It’s #WaveWatchingWednesday, and that means that you get all of last weeks wave watching Instagram posts compiled into one blog post. Enjoy!
I’m currently preparing a couple of workshops on higher education topics, and of course it is always important to talk about learning outcomes. I had a faint memory of having developed some materials (when still working at ZLL together with one of my all time favourite colleagues, Timo Lüth) to help instructors work with the modified Bloom’s taxonomy (Anderson & Krathwohl, 2001), and when I looked it up, I realized I had not blogged about it. But since I was surprised at how helpful I still find the materials, here we go! :-)
The idea is that instructors are often told to ask specific types of questions (usually “concept” questions), but that it is really difficult to know what that means and how to do it.
So we developed a decision tree that gives an overview over all different kinds of questions. The decision tree can support you in
- constructing questions that provoke specific cognitive processes in your students,
- checking what exactly you are asking your students to do when posing existing questions, and
- modifying existing questions to better match your purpose.
The nitty gritty details and the theoretical foundation are written up in Glessmer & Lüth (2016), unfortunately in german. But check out the decision trees below, I think they work pretty well on their own! We have four different versions of that decision tree, that guide you through both the cognitive and knowledge dimension until you reach the sweet spot you wanted to reach. Have fun!
- Abstract decision tree (most helpful for getting familiar with the general concept) [pdf English | pdf German]
- Decision tree with example questions (most helpful for constructing, or classifying, or changing questions) [pdf English | pdf German]
- Decision tree with example multiple-choice questions (most helpful as inspiration when working with multiple-choice questions) [pdf English | pdf German]
- Comparison of our decision tree with “conventional” types of questions (if you want to find out what a “concept question” really is when classified in the Bloom taxonomy) [pdf English | pdf German]
Any comments, feedback, suggestions? Please do get in touch!
This is a (admittedly terribly crowded — but I only had 1 A4 page and there are so many interesting #BergenWaveWatching stories to tell!) poster that I am presenting on behalf of myself, Kjersti Daae and Elin Darelius at the #FieldWorkFix conference next Tuesday (September 8, 2020). If you would rather listen to my poster’s voiceover than read the transcript below, please feel free to do that here!
Welcome to our poster!
The most important learning outcomes that, in my opinion, need to be achieved with a #FieldWorkFix are to enhance motivation and interest in concepts that are being dealt with theoretically in class, and in the students’ subject in general. When students are isolated in their homes and don’t have an inspiring community of learners in their field around them, it is so important to maintain a connection to their field of study! And one way to do that is by helping them realize that what they study is relevant and meaningful in the way that it helps them explain the world they see (even if they previously neither noticed nor felt the need to explain the waves on a puddle they accidentally stepped in).
There are different types of tasks that can help students achieve that level of observation and fascination with their subject (and if you are interested in what specific tasks can look like, check out the link on our poster, that will lead you to a blog post that links to all the different examples I am giving in the following, with tons of pictures).
For example, students could be asked to find realizations of a phenomenon in the world around them. It’s good to start with an easy example that they can definitely find in many different locations. In our case, “find a hydraulic jump” works well, because those can be generated artificially by turning on the tap in your sink, or can be observed near any weir, most rain gutters, and many rivers. These examples can be shared via the classes content management system or via social media – both work well and offer the added benefit of requiring some sort of description and explanation of what was observed and where, thereby practicing both note-taking and reporting skills.
Students could also be asked to observe a specific phenomenon in a specific place and discuss how the time of observation might have influenced what they saw, and how they would set up a schedule for observations that would be best suited to document the phenomenon. An example for that is looking at a tidal current underneath a specific bridge. Depending on what time and what day it is observed during the spring and neap cycle, the flow might be observed having different strengths or even going in different directions.
