Hydrodynamic effects of one-sided wart-type roughness on supercritical open-channel flows

Bed roughness plays an important role in controlling flow resistance, turbulence and energy dissipation in open-channel flows, particularly in high-velocity hydraulic structures such as spillways. The influence of wart-type roughness elements on supercritical open-channel flow dynamics was investigated through controlled flume experiments, comparing smooth and rough ( S = 20 cm spacing) bed conditions. Velocity profiles, shear stress, and specific energy distributions were measured along the channel using point velocity measurements in a 7 m long, 0.5 m wide Plexiglas flume with a 2% slope at the Ujigawa Hydraulic Laboratory, Kyoto University. Results show that the rough configuration significantly reduces mean velocities compared to the smooth bed, with pronounced velocity deficits near the bed. Wall shear stress ( τ ) in the smooth case ranged from 2.92 to 3.04 Pa, showing a slight Reynolds number (Re) dependence, while the rough case exhibited lower τ (1.88–2.28 Pa) with greater variability, suggesting reduced drag due to flow separation. Non-dimensional shear stress ( τ/(ρv 2 )) was nearly constant for the smooth bed and fully constant for the rough bed, indicating a transition to a fully rough regime. The logarithmic law of the wall was validated, with a consistent von Kármán constant ( κ = 0.41) and reduced intercept ( A = 5.09 vs. 5.5) for the rough bed. Specific energy distributions revealed enhanced dissipation near the rough bed, impacting hydraulic efficiency. These findings are limited to S/K = 5.71 and Fr > 1.8, but highlight the potential of this specific wart-type roughness for energy dissipation and erosion protection in high-velocity structures.