Experimental Velocity Measurements of a Low-Submerged Flow Coupling RIM and PIV in a Gravel-Bedded Laboratory Flume

The last decades witnessed a growing interest in the sediment faith in rivers, especially in relation to the operation of reservoirs. An efficient management requires a model for describing the interaction between the fine transported sediments and the armored gravel-bed. However, the definition of the streamwise velocity profile in gravel-bed rivers is still to be investigated, due to the inaccessibility of the deeper gravel substrate. We coupled the Refractive Index Matching (RIM) methodology with the Particle Image Velocimetry (PIV) technique to measure velocity inside the gravel voids. To that end, we filled a flume with gravel except for the measuring section, where we substituted the gravel with similar-sized hydrogel spheres. These spheres, originally similar to 2 mm sized, absorb water growing up to similar to 2.5 cm, keeping the same refractive index as water. Being invisible when submerged, they avoid optical disturbances. Such a set-up allowed us to measure velocity in low-submergence conditions in three longitudinal planes, from the flume bottom to the free surface. The resulting double-averaged velocity profiles show an inflection point above the crest level, usually associated with a mixing-layer type of turbulence. The mixing layer analogy is confirmed by the behavior of the Reynolds intensities, the skewness, and the ratio between the components of the Reynolds stress tensor. However, the mixing layer analogy alone does not describe the velocity profile from deep regions of the bed to the free surface, and our results indicate that a superposition of multiple laws better represents the entire velocity profile, coupling the surface and subsurface flows.