I am also a big fan of the more open “find something interesting to observe that is somehow related to the concepts discussed in class”, and being open to what students come up with. If you are worried about students not finding something interesting, I would encourage you to look at my Instagram @fascinocean_kiel, where I have almost 900 pictures of mainly waves (and a few other interesting oceanic phenomena) with explanations of what I saw. Once you start looking, there is physics everywhere!
The best thing about a collection like the one on my Instagram (or the one you are building by asking students to document their observations) is that they can be used for an indoor version of this #FieldWorkFix: Assigning pictures to students with the task to annotate and explain what they see. (Which is surprisingly difficult! I get often sent #FriendlyWaves; pictures of water with the request to explain what is going on there, and while it is very entertaining and educational, it is also really difficult because many of the relevant metadata does not come with a picture).
And finally, one could give the very open task to either come up with, or answer a given, research question by doing observations in the neighbourhood.
Depending on the social distancing requirements, all these tasks could be assigned to students working either in teams or by themselves. But if one of the learning outcomes is to practice working in teams, as it often is, this can be accommodated either way:
Several students can work together on the same research question and either do this together, or – which is most likely the mode they would choose in any case – divide work and take turns taking observations. This means they are also developing observational and collaboration skills: all have to be on the same page when it comes to what properties to observe by which methods and at what place and time, how to document, how much and what kind of metadata needs to be archived, how work is split between the students, et cetera.
Students could also be given complementing tasks that they each complete individually, knowing that they will ultimately have to put their results together like a puzzle. This, again, practices a lot of observational and communication skills.
The results of these tasks can be brought together either asynchronous, i.e. students report back in writing via the content management system or social media, or synchronous in video calls where students give presentations and there is a group discussion.
Lastly, one of the big learning outcomes often associated with field work is building students’ “identity as scientists”. Students come back from the field and talk about how we, meaning we oceanographers, or more generally, we scientists, do field work. Of course, the experience of a local field trip is not the same as a multi-day research cruise. But looking for phenomena related to ones field of study has an effect on how one sees the world. Very quickly, students will look at the world with different eyes, seeing physics where other people see the sparkly ocean or a fluffy cloud. This change in perception helps students feel like a specialist on their subject, as someone who has a deeper interest and wider knowledge than most people around them, and who looks at phenomena more carefully, trying to understand them. And this is a vicious circle: once hooked, it is difficult to stop looking at the world through that lens. Which is exactly what we wanted!
Thank you for your attention!
This is the longer version of the (A4!) poster that I am presenting on behalf of myself, Kjersti Daae, Elin Darelius, Joke Lübbecke and Torge Martin at the #FieldWorkFix conference next Tuesday (September 8, 2020). If you would rather listen to the voiceover than read the transcript below, please feel free to do that below! (Thanks to Torge, the voice over is about 1/3rd the length of the blogpost that I originally wrote to use as script :-D)
#KitchenOceanography: Bringing physical oceanography into students’ homes
Welcome to our virtual poster! I want to tell you about #KitchenOceanography: experiments that students can do at home, using common household items. Whether due to Covid-19, or institutional constraints like the lack of laboratory spaces or instructors, or simply because a hands-on experience would be useful with a certain concept, but it’s not on the syllabus – #KitchenOceanography is a great substitute for doing experiments in a laboratory course when that isn’t possible.
We use #KitchenOceanography when teaching physical oceanography and climate sciences. But the concept of home experiments can easily be transferred to other fields, and I therefore want to present the learning outcomes we can achieve on a fairly abstract level. If you would like to learn about #KitchenOceanography experiments in detail rather than just the general concept which I am presenting here, please follow the link on our poster to a blog post in which I have linked to tons of examples of different learning outcomes and experiments (and to all the experiments mentioned here, as well as 24 easy starter experiments).
