or not. So even if the surf forecast is correct, the perception of the surf conditions via the end user (and thus the perceived accuracy of the forecast) is also related to external influences like crowds, sand banks, tides, bluebottles, etc. 

So, for example, VL: what is your starting point when you say "a very strong offshore will keep a swell out or significantly delay the arrival of a swell"?

What's the evidence you have that the swell would have arrived, had there not been strong offshore winds?

indo-dreaming's picture
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indo-dreaming commented Sunday, 6 Aug 2017 at 10:32am

I think the effect happens to a much larger extent where waves move through a shallower water depth, for instance in in my area Kitty Miller bay is the classic example it gets super gutless when strong offshore and has much more push when no wind or light offshore or onshore even under the same swell period, other examples are any of the waves in the bay at Westernport on the island side and even more so the peninsula side.

Don't know if it delays a swell..i don't think id agree with that, i think it just weakens a swell a bit but don't expect it has as much effect on swells in deep water.

I agree it kind of goes against the science but i still swear it happens?

thermalben's picture
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thermalben commented Sunday, 6 Aug 2017 at 10:38am

That's a slightly different thing though (however, I think there are other factors at play here, rather than the wind being the sole contributor to an apparent size or energy loss).

VL's initial point was referencing swell events as a whole, rather than how swells respond at a specific beach level.

thermalben's picture
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thermalben commented Sunday, 6 Aug 2017 at 10:41am

Best way to test your theory ID (and this is hard, as we have no control over the wind) would be to compare surf "strength" during one swell event, if you're fortunate enough to see a rapid change in offshore wind strength during that session.

Of course, we'd need to rule out other factors such as tidal influence, phase of the swell, location choice etc. But it'd be a good start.

Nick Bone's picture
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Nick Bone commented Sunday, 6 Aug 2017 at 10:56am

Specifically talking kitty millers. Do you think that because your effectively surfing a far way out from shore, that a strong offshore has more surface to gain momentum or something in that ball park? Kitty Millers, like out back of Sorrento are unique in the sense that while they're in a bay of sorts, there actually far out to sea. Out back sorra is a no go zone pretty much regardless the of the sand quality during a strong offshore.

I am the bone

Clam's picture
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Clam commented Sunday, 6 Aug 2017 at 11:14am

Theres no science behind yet another old wives tale .

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Clam commented Sunday, 6 Aug 2017 at 2:04pm

Thermalben said ;
"and thirdly because surf forecasting (a third generational forecast product) can see large variations in size between small stretches of coast, related to incremental changes in period and direction. So a swell event across a small stretch of coast can be spot on at one location and completely wrong at a neighbouring break. "

Yeah ben .
swell events have a signature like a snowflake or dna . The key is in the signature imo .
What key unlocks Your wave .

Nick Bone's picture
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Nick Bone commented Sunday, 6 Aug 2017 at 12:47pm

Light SE and straight lines

I am the bone

Clam's picture
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Clam commented Sunday, 6 Aug 2017 at 2:05pm

"Light SE and straight lines"

Your missing the vital piece

eel's picture
eel's picture
eel commented Sunday, 6 Aug 2017 at 8:51pm

Bali's west coast blows offshore for pretty much 6 months of the year. It doesn't seem to stop the swell from coming on the west coast.

nathan666's picture
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nathan666 commented Monday, 7 Aug 2017 at 1:26pm

To be honest Ben, I was surfing the other morning at 13th from the crack of down, (sunday) the day you put photos up then about 10 guys all came down the steps in unison about 8:30 haha..
But in all seriousness, it was firing while the wind was lighter, consistent sets and bigger. The tide was on its way in, but from experience if the peak of the tide was at around 11, 9 oclock would normally see a little pulse in swell dying just before high until after high.
It basically lost size and dramatically lost consistency just after the NNW started howling.
My two cents from this exact swell

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 1:43pm

What time were you surfing 13th Sunday morning? Wind observations from Pt Lonsdale don't show any major strengthening trend, if anything there was a faint easing. 

Also, wind obs from Cape Otway show a slight easing trend between Saturday night and Sunday morning (keeping in mind CO is well exposed, and also at the entrance to Bass Strait, where the swells will start to wrap back into the Surf Coast), but in general a steady trend throughout the period.

Aireys Inlet had a significant easing between overnight Saturday and early Sunday morning, and then a minor re-strengthening mid-morning. Wind direction did back from the N to the NNW then eventually NW however. 

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 1:56pm

BTW, one key to this argument is: wind works on and above the surface. Waves work below the surface. In fact most of the energy is a considerable distance below the surface (i.e. the depth to which the energy of a 15 second swell period penetrates is around 576ft, or 178m). 

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nathan666 commented Monday, 7 Aug 2017 at 2:39pm

I got in the water just after 7.
That's interesting, because it definitely felt to me like it picked up, possibly just because it got more west in it there was less protection, and coinciding with the already west swell maybe this had an affect to blow it past even more?
To put it simple though, the swell just died in the ass around 9

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 2:58pm

Thing is, swells die in the arse. And they pick up. They fluctuate all the time, due to a couple of reasons we understand reasonably well (tides, location position, swell phase) and many reasons we don't understand well (specific characteristics of the parent storm, precise wind speeds, degree of capture fetch, length of active sea state, local currents etc etc).

