So today (and tomorrow and the day after) is the big event that I have been working towards all year in my not-so-new-anymore job: The GEO-Tag der Natur! If you are curious about what’s going on there, check out our Instagram account @geo.tag.der.natur that Kati is doing an amazing job with!
As you can imagine, the weeks running up to this weekend were quite busy and a little stressful, too. So last Sunday I went to the beach to hang out with friends and do some wave watching! Because nothing has a more calming effect on me than watching water…
For example below we see nicely the effect of the wave (and wind) breakers on the wave field. In the lee of the wave breaker, the water is completely calm, whereas towards the right of the bay waves form and grow larger and larger.
And below we see a pretty cool “diffraction at slit” example: Straight wave fronts reach the slit between two wave breakers, and as they propagate through the slit, they become half circles.
But to relax and get my thoughts away from my job, I tried something new: I created and posted my first ever Instagram story! I’m not quite sure it’s my format, but I definitely had fun! What do you think? Would you like to see more of those? (I only just realized the story is in german and my blog in English. Posting anyway… Would anyone like to see this kind of stuff in English? Then please let me know and I’ll see what I can do…)
(P.S.: Since I made this for Instagram, the format of the video was optimized for viewing on a mobile phone. Therefore it looks crap embedded in a blog. But some you win, some you loose…)
What do you do to relax and get your mind off of work? Wave watching and posting about it on social media? Have you ever tried that? Or what else would you recommend?
As someone living on the German Baltic Sea coast, I don’t spend a lot of time on the North Sea coast (except, actually, my week-long vacation after Easter with my godson and his family, and when my friend Frauke and I went to Sylt earlier this year, or when Frauke and I are going back to the North Sea next weekend. So maybe that’s actually not so little time on the North Sea coast compared to most other people?).
Anyway. I really like the North Sea, especially because I like the flat landscape where the highest points are dykes.
What I really dislike, though, is getting my feet muddy. But that’s pretty much the whole point of a North Sea vacation, according to my godson and his family.
On the other hand, though, having the opportunity to actively and directly influence water depth (or, as normal people would probably say, leaving footprints in the mud) makes for some pretty cool wave watching!
It’s a little hard to see, but if you look at the picture above, you see that the sun is coming from kinda behind my left shoulder, and the picture below is taken from a similar perspective (just telling you so you can interpret the footprints and resulting waves). So the left edges of the footprints are actually coming up and partly out of the water.
The wind is coming from the right, and you see the locally generated wind ripples and how they get defracted around the obstacles created by the foot prints!
Pretty cool, eh?
In the picture below, the wind is coming from the left and you see the muddy wakes of the fresh footprints! This I think is pretty awesome, especially because you at the same time see the refraction of waves around the obstacles.
What I also think is pretty cool are the little spaghetti piles of sand that the worms living in the mud leave behind.
And that, for each of the piles, there is a funnel somewhere close by, and a worm connecting the two inside the mud!
But then when the water is gone completely, it’s still pretty here, but also a liittle boring. Don’t you agree?
Ok, but it’s still pretty. But Wadden Sea and tides take the fun out of wave watching for quite substantial amounts of time every day, and I don’t approve of that ;-)
I took the selfie above mainly to send to my mom from my vacation in Dornumersiel on the German North Sea coast. But then when looking through the hundreds of pictures I took that day, I realized that not only was my hair parted on the wrong side because it was so windy (ha!), the wave fields to my right and left looked actually quite different, without the reason for that being immediately obvious. So let me show you a picture facing the other way.
Above, you see this wave breaker like structure, protruding into the sea. The wind is coming from the right side, thus the waves are a lot larger on the right side of the breaker where they are getting more and more energy from the wind as they come towards us, than the waves on the left, the lee side of the breaker, where they don’t get any new energy input and are just refracted around the breaker.
Looking the other way, towards the shore, the difference becomes even more clear (picture below) isn’t this fascinating?
I really like watching how waves interact with structures. Below, for example, we see that the wave crests are coming towards the wave breaker at an angle, and that they are reflected and traveling away from it, too. This contributes to making this side look a lot more choppy than the other side!
On the other side, the waves look smooth. I was still standing on the breaker when taking the picture below, and you see where the sea surface is still sheltered from the wind and where the fetch is long enough so the surface roughness increases and ripples and capillary waves form.
Since we are in the Wadden Sea, the shore has a very shallow slope going into the North Sea, so waves look super interesting when they are in the shallow water. Below you see many many almost-breaking wave crests behind each other, coming towards us. The water depth is clearly a lot less than a wave length, the waves are interacting with the bottom and thus have really long and uniform troughs and steep, short crests. (btw, for those of you wondering how I could say anything about water depth in my #friendlywaves post on Saturday: This is how. This is an example of waves in very shallow water, and you clearly see their shape being different, don’t you?).
I love looking at the details of where they hit the beach! All the sparkle, all the little Mach cones around the pebbles where the water is running off, all the small capillary waves!
