Why Anti-dunes move upstream?

Why Anti-dunes move upstream?

Anti-dunes

Anti-dunes are sand dunes that form in a moving fluid (such as water) and move in the direction opposite to the flow. This is in contrast to normal sand dunes, which form in a still environment and move in the direction of the wind.

Anti-dunes move upstream because of the way fluid flows over a bed of sediment. When fluid flows over a bed of sediment, it can create ripples and dunes, depending on the velocity of the fluid and the size and shape of the sediment particles.