A fluid flow drives the migration of a sand dune. Particles are entrained on the upstream slope and roll/jump to the crest where they avalanche or are ejected into the flow. The internal part of the dune behaves like a solid whereas the moving layer of particles is fluid-like.
Migration of an underwater sand dune —

A fluid flow drives the migration of a sand dune. Particles are entrained on the upstream slope and roll/jump to the crest where they avalanche or are ejected into the flow. The internal part of the dune behaves like a solid whereas the moving layer of particles is fluid-like.

https://fluids.ac.uk/files/07_Photo.1512131230.JPG