Category Archives: observation

Erosion

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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…

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We saw it in another post already:

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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… ;-)

Tides

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Another thing I can’t stop being fascinated by: Tides.

Sometimes you look towards Hastings Pier, and there is water all the way up to the sea wall. Those are the times I am too stunned to take pictures, unfortunately.

Other times, the sea retreats:

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Further and further.

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And then at some point I can’t zoom out enough to capture the building on the pier at the same time as the water’s edge…

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Standing waves

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One thing I find endlessly fascinating are – you might have heard it before – standing waves. At the waterfront in Kiel I saw some the other day:

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Watch the movie below and be fascinated, too! :-)

Isn’t it amazing how wave crests and troughs seem to appear out of nowhere and vanish again? When we are so used to seeing waves propagate, this is such an interesting variation of the theme! And it makes it somehow more easy to accept that waves transport energy, not mass, because if we can’t see which way they propagate, which way would they transport mass?

 

Waves transport energy, but not mass

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Usually we see wave crests propagating, and since the eye can’t stop following them, it is easy to assume that they transport water with them instead of leaving the water put and just transporting energy. But here is an example of a situation where bubbles as tracers for water “parcels” show that, despite large waves passing, the water itself only moves up and down, and a little back and forth, but isn’t really transported away:

Of course there is some Stokes drift, but compared to the wave speed the speed associated with that is tiny…

Waves break when they reach a critical steepness

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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:

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

Waves change their direction because of changes in water depth

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When waves run up a beach, they change their direction because for shallow water waves, phase speed depends on water depth (see this post for a nice little visualization for why the whole wave crest swings in towards the beach).

Unfortunately, it is really difficult to take good pictures of this phenomenon, but I tried:

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See below for an annotated version that shows the wave crests:

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It is easier to see in a movie, so here you go with a movie from that exact same spot:

How water changes coast lines

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

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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.

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Or, looking down towards the water:

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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.

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

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

The enormity of waves

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It’s easy to forget the enormous size and power associated with waves in the ocean. Yes, we see pictures of surfers on Hawaii or of ships in waves occasionally, but usually we don’t really think about how large waves out in the ocean actually are. Which makes it easy to underestimate the force that the ocean holds within, not only on days when natural disasters like storm surges or tsunamis occur, but on a daily basis, just in the swell of distant storms. So sometimes it is quite healthy to look at waves on the beach and compare them to other buildings.

For example here, waves and Hastings Pier:

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Those aren’t even particularly large waves, yet we see that their wavelength, peak to peak, is easily a lot more than 20m. This is what coastal structures deal with on a daily basis!

Waves in the clouds

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On my way back from London I had an almost equally interesting flight as on my way to London, which I talked about here and here. Except that most of the excitement this time round came from discovering that I wasn’t, in fact, sitting next to the person I thought I was, but that I was booked on a different flight from a different terminal. Which isn’t so terribly exciting in itself, but seeing that Terminal 5 is quite a distance away from the other terminals and the discovery itself happened at security some 20 minutes before boarding was supposed to start, it made for an interesting race across Heathrow.

But at least I ended up seeing pretty waves in the clouds:

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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.

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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.

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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.