Author(s): Rigden Yoezer Tenzin, Ana M. Ricardo, Rui M. L. Ferreira

Linked Author(s):

Keywords: Mountain rivers; Hydraulic conductivity; Log-law; Bed load transport; PIV; Double-Averaging Methodology;

Abstract: The effect of macroscopic properties of bed morphology and the effect of hyporheic/surface exchanges are of paramount importance to characterize the flow in rivers. Not many studies addressed the issue of the influence of hyporheic/surface interactions. This work addresses this knowledge gap. Its general objective is to study the effect of the hydraulic conductivity on mean flow and turbulence of open-channel flows. Specifically, it investigates the effects of hydraulic conductivity on the parameters of the logarithmic law of the wall. To fulfill the objectives, experimental tests performed in high conductivity beds (mono-sized glass sphere beads) are compared with the existing database of low conductivity beds of Ferreira et al. (2012), keeping constant the range of values of porosity, Shields parameters and roughness Reynolds numbers. The hydraulic conductivity is varied by changing the tortuosity (and the dimensions of the pore paths) and not the porosity, which results in an absolutely novel study. A new database of instantaneous velocities was acquired with Particle Image Velocimetry (PIV) and processed to gather time-averaged velocities and space-time (double-averaged) quantities, namely velocities, Reynolds stresses and form-induced stresses. The hydraulic conductivity was measured for both types of bed. The parameters of log-law obtained from high conductivity are compared with low conductivity of existing database, for mobile and immobile bed conditions. The main finding can be summarized as follows: i. hydraulic conductivity does not affect the location of the zero plane of the log-law relatively to the plane of the crests; ii. higher hydraulic conductivity is associated to a lower friction factor, (u*/U)2 and iii. flows over high conductivity beds appear drag-reducing even if roughness parameters do not change appreciably; consequently, the values of the von Kármán constant in flows over high conductivity beds are consistently smaller than those of flows over low conductivity beds.

DOI: https://doi.org/10.3850/38WC092019-0257

Year: 2019