Sand dunes are coherent sedimentary structures shaped by the shear stress exerted by the overlaying fluid. Natural dunes abundantly populate deserts, river bottoms, sea beds, and even snow fields. They rarely occur in isolation, but usually form extensive dune fields, which continuously evolve due to the flow-induced dune migration. The overarching aspiration of this thesis is to understand what controls the long term evolution of such a dunescape. To this end, we conduct a suite of idealised laboratory experiments within a subaqueous annular set-up.

The central part of the dissertation is dedicated to pairwise interactions between two discrete sand dunes. First of all, we study the physical underpinnings of a wake-induced dune-dune feedback. By quantifying the statistical properties of sediment transport, we uncover that the interaction mechanism is critically related to turbulent fluctuations of the velocity field. Secondly, we use a mixture of long-time experiments and simple mathematical models to explore the consequences of this coupling for the asymptotic behaviour of a two-dune system. Specifically, we demonstrate that, depending on the parameter regime, the attractive configuration may be either symmetric or asymmetric.

Apart from dune-dune interactions, we also study the problem of dune-obstacle interaction, which is directly motivated by the engineering challenge of sand dunes invading infrastructure. We show that a migrating dune can either cross an obstacle or come to an effective halt and we relate this transition to the structure of the flow.