Below, someone accidentally walked into my picture, but that’s actually a good thing, because it gives you a scale, and if you look at the little wave rings that were created when she put her foot into the water and it splashing forward a little. The wave rings actually have comparable sizes to all the other small stuff going on on the sea surface!
And what’s also pretty impressive: How the crests get refracted by changing water depth. Below it almost looks like parabolic shapes coming in from the right, right? The side of the parabola that is further away is actually the wave crest that is coming in from the open sea, and the rest, i.e. the actual curved part, is partly diffraction around the breaker and then refraction because of changing water depths. So cool!
Since I spent quite some time there, here is a picture later that day with a lot less water. Tides and all that… ;-)
And then another day with a different wind direction and less sun.
I think it looks really cool to see the fairly wide area to the right of the breaker, in its lee, where the surface is really smooth!
So far, so good. Gotta go now! Do you find this as fascinating as I do?
The other day, my friend and co-author Pierré sent me pictures he took during fieldwork in a Norwegian fjord. As I, sadly, wasn’t there, all I can do now is admire the pictures and wish I had been there. And, of course, do a #friendlywaves — an interpretation of a wave field that a friend sent me a picture of. Let’s see what he thinks about my interpretation!
So here we go. As you see, it’s a foggy day, and from the time he messaged me at, I know it was a foggy morning. The light seems to kinda be coming from a low angle which would support the morning (or evening) theory, but that’s always very hard to tell in the fog.
There are some waves on the sea surface, and below you see two distinct wave fields at a small angle to each other. What caused them?
I am guessing that the ones that look like sections of a circle are from some kind of point source, which would be located somewhere below and to the right of the picture’s lower right edge. Maybe something regularly dripping into the water, or a buoy being deployed. I think I’ve seen something like that when a CTD was coming up again and the wire was dripping as it went over a pulley. In any case, I am pretty sure the ship was on station as the picture was taken.
The second wave field, more or less parallel to the lower edge of the picture below, I would guess is the background field. Could be caused by anything, but nothing very close by: It’s not locally generated wind waves. If I had to guess it’s wind waves that have run for a little while. It might also be the ship gently rocking, radiating straight-ish wave fronts, but I doubt it.
As to what we can say about the spot the picture was taken in: There are no structures/shore lines really close by (otherwise we’d see reflections in the wave field), and the water depth is more than a meter or so — it’s definitely long compared to the wave length of the waves shown here as they can’t “feel” the ground (which we see from their shape — not shallow water waves).
Picture by Pierré de Wet, used with permission
The next picture, I am assuming, was sent to me to capture the mood. And to make me jealous. Yes, it worked ;-)
Picture by Pierré de Wet, used with permission
Below, we see that the ship is now moving. We are looking down and back and see the wake developing: The turbulent wake in the top left of the picture, one side of the feathery V-shaped wake on the right of the turbulent wake. The feathery waves are fairly steep, but that’s because of how they were generated, not because of any interference with the ground. The ground is still more than at least two or so wavelengths away (and it better had be, judging from the size of the ship).
There was hardly any wind when this picture was taken, the sea surface doesn’t show any locally created wind ripples.
Picture by Pierré de Wet, used with permission
I think it’s so fascinating to see the sharp line in the lower middle of the picture, separating the part of the sea surface that has been influenced by the ship from the one that hasn’t received any signal of the ship’s presence yet. If you think about the V-shaped wake as of the ship’s Mach cone, the outside of the V is where people would first hear the sonic boom after the ship has flown past!
The picture below is looking at a similar situation wake-wise. Now, though, there is a little wind: You can clearly see the enhanced surface roughness, and indeed individual capillary waves, in the bottom right corner.
Below is a third picture of the same situation. Now there are some small waves in the surface, however not locally produced, I think. Maybe they already sailed out of the spot (can you say breezy? It’s really not a windy spot) shown above?
Picture by Pierré de Wet, used with permission
What I find fascinating above is how clearly you see the one leg of the V-shaped feathery wake develop, and even in the foreground of the picture how you can see individual turbulence cells from where the bow wave broke as the ship sailed through the water.
What else do you observe? It’s not so easy to look at other people’s wave pictures and make sense of them! How did I do, Pierre?
Looking at Kiel fjord in the picture below, it is quite obvious from the shape of the waves that those waves are some ship’s wake.
Why is that obvious? Because the waves a) have a very short wavelength for their height, and b) are also all of the same wavelength. What I mean by that is a) on Kiel fjord, if we see waves that high that are driven by the wind, their wavelength is a lot longer since the waves have been building up over a long distance. For short waves to display such an amplitude, the waves would have to run up a fairly steep slope which I know is not the case in this location (and which would also lead to two or three high crests in the shallowest part of the water, not to as many as far out as we see here). B) we don’t see a spectrum of wavelengths as we would expect in a wind-driven wave field. In fact, the water surface doesn’t display any ripples or other evidence of wind at all.
