Turbulent junction flow characteristics upstream of boulders mounted atop a rough, permeable bed and the effects of submergence

While turbulent flow characteristics in the region upstream of wall-mounted obstacles (often called the junction region) are well described for many engineering systems, uncertainty remains about flow characteristics upstream of the large, immobile boulders that are commonly found in steep gravel-bed streams. This uncertainty is largely due to the unique bed features (e.g., relatively large bed roughness, a permeable bed) and hydraulic characteristics (e.g., variable submergence) present at boulders, which may affect flow characteristics. This study reports results of laboratory volumetric particle image velocimetry (PIV) experiments performed upstream of model boulders for fully submerged (FS) and partially submerged (PS) conditions. Several atypical junction flow characteristics were documented. Mean-flow reversal and spiraling streamlines (commonly associated with horseshoe vortices) were not documented, though limitations in near-bed measurement extents contributed to this result in several cases. Nevertheless, modest increases in local rotation rate (assessed via vorticity magnitude and swirling strength) were observed near where the horseshoe vortex is typically expected. Mean-flow data also suggest that notable mass flux into the permeable bed occurred upstream of boulders. Several effects of submergence on junction flow characteristics were also identified. Compared with the FS condition, the PS condition exhibited more rapid deceleration of the mean streamwise velocity, stronger downward velocities near the bed that extended over a wider transverse area, stronger increases of the near-bed turbulent kinetic energy, and oppositely signed streamwise-vertical Reynolds stresses near the water surface. The improved understanding of flow upstream of boulders provided here will aid future river engineering and restoration efforts.