Effect of Macrorough Sidewalls on Flow Resistance in Steep Rough Channels

Abstract

Previous studies showed that macrorough sidewalls increase the stability of step-pool sequences in steep narrow channels due to grain-sidewall interactions (i.e., jamming) and bed shear stress reduction due to sidewall friction. The main goal of this study was to quantify sidewall and bed shear stress for six types of macrorough sidewalls using physical model tests. Experiments were conducted in a tilting flume with initial slopes of 4%–10% with fixed rough beds without step-pool sequences and mobile rough beds containing step-pool sequences. A hydraulic geometry relation to predict mean flow velocity was extended with the relative sidewall roughness to account for friction effects of macrorough sidewalls, resulting in mean prediction errors below 8% for the mobile bed experiments. Furthermore, the study presents a method to estimate flow resistance caused by the macrorough sidewalls by applying a modified sidewall correction. For the fixed rough bed, the macrorough sidewalls accounted for 10%–70% of the total shear stress. In the mobile bed experiments, the bed freely adjusted to the flow leading to the formation of step-pool sequences. Consequently, sidewall friction accounted for 5%–40% of the total shear stress because the step-pool structures induced larger flow resistance compared to the fixed bed experiments. Overall, the results allow to determine bed and sidewall shear stress in steep channels with macrorough bed and sidewalls, thereby enabling future studies to better quantify the mechanisms responsible for the increase in bed stability in step-pool channels with macrorough sidewalls.