Effect of Macrorough Sidewalls on Flow Resistance in Steep Rough Channels

Abstract

Step-pool sequences typically form in steep mountain streams. Single step-pool units exist in steep channels with bed slopes exceeding 4% and a continuous step-pool morphology occurs in channels with bed slopes higher than 6%-7% (Church & Zimmermann, 2007;Recking et al., 2012). The flow regime in a step-pool channel is referred to as tumbling flow as the flow alternates between supercritical flow over the steps and subcritical flow in the pools (Whittaker, 1987). Thus, a large amount of flow resistance is attributed to the spill caused by hydraulic jumps (

Bed Stability of Step‐Pool Channels With Macrorough Sidewalls

“…The physical experiments were conducted in a tilting flume with a L tot = 9.6 m long test reach at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich. The same experimental setup was used as in Maager et al (2022), thus, only a short description is given herein focusing on additional instrumentation. The experiments were scaled according to Froude-similitude with a scale factor of 20.…”

“…A roughness parameter is assigned to both channel bed and sidewalls, that is, the Darcy-Weisbach sidewall and bed friction factors f w and f b are estimated. For macrorough sidewalls, Maager et al (2022) developed an empirical approach to estimate the sidewall friction factor f w :…”

“…with W m = mean channel width and σ w = standard deviation of the banks from the mean bank position, that is, a sidewall roughness parameter. Equation 1 is only applicable to RAF conditions (Meile et al, 2011) and within the parameter range indicated by Maager et al (2022), that is, relative submergences 0.49 < h/d 84 < 3.7, aspect ratios 1.5 < W m /h < 22.6, relative sidewall roughness σ w /W m < 0.052, and Froude number 0.37 𝐴𝐴 𝐴𝖥𝖥 𝐴 1.78. Following the sidewall correction procedure, Equations 2-4 are used to calculate the dimensionless bed shear stress θ Rb :…”

“…Therefore, the procedure developed by Maager et al. (2022) was applied, who expanded the Einstein‐Johnson sidewall correction procedure to steep rough channels with macrorough sidewalls.…”

“…(a) Bed shear stress is assumed to significantly decrease in experiments with macrorough sidewalls in comparison with smooth sidewall experiments, possibly explaining the increase in bed stability (first hypothesis). According to Maager et al (2022), sidewall friction contributed to 5%-40% of the overall shear stress but it remains unclear, whether this bed shear stress decrease fully explains the increase in bed stability. (b) More steps are expected in experiments with macrorough sidewalls as the narrowing regions caused by the macrorough sidewalls may enhance step formation (second hypothesis).…”