Category Archives: wave watching


For #WaveWatchingWednesday: A collection of pictures that I took (and shared on my wave watching Insta @fascinocean_kiel over the last week. For some reason with a lot more commentary on here than on Insta itself, don’t know why. Maybe because I am writing this while it’s still dark outside and I am waiting for it to get light so I can go do some wave watching? ;-)

Nothing makes me instantly as happy as looking at water. Especially blue water, but any water will do. This picture below? That’s what happiness looks like for me. No matter what else is going on in my life, this view instantly makes me feel calm and content. And happy. What is it for you that has a similar effect on your emotions?

And I discovered a new perspective that I am a little obsessed with right now. Right where I live, there is a big art piece out of metal that looks a little ship-wreck-y and very nautical (it’s called “Hafen 77” by artist Felix Fehlmann). It has circular holes in it, reminiscent of portholes, but it never occurred to me before to use them to frame pictures in. This is my fist attempt at it — what do you think?

Now that I have discovered that porthole view, I wanted to go back the next morning to take a picture with different lighting. Unfortunately I was 2 minutes late to catch the Sweden ferry through the porthole! I hadn’t thought about how that would make a great picture until I saw her and it was already too late. But luckily there are more ships going in and out Kiel port, so there will be another chance!

To keep my Instagram feed looking nice I couldn’t post the next porthole pic right after the first one, but luckily there was pretty cool wave watching that morning, too!

This is a picture of several ducks’ wakes: see how they are forming 2D Mach cones with the ducks at the tip? Taking pictures of waves is always best around sunrise and sunset, because then the contrast between a light sky and dark land helps show the wave structures as differently sloped parts of the waves reflect different parts of the high contrast surroundings.

Here is the mystery spot again (well, mystery for Instagram, I already wrote above that this is the “Hafen 77” art by Felix Fehlmann, so you already knew).

But this time also “zoomed out” so people can figure out where this is!

And then I had a brilliant day trip with Sara!

Sylt, an island on the german North Sea coast, is amazing for wave watching. But amazing waves come at a great cost: the nice sand beaches are threatened by coastal erosion and have to be protected and maintained with huge efforts.
On our mini-excursion, we saw many different measures for coastal protection, like tetrapods (those gigantic four-legged concrete structures) and different kinds of wave breakers.
I took 625 pictures (ok, mainly of waves, but also a lot of the ongoing coastal protection construction works!) that I won’t manage to sort through tonight, so this is your chance: which coastal protection structures should I write about first? (All the following pictures belong to this blog post until the next text interrupts the flow ;-))

The next post wasn’t a wave watching post, for #SciCommSunday I wrote about why I post selfies on my social media (and this picture is clearly not a selfie, it was taken by my colleague Sebi Berens ( / @sebiberensphoto)). But I like it a lot and was excited to post it on my Instagram using the excuse of #scientistswhoselfie ;-)

It’s really difficult for me not to watch the spectacle of the Oslo ferry making a U-turn in the narrow Kiel fjord before backing up into its berth. So difficult, that I took my conference call to the roof terrace and my colleague asked if I was swimming in the sea because apparently that’s what it sounded like on the other end. So ferry-watching was unfortunately cut short.

But watching the ferry wasn’t my main reason for visiting Geomar, see below what we were up to: Torge and I presented a seminar on “It’s always a great idea to play! Teaching ocean and atmosphere dynamics with rotating tanks” (or similar, can’t remember exactly ;-)) We gave a short presentation and then invited everybody “to play”. We had four rotating tables set up, each prepared for a different experiment. And people seemed to enjoy doing hands-on experiments a lot. So hopefully there is a lot more playing with a lot more players in our future! :-)

And then a little more about that amazing day trip to Sylt:

Why is there so much foam on the beach? Two factors are playing together here: Breaking waves trapping air under water that tries to get back up and out, and dissolved organic matter lowering the water‘s surface tension. Both have to be present at the same time: if the water was calm and no waves were breaking, there wouldn’t be a way to get air into the bubbles of the foam because no air would get underneath the water. And if the surface tension wasn’t lowered, the bubbles wouldn’t be able to exist, they would just collaps into drops of water.
Pretty counterintuitive that one has to lower surface tension to make bubbles that are stable, isn’t it?

