What are compound dunes and how are they related to smaller dunes?

Compound dunes, also known as mega-dunes or draa, are very large-scale features found in many of the world's great sand seas. These massive formations can reach heights of nearly 200 meters and are often spaced hundreds of meters to several kilometers apart.1

They are fundamentally related to smaller dunes through a hierarchical structure of bed forms. In this arrangement, large features are covered with smaller ones, which in turn are covered with even smaller features, such as ordinary sand ripples. Specifically, compound dunes are typically covered with a secondary pattern of smaller dunes. This relationship reflects a complex, multi-scale organization where different sizes of dunes and ripples coexist within the same sandy terrain, each influenced by variations in grain size and wind velocity.1