In gravel bed rivers, the microtopography of the bed exerts a
significant effect on the generation of turbulent flow structures.
Although field and laboratory measurements have indicated that flows
over gravel beds contain coherent macroturbulent flow structures, the
origin of these phenomena, and their relationship to the ensemble of
individual roughness elements forming the bed, is not quantitatively
well understood. Here we report upon a flume experiment in which flow
over a gravel surface is 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. The results
indicate that as the Reynolds number increases (1) the visual
distinctiveness of the coherent flow structures becomes more defined,
(2) the upstream slope of the structures increases, and (3) the
turbulence intensity of the structures increases. Analysis of the mean
velocity components, the turbulence intensity, and the flow structure
using quadrant analysis demonstrates that these large-scale turbulent
structures originate from flow interactions with the bed topography.
Detection of the dominant temporal length scales through wavelet
analysis enables calculation of mean separation zone lengths associated
with the gravel roughness through standard scaling laws. The calculated
separation zone lengths demonstrate that wake flapping is a dominant
mechanism in the production of large-scale coherent flow structures in
gravel bed rivers. Thus, we show that coherent flow structures over
gravels owe their origin to bed-generated turbulence and that
large-scale outer layer structures are the result of flow-topography
interactions in the near-bed region associated with wake flapping.