Tag Archives: beach

Christina’s #FriendlyWaves from Panama!

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Christina writes on Twitter: “#wavewatching from a plane, approaching #Panama. @Meermini, do you know what causes those regular ‘wrinkles’?” and how could I resist writing a blog post about what I think might be the explanation?

Below is the picture Christina shared on Twitter.

Picture by Christina Oettmeier @sulfurium

What I think we see here are basically two wave fields: The regular “wrinkles” and then a lot of small crinkle.

The small crinkle are boring: local, wind-generated waves. They are not what Christina asked about.

But the wrinkles are swell: Waves that were formed in a storm far, far away and that have propagated here over a long distance. While propagating from the area where they were formed to the beach where Christina took these pictures, the waves got sorted by wave length. The longer a wave, the faster it propagates in deep water. So long waves from a distant storm will arrive first, and over hours or days the wave lengths of the waves arriving at the beach will get shorter and shorter. The wave lengths we see here seem to be about the height of the high rise buildings we see on the shore. The highest high rise in Panama is almost 300m high, so the wave lengths might not be that long, but at least 100+m.

Why do they look so “wrinkly” and not like proper breakers? When waves are in water that is shallow compared to their wave length (so say water depth would be less than 50m for these waves if we assume they are 100m long, which I think are both reasonable estimates), their shape changes from the normal sine-shape that they would show in deep water, to steep crests and loooong troughs. You might have observed waves with this shape for example in the very shallow waters of a beach on the wadden sea coast or any other beach with a really small slope, where waves look like sausages or pool noodles that are being shoved onto the beach (compare for example to pictures in this post).

What makes me confident that we are really seeing what I’ve just described above? Mainly that I can see the interaction of the waves with the sea floor. If you look at the pic above, do you see the area where the waves bend? That’s where the water is shallower. I’ve tried to sketch that below: The red lines are — in first approximation — the wave crests (I’ve only drawn in every third or so for clarity). Red dashed lines are kinda the second approximation of the wave crests: Those are the deformations that I want to talk about. And those deformations are caused by a shallower area, which I’ve drawn in with the green dashed line. This little submerged headland slows that part of the waves down that runs above it (because in shallower water the wave’s speed only depends on water depth, not on wave length any more), but not the rest of the waves that propagate towards the beach with the straight crests intact.

It’s even easier to be confident when we look at the next two pictures that Christina shared with me. Now we are a little closer to the beach and can see the area where the waves break and where it is shallow enough that the wave lengths drastically decrease (since the waves are slowed town more and more the closer they come to the beach, waves that are further out are still faster and can catch up to waves in front of them). This is very typical for the parts of a beach where the depth changes rapidly.

Picture by Christina Oettmeier @sulfurium

And on the next pic, we see even more clearly that the waves change from pool-noodle shaped offshore to breaking waves close to the beach:

Picture by Christina Oettmeier @sulfurium

In case you don’t see what I am trying to point out, here an annotated version of the pic above. Green dashed circles: Smudges on the window, or possibly reflections on the window, but nothing to do with the waves. Red circles: Here we see foam on the back side of breaking waves, so there was definitely some wave breaking going on here. And blue circle: Cool structures in the flow of water that is retracting downslope from the beach, back into the ocean.

So much for now. No idea if that made any sense to anyone except myself. Please let me know! :-)

Influence of wind and water depth on a wave field (or: a beach vacation in Dornumersiel)

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I took the selfie above mainly to send to my mom from my vacation in Dornumersiel on the German North Sea coast. But then when looking through the hundreds of pictures I took that day, I realized that not only was my hair parted on the wrong side because it was so windy (ha!), the wave fields to my right and left looked actually quite different, without the reason for that being immediately obvious. So let me show you a picture facing the other way.

Above, you see this wave breaker like structure, protruding into the sea. The wind is coming from the right side, thus the waves are a lot larger on the right side of the breaker where they are getting more and more energy from the wind as they come towards us, than the waves on the left, the lee side of the breaker, where they don’t get any new energy input and are just refracted around the breaker.

Looking the other way, towards the shore, the difference becomes even more clear (picture below) isn’t this fascinating?

I really like watching how waves interact with structures. Below, for example, we see that the wave crests are coming towards the wave breaker at an angle, and that they are reflected and traveling away from it, too. This contributes to making this side look a lot more choppy than the other side!

On the other side, the waves look smooth. I was still standing on the breaker when taking the picture below, and you see where the sea surface is still sheltered from the wind and where the fetch is long enough so the surface roughness increases and ripples and capillary waves form.

