Reading ice on a river as tracer for flow fields

Ice on Elbe river in Hamburg. By Mirjam S. Glessmer

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.

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Small port on a tiny bay on the Elbe river in Hamburg. Look at the sheets of ice on shore!

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?

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Flow separation

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.

IMG_0901 IMG_0906 IMG_0923But 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! :-)


Tidal elevations and currents in Fowey, Cornwall

Tides in Cornwall.

The other day we talked about a very simplistic models of tides in a glass, and how the high tide and low tide travel as a wave around an ocean basin. This isn’t really a news flash for people reading this blog, I know. But it is sometimes hard to imagine how big the differences between high tide and low tide actually are, since the water rises and falls so slowly it is hardly noticeable.

Fowey harbor in Cornwall at high tide

On my most recent holiday (even though “most recent” means “some time during summer”, which is actually quite a while ago), A and I stayed in Fowey and had the best time. Anyway, we happened to stroll along the pier, and I happened to snap this picture.

Some more strolling happened (and we might or might not have had Cornish Cream Tea), and six or so hours later we were back in the same spot, to see this:

Fowey harbor in Cornwall at low tide

The water was gone! And I still find it absolutely fascinating.

Especially since at first glance the tides don’t seem to result in alternating currents. Which is really not possible.

Fowey harbor – incoming tide

But it took more than just a second look to realize that the tide in the picture above is coming in, whereas the one below is going out (Pictures taken from pretty much the same spot).

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Fowey harbor – tide is going out

You can only see that if you look at the moored sailing ships far across the water. The colorful boats always face out towards the sea – because they are moored between two moorings and are not turning freely around a single mooring as I had assumed they would. Duh! But for the yachts in the background it is clear they are only moored in one spot: They face right on the upper, and left on the lower picture. Yep, those are the kind of things that fascinate me while I’m on vacation! :-)