Diffraction

I find it really fascinating to watch waves being bent by objects in their way. See how the wave is spreading out from the little slit?

Or here how waves are coming in with straight wave crests that get bend once they run into the lee of the pier?

Surfactants 

When I was talking about foam stripes the other day, you might have noticed that the foam stripe wasn’t a foam stripe all the way, but became “just a stripe”. And since I have been thinking about surfactants a lot recently, I think that’s actually what we are seeing in places where the stripe is just a stripe: Some kind of film on the water.

Inside “the stripe” the water looks a lot smoother and waves are dampened out.

At first I thought that it was maybe due to being sheltered from the wind, but clearly that wasn’t the case. If anything, the stripe was in a location where it was more windy (see where it comes out underneath the pier? That should be funneling wind around that corner, not sheltering from the wind!). But surfactants would make sense if they collected in the convergence zone of the stripe.

See how the reflections from the sun are different where there is stuff on the water and the waves are dampened out?

Waves reflecting on a sea wall

I really like it when waves reach a sea wall at an angle, because the resulting criss-cross looks so cool :-)

And especially cool when you see it gradually building up, like below where the sea wall is partly protected by the gravel (or whatever you call those heaps of stones running in parallel to the sea wall?). The energy of waves hitting the sea wall at that part is dissipated, hence no reflected wave is sent off. However waves that hit the sea wall directly are reflected. Can you see how the reflections spread?

See it more easily in the movie below:

Foam stripes

One sunny morning, I noticed The Stripe again:

Running all the way up and down the coast.

In places where there is a large fetch the stripe is clearly foamy:

But looking downwind from the pier the picture above was taken from, the foam stripe becomes just a stripe!

And looking in the same direction, but from further out on the pier, we see that there is in fact a lot of foam on the water, somewhat organised in rows.

Later the same day the wind had picked up and it became even clearer:

See?

Playing with a ROV

The KiFo owns a ROV that — until now — has never been fully operational. But since I like a challenge (and have a really skilled research assistant who really deserves all the credit) it’s working again!

We first went to test it in a tiny lake on campus.

This was exciting enough, since it seemed to have been leaking on previous attempts.

But this time round it did not, and the lake wasn’t deep enough to test whether it was actually water proof even at increasing pressure.

So off to the Kiel Fjord we went!

And after some careful preparations…

…and a careful launch…

…it worked! :-)

Well, at least until the laptop battery died. But it’s a start! Thanks again for the great work, Nico!!!

Expedition learning

In July I will be involved in teaching an “expedition learning” course for a week. It will be all about coastal protection in the Kiel region, so two colleagues and I went on a private expedition to scout out what can be explored where. This is a very picture-heavy post, be warned! It’s more a note-to-self to document the different beaches we looked at than something I expect anyone else to be interested in.

We started out in Friedrichsort, where there were nice breaking waves to be observed. My part of the course, you might have guessed it, will be on observing waves…

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In Friedrichsort there is a lighthouse on a small headland, and there are sand banks around it that make for very interesting wave fields, like for example below, where the sand bank almost seems to filter out some wavelengths.

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Looking seaward over the sandbank, we see breaking waves over the shallow part, and waves being bent around the sand bank.

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A similar thing could be seen on a tiny headland: Can you see how one and the same wave crest gets wrapped around the headland?

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See? So cool!

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Btw, you might have noticed the weather changing a lot over the last couple of pictures. It’s April, I guess… But a couple of raindrops here and there make nice tracers for the time since the last wave washing up over the beach ;-)

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Always fascinating: When you can see wave-less spots that are shielded from the wind, and then local wind waves and others that are travelling in from further away. And breaking on a sand bank…

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Also, did you see how nice the weather was for a couple of minutes every now and then? ;-)

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And here is a close-up of the waves breaking on the sand bank.

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Oh, and looking back to where we came from: That’s the lighthouse on it’s headland right there! And my two colleagues figuring out what’s wrong with the GPS they brought. Their part of the course will focus on more geological things than mine…

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But I really like this view!

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See how nice and regular the waves are that reach the beach even though the local wind field is really messy (as you see a little further offshore) and the waves have gone over the sandbank?

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Oh, and always one of my favourites: When nice and regular waves hit a stone and it sends off wave rings. Love it!

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One more, because it’s so nice!

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And here waves bending around a wave breaker thingy.

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And this is a picture that really nicely shows how if you don’t have wind, you don’t have waves. The lagoon there is sheltered so well that you can actually see the reflection of the bird sitting on the edge!

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And here we have a very nice superposition of waves coming from different directions and with different wavelengths.

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And waves coming through the “slit” between sandbanks and spreading as segments of a circle. Nice!

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Oh, and more waves breaking on the sand bank.

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After a while, we reached Falckenstein:

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Not so far away from where we started out at that lighthouse over there:

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Another interesting superposition of wave fields.

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Oh, did I mention we did a lot of walking in the sand? About 20k steps. Well, I guess that isn’t even too bad…

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Below I really liked the criss-crossing of waves. It’s actually one wave crest crossing itself after being bend by the shallowing water.

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And those waves get deformed a lot, too!

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And here we knew that it was a matter of minutes until those rain showers would be where we were…

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Luckily, this shower went over quickly, too.

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And this is the kind of stuff the other courses will be dealing with: Awesome formations in the coast!

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Ha, another weather front:

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And this is my favourite geological feature: there are interesting features in the sand/soil/stone (however you call it?) and then erosion marks, clearly made by water, right below!

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A little bit further along the coast, there are weird wave breakers and if the wind hadn’t died down, we would probably have been able to see more interesting waves than these…

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But the waves below were really cool: There were the ones that you clearly see on the picture at an angle to the coast, and then there were waves that came in perpendicular to the coast (so the wave crests were parallel to the coast) and they washed the other waves on the beach and back into the sea. I should really upload the movie…

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So those waves above caused ripples in the sand which are parallel to the water line, even though in the pictures the other wave field is a lot more visible!

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See?

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We ended up in Schilksee and had a look around the marina. Apart from the typical wind / no wind resulting in waves / no waves, we saw……

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…this! Pretty cool, huh?

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And again:

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One last look at the coast near Bülk.

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At this point, only one of us still felt like exploring every nook and cranny…

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Even though there were some pretty nice wave fields, but we could see them from our vantage point without doing an extra step ;-)

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Actually, there were a couple of cool features on the beach still. What’s up with those little bays?!

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We ended the day with trying this very cool contraption to measure the coast with. It was actually a lot of fun!

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And you wouldn’t believe how much work it was to hold that ruler thingy in the wind!

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So yeah, that’s what we did. And how was your day in the office? :-)

More funny waves

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

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

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? :-)

Funny waves on Kiel fjord

The other day I noticed a funny wave pattern on Kiel fjord. Unfortunately I only took a picture where you see what caused the wave pattern:

A step that was just at water level and got flooded with every wave crest and surfaced at every trough. See also in the movie below: