24 Days of #KitchenOceanography — Surface waves and internal waves
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Tag Archives: waves
Irregular wave ripples, and some left on the beach when the water is gone
I’m getting more and more fascinated with wave ripples. I kinda understand how they form, but not enough to be able to explain as much about them as I would like to.
For example below: Why is the pattern so different where sand has been washed on top of the shallow stones? Yes, the water depth is different there, which will have an influence on the wave field, which will, in turn, have an influence on sediment transport. But HOW?
Here is another example. The wave ripples look choppy everywhere (and kinda cool!), but on that shallow, flat surface of the stone the wave length is completely different as is the orientation. And this is still all submerged, you can kinda estimate the water depth from the tips of the kelp stuff just reaching the surface.
Same day, slightly different area. Isn’t it cool to see how in the upper left there are no wave ripples but those streaks of larger pebbles?
And look at this. Utter chaos in the middle, little more orderly ripples to the sides! Why???
Or here. Steep wave ripple crests, long and shallow troughs in which larger stuff has been deposited (or is it the finer grain that has been transported away from the troughts into the crests and the coarser stuff got washed free and just stayed?). Estimates of water depth with help of kelp tips just breaking the water surface again…
Different day, more orderly wave ripples. But wavelength changes with distance from the sea wall. And weird things happen on the shallow stones…
On a low water day, parts of the sea floor got exposed. Now. I know for sure there were ripples all the way to the seawall. But at some point, the water retreated. When did they get smoothed out? The problem is that I can only really observe the seafloor when the water is calm, yet ripple marks form when there are waves. What happens during the transitional period? Or here, when the water level sinks?
Another interesting pics with some ripple marks that are still there, and then these little, smooth spots that recently fell dry (within maybe 15 minutes or so — I took pictures that same morning when there was still water on top and you could only see that there was a bump under the surface by “reading the wave field”. And then when I came back, the water level had fallen and this piece of mud had been exposed). Did the ripples there get smoothed out when there was still water on top, or at what point did it happen?
Or here where we have these interesting rip-current like structures right at the bottom of the sea wall:
Here is another thing I find fascinating: Ripples towards the sea wall, and then these streaks of larger stuff probably aligned with the main direction of the waves (I think the larger stuff is less dense than the sand, though, maybe pieces of broken shells?). What has to happen in order to transition between these two regimes?
Also note how there is no sand on the large flat stones today!
And same spot, different day: More ripples gone, and even less sand on the large flat stones!
So how do I figure out what is really going on here? I guess I would need to capture both the wave field and the sea floor over time. Web cams above & below water level, plus measure water depth? Any suggestions?
Reflections in waves
After doing a #friendlywaves post from a Norwegian fjord yesterday, let’s do another one from somewhere south of Bergen, by my friend Arnt. Very different mood today!
What I find super fascinating about the picture below is how clearly you see the ship’s feathery wake in the reflection of the street lamps on the bridge, as the ship is moving towards the left and you are looking out over it’s starboard side, more or less perpendicular to its direction of travel.
Towards the left of the picture, the street lamps reflect as straight lines towards you, as you would expect if the sea was calm and the surface more or less flat. (Why as a line rather than a single point source? Because, of course, the water isn’t completely calm, so in each wave there is an area that is positioned exactly such that the angle of the incoming light and the angle of the reflected, outgoing light (which have to always be the same) reflect the light from the lamp directly into the camera. As there are many waves that somewhere match that condition, we see light reflected in many spots even from a single lamp)
Towards the right, though, these lines of light get more and more deformed, bot as the lights get closer to us (so we notice the deformations more easily) and as there are waves caused by the ship.
Picture by Arnt Petter Både, used with permission
And isn’t it fascinating how the moon reflects as more of a smudge than a line? I think that is because its light is hitting the water at a steeper angle, so there would need to be much steeper waves further away if they were to reflect the light towards us.
Anyway, below there is another picture from the next night, I think. Here you very clearly see the ship’s feathery wake again, and here you can see how the slope of the waves is important for what gets reflected towards us and what doesn’t: Only where we have steep slopes (i.e. only at the wake) can we see reflections f the light from the land in the lower third of the picture. There is an area where the angles clearly don’t work between that and the clear reflections further towards the shore. Oh, and do you see the different surface roughnesses that make reflections seem clearer and less clear depending on whether the surface is flatter or rougher due to a local breeze?
Picture by Arnt Petter Både, used with permission
This is fun! Does anyone else have #friendlywaves for me? :-)
A #friendlywaves from a field trip in a Norwegian fjord
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?
