Sand dunes and their turbulent relationships*

Migrating dunes are sedimentary heaps which, due to the shear stress exerted by the overlaying fluid, propagate over the substrate as coherent structures. Natural sand dunes can be found in deserts, river bottoms, sea beds, and even snow fields. The shape and the dynamical evolution of sand dunes is a result of a closed feedback loop which couples the topography of the dune, the fluid flow around it, and the induced sediment transport. The physical nature and the consequences of this subtle interplay have been well understood for an

archetypal isolated dune migrating over flat substrate. Nevertheless, in nature sand dunes rarely occur in isolation, but they usually form larger collectives known as dune fields of dune corridor. Moreover, migrating sand dunes do not always rest on flat substrate, but they often interact with various natural human-made topographical features. Indeed, in deserts shifting sand dunes notoriously bury roads, rail tracks, or even buildings and under water they can block navigation channels or cover cables and pipelines. In this talk, I will summarize the key findings of my doctoral research dedicated to uncovering the fundamental physical laws governing how dunes interact with each other and their surroundings. First, I will discuss the hydrodynamic coupling between two neighbouring dunes and its consequences for the system- level dynamics of a dune field. Second, I will describe how migrating sand dunes interact with solid obstacles, focusing in particular on the role of fluid flow around the obstacle.