Tag Archives: wave breaking

A #friendlywaves from Cyprus

My friend Alice (of the awesome Instagram @scied_alice and the equally awesome blog, which you should totally follow) sent me a #friendlywaves from her trip to Cyprus. She said that this was a simple one, so I am looking forward to what else she has up her sleeve once I pass this test ;-)

So here we go with the pictures she send.

Clearly, she is on a boat trip, and she’s looking back at the wake of the ship. You see the one side of the feathery V of the wake, pretty much in the middle of the picture. On the “feather” closest to us, you can still make out the turbulent part of the breaking bow wave, where the water surface looks all crumpled up and not as smooth as it does further away from the ship. Actually, this is a really nice example to show that the waves are traveling away in the wake, but the water is not: All the other “feathers” further away have smooth surfaces as they have run away from the ship’s trajectory, while the turbulent wake traces out the exact path where the ship went (as long as there aren’t any currents moving around the water, which we’ll assume for now).

Picture by Alice Langhans, used with permission

The waves in the V-shaped wake are fairly steep, you can see them very slightly tipping over on occasion.

And Alice sent a second picture: Similar situation, except now it’s a little more windy. The turbulent wake is a little more foam-y than in the previous picture. This could be because the ship is sailing faster, or because it’s more windy. I would guess the first.

And when I say “sailing”, I am using this as the technical term for a ship driving. I am assuming that the boat Alice is on is not a sailboat. I’m thinking this because the wake looks fairly turbulent and sail boats usually don’t cause this much turbulence; also the little bit of the boat that I can see doesn’t really scream sailboat to me. We’ll have to wait to hear what she tells us, though!

Picture by Alice Langhans, used with permission

On both pictures, there is hardly any swell visible. Waves are usually not as visible when the water is deep as when they run up on a beach, and so far off shore we can assume that the water is fairly deep. But that also means that it isn’t very windy, hasn’t been very windy recently, and hasn’t been very windy anywhere near recently, either, so no large waves have traveled into the region.

So much for these #friendlywaves. How did I do, Alice? :-)

Same beach, different waves. Why?

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?

Wave watching: Refraction and diffraction of waves

A little more wave watching, today with a focus on how waves change direction when they run into shallow water. Let’s look at this beautiful wave and see what happens when it reaches the shallow shore.

Above, you see the wake of the pilot ship, consisting of many wavelets that propagate as parallel wave crests towards the shore.

Below, you see that the wave is propagating at an angle to the shore (something around 45 degrees, maybe?). If you focus on the wave crest that is just offshore of that little obstacle in the water (curious enough, a piece of brick wall), you clearly observe that angle. But then looking at the next wave crest in-shore, it is almost parallel to the shore! Assuming that both crests come from the same wave field, so that the second one was in the same position as the other one only moments before (which I know it was because I observed it), something clearly happened between then and now.

Refraction of waves

Why do waves change direction as the water depth changes? As waves run from deep into shallow water, at some point they start to “feel” the bottom, which slows them down.

Or, more scientifically speaking, the dispersion relation for shallow water waves is a function of water depth: The shallower the water, the slower the waves. That means that if a wave crest is running on a slope with one side being in shallower water while the other one is still in deeper water, it will change direction towards the shallow water because the shallow side of the crest is slowed down while the deeper side keeps on moving faster, thus forcing the whole crest around a curve.

But in this picture series there is more to see: See how the wave crest gets deformed after it has passed that obstacle?

Diffraction of waves

This is a process called diffraction: The change of direction after a wave crest has passed either through a slit and then starts radiating from that slit as circle segments, or, in this case, an obstacle. The wave passing an obstacle is, in a way, the same as the wave passing through two wide slits which are very close to each other, only separated by the obstacle: The edges of the wave crest at the edges of the “slits” also start radiating out as circle segments!

One spot, so many things to observe!

And there are, of course, ships. What I wanted to show on this picture is a close-up of the turbulent wake of the ship, but it’s really difficult to see so I’ll let that pass for today.

And the picture below shows so much cool stuff: Waves radiating from that pylon. Ripples on the surface by a gust of wind. Wave crests getting a lot steeper as they run up on the slope. And, my main reason for posting: I really like how the wave is spilling as it breaks! :-)

Waves break when they reach a critical steepness

You can quite accurately predict when a wave is going to break. When it will break depends on its steepness, which depends on the slope that it is running up on. So due to the funny beach shape caused by the differential erosion on the upstream and downstream sides of groynes we can watch one and the same wave crest break several meters apart depending on the side of the groyne we look at. Kept me fascinated for the better part of the three days we spent in Hastings, and would have kept me entertained for a lot longer, I am sure, had I not had to go to a conference (and win a poster award there, so it was all worth it in the end ;-))

Here we see a wave breaking on the right side of the groyne, while at the left side the previous wave is still breaking and the one breaking on the right side hasn’t even developed a clear crest yet:


It’s a lot easier to see in a movie. Have fun!