Influence of shallowness, bank inclination and bank roughness on the variability of flow patterns and boundary shear stress due to secondary currents in straight open-channels

Abstract

Boundary shear stress and flow variability due to its interaction with main flow and secondary currents were investigated under conditions that extend previous research on trapezoidal channels. Secondary currents that scale with the flow depth were found over the entire width in all experiments. These findings contradict the widespread perception that secondary currents die out at a distance of 2.5 times the flow depth from the bank, a perception which is largely based on experiments with smooth boundaries. The reported results indicate that a stable pattern of secondary currents over the entire channel width can only be sustained over a fixed horizontal bed if the bed's roughness is sufficient to provide the required transverse oscillations in the turbulent shear stresses. Contrary to laboratory flumes, alluvial river bed always provide sufficient roughness. The required external forcing of this hydrodynamic instability mechanism is provided by the turbulence-generated near-bank secondary currents. The pattern of near-bank secondary currents depends on the inclination and the roughness of the bank. In all configurations, secondary currents result in a reduction of the bed shear stress in the vicinity of the bank and a heterogeneous bank shear stress that reaches a maximum close to the toe of the bank. Moreover, these currents cause transverse variability of 10-15% for the streamwise velocities and 0.2u * 2 -0.3u * 2 for the bed shear stress. These variations are insufficient to provide the flow variability required in river restoration projects, but nevertheless must be accounted for in the design of stable channels.

Effect of the Junction Angle on Turbulent Flow at a Hydraulic Confluence

“…The reason behind the existence of such strong heterogeneity in Figure 3b derives, essentially, from a combination of two processes. In fact, near the lateral walls, the presence of the secondary currents is due to the amplification of the surface-corner vortex caused by the difference in roughness between the bank and the bed [36,37], whereas, along the width of the main channel, the turbulent secondary flows are attributed to the spanwise heterogeneity in roughness height, which causes low-momentum regions (LMRs) spanwise-adjacent to high-momentum regions (HMRs) (their spanwise positions are labelled in Figure 3). In fact, the layer of gravel was not uniformly glued on the channel bed and, therefore, different topographical scales were arranged in a highly irregular manner.…”

Recirculation cells in a wide channel

“…Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer. They appear in rapid granular flows 1 , in Rayleigh-Bénard-Poiseuille flows 2 , in the superficial layers of sea and lakes 3 , in straight tubes 4,5 , as well as in straight channels and natural rivers [6][7][8] .…”

Wall-Roughness Effects on Flow and Scouring in Curved Channels with Gravel Beds

“…Furthermore, only a few authors have studied the effect of the macroroughness of banks in bends, having concentrated mainly on the flow behavior, such as Atsuyuki (1992), Choi (2000), and Blanckaert et al (2010). Some examples of research on the influence of ribs placed in straight channel walls in the flow include Gairola (1996) and Rhodes and Senior (2000).…”

“…Blanckaert et al (2010Blanckaert et al ( , 2012 studied the influence of roughness at the outer bank in two distinct situations in a laboratory 193°channel bend (Blanckaert 2002), a rectangular channel, and a trapezoidal channel with 30°-inclined outer bank. For the rectangular channel, Blanckaert et al (2010) suggest that scouring close to the wall could be reduced with increasing wall roughness, whereas for the trapezoidal channel, depth-averaged downstream velocity over the bank toe is similar for all experiments regardless of the outer-bank roughness. Recently, in the very same laboratory installation, Dugué et al (2013) proposed a solution based on the installation of a bubble screen to reduce bend scour.…”

“…Examples of erosion preventing measures are utilization of riprap (Martin-Vide et al 2010); undulated, macrorough riprap (Chèvre and Schleiss 2005); installation of bottom vanes (Odgaard and Spoljaric 1986;Odgaard and Wang 1991;Voisin and Townsend 2002) or bank-attached vanes (Bhuiyan et al 2010); lining of the outer bank with concrete slabs (Sloff et al 2006;Roca et al 2007); construction of groynes or spur dikes (Przedwojski 1995;Sukhodolov et al 2002;Jamieson et al 2013b, a); construction of bendway weirs (Abad et al 2008); and construction of bandal-like structures (Teraguchi et al 2011). Blanckaert et al (2010Blanckaert et al ( , 2012 studied the influence of roughness at the outer bank in two distinct situations in a laboratory 193°channel bend (Blanckaert 2002), a rectangular channel, and a trapezoidal channel with 30°-inclined outer bank. For the rectangular channel, Blanckaert et al (2010) suggest that scouring close to the wall could be reduced with increasing wall roughness, whereas for the trapezoidal channel, depth-averaged downstream velocity over the bank toe is similar for all experiments regardless of the outer-bank roughness.…”