And what do you see when you look at water at night? :)
A little more wave watching, today with a focus on how waves change direction when they run into shallow water. Let’s look at this beautiful wave and see what happens when it reaches the shallow shore.
Above, you see the wake of the pilot ship, consisting of many wavelets that propagate as parallel wave crests towards the shore.
Below, you see that the wave is propagating at an angle to the shore (something around 45 degrees, maybe?). If you focus on the wave crest that is just offshore of that little obstacle in the water (curious enough, a piece of brick wall), you clearly observe that angle. But then looking at the next wave crest in-shore, it is almost parallel to the shore! Assuming that both crests come from the same wave field, so that the second one was in the same position as the other one only moments before (which I know it was because I observed it), something clearly happened between then and now.
Refraction of waves
Why do waves change direction as the water depth changes? As waves run from deep into shallow water, at some point they start to “feel” the bottom, which slows them down.
Or, more scientifically speaking, the dispersion relation for shallow water waves is a function of water depth: The shallower the water, the slower the waves. That means that if a wave crest is running on a slope with one side being in shallower water while the other one is still in deeper water, it will change direction towards the shallow water because the shallow side of the crest is slowed down while the deeper side keeps on moving faster, thus forcing the whole crest around a curve.
But in this picture series there is more to see: See how the wave crest gets deformed after it has passed that obstacle?
Diffraction of waves
This is a process called diffraction: The change of direction after a wave crest has passed either through a slit and then starts radiating from that slit as circle segments, or, in this case, an obstacle. The wave passing an obstacle is, in a way, the same as the wave passing through two wide slits which are very close to each other, only separated by the obstacle: The edges of the wave crest at the edges of the “slits” also start radiating out as circle segments!
One spot, so many things to observe!
And there are, of course, ships. What I wanted to show on this picture is a close-up of the turbulent wake of the ship, but it’s really difficult to see so I’ll let that pass for today.
And the picture below shows so much cool stuff: Waves radiating from that pylon. Ripples on the surface by a gust of wind. Wave crests getting a lot steeper as they run up on the slope. And, my main reason for posting: I really like how the wave is spilling as it breaks! :-)
As you know we are currently preparing for future wave riddles. So this afternoon I went out for a wave hunt again and found something beautiful for you!
The ship coming out of the Kiel-Holtenau locks into the Kiel Canal is making waves, but although those are pretty exciting, too, there are more things going on in the picture above…
Many processes can create waves
In addition to waves made by ships, seagulls, the locks opening and closing, and those waves being shaped by reflection, refraction, and all those other processes, most waves look actually pretty similar, and they are all formed by the same process.
Most waves are wind waves
In almost all situations it’s a safe guess that most of the waves you see are caused by the wind. Either locally, or by storms far away. Of course, the waves might look very different from day to day and location to location. But as a rule of thumb, the stronger the wind, and the longer it has been blowing, and the longer its way over water without any obstacles in its way, the higher the waves.
Usually the length of the fetch shapes the wave field
This uninterrupted stretch that the wind can blow over the water is called the “fetch”. And it explains why you don’t have really large waves on small ponds: if the fetch isn’t long enough, waves just don’t have enough time to build up from when they are generated at the upwind side of the pond until they have reached the downwind side.
Sometimes obstacles shape the wind field
Sometimes though, there are obstacles in the wind field that cause interesting wave phenomena. Below you see that the wind that has been coming across Kiel Canal is interrupted by those pylons. Upwind of the pylons the waves are fairly regular and pretty boring.
But remember your Bernoulli? If the area across a flow decreases, for continuity reasons the flow speed has to increase.
Since air is “flowing” in that sense, too, it’s accelerated where it goes in between and around those pylons since it has to squeeze through a smaller cross section than it had to its deposal further upwind.
The wind field is mirrored in the wave field — well, kind of
Do you see how the faster wind causes all these nice little ripples? Maybe “mirroring” the wind field isn’t quite the right way to express it, but you can definitely see where the wind speeds are different from the rest of the Kiel Canal just by looking at the waves! From there the waves then propagate as sectors of circles outwards and leave the areas of the high wind speed, but they quickly dissipate and vanish again.
Wave watching is awesome. Can you think of anything better to do on a Saturday afternoon? :-)
No matter how often I’ve seen it, I still find it absolutely fascinating how the tiniest structures can have a really visible effect on the downwind wave field. Like for example that pier below, leading to the little hut at the end. There is probably a meter and a half between the water surface and the gangway, which is propped up on really thin pylons. Yet, you clearly see that there are visibly fewer waves downwind of the structure. And the hut itself shades a huge area from wind.
Sometimes waves are very regular and mostly of the same length. Those are the ones that I usually talk about when I talk about interference of waves. But of course, other times, there are different kinds of waves with different histories and different lengths, and those do interfere, too. For example in the picture below, there are long swell waves caused by a distant storm, and then small wind waves on top of those, caused by a local breeze.
The really long swell you can’t even see in the picture, because waves with a couple hundred meters wavelength and just a dozen or so centimeters height are just really hard to photograph… But you get the idea!