Turbulent flows moving over a gravel bed develop large-scale,
macroturbulent flow structures that are initiated at anchor clasts in
the bed and grow and dissipate as they move upward through the flow
depth. This paper extends previous research in which we investigated
the influence of the Reynolds number on coherent flow structures
generated over a gravel bed by assessing the importance of effective
bed roughness. Here, we report on flume experiments in which flows over
beds of decreasing surface roughness have been quantified through the
application of digital particle imaging velocimetry, which allows study
of the downstream and vertical components of velocity over the entire
flow field. These results indicate that as the effective roughness
increases (1) the visual distinctiveness of the coherent flow
structures becomes more defined throughout the flow depth, (2) the
upstream angle of slope of the coherent flow structure increases, and
(3) the reduction in streamwise flow velocity and turbulence intensity
toward the upstream side of the structure becomes greater. Applying
standard scaling laws, these structures appear shear-generated and form
through a combination of both wake flapping and the reattachment of
localized shear layers associated with flow separation around
individual topographic protrusions. As the effective protrusion
decreases, the scale of these coherent flow structures also decreases.