Experimental study of a coarse-gravel river bed: Elucidating the nearwall and pore-space turbulent flow physics

High-resolution Particle Image Velocimetry (PIV) measurements were conducted near a highly permeable wall that served as a proxy for a coarse-gravel river bed. The measurements captured both the overlying flow as well as the flow within the permeable wall to explore the near-wall turbulence and the flow interactions across the permeable interface. The permeable wall was formed by two layers of cubically packed spheres cast in a transparent urethane. A novel refractive-index-matching flume was used to gain full optical access to the near-wall and subsurface flow. Results were compared with similar measurements of flow over an impermeable wall with identical roughness topography to highlight the influence of wall permeability on the near-surface flow. Single-point statistics provided a quantitative assessment of the structural modifications imposed by permeability in the near-wall region. An additional PIV experiment captured the subsurface flow within the pore space. The decay of the mean velocity and the turbulent fluctuations reported by earlier studies is observed in the current investigation.