'NERC-FAPESP' Migrating dunes over hilly terrains

Granular materials are universal and all around us, from the devastating powers of a snow avalanche to the fabrication of medicines, and from ground coffee beans to sand castles on the beach. Despite their prevalence in our daily life, granular materials are hard to characterize and understand from a physical point of view.

Dunes are manifestations of granular materials in our natural environment. Mobile hyper-arid desert dunes threaten settlements and infrastructure of as many as one billion people worldwide. In order to protect communities and infrastructure from desertification in parts of the world that become dryer each year due to climate change, we need to understand the dynamics of the evolution of a dune, from growth of a bedform to the steady-state processes of a mature dune. Sand dune movement and mobility demands controls on influencing, altering or even stopping the marching of desert dunes. This proposal focuses on a multifaceted approach to connect the avalanching processes at the grain-level with the large-scale dune marching characteristics.

The particular type of dune under investigation is a barchan dune - a fast-moving and crescentic-shaped dune present in unidirectional wind regimes with a limited sand supply. We choose to study barchan dunes as they form a unique system: they are small, scale linearly and move fast enough to study in experiments. The progression velocity scales inversely with height and therefore individual dunes chase or run away from each other in a variety of interactions. Physical scalings and mathematical models of dune migration have made huge steps forward in recent years, but are grounded on modelling migration on an idealized flat earth, which is not a realistic scenario.

In this collaborative project, we will investigate the effect of external landscape variations and topography on the speed and the shape of these fast-moving dunes. Specifically, we are investigating three different variations: dunes on (1) an inclining/declining slope, (2) a hilly topography and (3) from a polished to a bumpy landscape. We are using two different experimental facilities: in Cambridge, we mimic 2D dunes in a periodic system over very long times, while in Campinas we are able to create true 3D dunes, evolving in fixed-time in a finite channel. This combination of experimental facilities, and effective mobility of researchers between groups, will ensure success of our set goals.