Since we are in the Wadden Sea, the shore has a very shallow slope going into the North Sea, so waves look super interesting when they are in the shallow water. Below you see many many almost-breaking wave crests behind each other, coming towards us. The water depth is clearly a lot less than a wave length, the waves are interacting with the bottom and thus have really long and uniform troughs and steep, short crests. (btw, for those of you wondering how I could say anything about water depth in my #friendlywaves post on Saturday: This is how. This is an example of waves in very shallow water, and you clearly see their shape being different, don’t you?).

I love looking at the details of where they hit the beach! All the sparkle, all the little Mach cones around the pebbles where the water is running off, all the small capillary waves!

Below, someone accidentally walked into my picture, but that’s actually a good thing, because it gives you a scale, and if you look at the little wave rings that were created when she put her foot into the water and it splashing forward a little. The wave rings actually have comparable sizes to all the other small stuff going on on the sea surface!

And what’s also pretty impressive: How the crests get refracted by changing water depth. Below it almost looks like parabolic shapes coming in from the right, right? The side of the parabola that is further away is actually the wave crest that is coming in from the open sea, and the rest, i.e. the actual curved part, is partly diffraction around the breaker and then refraction because of changing water depths. So cool!

Since I spent quite some time there, here is a picture later that day with a lot less water. Tides and all that… ;-)

And then another day with a different wind direction and less sun.

I think it looks really cool to see the fairly wide area to the right of the breaker, in its lee, where the surface is really smooth!

So far, so good. Gotta go now! Do you find this as fascinating as I do?

Same beach, different waves. Why?

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Here is a puzzle for you.

Walking along a beach, first, the waves looked like this: One wave breaking at a time.

That’s the situation you also see in the foreground of the picture below, while in the background, a little further down the beach, something else starts happening.

If we look closely at that situation (shown in the picture below), there are several waves breaking at the same time, one behind the other.

And it isn’t just coincidence, it keeps happening throughout hundreds of pictures I took that windy Sunday. Why is that?

I think (and this theory would be easy enough to test if the water was warmer or if I wasn’t such a sissy) that the slope of the beach is just different on either side of this little jetty or whatever it is. The shallower the slope, the earlier waves of the same wavelength can “feel” the sea floor, or the shorter waves have to be to “feel” the sea floor at the same distance from the water line.

So I think the slope on the left of the jetty is shallower than on the right, making the incoming wave field that is the same on either side (I’m assuming, but give me a good reason for why it shouldn’t be?) behave differently.

Funnily enough, the only reason I ended up on that beach was that I wanted to go watch a cruise ship go through the locks and into the Kiel canal with a friend. And, funnily enough, the ship decided to not go through the Kiel canal, as it did the week before. So we decided that we should go to the beach. Very good decision! :-)

But here is a “before” pic from when I was still thinking I would be writing a blog post about the ship going through the locks. Isn’t the seagull hilarious, posing like that?

Roll waves in the sand dunes? Observing erosion

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On our trip to the west coast yesterday, I observed something really cool: Sand roll waves (I think!) in the sand dunes!

But before I get to that, this is the setting on Sylt. A sandy beach opening up to the North Sea, that is separated from the land by sand dunes which are overgrown with some kind of beach grass.

Yesterday was a windy day as you see from the waves, but neither was the water level very high, nor was the wind anywhere near as strong as it gets here during winter storms, so the erosion happening yesterday is not very strong compared to what it is like during more extreme weather conditions (and the process I am focussing on here is probably one of the least important ones).

In order to prevent erosion of the dunes which protect the inland from storm surges etc, it is crucial that the beach grass growing on the dunes isn’t stepped on by the hundreds of tourists visiting this beach every day (probably thousands during summer). Therefore there are these wooden staircases installed in regular, short intervals to bring people across the dunes without them doing any damage to the vegetation.

Therefore, in most places, the dunes look like this.

In some places, though, there is little or no grass growing on the dunes, so imagine what kind of damage strong winds can do here, let alone a storm surge!

And in one of these open sand areas I observed what I think are roll waves. Do you see what looks like a drag mark a little right of the center in the picture below?

Check it out in the movie below (it zooms in after 5 seconds to show it more clearly) — there is sand surging down this track! To me this looks very similar to roll waves, and I know roll waves have been observed in sediment flows and lots of other places, so why not in the sand of these dunes? What do you think?