Tag Archives: Kiel fjord

More funny waves

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I think I might need to find a new route to walk along the Kiel fjord. When I was walking — in the most beautiful sunshine! — with my friend over the weekend, she pointed out that there are funny waves and it looked like there was water dripping in, and I went without looking “no, there is a step right there that’s causing those“.

You see it in the picture below: Every wave crest washes over the step, and then when it retreats it sends off its own little waves.

It’s a funny thing with professionalized perception. What I notice walking along the Kiel fjord is really highly trained and specialised, I guess. But still a lot of fun! And it makes me really look forward to the excursion that I’ll do later this summer with a couple of high school students where they’ll learn to observe waves my way :-)

Nonlinear effects in shallow water waves

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I recently googled for something related to the shape of waves and came across a photo of a wave that caught my eye, and it took me to a journey that lead to the article “nonlinear shallow ocean wave soliton interactions on flat beaches” by Ablowitz and Baldwin (2012).

What’s discussed in that article is that while many wave interactions can be seen as (more or less) linear, sometimes there are nonlinear effects that can be replicated in a model. So far so not surprising. But I got fascinated because the phenomenon they look at I have seen over and over again and never really paid any attention to it: Wave crests forming X or Y shapes. But looking through my archives, I even had dozens of pictures of this exact phenomenon! (Actually, I didn’t have to look further back than to a beautiful day last November, when I also observed the wavelength dependency of wave-object interactions)

Take for example the picture below: Do you see the H shape in the waves closest to shore? (In the article they would probably call it a more-complex shape, since it’s a double Y shape…)

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Below I’ve drawn into the picture what I mean by H-shape in green, and the typical kind of linear wave interaction in red (all crests just move on without influencing each other except in the spot where they occur at the same time, there they just add to each other):

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Or below, I spot an X-shape:

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And here are several X- and Y-shapes

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And the picture below just to give you an orientation of where you are: Yep, it’s the same spot where we usually observe foam stripes, funny waves, or ice…

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—
Mark J. Ablowitz, & Douglas E. Baldwin (2012). Nonlinear shallow ocean wave soliton interactions on flat beaches Physical Review E, vol. 86(3), pp. 036305 (2012) arXiv: 1208.2904v1

More foam stripes

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As you might have noticed, I am getting a little obsessed with those foam stripes. Another day, a little more wind, looking up the coast:

And down the coast…

Do you notice the irregularities in the foam stripe in the pictures above? Those are the places where, in much calmer weather, you see the funny waves. I.e. there are steps that disturb the wave pattern and hence the foam stripe.

But remember we saw a foam stripe connecting one going in parallel with the pier to one going parallel to the sea wall? It’s here again:

Its taking a very similar path like it did last time, but this time it joins the one parallel to the sea wall, rather than forming a second stripe parallel to the first one.

And if we continue further down the coast, we see a similar phenomenon (we are now walking towards that edge in the background of the picture below).

Looking back, we see another foam stripe coming from the other edge of the pier, joining the one parallel to the sea wall.

See, this is how they meet at the sea wall?

And, funnily enough, a similar stripe can be seen going through the sailing harbour: Entering it through an opening in the pier and then going across the harbour and out the other end, until it finally joins the stripe at the sea wall.

This stripe finally convinced me: There don’t need to be convergences for the stripes to exist, at least not for those that aren’t running in parallel to the coast. Because I cannot imagine a convergence zone running in such a way through the harbour that is partly sheltered from the wind, has pylons in it, and just has completely different conditions than the open fjord. Or at least the mechanisms forming that convergence zone would have to be very different from those forming the other stripes. So now I am thinking those stripes are just advecting foam from places where it accumulated (in front of the pier) to new places where it accumulates some more.

And finally it just gets stranded on land:

See how nicely the foam stripe is going around the obstacle? :-)

Foam stripes mystery — closer to figuring out what’s going on?

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I think I might be getting closer to understanding the foam stripe mystery. Remember how we’ve always observed them going in parallel to the coast?