(Next four pics show different foam situations on the beach)

Recap of my #WaveWatching Insta for #WaveWatchingWednesday

Yay! Another recap of my wave watching Insta!

The year started off in the very best company — watching ships and waves and flowers with Astrid!

But of course there is also actual wave watching happening: Here we see a ship’s wake arriving. I find it fascinating how there are the stripes where you can look into the water and then those where you can’t! Total reflection in action. When I learned about that in physics class I never thought that was a phenomenon I would ever see in real life!

When the waves from the picture above meet the curved sea wall, they get reflected into this pretty pattern.

Another day, foggy and very windy! That day,  my focus was on how there are no waves in the lee obstacles. Only after sufficiently long fetch do waves start to grow.

See how the surface roughness changes with distance from the obstacle?

Oh, and then there was a sunny day! The Oslo ferry is leaving in the distance. See the stripes in different blue tones? In the foreground you can see how the colors are related to surface roughness. Areas that are more exposed to wind are rougher with more waves and different wavelengths, and look darker. More sheltered areas in the lee of structures have fewer waves and appear in a lighter blue.

Oh, and then I had a great day with my nephew in the port of Hamburg with harbour boat trips, walks on the beach, and tons of wave watching. He was very impressed by my skill to know how far each wave would run up the beach, especially since we saw lots of people who either ran away screaming or got wet feet :-D

Another picture from the same day with my nephew: Here is a wave train from a ship’s wake arriving at the beach. I love watching this kind of stuff!

And then one day, I went#WakeWatching! On all three pictures below, you see waves made by the same little ferry. On the right is the turbulent wake where the ship just moved through the water, and on the left some of the feathers that form the V.

Here the ship is turning so we see water that is disturbed by the ship moving through, but doesn’t have the „boiling“ like where the propeller stirred up everything.

Now we see how the wind waves seem to be bunching up at the boundary between the wake and the area that wasn’t affected by the ship moving through.

Then, for #SciCommSunday, I posted a picture of a book that I think is a brilliant introduction to the science of communicating science that I also wrote a blogpost on.

And surprise: Bonus pic for my dear readers that I didn’t post on Instagram but that I think is cool (will definitely experiment with this kind of pictures more! Except panorama mode does weird things to waves, so on second thought maybe not…)

What we are looking at in the picture above is a really low-water day in Kiel. Which leads to interesting wave watching opportunities!

See how waves that arrive with straight crests somewhere offshore get bent as they reach the shallow water? That’s because the velocity of a wave depends on water depth. The deeper the water, the faster the wave can move. The shallower the water, the more the wave is slowed down. Therefore, waves get slowed down first in regions where the water is shallower, and the parts of the wave crest that are still in deeper water wrap around the shallower part. Kinda like when you are slipping on an icy road, you fall in the direction of the foot that didn’t slip because your body spins in that direction.

When there is really low water in Kiel fjord, we can observe the influence of topography on waves much better than we usually can! For example here we see how on one part of the “beach”, there are several wave crests behind each other, all breaking, whereas on the other part on the other side of the headland there is only one wave crest at the water’s edge. Why is that?

Wave crests get steeper and start to break when the water is shallow enough for the wave to “feel” the bottom. On the left side of the picture, depth is increasing faster towards the open ocean: The wave only feels the bottom right before it has reached the water’s edge. On the right side of the picture, on the other hand, the depth changes very gradually. Therefore waves feel the bottom already much earlier and many wave crests are steepening, preparing to break and finally breaking at the same time.

And then I just thought this picture was fun :-)

And a rainy day at work!

I actually don’t mind all the rain: It makes the flow in all the storm drains so much more interesting! Here we see how water shoots out of a pipe into a little lake. Note that I’m saying the water is shooting rather than flowing: That means that it is moving so fast that any disturbance is washed away with the water.