One typical learning outcome in laboratory courses is the deepening of understanding of concepts that are theoretically dealt with in a lecture. If the concept itself cannot easily, affordably or safely be transformed into a home experiment, you could ask students to come up with a demonstration of an analogy with the concept instead. We have done that when teaching about processes that govern the El-Niño-Southern-Oscillationpattern in the Pacific Ocean. Of course, students cannot build a physical model that represents all the processes in the ocean and atmosphere that are relevant, but they can come up with demonstrations that show analogies of the cycle.
Another learning outcome in a laboratory might be developing intuition on the one hand, but also checking intuition against observations and explaining counterintuitive results. A great experiment here is to ask students to place ice cubes in two beakers with room-temperature water, one salt water and one freshwater. Asking students to predict which of the ice cubes will melt faster leads to 90% wrong predictions, and because it is really difficult to come to terms with a wrong intuition, it will lead to a lot of learning around experimental skills. Students will ask themselves if they maybe accidentally swapped the beakers because they didn’t take notes of which one was which. They might try to taste the water to test which of the beakers contains salt water (tasting in a lab of course being a big no-no). Even if the course is on a subject that is not related to ocean physics at all, this experiment still holds a huge potential to practice – and gaining appreciation for – laboratory skills.
A third common learning outcome in laboratory courses is for students to exercise curiosity and practice creativity. Using an experiment like the melting ice cubes one I just described ALWAYS works to do just that. Students will always come up with questions that they want to investigate. What would happen if the ice cubes weren’t floating in the water, but were forced down to the bottom of the beaker? Or if the ice cubes weren’t frozen fresh water, but had been made from salt water? In my experience, even students from other subjects that rolled their eyes when I told them they were going to do an experiment with water and ice in plastic cups get hooked and want to understand why their intuition was wrong and what more there is to explore.
Another learning outcome often connected to laboratory courses is to develop reporting skills. With the ice cube experiment I already showed the importance of taking notes even when experimenting only in your kitchen, and #KitchenOceanography lends itself to practicing writing lab reports: now many of the materials and conditions need to be described in a lot more detail because the cooking salt that I use in my kitchen might not have the same composition as the one that you are using, which might be kosher, or enriched in iodine, or reduced in sodium. So if we want to be able to compare results later on, all these things need to be written down. And of course, reporting skills might take a different form than a conventional lab report, especially when students are socially isolated, using for example social media or blogs as an outlet might provide them with community, feedback and recognition.
Lastly, a common learning outcome is to recognize problems and errors during experimentation. Since #KitchenOceanography is less supervised than a typical laboratory class, students will inevitably trouble-shoot more independently, and it’s a good idea to explicitly include reflection on what went wrong and how it was fixed in both documentation and discussion of the experiments.
So what would it look like to use #KitchenOceanography as #FieldWorkFix?
We have run #KitchenOceanography experiments in different instructional settings. Back in the day when we were still teaching in-person classes, in addition to using them as hands-on experiments within class, we gave them as homework. One task was to find a way to measure the salinity of a water sample the students were given, and came up with many surprising and creative solutions. In this setting, #KitchenOceanography was already done asynchronous: students did the experimental work whenever it suited them and report back. It can be done in exactly the same way, and reporting back can happen either in writing or in the class’ video call.
What we have had a lot of success with last semester, though, was a synchronous setup. In a video call, students did simplified versions of an experiment, and the instructor showed the full version of the experiment that students would have run in class, had that been possible. In our case, the experiments would ideally have been conducted on a rotating table to simulate Earth’s rotation. And while I have one in my home, not many students do. So we asked students to do the non-rotating version at home, while I presented the rotating version. The added benefit was that we took time to compare and contrast the two different versions and were thus able to isolate the effects of Earth’s rotation – something we would not have spent time on had students had the opportunity to work hands-on with the rotating tables themselves.
