Tag Archives: erosion

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?

Erosion

Erosion happens not only to sandy paths or beaches on the large scale, but also on a small scale to really sturdy structures like the groynes in Hastings, where each of the massive pillars has its own little dip around it, being bitten into the concrete little by little by the force of the waves, moving little stones around…

We saw it in another post already:

Yes, that’s why there are puddles exactly in the spots where one would like to sit, sheltered from the wind, the back resting against one of those pillars… ;-)

How water changes coast lines

One thing I have been noticing a lot recently is how water changes coastlines (or even just paths in parks).

Not really surprising, considering the enormous amounts of energy stored in waves that are released on the beach every couple of seconds. For example in Hastings, UK, the shape of the beach looked different after every high tide of the three-day mini break we spent there. groynes, built there to prevent erosion of the beach, get buried in pebbles on one side and totally hollowed out on the other side.

Or, looking down towards the water:

And this doesn’t only happen to one particular groyne that was built in a particularly nasty spot, but to every one for miles and miles along the coast line.

This gives the beach a see-saw-like edge:

And this is what it looks like when waves arrive at the beach and meet groynes. Enjoy!

The building of sand banks

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The eroding force of moving water can be seen in so many places when you pay attention. For example in a park where I sometimes go for walks, the really well-maintained paths are forever eroded and washed away by the heavy rains we’ve had recently.

In the picture below you see a green pipe opening into the pond, and what you can’t see is a second, larger pipe just to the right of the first one. Both pipes drain water from the park’s paths. Water then flows through the pond and eventually into Kiel fjord. And what happens is that all the pebbles and sand from the park’s paths end up in the drainage system and get washed into that little pond, where they get deposited in a sand bank.

Interestingly enough, water exiting the pipes seems to typically do so at such a high velocity that all the debris doesn’t get deposited right then and there, but carried downstream until the water has slowed enough that a sandbank can form. And on the sandbank you can see that larger rocks get deposited first while smaller ones are carried further with the current before they settle.