Visiting my friend’s office for some #wavewatching
Since we seem to be on the topic of wake watching these days, here are some pictures I took when visiting my friend Liz at the European Cruise Service’s offices in Bergen the other day. She had already told me about the awesome wave watching to be done from their meeting room, but see for yourself!
Above, you see a very nice example of the turbulent wake of that cruise ship substantially modifying the wave field even after the ship is gone (or, in this case, after the ship has turned to leave in the other direction. Doesn’t this whole picture look very Titanic? Or is that just me?). What is going on there is that the turbulence introduced in the water by the ship and its propellers moving a lot of water around sticks around for quite some time. While the water is still moving due to the turbulence, “normal” surface waves can’t propagate in the turbulent area. The water’s surface thus looks very smooth there, a lot less rough than in areas where there are wind waves. And the smooth areas reflect light similarly to a mirror, whereas the rough areas’ light reflections seem to resemble maybe a disco ball?
Below, you see both parts of a wake quite well: The turbulent wake right behind that ferry (which will show up all smooth from a distance), and then the feathery V wake (with the ferry at its tip) that spreads on either side of the turbulent wake.
And you see some more old turbulent wakes in the picture above, for example one that the ferry in the foreground is following closely (see how it stretches out before the ferry?) and one that turns left to go towards the invisible Askøy bridge (you can still make out the ferry where the wake begins).
Do you see the potential of this wave watching spot? I definitely have to come back!
Even if you are not into wave watching, it’s a super interesting place to visit because it gives quite an interesting look on the city, even with Ulriken being disguised by the low clouds that day…
Would you be interested in a wave watching tour when you visit Bergen (or Kiel, or any other place)? If so give me a shout, we might be able to arrange something ;-)
I’m explaining your wave pictures: A #friendlywave from the Île d’Yeu, France
The whole #friendlywave thing (where I explain your wave picture) is starting off great! Here is one that reached my via my Twitter; link to thread here.
What’s going on in the north east of Île d’Yeu, France? Here are four pictures from the Twitter thread that got me intrigued: Because of the awesome waves they were displaying, but also because they introduced me to ESA’s EO browser which is so amazing that I don’t think I will be able to stop playing anytime soon!
First, a true color image of the Île d’Yeu and, more importantly, the wave field around it (Click on all pictures to enlarge).
And this is what the topography in that area looks like:
Zooming in on the area north of the eastern tip where something interesting is happening…
…this checkerboard pattern of waves! Now the question is what causes those waves. Well, let’s find out, shall we?
I couldn’t figure out exactly where the image above was from, but I am seeing a very similar pattern in the pictures that I saved off the EO browser myself.
First, here is a true color image again (click to enlarge, or click the link to see it on the browser to play yourself)
True color image of Île d’Yeu and surrounding ocean, acquired with EO browser, January 28th, 2019.
Here is the same image, except with my annotations on it. I have marked a couple of wave crests to show what I think is going on. What I see here (and please let’s discuss this! I’m super curious to hear what you think!) is a wave field coming in from west northwest-ish (see straight-ish fronts on the top left). When this wave field encounters an obstacle in its path (the island), it gets diffracted, kind of as if there were two very wide slits on either side of the island (a very simple example of that here). It’s difficult to follow the wave crests that pass the island on its north side, but the ones that go round the south side are clearly visible as they turn around the eastern tip of the island.
Zooming in to look at it more closely:
True color image of Île d’Yeu and surrounding ocean, acquired with EO browser, January 28th, 2019.
And here is my annotated version of the wave field. You recognise the wave crests that were propagating along the southern side of the island, then turned around the eastern tip and are now spreading northward. And you see the wave crests of the waves that travelled along the north coast all along. Notice how they are crossing in a crisscross pattern?
The area with the really dense red checkered pattern is the one I think was shown on the original picture on Twitter. So my interpretation is that it’s an interference pattern of waves, all originating in the same wave field, being diffracted l’Île de Yeu. What do you think? Do you agree?
What I find quite interesting is that it’s very easy to follow the crests that propagate northward around the eastern tip, but a lot more difficult to do the same for the ones propagating southward. I could imagine that the explanation is the topography: The waves propagating in the north of the island were in shallower water for pretty much the length of the island, so they might have lost a lot of their energy already, whereas the ones from the south only run into shallower water once they’ve turned around the eastern tip of the island.
Thanks, Rémi, for pointing me to ESA’s awesome EO browser and to your super interesting Twitter!
