Atypical turbulent junction flow characteristics upstream of model boulders mounted atop a rough, porous bed

Boulders are common flow obstacles in steep, gravel-bed streams that affect local sediment bed morphology via their influence on local hydrodynamics. While turbulent flow characteristics in the junction region, which occurs where a boundary layer meets a wall-mounted obstacle, are well described for many engineered flow obstacles, uncertainty remains about flows upstream of boulders largely due to the uncertain effects of the rough, permeable gravel bed and variable submergence. This laboratory flume study provides insights to turbulent flow characteristics upstream of cylindrical model boulders via new volumetric particle image velocimetry (PIV) tests conducted at fully and partially submerged conditions. While near-bed, mean-flow separation is a prominent feature of most junction flow studies, a striking and atypical finding here is that flow separation was substantially delayed and was not observed in measurements (made within 0.1 diameters upstream). In addition, no clearly discernable mean horseshoe vortices were observed via mean streamline and vorticity plots, which are typically found in junction flows. These findings suggest the presence of atypical junction flow characteristics at boulders, which should be considered in river engineering and stream restoration efforts.