Flow structure in large bedrock‐channels: The example of macroturbulent rapids, lower Mekong River, Southeast Asia

Abstract

The rate of bedrock channel incision is key to the understanding of landscape evolution. Theoretical models relate channel incision to sediment transport; the latter conditioned by the bed shear stress. However, theory is deficient in an appreciation of the transverse and vertical flow structure that mediates shear stress for deep, narrow inner‐channels, which often characterize large bedrock rivers. Here we present the detail of the structure of high Reynolds number flows for bedrock‐controlled rapids of the Mekong River, SE Asia. Distinct filaments of high‐velocity flow, separated by regions of slower flow, occur across channels; the numbers of filaments scale linearly with channel width to maximum‐depth (B/hmax) ratios. Inner‐channels with low ratios (B/hmax < 20) exhibit wall‐effected flow structure. Effects include suppressed maximum velocity filaments due to: (i) significant channel‐transverse flow; (ii) strongly‐sheared vertical flow structure; and (iii) significant underflows. Such complex water column flow patterns largely defy theoretical description. Nonetheless near the bed, the vertical velocity distributions often conform to: (i) ‘law‐of‐the‐wall’ logarithmic theory; or (ii) profiles in the near‐bed region and within the transition to outer flow can be described using a log‐wake function. Consequently, for selected velocity profiles it is possible to derive hydraulic parameters suitable for input to incision models. Chezy‐C values are high, indicating low flow resistance, while bed shear stresses remain competent, even during low‐discharges, to transport cobble bedload across low roughness bedrock surfaces. Thus, as competence is high, no blanketing sediment deposits can develop within inner channels to prevent bedrock erosion. Consequently, within similar high competence systems, incision is probably progressive as long as sediment supply is sustained to abrade the bedrock. The landscape modelling implication is that abrasion in this system is supply‐limited and not limited by flow competence. In contrast, a transport‐limited system is likely to evolve to exhibit an alluvial bed. © 2018 John Wiley & Sons, Ltd.

Amplification of Plunging Flows in Bedrock Canyons

“…The lateral flow enhances lateral erosion downstream of a constriction, causing bank erosion that makes the channel wider (Venditti et al., 2014). This phenomenon of high flow velocity being driven toward the bed by secondary flow cells in meander bends and straight channels is referred to as the velocity dip phenomenon (e.g., Blanckaert, 2011; Carling et al., 2019; Chauvet et al., 2014; Stoesser et al., 2015; van Balen et al., 2010; Yang et al., 2004). Plunging flows in bedrock canyons are novel when compared to the velocity dip phenomena due to the strength of the secondary circulation, the absence of sinuosity in the channel, and the greater magnitude of the velocity near the bed.…”

Bed and Bank Stress Partitioning in Bedrock Rivers

“…Black Canyon, for example, has six CPW sequences with along channel length scales of ∼1,000 m and smaller wall forms with length scales of ∼50 m (Curran, 2020, Figure 11). These wall forms cause flow separation, secondary flow cells and the formation of large scale coherent flow structures (Ansari et al., 2018; Venditti et al., 2014) that effectively reduce the mean velocity and increase the form drag on the walls (Carling, 1989; Carling et al., 2019; Carter & Anderson, 2006; Wohl, 1993; Wohl et al., 1999; Yang, 1971). The walls may also be rougher than the bed because the bed might be smoothed by alluvium filling depressions.…”

“…We suspect that the negative result is because our estimate of the hydraulic radius is not commensurate with the resolution of the bed shear stress observations. It is also not clear that hydraulic radius calculated from cross‐sections is capable of capturing the variation in the banks that cause the high stress on the walls (Carling et al., 2019).…”

Impact of dams and climate change on suspended sediment flux to the Mekong delta

“…In the present study, we model more than 100 years over a large area. It can be assumed that no significant changes to the riverbed morphology are expected aside from deposition and remobilisation within this time frame and at this scale, especially because many previous studies point out that many sections of the river are bedrock-constrained (Carling et al, 2019;Kondolf et al, 2014;Meshkova and Carling, 2012;Nie et al, 2018;Van et al, 2012) and thus significant geomorphological changes are expected to take much more than just a century. This might not be true for the delta part of the river, however, as pointed out previously, this paper focuses on the inland part of the river and estimates the influx of sediment at the apex of the delta.…”