Walking along a beach before sunrise I saw a piece of brain coral had washed up at night. I have no idea what are the uses of the intricate patterns on the coral. The valleys between the ridges are where the feeding tentacles of the coral rest when they are not waving around. But why is are the ridges so complicated in shape? I had no clue about this. But I thought I should be able to understand the pattern in the sand around the piece of coral.
As the tide recedes, waves break near the shore, and the water runs up the sand. Then, the water drains back. As you can see in the photo above, the water breaks into little eddies as it drains, and the eddies leave the pattern in the sand around the piece of coral. You can see a pattern of parallel grooves.
The spacing between them is determined by the viscosity of water. It turn out that if the water tries to drain away with a speed of about a meter per second, then turbulent eddies of size of about 5 millimeters should be seen. That size is about the smallest scale that you can see in the pattern. Interestingly, this gives us a way to see how fast the water drained away after the water has gone. Look at the smallest structure in the pattern left by the water, and the speed should be inversely proportional to this. This means that the finer the pattern, the faster the water drained!
This is all about very small scale patterns. There is much more to the formation of patterns in the sand, as another blog discusses. As I walked about, the horizon began dipping towards the sun, and I was distracted by the easier pattern of the daily sunrise.