Coherent flow structures in a depth-limited flow over a gravel surface: the role of near-bed turbulence and influence of Reynolds number

In gravel bed rivers, the ratio between the mean flow depth to roughness height seldom exceeds a value of 10 for normal flow conditions, and thus the detailed microtopography of the bed exerts a significant effect on the generation of the turbulent flow structures. Past field measurements and laboratory visualization have indicated that shallow flows over gravel beds contain coherent macroturbulent structures. However, the origin of these macroturbulent 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 measured heterogeneous gravel surface is quantified through the application of 2D digital Particle Imaging Velocimetry. This methodology allows study of the downstream (u-) and vertical (w-) components of velocity over the entire flow field at a spatial resolution of 0.002 meters and a temporal resolution of 15 Hz. These dPIV measurements were linked to high-resolution quantification of the bed surface topography obtained using digital photogrammetry. This experimental set-up enabled study of the generation and evolution of coherent flow structures over a known bed topography that allowed the potential to identify: i) the topographic characteristics required for generation of macro-scale turbulent flow structures; ii) the geometric shape of the flow structures; iii) the temporal length scales of the flow structures, and iv) how these characteristics change with flow Reynolds number.