Roughness influence on secondary circulation and momentum distribution in an ice-covered river bend

Ice-cover roughness perturbs velocity and momentum distributions, leading to secondary flow redistribution within river channels. Particle image velocimetry measurements examined the effects of ice roughness on secondary flow at the apex of a 135° bend under two different simulated ice covers. Both flows produced a two-layer flow structure, with roughness strongly influencing cell position and intensity. Under rough ice, the top-layer inner-bank cell (IBC) observed in the smooth ice case merged with the primary vortex, forming a single dominant vortex that was shifted toward the inner bank. While the total circulation in the ice-layer decreased, the bottom-layer vortex circulation increased with roughness. A three-dimensional (3D) momentum analysis showed vertical momentum with intensified downward flux from the ice layer and enhanced upward flux from the bed. Cross-stream momentum was characterized by concentrated high-intensity zones near the inner bank, indicating roughness-induced lateral redistribution. Roughness had a greater effect on the vertical and cross-stream components of momentum than on the streamwise momentum. The rough ice cover induced greater turbulence across the cross section from outer to inner bank, with a localized decrease near the bed layer adjacent to the inner bank. Increased turbulence under rough ice facilitated the observed merging in the ice layer of the IBC with the primary circulation cell. Momentum and turbulence responses suggest that IBC formation and stability are shaped by boundary morphology and, under rough ice, involve structural reorganization and integration into the primary circulation.