Tag Archives: tidal current

The mystery of “51”, or: Tidal current at Straume Bru

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My friend Pierre, who I went to Saltstraumen with in 2012, wrote me a text about a year ago and asked me to remind him to tell me about 51 next time we met. We met and, as we do, geeked out about some hydrodynamics stuff. And he told me about 51. It turns out that on his way to work, Pierre crosses Straume Bru on the 51 almost daily, and watches the strong tidal currents and whirlpools that form there. But it wasn’t until a couple of days ago that we managed to take bus no 51 to this specific spot — Straume Bru — and geek out about it together!

I got there about an hour before Pierre did. Not because he was late, but because I was so excited to finally see that tidal current I had heard so much about! And also because I had looked at the tidal forecast and wanted to make sure I would see the strongest current and not miss it by an hour.

Turns out that when I arrived, the current was indeed quite strong. But look at the water level relative to the structures. Pretty much half time between high water and low water, even though the current is strong, there is still a lot of water left in the reservoir!

I love watching the waves radiating from the edge in that wall, and the wedge of eddies that is separating the fast flow from the boundary.

Also look at how the waves are being deformed by both the eddy and the current in the picture below! Especially in the top left corner, where the “wake” of the edge follows the meanders.

As had been forecasted, it started to rain while I was wave watching. With the surface a little rougher now, I noticed these two long streaks. Not sure what was going on there?

It didn’t look like oil or any surface film, but I can’t really think of anything else right now. I was briefly considering Langmuir circulation, but I don’t think the wind was steady enough and also I don’t know if that would combine well with a strong current. Any ideas, anyone?

In any case, the stripes weren’t visible any more when the rain stopped. But look at all these amazing waves!

And, now looking downstream, some more eddies and whirlpools!

Looking upstream towards the bridge, we see the glossy V that is formed upstream of where the wakes meet that are formed by the walls on either sides of the outlet.

And downstream again — how awesome and cute are those little eddies? And how amazing is it that they can persist over long distances while maintaining a dip in the surface that is probably as long as my thumb?

And not two seconds look the same!

Below is a closer look of the two wakes of the sides of the outlet coming together and overlapping.

And here is another picture of the wedge of eddies that forms, separating the strong current from the more stagnant, turbulent boundary layer. Look at how irregular the wedge is, formed of eddies of different sizes that are being advected downstream! And also look at the waves that are being pulled with the current, leading to stripes along the current!

Here is another look at the wedge and the stripes of the waves that are being deformed by the current.

And another one, because the scenery is actually really pretty, too! Which I hardly noticed until I had taken about 500 pictures of the water ;-)

More eddies in the wedge.

Interesting how one side of the outlet forms a wedge while the other “just” forms a wake, isn’t it? I think it’s because the the left one, the one with the wedge, restricts the current a lot more.

And here is a new perspective: Looking at the wedge of a second, parallel outlet. You see really well how the boundary layers from both sides come together!

Looking upstream, the standing waves in the foreground give you an idea of how strong the current is!

And another beautiful wedge!

And more turbulence looking downstream. Funny how parts of the surface look so glossy and smooth, isn’t it? I think those are the areas outside of the current that aren’t turbulent.

Now a final excursion to the other side of the bridge, to look at the wakes of the structures. Notice how much less water there is now!

And here is the upstream part of the V.

And the beginning of the wedge.

Walking a little around the corner, we see that the wake begins already upstream of the corner!

And a final look at the wake.

This is how happy wave watching makes me, even when it’s cooold and raining! At least occasionally, not the whole 1.5 hours I was there… The rain, I mean. Happy I was all the time :-)

Two days later, I actually took the bus across the bridge (after another adventure with Pierre, more on that soon!) and the current went the other way! As it should, but it’s always nice to confirm theory.

Anyone taking this bus regularly? You should start taking pictures for a time series! :-)

Fantastic wave and current watching at Piel Island

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We saw Piel Island with a very cool castle ruin across the bay when visiting the South Walney Nature Reserve the other day,  and were intrigued by it. Depending on the tides, you can drive over by car, walk, or take a ferry, which is what we did.

Arriving at the spot where the ferry was supposed to leave, we were greeted with this beautiful sight: A pier going down into the water, creating beautiful wave pattern in the strong incoming tide! We see a hydraulic jump similar to the one we saw on Walney Island, except that this one is even cooler: It happens in the area where the pier is just below the water surface, but a strong current goes underneath the pier on the land side as soon as it is above water level.

Above, you see the current going left-to-right, creating a lot of turbulence where water comes out from underneath the pier (see those eddies where the water looks as if it was boiling?). You also see the waves hitting the pier on the left side, and then standing waves towards the right of the pier, locked in place because they are propagating upstream with the current’s exact velocity, thus staying in place (aka “standing”).

This is super fascinating! To me, anyway ;-)

Once on Piel Island, there is a lot of really cool wave watching to be done, too.

Below, you see waves reaching the island and “wrapping around it” — i.e. being refracted towards regions of lower velocities, which means that they will be bent towards the shore, no matter which direction they originally came from.

You can observe this for quite a big part of the island as you walk around it! The original wave direction is the one seen in the bottom left, all the rest of the wave field has been refracted by the change in water depth!

But obviously there is a limit to how long you can play this game. Below, you still see waves wrapping around the island, but they aren’t reaching the shore more or less parallel to it.

But even just watching all these crests break, one after the other, along the shore looks pretty cool!

But, obviously, if waves get wrapped around an island, but not completely, there must be areas where wave fields going around either side of the island meet up at an angle to each other. Like here:

And once again, this time moving:

And another very good spot to see this kind of pattern is a little headland like below:

Can you spot the distinct checkerboard pattern of the waves, and see how they break where a crest meets another crest?

I can watch these kind of things forever without getting tired of it!

And once more, as a short movie, because waves are even more awesome when they are moving:

Or waves more generally, especially breaking waves.

How beautiful is this?

I can really watch waves for hours without getting tired of it.

But anyway, walking further around the island, here is a spot with fewer waves: Here we are in the lee of the island, the area that is sheltered from the wind by the island itself.

Oh, and this is the ferry that got us over to the island. As you might notice below, the current has turned and is now going out — unfortunately I didn’t take another video or even good picture! But you see the edge of the jetty in the lower right, and the current downstream of the obstacle with a very different surface texture than the surrounding water. And then there is always next time… ;-)

On our way back home, we stopped for scones and coffee (sorry, no tea) and had the amazing views you see below. These channels don’t look dangerous by themselves when they are empty, but thinking back to how quickly the tide comes in around here they don’t seem as harmless any more, do they?

But oh so pretty! :-)

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

Reading ice on a river as tracer for flow fields

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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

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On the way to the pool I cross over the Elbe river on this pretty bridge.

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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

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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.

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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:

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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.

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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! :-)