Experimental study on the velocity characteristics of adverse-slope open-channel flows on spacing-roughed bed

To explore the influence of spacing-roughened beds on velocity distribution in adverse-slope open channels, flume experiments under nonuniform flow conditions employing particle image velocimetry (PIV) were conducted. These experiments encompassed both positive and adverse slopes. A meticulous comparative analysis was performed on the mainstream velocity distribution and the structure of the near-bed flow field for various spacing ratios (the ratio of longitudinal spacing λ_x to the height h of roughness elements), spanning from 2 to 16. Additionally, the variations in the zero-plane displacement z₀/h and the equivalent roughness height k_s/h were explored. The results show that: ① The double-averaged (time-space averaged) velocity distribution tends towards a logarithmic distribution above roughness elements, with near-surface velocity escalating as the slope transitions from adverse to positive. Below the roughness elements, the profile exhibits a linear distribution. ② Specifically, at λ_x/h = 2, the positive-slope case scenario displays a significant reduction in near-bed velocity due to cavity recirculation. Conversely, the adverse-slope situation showcases no recirculation, resulting in a weaker velocity reduction effect. For λ_x/h ≥ 4, separation zones of 1.1h to 1.4h emerge in both positive- and adverse-slope instances. ③ The zero-plane displacement z₀/h diminishes logarithmically with augmented roughness spacing and intensifies as the slope transitions from adverse to positive. ④The k_s/h peaks at λ_x/h = 4 and undergoes a significant increase when the slope changes from adverse to positive. These findings offer valuable theoretical underpinnings for enhancing the conveyance capacity of water diversion projects.