Turbulent Flow Around Fully and Partially Submerged Boulders and Implications to Sediment Movement

Mountain streams are characterized by steep gradients and wide grain size distributions that include fine, more mobile sediment, and large immobile boulders. These large boulders are partially submerged for most of the year, but become fully submerged at higher flows, exhibiting low and high relative submergence, respectively. Through their interaction with the approach turbulent flow, the boulders modify the surrounding bed shear stress field, thus altering the timing, magnitude and pathways of bedload patterns. The goal of this study is twofold: (1) to identify the dominant vortex structures around the boulders at high and low relative submergence; and (2) assess the effects of these vortex structures on the surrounding bed shear stress field. Particle Image Velocimetry (PIV) was used to interrogate the turbulent flow field around a spherical boulder mounted atop a flat rough bed under high and low relative submergence conditions. Under high relative submergence, a pair of arch vortices and inboard vortices with the same sense of rotation dominated the boulder wake. For the low relative submergence, a pair of strong von-Karman vortices developed within the boulder wake region. The arch and von-Karman vortices observed for the high and low relative submergence conditions affected the magnitude and the directionality of the bed shear stress vector in plan view, by directing it towards and away from the boulder centerline, respectively. The reversal of the bed shear stress vector directionality likely promotes a reversal of incoming sediment deposition from the boulder wake to the stoss region for high and low relative submergence, respectively. This reversal was verified by prior observations of bedload particle deposition around the boulder. The findings of this research underline the effects that large, immobile boulders have on the surrounding turbulent flow and bed load particle movement with important implications on sediment management in mountain streams.