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.