We had three modes of presenting the “full” version of the experiment: using pre-recorded videos (which is definitely the more error-proof way to do it!), running the experiments as a demonstration in real time, or asking students to “remotely control” me doing the experiment by telling me what parameters to modify to which values. This worked by me joining the video call with two devices: One that was recording myself and my experimental setup, looking into the tank from the side, and one that was mounted above the experimental setup and showed the top view (which was relevant for the experiment we were doing). Students shared their experiments via video stream when they chose to. The class was taught by a second instructor, which is what we would definitely recommend: Having one person host the meeting and deal with questions and difficulties as they arise, and have a second person focus on doing the experiment.
All in all, despite the unavoidable tech problems, doing these video conferences where we all did experiments together, were a lot of fun for all involved, and definitely helped make the somewhat sad and lonely experience of learning alone at home, instead of hands-on with a nice group of people, less lonely and a lot more fun.
Thank you for your attention.
Über eine Lehrinnovation, die auf Kleingruppenarbeit und „hands-on“-Praxiselemente setzte – und was in Zeiten von Covid-19 daraus wurde.
In der Lehrveranstaltung „Atmosphären- und Ozeandynamik” im Bachelorstudiengang Physik des Erdsystems wird das theoretische Grundgerüst zum Verständnis der globalen Bewegung von Luft- und Wassermassen erarbeitet, welches zum Beispiel Wetter- und Klimavorhersagen ermöglicht.
Vor der Lehrinnovation von Dr. Torge Martin (GEOMAR) und Dr. Mirjam Gleßmer (fascinocean) geschah dies vorwiegend theoretisch auf Papier und an der Tafel. Die Verknüpfung der Theorie mit beobachtbaren Phänomenen der realen Welt kam dabei oft zu kurz. Um die Theorie begreifbar zu machen, haben wir praktische Experimente in rotierenden Wassertanks und am Computer eingebettet. Diese werden von den Studierenden gemeinsam durchgeführt und das Verständnis durch in Gruppenarbeit erarbeitete Blogposts vertieft. So zumindest im ersten Semester der zweisemestrigen Lehrinnovation…
Das erste Semester – der Plan geht auf
Schon in Vorbereitung der Antragstellung bei PerLe konnten wir nicht länger warten – wir mussten uns einfach privat einen rotierenden Tank für zuhause anschaffen und die Experimente schon mal probieren! Was normalerweise viele Hundert Euro kostet, ist Dank der Bauanleitung des DIYnamics Teams und der Verwendung einfachster Bauteile (wie zum Beispiel eines LEGO Motors) auf einmal erschwinglich. Und das Wissen, dass eventuelle Fehler nicht furchtbar teuer werden, lässt uns – und auch unsere Studierenden – viel unbeschwerter und kreativer experimentieren!
Rotierende Tankexperimente durchzuführen ist zeitaufwendig: Bis der gesamte Wasserkörper in gleichmäßiger Drehung ist und die Durchführung des eigentlichen Experimentes starten kann, vergehen schon mal 30 Minuten. Die Finanzierung unseres Lehrinnovationsprojektes durch PerLe ermöglichte uns, vier rotierende Tanks anzuschaffen – genug, dass Studierende in Kleingruppen experimentieren können und so vier Experimente gleichzeitig vorbereitet und je nach Anwendung direkt oder nacheinander durchgeführt werden können. So können Entscheidungen über Parameter individuell in den Gruppen oder gemeinsam besprochen und getroffen. Im Seminarraum entsteht so eine angeregte Diskussion über Effekte und Theorie, wie sie zuvor durch Vorrechnen an der Tafel nie entstand. Und noch etwas haben wir gemeinsam erfahren: Bloß weil zwei Gruppen die gleichen Parameter ausgewählt haben, werden zwei Experimente noch lange nicht gleich aussehen! Diese Erfahrungen zu machen und zu diskutieren war sehr wertvoll und nur durch die vier parallellaufenden Tanks möglich.
Der zweite Aspekt unserer Lehrinnovation – frei nach dem Motto „Lernen durch Lehren“ – war ein Kurs zum populärwissenschaftlichen Schreiben, den Dr. Yasmin Appelhans durchgeführt hat. Die unglaublich kreativen Ergebnisse kann man auf unserem Blog „TeachingOceanScience“ bewundern! Es sei nur ein Beispiel herausgehoben: der beeindruckende Comic von Johanna Knauf. In dem Comic behandelt Johanna nicht nur fachlichen Inhalte, sondern hebt auch hervor, dass wir auf Lehrenden- wie Studierenden-Seite die Lehrinnovation mit Enthusiasmus und Spaß – und ganz viel Spielen! – durchgeführt haben.
Alles Feedback, das wir bekamen, war also uneingeschränkt positiv. Doch dann kam Covid-19.
Das zweite Semester – hands-on und digital
Was tun, wenn auf ein Mal genau der enge Kontakt zwischen Studierenden, das gemeinsame Spielen und Beobachten, die das Herzstück unserer Lehrinnovation waren, nicht mehr möglich sind und alle Lehre digital stattfindet? Idealerweise hätten wir allen Studierenden einen eigenen rotierenden Tank zur Verfügung gestellt, aber das ging natürlich nicht. Aber da war doch ein privater Tank irgendwo zuhause…?
Der Einfluss der Erdrotation auf Meeresströmungen und atmosphärische Winde ist nicht gerade intuitiv. Um diesen gut zu verstehen, ist es oft hilfreich, ihn direkt mit dem analogen nicht-rotierenden Experiment zu vergleichen. Und so gelang es uns, auch in der virtuellen Lehre die hands-on Komponente zu erhalten: Die Studierenden führten bei sich zuhause die einfachen, nicht-rotierenden Fälle durch, und für die rotierenden Experimente kamen sie kurzerhand virtuell in Dr. Gleßmers Küche.
Bei dieser virtuellen Exkursion konnten Studierende durch Zuruf direkt das rotierende Experiment beeinflussen. Über zwei Endgeräte konnten sie das Experiment von der Seite und von oben beobachten und die Ergebnisse mit ihren eigenen, nicht-rotierenden Experimenten vergleichen. Als Backup, Vor- und Nachbereitung haben wir die Experimente mit dem gleichen Setup gefilmt und online zur Verfügung gestellt.
Unser Fazit? Für eine spontane Lösung ist uns das ziemlich gut geglückt. Auch hier steht am Ende die Erfahrung, dass es für einige Studierende eine wichtige, in der Vergangenheit oftmals vernachlässigte Komponente ist, Theorie tatsächlich „begreifen“ zu können. Mit einfachsten Mitteln lässt sich zuhause zumindest die Motivation für die nächste online Vorlesung deutlich steigern. Aber wir freuen uns auf die Zeiten, wenn wir mit unseren Studierenden wieder gemeinsam in einem Raum experimentieren können!
It doesn’t feel like it, but today marks the 7-year-anniversary of my first blog post on my “Adventures in Oceanography and Teaching”! To celebrate, I sent out this call to action (and please feel free to respond, no matter when you are reading this):
Below, I am sharing the pictures that people sent me plus my thoughts on them, newest on top. Pictures that reached me after August 28th 2020 will be posted in follow-up posts! (Keep them coming, I love it!)
23:58 — Kristina
21:55 — Phil (San Francisco)
21:06 — Clark (Bay of Fundy)
Clark wrote an entire thread explaining this awesome observation in the Bay of Fundy. You should totally check out the whole thread & explanations on Twitter, but I had to share this video so you can see what an exciting situation it is!
20:51 — Elin (Bergen)
18:14 — Simone (Hamburg)
15:43 — Dong
15:18 — Nena (Bodensee)
15:13 — Jeffrey (Boulder)
Wow, this video is super tricky! Please check it out — volume up!
