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:

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

Interference of one wave field with its reflection and a second wave field

All you regular readers of my book and my blog surely recognize what’s going on below?

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Yes! A wave field comes in at an angle to the pier and gets reflected, leading to a chequered pattern. And a second wave field comes in with wave crests pretty much parallel to the pier, adding a little more interest to the pattern.

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I love watching these kinds of waves! But it is really difficult to take good pictures. Sorry about the over-exposed background…

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But it is so beautiful!

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Wave-watching

Do you know the feeling when you, even on the most beautiful of days, want to get out of the pretty parks as quickly as possible so you can finally see the water?

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Especially when it’s foggy?

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And it is so worth it, there is always something to see. For example on that day: what a nice field of shallow water waves!

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And what an awesome criss-crossing of waves being reflected on the sea wall on which I was standing when taking that picture.

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And how sad that this lake was frozen over! :-)

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P.S.: Still looking for a christmas present for your nerdy friend, your niece, anyone who should spend more time looking at water? Check out my book :-)

Wavelength dependency of wave-object-interactions

Wavelength dependency of wave-object-interactions. What a title! :-) But that is exactly what I observed over the enormous timespan of three full minutes (as shown by the time stamps of my camera) when I went strolling along Kiel Fjord one Saturday morning.

First, I saw this old, overgrown tyre in a wave field that was dominated by small wind waves. We clearly see how they are diffracted around the tyre and how there is a nice interference pattern downwind of the tyre (to the left in the picture below).

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Tyre in the Kiel Fjord and diffraction of small wind waves around it, leading to a beautiful interference pattern to the left of the tyre

Also I quite like how there are absolutely no waves inside the tire, where the wind is shaded off by the tyre and the stuff growing on it.

Then, a really short while later, the wave field was dominated by longer waves running in from the distance. Below, we still see remnants of the old interference pattern to the left of the tyre, but also how the longer waves run around it. In the picture below, the wave crest that was broken up by the tyre is about to rejoin.

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And then, only an instant later, this is what the wave field looked like. Hardly and ripples caused by local wind, but many short waves. No real interaction between tyre and waves visible any more.

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Isn’t that fascinating? And it all happened within three minutes! :-)

P.S.: Still looking for a christmas present for your nephew, your friends’ kid, your geeky friend? Check out my book! :-)

If waves spread equally in all directions, then how come we see linear wakes?

If waves spread equally in each direction along the water’s surface, then how come ships (or ducks) have wakes that are just those long lines of waves and not circular at all?
So. Kids are typically familiar with what it looks like when you throw stuff in the water (for proof see below: my godchild on a “Tour de Ruhr” where I learned tons of stuff about mining in Germany. I had no idea that stuff was so interesting! Anyway, I digress. Obviously you had to throw stuff in the water when the reflections are this awesome!)
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But then wakes are seemingly behaving in a very different way. For a nice example of a wake, see the movie below. In that movie, you are looking backward from a boat at its starboard wake. The boat has been sailing straight ahead for a bit after turning to the starboard side (and you will see the resulting curve in the wave in the movie).
Even though slightly curved due to the ship’s change in heading, that wave clearly doesn’t look like a ring around the boat (from where I found the video on my phone I think it must have been a touristy boat in Bergen that I went on with my friend Leela).
So. Good question, isn’t it? Why does the wave look straight? Now don’t tell me it has something to do with interference and stuff, because I need to explain it to a young kid.
I have attempted an explanation, but I am really not sure if it works. What do you think? Check it out and let me know!
The image below shows a sketch of what it looks like if you throw a pebble into the water (or the pattern a raindrop would make). Ideally, we would only see one ring, but since a secondary drop is typically thrown into the air (and sometimes a tertiary) let’s work with three concentrical rings of waves so that the pattern looks as much as possible like what the kid would be likely to observe. The fading colors indicate that the second and third ring have a smaller amplitude than the first one (whose amplitude should be decreasing as time goes on, but let’s not get too technical here…).
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So now how to go from the pebble to the wake? Continuous pebble drops!
From this we have the bow wave and the choppy water inside those two rays of waves. Of course, there we would also have turbulence due to the ship’s propeller or the duck’s feet etc, but maybe this is enough for now?
Except to add that those kind of waves are shock waves (the source of the waves traveling faster than wave speed) — in 3D and in air, the same physics would lead to sonic boom! :-)

Wave hunt expedition. You don’t need to live close to the coast to observe all kinds of wave phenomena!

Waves on Aasee in Münster. By Mirjam S. Glessmer

A 1.5 hour walk around a lake — and 242 photos of said lake — later I can tell you one thing: You definitely don’t need to live close to the coast in order to observe wave phenomena!

The idea to go on a “wave hunt expedition” is actually not mine (although it definitely sounds like something I could have come up with!), it’s Robinson’s idea. Robinson had students go on wave hunt expeditions as part of their examination, and present their results in a poster. I was so impressed with that, that I had to do it myself. Obviously. So the second best thing about work travel (right after the best thing, again, obviously!) is that I find myself in a strange place with time on my hand to wander around and explore. Not that Münster might not have been a nice city to explore, but the lake…

Anyway. I only want to show you 53 out of the 242 pictures. I was going to annotate all of them so you actually see what I mean. And I started annotating. But since I am giving a workshop tomorrow (which is all prepared and ready, but I do need my beauty sleep!) I only drew the key features in the pictures, and you will have to come up with the correct keywords all by yourself (have your pick: refraction! diffraction! fetch! interference! :-)) So click through the gallery below and see first the original photo and then one that I drew in. Do you spot the same stuff that I saw, or what else do you see? Let me know!

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If you think it would be useful to see all those pictures with proper annotations and descriptions at some point please let me know. I might still be excited enough to actually do it, who knows…

P.S.: I actually really enjoy work travel for the work parts, too. For example, I went to a great workshop in Dortmund earlier this year to learn about a quality framework for quantitative research, and that workshop was amazing. And a week ago, I went to Stuttgart for a meeting with all the fellows of the Stifterverband für die Deutsche Wissenschaft, which was also great. And now I am giving this workshop in Münster, that I am actually really excited about because I managed to condense pretty much all I know about “active learning in large groups” into a 2.5 hour workshop. Just so you don’t get the wrong idea about my priorities. Obviously water comes first, but then work is a very close second ;-)