Abstract
We used a hydraulic flume to conduct analog model experiments to elucidate the effects of alternating orientations of bidirectional flows (intersection angle θ) on the morphology and evolution of isolated sand dunes. An initial cone of fine-grained quartz sand was exposed to bidirectional water flows with a range of intersection angles (45°, 90°, 135°, and 180°) for 20 cycles of flow alternation, with a cycle duration of 2 min. Experimental trials showed two types of deformation of the crest line: (1) “independence type”, in which a new crest line formed at a different location from the existing one; and (2) “reverse type”, in which an existing crest line reversed its migration direction and a new slip face was generated on the new stoss side of the existing crest. The independence type was observed at θ = 45° and the reverse type was observed at θ = 180°. At intermediate values of θ, the two types occurred contemporaneously. Due to the cumulative effect of deformation of the crest line, three types of topographies developed, with features characteristic of dome, longitudinal (seif), and reversing dunes. These results yield insights into the processes and conditions required for the formation of sand dune types.
