Time-Averaged Vortex Structure Around An Immobile Boulder

Large immobile boulders, which are typical features in mountain stream reaches, modify the turbulent flow patterns and alter the bed shear stress distribution in their vicinity, thus affecting the magnitude and timing of bedload transport events. The goals of this study are: (1) to identify the dominant vortex structures developing around the boulders at different relative submergence conditions; and (2) to assess the effects of these vortex structures on the bed shear stress field around a boulder. The flow field around a single boulder under conditions of high and low relative submergence was interrogated via Digital Particle Image Velocimetry (DPIV) in a laboratory flume. The experiments revealed the key vortex structures that develop around the boulders under high and low relative submergence conditions. These vortex systems were found to exert an influence on the bed shear stress distribution around the boulders, which was in agreement with observed sediment deposition patterns around the boulders.