15 October 2014

"Cross sea" and "cross swell" - updated re the Écluse à poissons

"In surface navigation, a cross sea is a sea state with two wave systems traveling at oblique angles. This may occur when water waves from one weather system continue despite a shift in wind. Waves generated by the new wind run at an angle to the old, creating a shifting, dangerous pattern. Until the older waves have dissipated, they create a sea hazard among the most perilous."
A cross swell is generated when the wave systems are longer period swell, rather than short period wind generated waves
Amazing.  I didn't even know this could happen.  You learn something every day.

Text and image from Wikipedia, via The Soul is Bone.

Addendum: Reposted from December 2013 to add this video sent in by reader Dominique:


The video provides a brief tour of Ile de Re, the tip of which is shown in the photo at the top. I tried to set up the video to start about midway, to focus on the enormous tidal fish trap constructed there but it didn't work, so you can skip to about the 3:45 mark to see the relevant portion.

I have seen other weirs and fish traps over the years, but never one as massive as this.  After watching the video I went to Google Maps and was pleased to see that the structure is visible in the satellite view:


It extends out from the lighthouse and the beach (light brown in the satellite image); the scale can be appreciated by comparing it with the buildings at the bottom.

There is more information hereAnd here.  Apparently the structure (or its earliest incarnation) dates to about the 14th century.  Amazing.

16 comments:

  1. That looks absolutely terrifying. I can't imagine trying to navigate in that.

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  2. I've seen that sort of thing when out surfing: it's common to have two different wave sources when weather is changing, or if there are two storms following each other up the coast. I've definitely never seen it as clear and as right-angled as in the above image, that's pretty cool.

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  3. Wow! I don't think I've ever seen anything like cross seas or cross swells. Sure wouldn't want to have to navigate it. BTW, that photo is just amazing.

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  4. I agree with the above commenters. That picture is very striking!

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  5. I never would have imagined that possible. A beautiful photo as long as you don't have to be in the water.

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  6. Clearly this is the result of chemtrails. [/sarcasm]

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  7. ? But what is the big dark object at the left view of the beach ? A submarine ? A Loch Ness type creature ? An internet delivery tube ? A SEWER ?

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    1. None of the above. It is a man made structure intended to slow down the erosion of the beach. There are several of them around the western tip of the island. Search for "Phare des Baleines".

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    2. Ahem...
      It is an "ecluse à poissons", a half circle wall of stones (100 to 500 meters wide, no cement) covered by the sea at high tide. The fish get caught at low tide and can then be fished more easily.
      Their history goes back to the middle-ages, the construction was ruled by Colbert in 1681 and they needed 10 to 20000 hours to build.
      If you make a Google image search with "ecluse à poissons ile de Re", you will recognize the place at low tide.

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    3. Thank you so much, anonymous person. The English equivalent would be a fishing "weir," and the one in the photo must be the largest I have ever seen. Here is a link for some smaller ones in Asia -

      http://tywkiwdbi.blogspot.com/2009/03/stone-fishing-weirs.html

      - and here's a video of one made of sticks, found underwater off the coast of Sweden:

      http://tywkiwdbi.blogspot.com/2012/06/mesolithic-fish-trap-9000-years-old.html

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    4. To fully appreciate the size of this écluse, have a look at this video : https://www.youtube.com/watch?v=7ffNpdfXAJ4 (skip to 4:00, the guy is a bit slow ;-))

      And sorry for the anonymous post !

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    5. Incroyable! And blogworthy ("blogdigne?"). Bookmarked for blogging. Tx, Dominique.

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    6. Thanks to you Stan. Without this post about cross swell I wouldn't have noticed these waves when I was there two weeks ago ;-)

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  8. Here's an article on fishing-weir-like structures in Bolivia that you might find interesting:
    http://www.theatlantic.com/magazine/archive/2002/03/1491/302445/

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  9. I took this video today,it shows a cross sea occurring! 12-22 sec into the vid at the bottom of shot!https://m.youtube.com/watch?v=L4VQSBRb9c8

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    Replies
    1. Absolutely. The one you document is occurring so close to shore that one wonders whether there might be some shore-based mechanical/industrial basis for the creation of the waveforms.

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