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Old 24-10-2016, 21:09   #1
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Dynamic Fetch

Hi Forum,

I'm presently enjoying some quality time out at 57N 174W. We have a force 10 storm happening here at the moment, with some gusts into the mid-60's.

After watching the sea go from 10 feet to 25 feet in a matter of a couple of hours, it got me to thinking about Dynamic Fetch. While I don't actually attribute our rapid increase in sea size to this phenomenon, it is a worthy topic for those who consider weather routing offshore.

Generally, DF can develop on the S/SW side of a low, the opposite in the Southern hemisphere, of course. If the low is moving at generally the same speed as the seas below, the stage is set for this to occur. I remember once being out in the Gulf of Alaska in December, and we saw seas (verified by buoy) in excess of 40 feet in just a 45 knot blow.

Anyway, we're just out here laying to waiting for the worst of this one to pass by, it's too dangerous out on deck for the lads, and I got to thinking about this topic. It seemed appropriate.

For those of you routing passages in the higher latitudes, it's worth looking into if you haven't already.

Good sailing, TJ
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Old 24-10-2016, 21:33   #2
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Re: Dynamic Fetch

I can't speak directly to the terminology that you're using being 100% correct by definition. But as to the waves, perhaps what you're seeing is ones which are part of the weather system arriving in your location, after they've built over hundreds of miles in the open ocean?

As I can't see 15m waves building up rapidly from wind alone. Even in shallow coastal waters. Waves of that size aren't exactly common, & folks tend to keep tabs on them. Surfers especially, & the top guys will fly in from half way around the world just to ride such monsters when they're forecast.

Even 25'ers are huge waves, & it takes a bit of energy & time for them to form, non?
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Old 24-10-2016, 21:44   #3
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Re: Dynamic Fetch

This is sort of why I brought it up. Seas of that size indeed shouldn't happen out in deep water far from land from wind alone, but they do under certain circumstances. Far in excess of what the normal wind speed/duration/fetch calculations would indicate. I've been around for one or two of these events over my career up here, and it's quite stunning when it does happen.

I recall another one out in the Western Aleutians in November one year. This was after the phenomenon had been identified. The official forecast just gave up and called for seas 'greater than 45 feet'. I can confirm the accuracy of their forecast. We didn't get into the lee of Amchitka in time, and we got quite a pasting out of the deal.

Cruising world did an article about it a number of years ago, after it had been directly observed a few times by scientifically reliable instrumentation. The oceanographers were extremely surprised by what they were seeing. It didn't fit any known model, but there it was.

It's of some significant interest to weather geeks.

For my part, it's not Dynamic Fetch happening out here. It's just very, very, windy.

Just another day at the office...

TJ
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Old 25-10-2016, 03:41   #4
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Re: Dynamic Fetch

A facinating subject. I had something like this happen once, no so much a sudden increase in size, but the benign but large swell, quite suddenly became dangerous steep breakers as the winds built to 45 knots or so, much quicker than they should have.

I am not sure of the cause, prehaps a combination of dynamic fetch, and a backeddy in the current. Prehaps it was just the way the waves and swell interacted as they got to a critical size?

But it was scary how quickly things went from fun to scary. You could hear the breakers rolling like thunder in the distance. Of course it slowly got worse overnight, but the suddenness of the inital change has stuck with me.
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Old 25-10-2016, 04:22   #5
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Re: Dynamic Fetch

It's a very real phenomenon, and I guess that this is exactly what you are seeing.

I have noticed similar things in our waters.

It is caused by resonance, and the exaggeration of wave height happens at a particular system speed.

See: ftp://ftp.wmo.int/Documents/PublicWe...tations/H1.pdf

Pretty scary, actually.
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Old 25-10-2016, 04:35   #6
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Re: Dynamic Fetch

Also known as “Trapped Fetch Waves”
http://journals.ametsoc.org/doi/pdf/10.1175/WAF849.1

ftp://ftp.wmo.int/Documents/PublicWe...tations/H1.pdf
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Old 25-10-2016, 04:49   #7
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Re: Dynamic Fetch

^^ A good link Dockhead, thanks.