As the water shoots into the much slower flowing lake, it pulls water from the sides with it. This water obviously needs to be replaced from further away, so a recirculation is set up.

Here is an annotated version of the picture to see the recirculation going on:

So that’s what has been going on over on my wave watching Insta @fascinocean_kiel! :-)

#WaveWatching Insta recap: December 2019

There was a lot of wave watching going on on my Insta @fascinocean_kiel this last December! Partly because I had decided that I wanted my feed to consist of both my 24 days of kitchen oceanography advent calendar and my usual wave watching content. And once I had started the pattern, I wasn’t going to break it… But here I will only recap the wave watching part, check out the advent calendar on its own page.

December started off with what has become a bit of a pattern recently: wave watching from the train. It’s always surprising to me how much you can actually see from a train window! Below, for example, the reflection of the bridge in the water helps see the prevalent wave lengths.

Next: wave watching on my way to work! My commute is pretty awesome for sight seeing!

Here we see a wake hitting the pier.

In early December, I went to Nuremberg for work. But of course I managed to squeeze in a little wave watching there, too! The river Pegnitz is very much tamed inside the city, which leads to very pretty flow pattern. For example this submerged hydraulic jump below.

Doesn’t it look spooky how the reflection of the houses is deformed where the water shoots down the weir?

And here is a different hydraulic jump. I love the little wakes that show up around the disturbances on the weir!

My friend Nena took this picture of me wave watching in Nuremberg. You bet I will manage to find waves!

And then some more conventional sight seeing…

Getting distracted by drops falling into the Pegnitz… Isn’t it cool how the wave rings are being advected with the current, so that despite the drops all falling on the same spot in space, the wave rings don’t end up concentric?

And some rapids — with hydraulic jumps! — on the Pegnitz.

Back in Kiel!

The different colors in the sky make it very easy to observe pattern in the waves. The sides of the waves that are sloping towards us reflect the bright sky above us, the slopes facing away from us the dark clouds.

Windy day! See the foam that has been blown with the wind and collected at the shore?

We also see the stripes that form when strong winds blow in a consistent manner for a long period of time: Langmuir circulation!

Then, one day, I saw really cool ripple pattern in the sand. At the very top, ripples in the waves. At the very bottom, ripples in the sand. In between, slightly larger breaking waves, and at the top right stripes of darker matter aligning with the waves‘ direction of travel. I wish I knew more about the transition between the light material being sorted into stripes and the the whole sandy sea floor being shaped into ripples. I guess it’s a function of water depth as well as wave length (so how much energy actually reaches the bottom), can anyone explain it?

And I got this comment by @coastalgeology.kiel: “Dear Mirjam, the beautiful ubiquitous regular bedform structures sure are fascinating in their regularity. Note the long symmetric ripple crests that merge and diverge (=bifurcations). This is a nice question about stability: The sea bed moves if it can… And fine sand may form symmetric ripples if the back and forth motion of the waves is strong enough to move, but not too strong to wash away the structures. The wavelength of the ripples scales with the orbital velocity and the period of the waves and the bed sediment characteristics (grain size and roughness). If no sand is available, if it is too coarse, or the bed and wave characterists change, ripples cannot form. In your photo this is the case in the vicinity of the algae, and in the region of shell hash, which is transported (partly in strikes) but does not form ripples itself…”

Next day: Watching swans and their wakes.

And then: back to work. Watching the rain on a puddle…

And I guess the next day I wasn’t feeling like grey Hamburg weather, so I dreamt myself back to sunny Bergen!

Then, more wave watching from the train. Today with a ship’s wake as well as the wake that forms at the bridge’s pylon because there is a strong current on Elbe river.

And then, I went on a trip with my godson & his family to Möhnesee. We’ve been there before (you might remember the very cool hoar frost we saw there), but it’s always worth a trip!

It was a very windy day, so we got to see some nice wave action.

