Three-dimensional reconstruction of the oscillatory free-surfaces of a flow over antidunes: stereoscopic and velocimetric techniques

The authors present imaging methods developed to characterize the oscillatory free-surface of rapid flows and apply them to torrential currents propagating over sediment antidunes. The aim is to obtain high-resolution relief maps of the free-surface topography, in order to highlight the regular spatial patterns associated with the bedforms. Two measurement principles are outlined and tested, both based on the imaging of floating tracers dispersed on the rapidly flowing surface. The first relies on direct stereoscopic measurements obtained using two cameras, while the second exploits an original velocimetric principle allowing to derive elevation from the velocity field acquired using a single camera. The measurement procedures and image analysis algorithms are introduced for the two methods, along with the physical assumptions underlying the velocimetric principle. The results of the two techniques are compared for different free-surface patterns and good correspondence is obtained. The obtained relief maps vividly depict the variety of motifs that can evolve as a result of interaction between shallow flows and loose sediment beds.