Experimental Modeling of Ice Cover Effects on the Secondary Flow Formation in Meander Bend

Abstract

When a river surfaces freezes, the ice cover introduces a rigid upper boundary that alters flow structure. In straight reaches, this results in a two-layer system governed by shear at both the bed and the ice-water interface. In bends, the combined effects of channel curvature and the ice boundary generate counter-rotating secondary circulation cells near the bed and the ice underside. These processes remain poorly understood due to limited observations under ice-covered conditions, and because many existing models assume an idealized open-channel condition with negligible surface shear, thereby neglecting ice-induced roughness and resistance. This study investigates the mechanisms governing secondary flow in ice-covered bends, hypothesizing that the observed flow structure arises from the interaction between channel curvature and the rigid ice boundary, although the underlying dynamics are not yet fully resolved.

The central focus of this thesis is advancing the understanding of how these circulation cells form and develop under varying flow and roughness conditions. The effects of flow rate, flow depth, and the ice cover surface roughness on the flow structure in a simulated ice-covered bend are systematically examined. A comprehensive set of experimental investigations was conducted using uOttawa’s laboratory bend flume, where Particle Image Velocimetry (PIV) technology was applied to capture 3D velocity components. The data was used to carry a detailed examination of flow characteristics under ice-covered conditions. These measurements provided unique insights into the spatial variations of velocity, turbulence, and secondary flow circulations in ice-covered confined channel bends where the flow displays strong three-dimensional characteristics.

In particular, it was found that variations in flow rate (i.e., Froude number) distinctly influenced turbulence distribution patterns beneath the ice cover in the channel bend. As discharge decreased, the normalized mean kinetic energy increased, indicating more structured flow and greater relative turbulence at lower flow rates. Results related to increasing ice-cover roughness revealed a reduction in total secondary circulation within the ice layer, while circulation near the bed was enhanced. The three-dimensional momentum response under varying roughness conditions was also examined, revealing directional sensitivity differences: vertical and cross-stream momentum exhibited greater sensitivity to ice roughness than streamwise momentum, leading to a reshaping of the secondary vortex structure under increased roughness. The aspect ratio variation under the ice cover additionally showed a marked change in the spatial configuration and strength of secondary circulation vortices. Clear variations in the height of maximum velocity were observed along the channel, a related but distinct trend driven by local turbulence as the aspect ratio changed. Such data are rare (if not entirely absent) in existing experimental studies, especially for meander bend river channels. These findings provide new insights into the influence of ice cover on turbulent flow fields in meandering rivers and offer valuable guidance for hydraulic engineers and researchers working in cold-region water systems.