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? :)
In the gif below, I have drawn in several things. First, in red, the “weird” tracks that we are trying to explain. Then, in green, the crests of two different wave fields that are at a slight angle to each other. I’m first showing one, then the other, then both together. Lastly, I am overlaying the red “tracks”.
So this is what those tracks are: They are the regions where one of the wave fields has a crest and the second one has a trough (i.e. where we are right in the middle between two consecutive crests). What’s happening is destructive interference: The wave crest from one field is canceled out exactly by the wave trough of the other field, so the sea level is in its neutral position. And the wave fields move in such a way that the sea level stays in a neutral position along these lines over time, which looks really cool:
And even though these weird neutral sea level stripes are parallel to the bright stripes on the sea floor, I don’t think that the latter one is caused by the first. Or are they? Wave lengths seem very different to me, but on the other hand what are those stripes on the sea floor if they aren’t related to the neutral stripes in the surface??? Help me out here! :-)
After talking a lot about turbulent wakes this week, here are two pictures of different kinds of wakes. They are of course turbulent, too, but on a very different scale.
In the picture above, see how you see pairs of eddies on either side of the row boat’s wake? That’s where the oars were in the water! But this wake stays visible only for seconds, maybe a minute. Nothing you would be able to see for a long time from afar!
Same for the waves the birds made. Can you still spot that they were swimming in the same direction as the row boat and then made a 180° turn? Maybe you can, maybe you can’t, but that’s how quickly those wakes vanish.
Now below: This is a very interesting wake. Since the SUP board is pretty much flat on the water and doesn’t displace a lot of water while moving through the water, it pretty much only creates the V-shaped wake, not a turbulent one the way a ship does when it’s displacing a large volume of water in order to get forward.
Think I’ve said everything there is to say about waves? Well, then just enjoy this one from the ColorLine ferry that sailed past a couple of minutes ago… How beautiful is this? :-)
You might have noticed that in today’s first post there was a lot less water in the Kiel fjord than in yesterday’s post (starting this year strong on the blogging front! I like it! And don’t worry, I won’t be keeping up this pace :-D). But look how little water there actually is!
In the picture above you see two navigation signs that are usually necessary there, because the rock in the middle is submerged far below the water surface. But not today!
And also in these locations you would typically see water coming all the way to the sea wall and sometimes even higher than that. So what happened? Strooong winds!
And the even more interesting thing will happen in a day or so, when the winds die down and all the water that got pushed out into the Baltic Sea comes rushing back into Kiel Fjord! Unfortunately I will most likely not be able to document it due to travel. Someone should pay me for documenting important oceanographic events in Kiel Fjord all day every day :-)
Just kidding. Below you see a movie of a neat interference pattern I observed this morning. The situation is similar to yesterday in that the ferry has sailed past and the wake runs up on those bathing steps. But: today it’s quite windy and the wind waves’ crests are perpendicular to the crests of the ferry’s wake. Check it out:
That’s the kind of stuff I loooove watching! Happy New Year, everybody, may there be plenty of wave watching in 2019!
P.S.: Am I the only one who always wants to write fairy when writing about ferries? :-D
Ending 2018 in style and exactly the way I want to continue in 2019: wave watching and dipping into Kiel fjord!
2018 has been an exciting year and a lot of changes that will shape 2019 to be very different from anything I have ever done before have already been set in motion. But despite all the new adventures, some things will stay the same: Stay tuned for ever more adventures in oceanography and teaching that I look forward to bringing to you!
Last night it rained a lot. So the first thing to do this morning was to check what that had done to my green lake!
The dye is now a lot more diluted, but overall it still looks surprisingly green seeing that there is a lot of rain water draining into the lake. To give you an idea of how much more water is going through now than when I last showed pictures of the green stream: Look at how clearly you see the inflow into the lake in the picture above! And remember the little waterfall in the picture below? There is a lot more flow now.
Another thing that has gotten a lot easier to see now is where the dye goes into the Kiel fjord. Because the flow rate is a lot higher, so the flow itself is clearly visible, independent of the tracer, but also because … well, there isn’t a lot of water left in Kiel fjord!
This is what it looks like this morning: That little stream is water from the lake going into the fjord. Usually there is about a meter more water here!
It looks actually pretty cool to see exactly what the sea floor looks like.
Even though there are no tides in the Baltic (well, hardly any), we do have some large changes in water levels sometimes. They are due to changes in wind or pressure; in this case there was a lot of wind last night that pushed a lot of water out of the Kiel fjord into the Baltic.
