Flow processes near smooth and rough (concave) outer banks in curved open channels

Abstract

[1] Flow processes near the (concave) outer bank in curved river reaches are investigated in a laboratory flume, with focus on the influence of the bank roughness. An outer-bank cell of reversed secondary flow occurs for all the investigated roughness configurations of the outer bank. The cell widens the outer-bank boundary layer, which reduces the flow forcing on the bank, but also advects high-momentum fluid toward the lower part of the bank, which enhances the flow forcing on the bank. Increasing the roughness of the outer bank causes a considerable widening and strengthening of the outer-bank cell, which amplifies both effects on the flow forcing. The widening of the outer-bank boundary layer induced by the outer-bank cell reduces the effective width of the channel, i.e., the width where most of the discharge is conveyed, which increases significantly the sediment transport capacity in the central part of the channel. The near-bank processes are qualitatively similar but show considerable quantitative variations around the bend. The outer-bank cell reaches its maximum strength in the zone that is most vulnerable to bed scour and bank erosion, which corroborates its morphological relevance. The outer cell is mainly generated by reversed near-surface gradients in the profile of the streamwise velocity. The anisotropy of the cross-stream turbulence is the major mechanism opposed to the outer-bank cell. Increasing roughness of the outer bank amplifies all dominant mechanisms but does not modify their relative importance. The relevance of the experimental results for natural curved river configurations is discussed.

A parametrical study on secondary flow in sharp open-channel bends: experiments and theoretical modelling

“…Table 2 Non-intrusive velocity measurements were performed with an Acoustic Doppler Velocity Profiler (ADVP) at the centreline at arc lengths of 30°, 60°, 90°, 120°, 150°and 180°in the bend. Data measured in various cross-sections around the bend in the same laboratory flume that illustrate a more complete picture of the spatial evolution of the hydrodynamics, and that demonstrate the representativeness of centreline data have been reported by Zeng et al (2008) and Blanckaert et al (2012). The ADVP measures vertical profiles of the three-velocity components with high temporal and spatial resolution.…”

“…Understanding of secondary flow in sharply curved reaches is still hampered by the paucity of experimental data. Only a limited number of experiments in the high-curvature range have been reported (Blanckaert, 2009(Blanckaert, , 2010Blanckaert et al, 2012Blanckaert et al, , 2013Frothingham and Rhoads, 2003;Jamieson et al, 2010;Nanson, 2010). Moreover, the influence of the bed roughness and the Froude number have never been systematically investigated.…”

Hydrodynamic processes, sediment erosion mechanisms, and Reynolds-number-induced scale effects in an open channel bend of strong curvature with flat bathymetry

“…Field experiments are not subject to scale effects but hindered by irregular geometries, unsteady conditions, interactions with ecological processes, and an inherently low spatial resolution of the measurements [e.g., Bathurst et al ., , ; Thorne and Rais , ; Thorne et al ., ; Odgaard and Bergs , ; Ferguson et al ., ; Frothingham and Rhoads , ; Sukhodolov , ]. Although subject to scale effects, laboratory experimental studies allow flow processes to be isolated, accentuated, and measured with a greater accuracy than is possible in a field study [e.g., Blanckaert and Graf , ; Blanckaert and de Vriend , , ; Abad and Garcia , , ; Blanckaert , , ; Jamieson et al ., ; Keylock et al ., ; Blanckaert et al ., , ]. During recent years, high‐resolution eddy‐resolving numerical simulations such as 3‐D large eddy simulation (LES) and detached eddy simulation (DES) have been shown to provide a powerful tool to investigate the physics of flow in curved and meandering channels of medium and high curvature at least for flow conditions corresponding to laboratory studies [e.g., Moncho‐Esteve et al ., ; van Balen et al ., , ; Constantinescu et al ., , ].…”

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

“…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.…”