Upstream-migrating antidunes are a particular type of bedform that develops at the interface between fluid and granular beds in motion. The mechanisms of their seemingly counterintuitive movement in the opposite flow direction are not well understood yet. We present a numerical approach to simulate antidune formation and propagation with geometrically resolved particles. The model is based on a coupled discrete element and free-surface lattice Boltzmann method, the latter of which we will introduce in more detail in this talk.