Explanation for strange pattern appear on Bath tub with sand in it

We have a round bathtub with a surface radius of about 15 cm or greater. It so happens that the water that came from the tab has a little sand in it. Today when the water was about half-filled, I just seen this pattern made up of sand.

At a first glance, I haven’t think too much, just put my hand inside and make the water round and round so that after a little time sand seems to sit in the middle. But After $15$ or more minutes later the same crazy pattern reappears.

So Is there any particular reason for this particular pattern to appear?