HOW DO DUNE FORMS MODULATE SEDIMENT ADVECTION IN SPARSE SEDIMENT COVER?

Wind advects sediment downwind, commonly yielding bedforms (dunes). In turn, dunes modulate the advection of sediment. This coupling is poorly acknowledged, yet central to understanding the translation of sediment across landscapes, the dynamic stability of bare dune fields, aeolian depositional records, and landscape evolution.

Here, we explore the effects of dunes on dune-field wide sediment advection. We compare sediment advection on a flat sand sheet to a variety of common dune field configurations. There are (at least) four major mechanisms that act to modulate sediment advection: (i) dune roughness influences the boundary layer (reducing sediment advection), (ii) dunes situate sediment higher into the airstream (increasing advection), (iii) dunes expose non-erodible substrate (reducing advection), and (iv) dunes shadow a portion of the surface in separation zones (reducing advection).

We use real topographic data to develop estimates of the local sediment advection following previous fluvial work. We then relate advection to equivalent sediment thickness (a spatial average of sand volume per unit area) and sediment cover. Spatial differences in sediment advection lead to changes in the equivalent sediment thickness, which feeds back into changes in sediment advection. We explore the potential for dynamic instabilities that could explain widely seen ‘clustering’ of barchan/transverse dune forms. A method for quantifying the advection of sediment provides new insight into the long term stability of dune fields and allows advances in inverting topographic records to infer formative environmental conditions, from the Sahara to Mars and beyond.