The gusts of wind ran over the surface of the lake, making the weirdest pattern.

Fun to see how in the lee of the land the water surface is absolutely flat, but the further out of the lee you move, the more wave action happens!

Also really interesting to see how little water there is in the reservoir, and how previous water levels are still archived in the stripes of debris on the shore.

And: A foam stripe parallel to the wall!

This is always such an interesting place to visit!

Here the downstream side of the reservoir. Quite impressive!

And then: Some gold fish watching!

I didn’t use any filters or anything on these pictures, the fish were really that colorful! Here with some waves after a pebble happened to drop into the pond…

And some wind waves.

And the super spooky reflection of the dark, naked tree skeletons in the back…

I love how there are so many layers visible here from the reflection at the surface to fish at different depths. This might actually be one of my favourite wave watching pics of all times!

But then I of course always love water running into water, making waves…

Some more layers here: Reflections, surface waves, stuff floating in the water, ripples on the ground…

Oh, and then the one day where I only managed to see the water by proxy of seeing the cranes at the port of Kiel across the city.

But making up for it with my porthole back home, and my Christmas tableware that I found a couple of years ago and that I loooove despite (or because of?) its horrible kitschyness.

But: Back to proper waves!

I love how you can see all these different stages of wave breaking at once: Waves that are still far out don’t seem to have distinct crests, but the closer they get to shore, the more pronounced the crests become. Steeper and steeper, until they finally break and a water-and-foam mixture is pushed up on the sand.

Below, what I find most fascinating is this really smooth-looking stripe parallel to the wave crests, in between the one that is breaking and the one that is about to break. Of course there can’t be wind waves on water that has just been through a wave-break, but still! Fascinating!

Similar thing here.

I also think it’s super cool to watch the foam vanish on the sand. It doesn’t get pulled back into the sea, it just seeps down into the shore.

Looking back at these pictures makes me really happy. Such a great day :-)

Below, you can see the little foam-free circles, where large bubbles just popped.

More wave stages: In the very front the left-over foam, then foam just being pushed on shore, and then a wave juuuust about to break. So cool!

And now properly breaking!

So cool how far the water is being pushed up the beach even after the wake already broke!

It’s just sooooo beautiful!

Meeting up with some scicomm friends! Of course this had to include some wave watching…

What I find super fascinating here is how a sharp gradient in the sea floor makes waves break instantaneously, leaving this turbulent pattern that looks so much like lace!

More wave watching from the train: The bridge’s pylon’s wake!

And water wrapping around a stone in a very cool way.

Rainy Christmas morning.

And: Throwing a pebble into a small stream. See the first splash?

And then the pillar?

And then all the wave rings?

And growing wave rings?

And even larger wave rings?

And a wave riddle for you: Why is there such a bow wave on a ship that is clearly moored?

More port of Hamburg!

And a little wave watching in the dark. See how the reflection of the Christmas tree looks very different depending on how many waves there are on the water’s surface?

More wave watching from the train!

Beautiful wake!

And here we go with my Instagram’s “top 9 of 2019” pictures (according to how many likes they got):

  • two pictures of the port of Hamburg (thanks to my beautiful commute!)
  • two pictures taken from the ferry on my way home after 2019’s Bergen month
  • and a lot of Kiel fjord pics with wave watching, cloud watching, and ship watching!

Funnily enough, a different algorithm (because I was too impatient to wait for the first one I tried) came up with a slightly different selection. Love that seagull!

So here we go, that was my December 2019 wave watching Insta!

October #wavewatching: reposting my Instagram posts

My scicomm Instagram @fascinocean_kiel is back! As in it’s something that I am putting more thought into again. While it started off strong almost two years ago with daily posts written specifically for Instagram, I’ve been in a bit of a slump. For a while, I just posted pictures from current blog posts with a description like “read more about this on my blog”, but this didn’t feel satisfying. It also meant that I had a lot of #wavewatching posts on my blog, which I felt were taking over what I want the focus of this blog to be on. So the current compromise is that all the #wavewatching stuff happens over on Instagram (in German, but they have a really good translator at least to English), and only the most outstanding highlights will get their own post on here. But there will be a summary post of what went on on Instagram every month or so. Or at least that’s the plan for now!

