Actually, there is no need to guess. If you tilt your head 45 degrees to the left, you are looking at Hamburg the way it would be shown on a map, North up. The Elbe river, which you see in the foreground, flows east-to-west into the North Sea. And now there are at least two spots in the image below where you can see fronts in the water, more turbid water in the main river bed, clearer water in side arms and bays. Those fronts always start at upstream headlands and go downstream from there, therefore it must be ebb tide, with the water going out into the North Sea. Easy peasy :-)
Funny how “upstream” and “downstream” make so little sense in a tidal river, yet everybody knows what I mean…
Would be interesting to see if you can see fronts when the tide is coming in, too, when the muddy river water is pushed into the more stagnant side arms and bays. I expect so but don’t actually know. Maybe I will be able to observe it on some future flight?
Disclaimer: This post might well be called “fun with tides” similar to Sheldon Cooper’s “fun with flags” — it is super nerdy, but at least I am having fun!
There is some really cool art around Hamburg, and the one I want to talk about today is called “four men on buoys” by Stephan Balkenhol: Four wooden statues of approximately live-sized men, standing on little floats, moored in four different spots all over Hamburg. One of them happens to be on the Elbe river, visible when you cross the bridge from where I work over to the city center. You ca see the scene below: The train going across the bridge, and the guy (in the white shirt) standing on the river.
What you can sort of see in the picture above from the yellow buoy being tilted to the right: There is quite a strong current in that river. And what you can’t see in the picture, but will find out below: It’s a tidal current, hence its direction reverses regularly.
You can guess what that means for anything moored in the river: Yes, it will change its position following the tides!
This is where my nerdy self comes in. Whenever I take the train across that bridge, I try to snap a picture of the guy on the buoy. It is quite a difficult endeavour — the train usually goes pretty fast, and I never know where exactly the guy is going to be (well, I guess I could look at a tide table beforehand, but I’ve never done that) and taking pictures out of a train window is not that easy in itself. But sometimes it works out beautifully to show both the position of the guy and the currents:
As you see in the image below, the wake is in the direction towards the viewer. This means that the water is flowing towards the viewer, i.e. downstream. You can see that the current is fairly strong because the wake is very pronounced (“very” at least relative to some other pictures you’ll see later).
For this post, I checked my phone and found a collection of 16 pictures of that guy. So clearly I had to see when they were taken relative to the time of high water that day. In the image below, each tick marks the time of one of my pictures relative to zero, the time of the nearest high water.
As you can see, I seem to be on the train more when it’s close to high water than close to low water. Funny!
Now, when I show you all my 16 data points, let’s remember that we are now only looking at time before/after high water. We are neglecting important things like where exactly the picture was taken from (I’m excited to catch the guy on the buoy at all from a fast train!) or where we are in the spring / neap cycle. Plus the different times of day when the pictures were taken and the different weather conditions make comparison hard. Yet, it’s fun to see how the strength and even direction of the current (which you can see by looking at the wake and the position of the guy relative to the bridge) is changing!*
Before I show you the pictures, a CALL TO ACTION: If you happen to be on that train, snap a picture and send it to me! I’ll happily compile a better series with more data points! I’ll continue taking pictures, too, that’s for sure! :-) Imagine how you could use this kind of data in teaching! If I were to teach a class on tides at this university, I would have students collect this type of data and use it to say something about the tides on Elbe river. If there is enough data (which should be easy enough to get with many students commuting across this bridge every day), I am sure one could learn a lot from this case study! And working with data students collect themselves is always more fun than looking at some data set in a text book anyway. Plus how much more exciting would commuting get for those students once they start observing in this way, and starting to think about water, instead of just being bored on the train? There are actually a couple more times where you see the river quite well on the train journey between university and the city centre, so there might be many more case studies easily done if only people started looking for them…
And here are all 16 pictures, in the order going from low water to high water to low water. The caption includes the time before/after high water. Enjoy!
*Btw, sometimes you see that my mapping is clearly not right (for example, when the wake is in the direction away from the viewer, we cannot be past high water already, since the current is clearly still going up the river, so the tide hasn’t turned yet). These errors might be due to me not taking enough care when looking up the tidal data (yep.) or the tide tables that were used not accounting for factors that might have influenced the tides other than the classical tidal components, like for example wind conditions. I could, of course, go back and look at actual data and/or double-check, but I am happy with what I can see from the data already. If you are not, please knock yourself out and I’d be happy to host your guest post with corrections of my post! :-)
You know I like to point out where you can spot hydrodynamics concepts in your everyday lives (at least if your everyday lives include strolls along rivers and generally a lot of water)
A while back we went to Geesthacht. We were hoping for more ice on the Elbe river, but sadly there was none. But! In Geesthacht they have a weir, combined with locks. They keep water back to bring the level of the Elbe upstream of Geesthacht up to 4 m above sea level for shipping purposes. But then they obviously need a lock to get ships up and down this sill. But the coolest thing is the weir:
Do you see the three different states the fluid in the picture above is in?