I hadn't heard of the critical importance of the exact system speeds being so crutical for extreme wave heights. It makes sense though.

So four terms seem to pop up, dynamic fetch, trapped fetch, resonant fetch and wave fronts. They all seems to be related, are they basically talking about the same thing, or are there subtle differences?

I guess the wave front is more about the point where the waves suddenly increase in height, and the others are more the mechanism that causes this?
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Old 25-10-2016, 05:38   #8
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Re: Dynamic Fetch

Quote:
Originally Posted by Snowpetrel View Post
... So four terms seem to pop up, dynamic fetch, trapped fetch, resonant fetch and wave fronts. They all seems to be related, are they basically talking about the same thing, or are there subtle differences? ...
Dynamic Fetch (DF) occurs when the generating area moves with the waves it generates. In a DF all waves moving in the same direction as the generating area will be amplified. But the waves experiencing the greatest amplification will be those whose group velocity is equal to the speed of the fetch. Since Cgis a function of period, DF results in selective amplification of waves having a specific period. This is a kind of resonance phenomenon based on the group velocity, so DF is sometimes called “group velocity
quasi-resonance” [Dysthe, 1987]. In a DF, fetch and duration are effectively unlimited - until the fetch changes speed or direction, or until the waves reach a coast.
http://www.waveworkshop.org/7thWaves/Papers/Bigio.pdf

These “trapped-fetch” waves have been a point of close study and modelling at the Canadian Hurricane Centre since Hurricane Luis (1995). Luis created phenomenal waves which were measured by both a Canadian NOMAD Buoy and the QEII Luxury Liner. The maximum wave height measured was over 30 metres tall (100 feet).

One of the best examples of “trapped-fetch” waves was with Hurricane Danielle (1998). The “significant waves”, or the average of the highest one-third of waves, measured in Danielle by a NOMAD buoy, grew from 7 m to 16 m 50 + feet) in only two hours, arriving with the storm almost like a “wall of waves”.
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Old 25-10-2016, 06:52   #9
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Re: Dynamic Fetch

Saw this making a crossing from Canaries to St. Lucia. Storm off the African coast caught up with us two days out and wave heights went from 3-4 meters to 15 meters in a matter of hours. Made for one hairy night...no moon, winds built to 50 kts, running bare poles and debating turning and tossing out the drogue as we did not know how much worse it would get. Wind vane totally paid for itself.
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Old 25-10-2016, 07:24   #10
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Re: Dynamic Fetch

Quote:
Originally Posted by Snowpetrel View Post
. . . I guess the wave front is more about the point where the waves suddenly increase in height, and the others are more the mechanism that causes this?
Yes, and I think what is most terrifying is not the height of the waves, but their shape. The wavelength is shortened like wind against tide.

I have experienced this. It happens only in a narrowly defined set of circumstances, but it can kill you for sure.

It's one reason why I realize I need to finally get around to making a series drogue for my boat.

I got knocked down in the North Sea last year (not from this phenomenon) and it changed my whole outlook on life. No more stupid overconfidence about sea states. Ignorance is bliss, and the age of bliss is now officially over for me.
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Old 25-10-2016, 07:57   #11
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Re: Dynamic Fetch

I wonder if this is what gave us a bit of a fright in January. On the evening of the 28th it was rough but not to bad. On the morning of the 29th ( about 1200Z) the SW sea grew quite alarmingly between breakfast and morning tea and gave us a severe knock that removed the aux rudder.
24 hours later we were motoring in a calm and the sea was flat enough to transfer fuel from drums on the side deck.
We were in 44*50'S, 136*45*W, A low of 950mb was about 800 miles away in about 55*S,120W

This archive function on the BOM site is rather good.. Analysis Chart Archive
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