What typically happens now is that this water doesn’t stay away indefinitely, but once the winds stop, forms a “seiche”, a standing wave, with a period of a little more than a day.
Of course I am going to check if there is water back by tonight, and then gone again tomorrow morning! Assuming, of course, that the winds stay calm. Otherwise that would influence where the water goes, too.
What I found really interesting, too, is that I saw a lot of herons now that I’ve hardly ever seen in this part of Kiel fjord before. It makes sense — usually there is too much water so they have nowhere to stand — but it was still weird to see five at once, and more as I walked along the fjord.
And — at last! — it was possible to see from land what those two sticks in the water are warning about: The stone in the middle! I had never actually seen that before. Now I know! And now the water can come back; wave watching is more fun when the waves have slightly shorter periods than the seiche’s 27 hours… ;-)
…Update in the afternoon…
After more rain throughout the day, we now actually see a clear plume of the rain water going through the green lake, with a little mixing on the sides as the green water is entrained!
And some water is back in Kiel fjord. Phew. So there is wave watching to be done right away:
Below, we see a really nice example of waves changing their direction as they run into shallow water, since their phase velocity depends on water depth (more about that here).
The German Society for Chemistry and Physics Education (GDCP)’s annual conference started out with a 2 hour cruise on Kiel fjord, during which the participants had the opportunity to choose between enjoying the sunshine and just doing whatever they liked, and several “guided tours” on either the sights of Kiel in general, or the biggest sight in Kiel, the water :-)
Preparation is everything: charts to learn to observe waves
I had prepared laminated charts to be used in case for some reason the weather wasn’t cooperating, and they definitely came in handy even during beautiful sunshine. On those charts, I used pictures that you’ve seen on this blog before, and contrasted them with typical physics text book illustrations, either sketches or ripple tank photos.
Since I was fairly busy talking, I only snapped two pictures:
The Oslo ferry right after turning inside the Kiel fjord, right when it starts backing up towards its berth, and, more interestingly, the turbulent wake. See how messed up the wave field is? It’ll take quite a while for all that turbulence to dissipate and for the sea surface to look as if nothing ever happened there!
And then our ferry’s wake. Here we see the turbulent propeller wake and one side of the feathery wavelets of the V formed by the wake.
Beautiful day to be on a ship!
Now, if you’d like to do a guided wave watching tour, you know where to find me… ;-)
So many people are surprised when I speak of wave watching as of a “real activity”. But to me it is! So I am going to talk you through a couple of minutes I spent looking out on the water where the Kiel Canal meets the Kiel fjord, right outside the locks at Kiel Holtenau.
A light breeze across the fjord
The “light breeze” part is fairly easy to observe: There are ripples on the water, but no actual waves. “Across the fjord” is also fairly obvious if you look at either side of the wave breaker: On the fjord side, there are ripples, on the shore side, there are none (or hardly any), indicating that the wave breaker is sheltering the shore-side from the wind (and dampening out the waves that come across the fjord).
And then: A ship sails into view!
We watch the ship sail past, dreaming of foreign countries and exciting adventures.
A ship leaving a wake
Behind the ship, the water looks very different from what it looks like everywhere else. The wake is turbulent and waves radiate outwards like a V, with the ship always at its tip.
Then, the ship is gone. But we can still see where it went.
There are no waves in the tubulent wake
The ship’s path is completely smooth. No waves have invaded the turbulent waters of the wave just yet, claimed them back. However, the waves the ship created in that V are about to reach the wave breaker.
Also the wind has picked up a little, as evident from the less smooth water surface shore-ward of the wave breakers.
Diffraction at a slit
Right after the waves from the V reach the wave breaker, they reach the opening at the end between the pylons. And what happens now is that the waves get diffracted at a “slit”: they propagate outwards as semi circles, even though the wave fronts were straight when they reached the slit.
How awesome is that? And all of this happening in a matter of minutes!
The weather changes
I said earlier that there was hardly any wind. Later that afternoon, it still wasn’t very windy, but the wind direction had changed: now the smooth and sheltered part has moved to the other side of the wave breakers. There are a lot more waves on the shore side of the wave breaker now, the ones with crests parallel to the wave breaker due to it moving, and the ones with crests perpendicular to it generated by wind. And you see gusts of wind on the sea side of the wave breakers in the different surface roughness.
So if you were wondering, too: That’s the kind of stuff I look at when I am wave watching. And I still find it super fascinating and relaxing at the same time! :-)