An investigation of the modification of airflow by a discrete, semi-circular roughness element (in this case, vegetation), the resulting flow structure, and the formation of pyramidal-shaped shadow dunes is presented. Two erect pioneer sand dune grasses, Festuca littoralis and Ammophila arenaria , were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities (4-20 m sec (super -1) ). The three-dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake. A vertical separation envelope is also formed over the vegetation. Pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs. Shadow dune height is dependent on element basal width and repose angle of the sand. Shadow dune length is dependent on element basal width and wind velocity. The results are extrapolated to include all semi-porous to non-porous, semicircular, discrete roughness elements. The role of shadow dune formation in the development of foredunes and related environments is examined.

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