# Lüneburg — how a few centimetres in distance separate two seemingly completely different bodies of water

Sightseeing is best when it involves a little water watching, like for example last weekend in Lüneburg.

Doesn’t it look intriguing below, the change from a calm, mirror-like surface to something a lot less regular on the other side of the bridge?

Take it in: so peaceful! Although, judging by the plants growing in the water and by how they look like someone took a rake and put them in order, there must be a substantial current going through underneath the bridge.

And turns out there is: The bridge is a weir and there is a waterfall on the other side!

I find it so fascinating how the appearance of water can change literally over the distance of a few centimetre. So calm on one side, and boiling, spraying, turbulent on the other!

And then just a couple meters further downstream, we are back to mainly calm and only a few bubbles floating along give you an indication of what just happened upstream…

And again, no matter how peaceful everything looks here, the water plants tell us that there is still a lot of water moving, bending the leaves with it.

Do you look at this kind of things when sightseeing, too?

# Observing hydrodynamics on a very large scale

You know I like to point out where you can spot hydrodynamics concepts in your everyday lives (at least if your everyday lives include strolls along rivers and generally a lot of water)

A while back we went to Geesthacht. We were hoping for more ice on the Elbe river, but sadly there was none. But! In Geesthacht they have a weir, combined with locks. They keep water back to bring the level of the Elbe upstream of Geesthacht up to 4 m above sea level for shipping purposes. But then they obviously need a lock to get ships up and down this sill. But the coolest thing is the weir:

Weir on Elbe river near Geesthacht

200 m of pure hydrodynamics! You know I love a good hydraulic jump

Weir on Elbe river near Geesthacht

Do you see the three different states the fluid in the picture above is in?
Looking from right to left (i.e. with the direction of the flow), we first see normal flowing water. You can see that there are waves and ripples going in all directions. Then, the middle part of the picture, all disturbances on the water surface are clearly oriented right-to-left. That is because here the water is shooting (meaning flowing faster than waves can propagate), and all disturbances get deformed by the flow rather than spread by themselves. And then on the very left, we have a submerged hydraulic jump (which we cannot see, because, as the name says, it is submerged) and above massively turbulent water.

Weir on Elbe river near Geesthacht

I just love the look of it!

Watch the video below to see the whole thing in motion.

# Observing hydrodynamic phenomena on a creek

Looking at a creek on a Sunday stroll, and seeing lots and lots of concepts from hydrodynamics class.

For example below, you see waves radiating from each of the ducks. And you see interference of waves from all those ducks.

What happens if the ducks bring their waves closer?

At some point, all those waves from the ducks are going to hit the weir in the picture below.

And there, they are going to somehow react to the flow field caused by the changes in topography.

And you can spot so many different phenomena: Standing waves, hydraulic jumps, and lots more!

Watch the movie below to see the whole thing even better!

Btw, you might remember this spot, I have talked about standing waves from right there before. Interestingly, the wave pattern in the other post looks really different, probably due to different water levels or changes in topography (maybe someone threw in rocks or they did some construction work on the weir?). But it is still just as fascinating as last time :-)

And for those of you who like to see a “making of”: