Tag Archives: sediment

Watch how sediment is transported with the flow while surface waves above move in the same direction but much faster

When I was on my way down to Kiel fjord earlier this day I was in a bit of a weird mood. I was thinking about how the weather was grey and gloomy. And how that meant that there wouldn’t even be a nice sun raise to take pictures of. And how I might already have said everything there is to say about waves in that area of Kiel fjord.

But then, this happened.

Thanks to people working on the drainage system on the other side of the road, there was a lot of sediment dissolved in the water dripping into Kiel fjord! And watch how it spreads into half circles from where it enters the fjord. How pretty is that?

And then, even better, when you look at the surface waves, they propagate a lot faster than those clouds of sediment do. So this is a really nice example of how wave motion and transport of matter in the ocean are independent of each other (as a first order approximation at least).

We do see an influence of waves on the sediment rings further out as they get deformed by the wind waves on Kiel fjord.

Actually, looking at the whole thing from a bit further away and not from directly above, we see that the sediment forms a plume parallel to the coast rather than spreading out in half circles further. And this I do believe is due to wave action. But let’s focus on the area right around the outflow… ;-)

I found this pretty cool to watch and was very glad I stopped my run to take pictures and movies when I first saw it, because on my way back it was all gone already. Lesson learned: Always stop to observe the cool stuff instead of pushing through with the exercises you planned on doing ;-)

Roll waves in the sand dunes? Observing erosion

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?