At first, I thought that the periodicity was set by eddies shedding periodically after water had washed over the obstacle. But after about the 50th time I looked at the video, the obstacle (is it driftwood?) seemed to start moving. If it is actually moving, the periodicity makes sense: The wood is trapped in place (you see that on the far side of the river) but it can move a little. It’s bopping on the water, floating at whatever height the waterlevel is at, but at the same time acting as a dam and trapping water on its upstream side, thus influencing the waterlevel. So this is basically a recharge-discharge oscillator. Maybe. Or maybe not. Any ideas, anyone? This is really tricky!
12:38pm — Gabriela (Lüneburg)
11:49am — Gabriela (Lüneburg)
11:39am — Gabriela (Lüneburg)
11:20am — Katharina (Hamburg)
I guess I said I liked a challenge… Screenshots with comments below! And check out the sound in the movie! Volume up!
10:46am — Astrid (Hamburg)
10:38am — Sara (Klein Waabs)
10:37am — Florian (all over the world!)
9:09am — Gabriela (Lüneburg)
Honestly, what jumps at me most is my ADORABLE niece who’s saying Kaffefoto (“coffee pic”). But then there is also the puzzle of why the coffe coming out of the machine looks so much lighter than when it’s in the mug (underneath the foam)? Well, the foam is the clue here! When there are a lot of airbubbles in the coffee still, they reflect light differently (i.e. from all different directions, making it look white, rather than directional, showing either the color of the coffee or a reflection) than when the coffee has settled down and the air bubbles have gone away.
9:01am — Kristin (hiking somewhere near Bergen)
9:01am — Siddharth (Sadashivnagar)
Oh I love this! I think that what Sid doesn’t show us on the very right is a narrow connection to a second body of water, on which waves are generated by wind. (Alternatively, there might be something there at the very right just outside the frame that is making waves, such as a bird or a fountain, but I don’t think that’s the case. Birds usually don’t move this regularly for long enough to generate this wave field even before you started filming and then throughout the whole movie. Fountains usually generate concentric waves (unless there are several fountains, in which case this would be a trick question ;-))) So let’s assume that wind-generated waves from a second body of water pass through a narrow inlet onto this pond. As they pass the narrowest part, they start spreading to all directions, forming concentric waves that grow over time. Well, almost concentric, because the narrowest part isn’t a perfect point source. Therefore we don’t see diffraction at a slit, but rather at a wider opening.
8:58am — Torge (Kiel)
Not a picture, but even better: He managed to fix the problem we had been having with the co-rotating video of our rotating tank. Super excited! If I wasn’t so busy today (slightly underestimated how many pics my dear friends would send me!) I would go try it out right away!
8:51am — Sam (Manchester)
8:28am — Ronja (Nordsee)
8:13am — Elsa (Bergen)
7:26am — Kati (Schönbrunn, Wien)
7:07am — Marisa (Hamburg)
6:26am — Désirée (Möhnestausee)
6:17am — FrozenBike (Khajoo Bridge in Isfahan)
I’m posting a couple of screenshots from that video to make it easier to discuss…
6:06am — Chirine (Kiel)
Last year in pre-social distancing times, Torge and I brought hands-on rotating tank experiments into his “atmosphere and ocean dynamics” class. The “dry theory to juicy reality” project was a lot of fun — the affordable DIYnamics rotating tables are great to give students hands-on experiences in small groups and to see — by running the same experiment on four rotating tables in parallel — how the same experimental setup can lead to very different realizations because of tiny differences in boundary conditions.
Instead of a classical lab report, we asked students to write a pupular science text about an experiment of their choosing. We got lots of great results (see all of them on our blog “Teaching Ocean Science“), but there is one that particularly stood out to me, and the author, Johanna Knauf, kindly agreed to me publishing it here. Enjoy!
I am super impressed with this comic, and also increadibly flattered and touched. This comic is the most meaningful feedback on my teaching and science communication I ever got and that I can possibly imagine. Thank you, Johanna!
P.S.: Curious about how we modified the project to work with social distancing? Check it out here!
