Author(s): Giulia Stradiotti; Giuseppe Roberto Pisaturo; Markus Noack; Maurizio Righetti

Linked Author(s): Giulia Stradiotti, Giuseppe Roberto Pisaturo, Markus Noack, Maurizio Righetti

Keywords: Refractive index matching; Particle image velocimetry; Gravel-bed rivers; Streamwise velocity profile

Abstract: The last decades witnessed a growing interest in the sediment faith in rivers, especially in relation to the operation of reservoirs and their downstream impact. An efficient management requires a model for describing the interaction between the fine transported sediments and the armored gravel bed. However, even the definition of the streamwise velocity profile in gravel-bed rivers is still an open question, mainly due to the technical issues related to the inaccessibility of the deeper regions of the gravel substrate. In this work, we combined the RIM (Refractive Index Matching) methodology with the PIV (Particle Image Velocimetry) technique to perform velocity measurements inside the gravel voids. To that end, we filled a laboratory flume with gravel for its entire length, except for the measuring section, where we substituted the gravel with similar-sized hydrogel spheres (made of crosslinked modified acrylic polymer). These spheres, originally ~2 mm sized, absorb water growing up to ~3 cm, and they keep the same refractive index as water, hence being completely invisible when submerged, avoiding any optical disturbances. Such an experimental set up allowed us to measure the velocity field in low-submergence conditions in three longitudinal planes, from the bottom of the flume to the free surface, and to obtain a time- and space- averaged velocity profile. The double-averaged velocity profile shows an inflection point above the crest level, usually associated with a mixing-layer type of turbulence in the interfacial region. 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 overall velocity behavior from the deepest region of the bed, to the free surface. Rather, the profile seems to be better represented by a combination of models.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022787

Year: 2022