Dynamics of migrating sand dunes interacting with obstacles

Wind- and water-driven migrating sand dunes frequently interact with elevated natural and artificial topographical features. Generically, dunes interact with obstacles either by `crossing' over the obstacle or by being `trapped'. We study this problem in an idealized quasi-two-dimensional laboratory experiment. We show that the outcome -- crossing or trapping -- depends on the size and shape of the obstacle and we relate these observations to the flow structure in the immediate vicinity of the obstacle.