Welcome to another #WaveWatchingWednesday — all my wave watching Instagram pictures in one place! Read all about it (and look at a lot of pretty pictures!) below the cut.
Florian sent me a #friendlywave — a wave picture he took, with hopes that I might be able to explain what is going on there. And this one had me puzzled for some time!
This is what the picture looks like:
What I knew about it: Florian was on the ferry from Wisschafen to Glückstadt, crossing the Elbe river.
In the picture itself, there are several features that jumped at me. First, drawn in with the lightblue line below: A sand bank parallel(-ish) to the island’s coast line.
Then, the ship’s wake (shown in red) breaking right near the ship (orange) and turning (green) and breaking (yellow) where it runs on the sand bank.
Florian wrote he was watching the ferry’s wake and noticed something curious: There seemed to be a shallow part, where the waves suddenly became a lot faster! And could I explain what was going on?
Looking at the picture, there were two possibilities for what he might have meant (and, spoiler alert — I completely jumped on the wrong one first!).
Below, I’ve drawn in the part of the wake that is running on the shallow sand bank (green) and how those wave crests continue on the other side of the sand bank (red). I’ve also drawn in some mystery wave crests in blue. Those were the ones I chose to focus on first, since Florian had written that he noticed waves behaving weirdly and suddenly becoming much faster. So if we are talking fast, we are talking really fast, right?
So how do we explain those blue wave crests?
There is a limit for the maximum speed a wave can have. That limit depends on the wave’s wave length: The longer a wave, the faster it travels. In deep water, i.e. water deeper than 1/2 the wavelength, the wave travels at this maximum speed (see green lines in the plot below).
But as it comes into shallower water, it gets slowed down (see black lines in the plot below — those are just a quick sketch, there are complicated equations to calculate it exactly).
In shallow water, i.e. water that is smaller than 1/20th of the wave length, the phase speed only depends on water depth: The shallower the water, the more the wave is being slowed down (see the red lines in the plot below).
So looking back at Florian’s picture, for the blue waves to have been caused by Florian’s ferry, there are two options:
A) they would have to have wave speeds faster than the ferry’s bow wave and wake
B) the ferry would have had to come from the direction of the island, so that the waves propagated in that deeper channel behind the sand bank before the ferry made its way around the sandbank.
Option A is impossible, because wakes travel at maximum wave speed (similar to a sonic boom in the atmosphere, where sound is travelling at maximum speed, forming a cone with the air plane at its tip, only here it’s a 2D version, a V-shaped wake with the ship at its tip). So if the wake is traveling at maximum speed already, then the blue waves can’t go faster than that.
For option B, looking Florian’s ferry up on a map, I saw that that ferry goes around a small island, which is the land you see in his picture. But a quick glance at the map shows that even though the sand bank seems to end where the ship would have had to have gone in order to create those waves, the island is still very much in the way. So this can’t be the solution, either.
So let’s take another good look at the original picture.
Remember those wave crests that I marked in blue? Well, upon closer inspection it turns out that they are tidal gullys and not wave crests! (Which is what Florian confirmed when I asked whether he remembered the situation) Guess I have been barking up the wrong tree all this time!
So back to the wave crests that I marked in red:
What we see here is exactly the depth dependence of the phase speed that I plotted above. Right at the sand bank, the water is shallowest and waves are slowed down (we see that both in the green wave crests that seem to be falling back and start breaking as they get closer to the sand bank [both indicating that the water is getting shallower], and in the red wave crests right at the sand bank). But as the water gets deeper again on the far side of the sand bank (which depth measurements in the map above seem to confirm), the phase speed picks up again (as it has to — see my plot above) and the wave crests accelerate again. Hence we have the weird phenomenon of waves suddenly speeding up!
Very long explanation, I know, but still pretty cool now that we solved it, right? I love #friendlywaves — if you have any mystery wave pictures, please do send them my way! :-)