I started out posting on Instagram again at the end of my month-long trip to Bergen, Oslo, and finally Gothenburg, when I got the exciting opportunity to “meet” Anna Wåhlin’s AUV, Ran, and take part in a short cruise on RV Skargerak to see her in action.

Btw, the reason I am posting more selfies now is that I’ve been thinking about the research around #ScientistsWhoSelfie that shows that showing selfies is beneficial for being perceived as warmer, more trustworthy and also as reducing gender-related stereotypes about who can be a scientist (at least if you are a woman posting selfies). And I think that it’s a very easy contribution to make if it helps achieve scientists in general being perceived as more trustworthy and also helping people to see that scientists are not always old, white men with beards and messy hair in lab coats. Sometimes it’s also me, sitting in the rain, grinning because I get to see a cool AUV up close! :-)

On that cruise, there was of course also CTD work going on.

And work doesn’t stop just because it gets dark…

Then this is the library in Gothenburg — a beautiful building that I passed several times on the way to the institute, and always admired.

And then I was on the ferry to go back home! In a cabin with sea view! That made me so happy. I was so tired after that exciting month, so sitting in my bed instead of standing on the cold, wet and windy deck felt like heaven :-)

And then we were home! Or at least almost. The Kiel lighthouse is situated offshore in Kiel bight, and it’s where the pilot station is (you see the pilot ship returning to the lighthouse in the picture below)

And this is the ferry arriving in Kiel port. I always love watching how these big ships are carefully maneuvered into port!

And this is another lighthouse on our way into Kiel port, and the ferry’s wake.

And then we had a little throwback, reminiscing of Pierre & my adventures on Straume Bru.

And then we are back to wave watching in Kiel! Reflections on the sea wall, and total reflection.

And what was going on here with all that foam? And is foam actually a passive tracer, or is the distribution also influenced by surface tension or other stuff?

Here is where the foam ends up in Kiel fjord. And what I find fascinating is how towards the upper edge of the picture, the ring waves of the water draining into Kiel fjord are really visible, whereas in the lower edge the picture seems dominated by wind waves. Even though both types of waves are probably fairly equally present in all parts of the picture, just the different angle makes one or the other appear more prominently.

It’s really fun to bring this collection of screenshots of my Instagram together. I’m a little impressed by how many wave pictures I post!

Here we are looking at a ship’s wake that is reaching the beach.

And I just love the sound of waves on a stony beach!

Not every day is wave watching day, sometimes we just have to be content with water.

And autumn leaves…

But then there is of course always the opportunity to make the waves you want to see (and be the change you want to see ;-)). Here we are looking at hydraulic jumps in the sink at work.

Although with these views it seems almost silly to go to a sink for wave watching opportunities…

But now a wave riddle. What’s happening between this pic…

…and this one? Any ideas, anyone?

And then there was the day where I went to play with four rotating tanks simultaneously in the morning, and then later to a conference on “Screening the Sea”, audiovisual media and the sea. That was one exhausting day!

The next day, walking down to Kiel fjord, I was in a bit of a gloomy mood and thought that there might not possibly anything going on that I hadn’t taken a million pictures of already. And luckily I was so wrong! My faith in daily wave watching was completely restored when I saw the sediment clouds and how they behaved in comparison to surface waves.

And then there was the day with very low water, where we could really nicely see how the shape of the ground influences the waves.

Shallow water waves always look a bit ridiculous, don’t they? Like sausages moving onto the beach.

And then I finally combined a lot of selfie movie clips about meeting Ran, the Autonomous Underwater Vehicle, into a 5-min-movie and advertised that on my Insta, too.

When I went to go vote, I happened to wave watch in a spot that I don’t usually go to. Also fun! And nice to see how this floating bridge is sheltering parts of the lake from the wind. Although some waves come through, as you see in the deformation of the reflections of the hand rails.