P.S.: Speaking of topography: Of course the change in water depth could also have an effect on the wave field by refracting the waves towards the slower medium, i.e. the shallower water. But I don’t think that’s the case here. Do you?
Commissioned wave watching: Eckernförde edition on a beautiful calm and sunny Sunday!
Recently, more and more of my friends send me pictures of waves they spotted when walking along a lake side or taking a ferry ride. I love how contagious wave watching is, and I love sharing my fascination with you! :-)
Here are some pictures that Fred sent me of his lovely Sunday walk today. There are at least five interesting things that I notice in the picture below. How about you?
- Look at the beautiful interference pattern where two wave fields are almost perpendicular to each other, creating the checkerboard pattern! As you see in the picture below, there is one wave field coming in at a 45ish° angle to the sea wall, so its reflection is at 90ish° to the original wave field.
- In the background you see the surface roughness changing and the water seeming darker where there is a breeze going over the water, creating small ripples that reflect the sky in a different way than the smooth surface closer to us.
- See the waves the seagull made where it landed on the water?
- Looking at the foreground, do you see the tiny ripples that show up not so much on the surface of the water, but rather at the sandy ground, because they focus the light?
- And notice how you can look into the water in the foreground but not in the background? That’s the awesome phenomenon of total internal reflection where, if you look at water at an angle that is smaller than a critical angle, you cannot look into the water any more but just see light reflected at the surface! One of the things I never understood we had to learn about in school, but that I find super cool now.
And in the picture below, what do you see?
What I find most interesting in the picture above is how the reflection of that storehouse tower looks different in areas with different surface roughnesses. Where there is a breeze on the water in the background and in the foreground, it’s a lot more spotty than in the calm and smooth surface in between. And the checkerboard waves pattern (now you see the seawall that created the reflection, btw) carries through to the reflections, too, with the blue crisscross going into the white area where a cloud is reflected.
And then the phenomenon of total internal reflection is really clearly visible here with a lot of reflections on the water (or just more interesting things to reflect than just a blue sky in the previous picture) and a view down to the ground only in the very foreground of the picture.
Thanks for sharing these beautiful pics, Fred!
Visiting the ruins of a wave power plant — waves running up a funnel to fill a reservoir
Using wave energy to generate electricity sounds very attractive, after all there are tons of waves and all they do (in addition to looking pretty) is eroding coast lines. But that’s exactly the problem: There is a lot of energy in waves, so wave power plants have to be extremely tough.
Here is another post about the ruins of the wave power plant I visited on Toftøy. For an idea on the size of the waves on this not-very-windy day with fairly moderate waves, check out the movie at the end of this post (there are two people that you might be able to spot on the rocks on the other side, and those pillars used to carry a bridge).
Below you see the waves entering a funnel that will lead them slightly uphill…
…so the water can fill up reservoir which is located higher than sea level…
…in order to drive turbines when the reservoir is emptied out again into the sea.
You already see the huge amount of energy stored in those waves, and looking at how little is left of the power plant, it’s definitely safer to stay well clear of those waves!
Check out in the movie below what it looks like when waves enter this power plant (and pay attention to the two people on the rock on the other side — they clearly didn’t expect that much energy in the waves! :-D)
Visiting the ruins of a wave power plant — waves driving a turbine
After posting about how longer fetch leads to higher waves yesterday, here is why I was in that exact spot in the first place: To visit an old wave power plant on Toftøyna! The power plant was built in the 80s but destroyed only a couple of years after it had been built, so all there is to see now are some pretty exciting ruins!
Below, you see a cylinder that is a couple of meters high and some meters across, and that connects the air above the water with the water below. There used to be a turbine sitting at the top of that cylinder that used to be driven by the air column moved by waves at the base of the cylinder. The turbine is long gone, but what still happens is waves putting the water inside the cylinder into motion. And that looks pretty impressive as you see in the movie below!
Looking at those fountains shooting out of the cylinder, it’s not difficult to imagine what enormous kinds of forces the turbine had to endure before it got destroyed. Super impressive!
But what’s similarly impressive to me is how there are tiny flowers growing in this harsh environment. I guess it’s true: “life, eh, finds a way” :D
Same wind, different waves, or: the influence of fetch length on the size of waves
I just found this picture that I took back in May near my friend Elin’s cabin on an island in western Norway, and it’s a really nice illustration of how the same wind will cause very different waves depending on whether it’s blowing over the sea for many kilometres, or over a puddle for only a couple of centimetres.