Yesterday I saw this again, looking up the coast in one direction…

…and down the other direction. I’ve had the hypothesis that they might be somehow related to Langmuir circulation, but in any case there must be some kind of convergence zone there.

But let’s move closer to that pier we see in the background of the picture above. Here we see a foam stripe parallel to the pier, but at a 90 degree angle to the see wall that I am standing on and that has a foam stripe running in parallel, too! And even more curious: at the edge of the pier, the foam strip detaches and runs toward the coast! See?

Looking down the coast again, we see that foam stripe coming in at an angle, and running in parallel to the coastal stripe in the far back.

Looking up the coast from the pier right where it meets the sea wall, we see both foam stripes running in parallel (as we saw in the picture above):

I think what is happening here is that the foam of the foam stripes doesn’t form locally (which was an implicit assumption I had whenever I was staring at the water, trying to observe more wave breaking there than in other places). Instead, foam forms somewhere else (probably pretty much all over the place) and just accumulates in those stripes. That’s actually pretty likely if we think back to the eel grass or leaf stripes: the eel grass and leaves were clearly advected from somewhere else, too. And actually that’s the same with Langmuir circulation, too: stuff just accumulates in convergence zones but isn’t formed there.

So for some reason there is a convergence parallel to the sea wall as well as the pier, and foam just accumulates there. And as for the part of the stripe that detaches from the pier and runs to the coast? It is going more or less downwind. So it’s probably just part of the stripe parallel to the pier that gets advected around the corner and blown toward the coast.

Why does that stripe end up in parallel to the one at the coast rather than joining it? I don’t know yet. But at least now I only need to figure out why there are convergences in some places and I can let go of the obsession with foam formation in the stripe itself :-)

Do you have any idea that might explain those foam stripes? I’d love to hear from you!

Ice and waves

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Waves going through very fresh, still flexible ice. What an exciting topic :-) We had a first look at the picture below in the last blog post already, when we looked at stages of ice formation. Can you see how the pancakes are deformed by waves going through?

It’s a little more difficult to see in the picture below, where a storm drain drips into the fjord. Can you make out how the wave rings are spreading through the thin ice?

And one thing that I found super fascinating was that ducky ice-breaking: You see the open water bit just behind it, and there are a lot of waves. But then you also see those waves spreading into the ice! Poor ducky must have put a lot of energy into this…

My favourite picture, though is the one below: A crisscross of waves and an ice floe that moves (obviously) with both wave fields!

This even works if there is more ice than just one lonely flow:

How awesome is that??? :-)

Ice forming on Kiel fjord

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One thing I really like is watching ice form. Well, maybe not watching the actual freezing (don’t have the attention span for that) but looking at all the different stages.

At first, you have all the small, individual needles that still slush around.

Then, the needles start sticking together, and if there is a little wave action, the ice breaks apart into individual pancakes, which bump into each other and start piling up along the edges.

If the wave action isn’t too bad, those pancakes can freeze together, forming a closed ice cover.

Sometimes you can also see several stages at once, like in the picture below: Open water out on the fjord, slush a little further in, then smaller pancakes and then larger pancakes. Not very surprising: Most wave action in the middle of the fjord, and the further you get towards the shore, the more waves have been dampened by the ice, so the larger the ice floes can grow.

Here you can actually see waves going through the ice-covered area, giving you an idea of how flexible the pancakes still are. More on that in the next post… ;-)

Here you see all the stages in one pic again. Together with the raising sun it makes for very pretty pictures! :-)

Wind waves

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No matter how often I’ve seen it, I still find it absolutely fascinating how the tiniest structures can have a really visible effect on the downwind wave field. Like for example that pier below, leading to the little hut at the end. There is probably a meter and a half between the water surface and the gangway, which is propped up on really thin pylons. Yet, you clearly see that there are visibly fewer waves downwind of the structure. And the hut itself shades a huge area from wind.

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This kind of stuff is so cool to watch! :-)