And then I went to Hanover to give a workshop at their university. But first, I had to go do a picknick with Frauke :-) And, of course, a wave riddle. Can you guess who or what made the waves in the picture below?

Frauke is the best. She brought soap bubbles to our picknick! And there is so much physics in soap bubbles. From the films that change color as the bubble ages, to the shapes they form, to how their size is related to how hard you blow when making them. So cool!

Also super interesting to watch how the soap is sliding down the soap bubble, leading to a discoloration starting at the top, sliding down, until it finally bursts when enough water has evaporated.

And now for some drop photography! Water is dripping down from somewhere fairly high up, so the crown of droplets that is thrown up into the air by the surface bouncing back after each drop is quite large! As I was watching, someone moved a window in the office building right there, so that the sun’s reflection lit up the point of impact perfectly.

Then a morning walk with my sister in Hamburg. There was some fog in the shadow-y bits of the river but it’s really hard to spot…

And then back home in Kiel! Perfect wave watching, right?

Later that same day my favourite spot at the Holtenau locks.

I just always love it there!

And then: A day full of jellyfish watching! They are SO beautiful! (For more jellyfish pics, see this blog post)


And one last selfie with jellyfish.

So this is where we are at with my Instagram @fascinocean_kiel now: 444 posts! And the last three all featuring jellyfish :-D

And I am thinking about switching it back to English again. So much for knowing my goals with this profile… :-D

#WaveWatching, beautiful jellyfish, miniature rip currents, double-diffusive mixing in a Latte

So today I spent a perfect day at the beach in Schilksee, at the mouth of Kiel Bight.

Not a lot of wind (you see the gusts of wind further offshore) but perfect wave watching nevertheless. Look at this beautiful wake!

Here I wanted to take another picture of a wake, but the seagull had already taken off by the time I had my phone ready. Still beautiful though!

And in this picture I just love the geometry.

I have never seen as many jellyfish as today. They were everywhere! So I had to take a selfie.

They look so beautiful, and give such a nice depth to the picture below!

We were watching for quite some time.

And more selfies. But isn’t the jellyfish just amazing?

In the gif on top this post you see it swim, here you have to admire each picture individually :-)

I think they are sooo beautiful!

Coffee break: Double-diffusive layering in a Latte! Super clear layers, so I totally invaded someone else’s personal space AND took pictures of their drink. The things we do for scicomm ;-)

On the way back. It was only around 2:40 when the picture was taken (as you can confirm if you recognize the ferry in the picture), but the light already looks like it’s the evening. And another beautiful wake!

And then, more exciting stuff: Miniature rip currents!

When the water is pulling back, they really become quite strong and impressive and it’s easy to imagine that they get super dangerous really quickly when they are a little larger.

Now with the small waves, they just look beautiful.

And did I ever mention that I looooove waves just before they break?

Have a great evening, everybody!

Watch how sediment is transported with the flow while surface waves above move in the same direction but much faster

When I was on my way down to Kiel fjord earlier this day I was in a bit of a weird mood. I was thinking about how the weather was grey and gloomy. And how that meant that there wouldn’t even be a nice sun raise to take pictures of. And how I might already have said everything there is to say about waves in that area of Kiel fjord.

But then, this happened.

Thanks to people working on the drainage system on the other side of the road, there was a lot of sediment dissolved in the water dripping into Kiel fjord! And watch how it spreads into half circles from where it enters the fjord. How pretty is that?

And then, even better, when you look at the surface waves, they propagate a lot faster than those clouds of sediment do. So this is a really nice example of how wave motion and transport of matter in the ocean are independent of each other (as a first order approximation at least).

We do see an influence of waves on the sediment rings further out as they get deformed by the wind waves on Kiel fjord.

