The geomorphological significance of airflow patterns in transverse dune interdunes

The interdunes between aeolian dunes have been relatively ignored when compared with the research attention on the morphodynamics of the dune bodies themselves. This neglect is in spite of the possible significance of interdune dynamics for the geomorphology of the sand dune system as a whole, especially with regard to dune spacing. This paper considers the mean airflow within four relatively simple transverse dune interdunes. The study locations were chosen in order to sample interdunes with different size and surface characteristics, the dynamics of which were investigated for when incident flow was normal to the upwind crest. The findings confirm existing models of flow reattachment length and recovery for aeolian dune lee-side flow, and show a consistent pattern of increasing near-surface velocity downwind of reattachment that supports a mechanism for interdunes as sand-free features. Flow dynamics are characterised for the different types of interdune observed, where two groups are recognised. The flow patterns in relatively short interdunes (where dunes are closely-spaced) with a sandy surface were accordant with those of the flow response model. In ‘extended’ interdunes, where bounding dunes were spaced with a length well over that for flow separation, evidence at the downwind edge of the interdunes suggested that flow reacted to the subsequent dune. For the case of these ‘extended’ interdunes, a new descriptive model is presented to characterise their dynamics. In this model, the variation in near-surface flow allowed process zones to be identified through the interdune. The geomorphological significance of the processes dominating each zone is discussed, and comparisons are made between the flow response case and the new interdune model from this study. In a discussion on the controls of spacing between dunes, where reattachment length exerts a fundamental control, the role of sediment availability is also highlighted as a significant factor. The presence of a sandy bed can, in some circumstances, determine whether dune development, and therefore spacing, is controlled primarily by elements of flow response.