Lateral bed-roughness variation in shallow open-channel flow with very low submergence

Abstract

Quantifying turbulent fluxes and secondary structures in shallow channel flows is important for predicting momentum and mass transfer in rivers as well as channel capacity and associated water levels. Here, we focus on the flow over a lateral bed-roughness variation with very low relative submergence of the roughness elements, h∕k = {3, 2, 1.5} , where h is the flow depth and k is the roughness height. Measurements were performed in a 1.1 m wide and 26 m long glass flume whose bed was fitted with cubes arranged in two regular side-by-side patterns with frontal densities f = 0.2 and 0.4 to create a rough-to-rougher variation. Measurements were performed using stereoscopic PIV in two orthogonal planes, in a vertical transverse plane spanning the two roughness types, and in a longitudinal one at the interface between the roughness types. The results show that the bulk velocity difference between the two sides of the channel increases with decreasing h/k. Also, contrary to what is observed at high relative submergence with smooth-to-rough transitions, higher bulk velocities occur on the side with higher roughness. This difference is increasing as the flow becomes shallower and is shown to be due to increasing effective depths ratios, leading to increasingly lower friction factor ratios with lower friction factors on the high-velocity but rougher side. Although increasing streamwise momentum transfer at the interface is needed as h/k decreases, the turbulent and secondary circulation transfer of momentum is increasingly inhibited. A globally-driven secondary-circulation at h∕k = 3 ceases for lower h/k and roughness-scale circulation becomes dominant. Also, even the increased global shear does not lead to large-scale Kelvin Helmholtz instabilities structures. However, the relative importance of the roughness difference on the flow is augmented as the flow becomes shallower and momentum transfer due to lateral dispersive stresses increases.

Compound open-channel flows: effects of transverse currents on the flow structure

“…The criterion of λ 0.3 for the emergence of KHCSs was recently found to be confirmed in the experiments of Caroppi et al (2019) that focused on water-vegetation interfaces in open-channel flow. On the other hand, the experiments of Akutina et al (2019) show that, in the case of a lateral bed-roughness variation 10 3 10 2 10 1 10 0 k (rad m -1 ) 10 -1 10 3 10 2 10 1 10 0 k (rad m -1 ) 10 -1 10 3 10 2 10 1 10 0 k (rad m -1 ) in shallow open-channel flow with very low submergence, this criterion is no longer valid. This might be due to the effect of the bed-induced strong turbulence generated at the top of the roughness elements that would prevent emergence of the transverse turbulent motions in the case of a very low submergence of the roughness elements.…”

Interaction between a rough bed and an adjacent smooth bed in open-channel flow

“…2011; Akutina et al. 2019): (i) a net bulk transverse flow between the two flow regions (for non-uniform flows); (ii) secondary currents; (iii) the lateral turbulent shear stress; and (iv) the lateral dispersive shear stress. Generally, the dominant contribution is the turbulent shear stress.…”

Fine‐Sediment Erosion and Sediment‐Ribbon Morphodynamics in Coarse‐Grained Immobile Beds

“…The effective water depth H e accounts for the volumetric displacement induced by the spheres with ϕ the porosity of the bed given by ϕ = $$\int \nolimits_{-k}^{0}\left[1-{\lambda }_{p}(z)\right]dz$$ and λ p = 2 S p ( z )/ S r the planar density of a protruding sphere, as in Akutina et al. (2019). The ridge spacings Λ l , Λ r and Λ c , all normalized by the effective water depth $${\langle {H}_{e}\rangle }_{y}$$, are shown in Figures 8c and 8d for the upstream and downstream measurement locations, respectively.…”