Actually, looking at the whole thing from a bit further away and not from directly above, we see that the sediment forms a plume parallel to the coast rather than spreading out in half circles further. And this I do believe is due to wave action. But let’s focus on the area right around the outflow… ;-)

I found this pretty cool to watch and was very glad I stopped my run to take pictures and movies when I first saw it, because on my way back it was all gone already. Lesson learned: Always stop to observe the cool stuff instead of pushing through with the exercises you planned on doing ;-)

Lille Lungegårdsvannet

The geometric shape of Lille Lungegårdsvannet makes for perfect wave watching conditions. Not only when wanting to look at waves from all sides, but also when you are just fascinated by reflections and geometric wave pattern.

And also by rainbows. Am I the only one who, when the sun is out and at a good angle, walks around Lille Lungegårdsvannet to see the rainbow that you know must be there?

Visiting a wave power plant on a no-waves day

You might remember earlier posts on a wave power plant I love to visit in Øygården, where it almost always looks like in the movie below (That movie is part of this blog post, but I also have a blog post on the wave power plant where waves used to drive a turbine or the one where waves run up a funnel to fill a reservoir).

Anyway, when I went there a couple of days ago, things were different. And while I still love visiting ruins of industrial buildings, especially in great weather, the water was … flat.

As in “flat as a mirror”. Below, you see the pillars that a bridge used to rest upon when the power plant was still in operation. And I have never been able to get this close to the funnel, whenever I have been here before, there were waves splashing everywhere and I wouldn’t have dared to go anywhere near that area.

Below you see the funnel that waves usually run up in and splash spectacularly. On the day we were there, we could walk up all the way to the funnel and even look down into it. See how the floor isn’t even wet in some places close to the funnel? The largest waves we saw were the size of the one below, just barely making it up into the mirror-like pond you saw in the picture above.

But luckily there is interesting stuff to watch there even when there are no waves, for example this happy fishy which just looks so content enjoying the view from the top of the cliff.

Or corroded steel rope. Did you know there is just ordinary rope in the middle? I did not. And how interesting that that middle bit is all that is left of the steel rope in places!

And also I always enjoy seeing different wave fields on bodies of water that are located close to each other, like here where the upper reservoir is sheltered from the wind whereas the lower isn’t.

So a nice trip all in all, just not quite the wave watching I had been hoping for! But I will be back! :-)

Phase and group velocities in deep and shallow water

When Tor came to visit me in GFI’s basement lab a couple of days ago, he told me about an experiment he had seen in Gothenburg in the seventies. So Elin and I obviously had to recreate it on the spot. Therefore today, we are comparing phase- and group velocities in deep and shallow water!

Waves are excited by means of an oscillating, hand-helt beer can, curtesy of the beer brewing club at GFI. The experiments are filmed and wave lengths and phase velocities are determined from the videos, which is a lot easier than measuring them directly while the experiment is being run.

Shallow water waves

For shallow water, we are using a water depth of 10 cm. Waves are very easy to see and phase velocities are equally easy to measure.

There is another experiment on (standing) shallow water waves being run at GFI the year before students attend GEOF213, which I described back in 2013.

Deep water waves

For deep water waves, we use a water depth of 42.5 cm (the exact number only matters when the tank filling is also used to fiddle with the dead water experiment, as I had been when the idea for this experiment came up).

Typical wave lengths that are easy to do are between 10 and 25 cm (wave lengths obviously have to be short enough that the water is still “deep”, i.e. H>>wave length) — Elin’s instruction to me for the kind of waves she wanted was “Allegro!” :-D Elin, you are really the coolest and most fun person to play with tanks with!

In deep water, we now have the added difficulty that the phase speed is twice as fast as the group speed. This makes observing the whole thing a lot more difficult. Also amplitudes are a lot smaller now, since the tank was so full and we wanted to keep the water inside…

Here is t0 — Elin has just dipped the beer can into the water for the first time

t1 — can you see the wave signal has propagated up to where the red arrow is pointing to?

t2 — the signal has reached my thumb at the left edge of the picture.