Looking from right to left (i.e. with the direction of the flow), we first see normal flowing water. You can see that there are waves and ripples going in all directions. Then, the middle part of the picture, all disturbances on the water surface are clearly oriented right-to-left. That is because here the water is shooting (meaning flowing faster than waves can propagate), and all disturbances get deformed by the flow rather than spread by themselves. And then on the very left, we have a submerged hydraulic jump (which we cannot see, because, as the name says, it is submerged) and above massively turbulent water.
I just love the look of it!
Watch the video below to see the whole thing in motion.
For most of my readers it might be pretty obvious what the movement of floating ice says about the flow field “below”, but most “normal” people would probably not even notice that there is something to see. So I want to present a couple of pictures and observations today to help you talk to the people around you and maybe get them interested in observing the world around them more closely (or at least the water-covered parts of the world around them ;-)).
For example, we see exactly where the pillars of the bridge I was standing on are located in the river, just by looking at the ice:
What exactly is happening at those pillars can be seen even more clearly when looking at a different one below. You see the ice piling up on the upstream side of the pillar, and the wake in the lee. Some smaller ice floes get caught in the return flow just behind the pillar. Now imagine the same thing for a larger pillar – that’s exactly what we saw above!
And then we can also see that we are dealing with a tidal river. Looking at the direction of the current only helps half of the time only, and only if we know something about the geography to know which way the river is supposed to be going.
But look at the picture below: There we see sheets of ice propped up the rails where the rails meet the ice, and more sheets of ice all over the shore line. As the water level drops due to tides, newly formed ice falls dry and that’s all the sheets of ice you see on land.
The bigger ice floes in the picture have likely come in from the main arm of the Elbe river.
It is actually pretty cool to watch the recirculation that goes on in all those small bays (movie below picture). Wouldn’t you assume that they are pretty sheltered from the general flow?
And the current and the sun glint were perfect for this kind of photos:They almost look like schlieren photography images in those super old papers, don’t they? And I find it extremely fascinating how you can see the boundary layer between the flow and the stagnant water, and how wind waves don’t manage to cross that boundary. See the tiny capillary waves on the right side of the boundary? Those are locally generated because the larger waves on the top left just don’t make it over the strong shear.
You want to watch a movie? Sure!
And another thing I love on those early morning trips? Being completely alone in a pretty park, with dew on the grass and flowers in the sun :-)
I remember being on a looooong walk on some Danish dike when my sister was small and really didn’t want to walk any more, telling her about how phase velocity of shallow water waves depended on water depth and how you could observe that when waves are refracted towards the coast (assuming the sea floor has the right slope). And whenever I see this happening now I have to think of that freezing cold and windy day a long time ago.
Watch how the angle of the wave fronts changes as they come closer to the shore:
Today, we are back at the same pylon, only that this time the tidal current is a lot less strong, but there is a lot of wind, so our focus is on wind-generated waves.
It might be admittedly a bit hard to see, but if you watch closely and use your imagination, you might be able to see the waves propagating towards the pylon and then being reflected and radiating radially outward from where they hit the pylon. Pretty fascinating!
Can you see the locally generated waves to the left of the pylon? All those tiny waves where the wind is funneled around the pylon?
On the way to the pool I cross over the Elbe river on this pretty bridge.
Which is pretty spectacular, just because the structure itself is so amazing.
But what is even more spectacular is how every time I am there I see new things in the flow field. And the example I want to show to you today is the flow field around one of the pylons of the bridge that runs in parallel to the one I am on.
In the movie below you see a classical flow separation, similar to what might happen at an airplane’s wing. The water flowing towards you under the bridge arrives (pretty much) laminar, but then on contact with the pylon turbulence develops, eddies form and the flow separates from the boundary of the obstacle. Nice! :-)
Did you ever notice how when certain ferries dock, they stop, already parallel to the dock, a couple of meters away from the dock and then just move sideways towards the dock? Usually they don’t even move passenger ferries any more, just use thrusters to keep them steady while people get on and off.
But why this weird sideward motion?
One reason is the Coanda effect – the effect that jets are attracted to nearby surfaces and follow those surfaces even when they curve away. You might know it from putting something close to a stream of water and watching how the stream gets pulled towards that object, or from a fast air stream that can lift ping pong balls. So if the ship was moving while using the thrusters, the jets from the thrusters might just attach themselves to the hull of the ship and hence not act perpendicularly to the ship as intended.
But I think there is a secret second reason: Because it just looks awesome :-)
Do you use a tide chart to find the best time for your Saturday walk, too?
I showed you a vortex street on a plate formed by pulling a paint brush through sugary water as an example. Now today I want to show you the real thing: Instead of stagnant water and a moving object, I bring to you the flowing Elbe river and a bollard!
Watch how vortices with alternating spin are shed every three or four seconds!