More than likely you saw a temporary period of easing surf, which simply coincided during a period when the wind picked up locally.

Here's the Pt Nepean buoy trace from the last 48 hours - this is the closest location to 13th Beach and usually has a pretty strong correlation (trend wise, at least). Eyeballing the data and there's no discernible change from overnight Saturday through much of Sunday.

At a scientific level it just doesn't add up either. The swells you were riding when you first got in the water around 7am were generated thousands of kilometers away, and would have passed through all kinds of sea states within the Southern Ocean - no wind, a bit of wind, a lot of wind. From many directions too.

But swell energy (particularly long period swells) keeps on travelling through these disturbed sea states regardless of what it encounters - unless it's related to bathymetry.

So it doesn't make any sense that a swell that's travelled for a couple of thousand kays was producing perfectly acceptable surf at 13th Beach, until it interacted with a strong offshore breeze near the coastline - in which case the swell suddenly stopped in its tracks (where? Just offshore? Somewhere in the middle of Bass Strait? Further west of Cape Otway in the Southern Ocean?).

And then if the offshores were to suddenly abate, that the swell energy would pick up where it left off, and continue along as if nothing had happened.

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 3:04pm

Also interesting is the swell direction reported at the Pt Nepean buoy this weekend, under what was a pretty strong (regional) W'ly swell direction. For the most part, the buoy recorded anywhere from the SW thru' almost S; mainly S/SW.

Craig and I wrote an article over four years (!) ago which touched on the influences of the Pt Nepean swell direction:
https://www.swellnet.com/news/swellnet-analysis/2013/04/11/buoys-follow-...

crustt's picture
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crustt commented Monday, 7 Aug 2017 at 3:18pm

Maybe you need southey to explain it to them again! :))

Once was enough though, thanks.

nathan666's picture
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nathan666 commented Monday, 7 Aug 2017 at 3:58pm

Very interesting mate, I guess data doesn't lie does it!
Although looking at that chart, at around the time I mentioned, it did take a pretty sharp drop in size, plus an increase in period which I'd think be exactly why it seemed to die in the ass when I was out there.
I was never for, or against your subject, just very curious as to add in to the topic to get to the bottom of it as I was there experiencing it first hand. Plus I and am also very interested in weather, swell and wind predictions.
One thing I know is we will never be able to predict with 100% accuracy how swells are going to act at each beach until eye ball it at the time.
Hence nothing beats a drive down to the beach at first light to check it.
I'll have a look at that article.
Cheers

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 4:08pm

"it did take a pretty sharp drop in size, plus an increase in period"

I don't see any drop in size (top graph) and very little change in period (bottom graph) - though significant period did ease during this period.. not sure how you're seeing the opposite?

The drop in significant period - or Tsig - around that time is indeed related to the wind - that's because Tsig is the average period of the highest 1/3 of all recorded waves. So, it can become contaminated by windswell, especially during inconsistent swell events.

This occurs when local wind speed picks up, generating a large percentage of new low period windswell.

See the image below - this is the recorded wind speed and direction at Point Lonsdale through the same time period - the circled area just before midnight on Saturday was averaging 25kts (gusting almost 40kts), which coincided with an even larger temporary drop in swell period.

When looking at buoy data you have to be careful not to cherry pick specific data points, and instead look at broad overall trends. 

carpetman's picture
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carpetman commented Monday, 7 Aug 2017 at 4:21pm

I'm only talking nearshore. Most of the energy is below the surface until the wave breaks - or is about to break. Swell steepness is no longer small at this stage, and due to the low gradient of the ocean floor around some areas of Victoria waves can stand up for a long period of time before finally pitching.

All your points are fine for offshore but like everyone else here its nearshore we're interested in. Surely there's a paper somewhere that proves the theory for nearshore?

nathan666's picture
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nathan666 commented Monday, 7 Aug 2017 at 4:37pm

I was also talking about the peak wave heights, not significant. Maybe I am reading the graph wrong, because its pretty small on my screen.
To the left of the box you drew, the max wave heights were much closer to 3m, while the peak period was close to 15 seconds. The peaks then dropped to 2.5m with 18s period peaks. What this says to me is the sets lost size, and frequency..?
Now because when I surf, no matter how big it is (in my own limits) I wait for the best and biggest set waves, which in my eyes would be the peak waves on a graph, not the average dribble in between.
But I haven't been able to read a graph so I guess that's wrong, just my logic.

thermalben's picture
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thermalben commented Monday, 7 Aug 2017 at 4:50pm

CM - I'm not sure what has been written in this field, but I'll look for some papers to see if there's anything around.

Nathan - my point is, don't look at the data over short time scales (i.e. half hourly to half hour). Swells simply don't behave linearly in this fashion. 

Clam's picture
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Clam commented Monday, 7 Aug 2017 at 5:01pm

You have to ride an alternate craft . simple

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Clam commented Monday, 7 Aug 2017 at 5:15pm

Quite a lot of water in the lip of the wave is actually removed whilst breaking against headwinds .
Thats the opposite situation of an onshore tailwind which would add to the lip.
Wind resistance is the enemy because its harder to get speed standing on your board going into it