From timing this, we can calculate the group speed. We can also measure the wave length on the video and then calculate a theoretical phase speed from that. For the experiments Elin and I did, the results were pretty good, as in phase speed was usually about twice as fast as group speed. And I am curious to hear how well this works out when the students run the experiment!

The mystery of “51”, or: Tidal current at Straume Bru

My friend Pierre, who I went to Saltstraumen with in 2012, wrote me a text about a year ago and asked me to remind him to tell me about 51 next time we met. We met and, as we do, geeked out about some hydrodynamics stuff. And he told me about 51. It turns out that on his way to work, Pierre crosses Straume Bru on the 51 almost daily, and watches the strong tidal currents and whirlpools that form there. But it wasn’t until a couple of days ago that we managed to take bus no 51 to this specific spot — Straume Bru — and geek out about it together!

I got there about an hour before Pierre did. Not because he was late, but because I was so excited to finally see that tidal current I had heard so much about! And also because I had looked at the tidal forecast and wanted to make sure I would see the strongest current and not miss it by an hour.

Turns out that when I arrived, the current was indeed quite strong. But look at the water level relative to the structures. Pretty much half time between high water and low water, even though the current is strong, there is still a lot of water left in the reservoir!

I love watching the waves radiating from the edge in that wall, and the wedge of eddies that is separating the fast flow from the boundary.

Also look at how the waves are being deformed by both the eddy and the current in the picture below! Especially in the top left corner, where the “wake” of the edge follows the meanders.

As had been forecasted, it started to rain while I was wave watching. With the surface a little rougher now, I noticed these two long streaks. Not sure what was going on there?

It didn’t look like oil or any surface film, but I can’t really think of anything else right now. I was briefly considering Langmuir circulation, but I don’t think the wind was steady enough and also I don’t know if that would combine well with a strong current. Any ideas, anyone?

In any case, the stripes weren’t visible any more when the rain stopped. But look at all these amazing waves!

And, now looking downstream, some more eddies and whirlpools!

Looking upstream towards the bridge, we see the glossy V that is formed upstream of where the wakes meet that are formed by the walls on either sides of the outlet.

And downstream again — how awesome and cute are those little eddies? And how amazing is it that they can persist over long distances while maintaining a dip in the surface that is probably as long as my thumb?

And not two seconds look the same!

Below is a closer look of the two wakes of the sides of the outlet coming together and overlapping.

And here is another picture of the wedge of eddies that forms, separating the strong current from the more stagnant, turbulent boundary layer. Look at how irregular the wedge is, formed of eddies of different sizes that are being advected downstream! And also look at the waves that are being pulled with the current, leading to stripes along the current!

Here is another look at the wedge and the stripes of the waves that are being deformed by the current.

And another one, because the scenery is actually really pretty, too! Which I hardly noticed until I had taken about 500 pictures of the water ;-)

More eddies in the wedge.

Interesting how one side of the outlet forms a wedge while the other “just” forms a wake, isn’t it? I think it’s because the the left one, the one with the wedge, restricts the current a lot more.

And here is a new perspective: Looking at the wedge of a second, parallel outlet. You see really well how the boundary layers from both sides come together!

Looking upstream, the standing waves in the foreground give you an idea of how strong the current is!

And another beautiful wedge!

And more turbulence looking downstream. Funny how parts of the surface look so glossy and smooth, isn’t it? I think those are the areas outside of the current that aren’t turbulent.

Now a final excursion to the other side of the bridge, to look at the wakes of the structures. Notice how much less water there is now!

And here is the upstream part of the V.

And the beginning of the wedge.

Walking a little around the corner, we see that the wake begins already upstream of the corner!

And a final look at the wake.

This is how happy wave watching makes me, even when it’s cooold and raining! At least occasionally, not the whole 1.5 hours I was there… The rain, I mean. Happy I was all the time :-)

Two days later, I actually took the bus across the bridge (after another adventure with Pierre, more on that soon!) and the current went the other way! As it should, but it’s always nice to confirm theory.

Anyone taking this bus regularly? You should start taking pictures for a time series! :-)