Impact of bed roughness and submergence on velocity profiles and flow structures in hydraulic jumps

Abstract Hydraulic jumps downstream of spillways exhibit high near-bed velocities that can induce cavitation and scour. While bed roughness is known to modify velocity distribution and roller characteristics, the underlying flow physics governing velocity transition, turbulence dissipation, and vortex evolution under varying submergence conditions remain insufficiently understood. This study presents an integrated experimental and numerical investigation of free and submerged hydraulic jumps over smooth and corrugated beds to elucidate these mechanisms. Experiments were conducted at discharges of 11, 13, and 15 l s⁻1 for submergence ratios ranging from 0 to 0.4, and complemented by three-dimensional CFD simulations using a RANS-based RNG k–ε model in FLOW-3D. Detailed analyses of velocity similarity profiles, boundary layer development, turbulent kinetic energy (TKE), dissipation rate, total energy loss, and Q-criterion based vortex structures were performed. The results reveal that bed corrugation accelerates boundary layer development, enhances turbulence breakdown, and increases energy dissipation compared to smooth beds. Submergence shifts the turbulence generation zone downstream and increases roller length; however, corrugated beds consistently confine coherent vortices and reduce near-bed velocity under all tested conditions. The study provides new physical insight into turbulence–roughness–submergence interactions in spillway-induced hydraulic jumps and offers design guidance for improving stilling basin performance and reducing downstream scour risk.