The impact of ice cover on the secondary flow structures at the bend apex*

River ice plays a significant role in river flow characteristics in winter. However, the impacts of the frozen surface on river hydrodynamics are still unknown. In this work, we investigate the impact of the ice cover on the secondary flow patterns by (1) conducting field surveys, and (2) performing large-eddy simulation (LES). Measurements are carried out using Acoustic Doppler Current Profiler (ADCP), Sontek M9, in a river bend of Red River in Fargo. A minimum of six ice holes are opened at each designated cross-section at the bend apex during the Winter seasons of 2021 and 2022. The classical rotation-based Rozovskii method is employed to generate secondary flow patterns over the cross-sections. Measurement and LES results show that the existence of ice cover alters the secondary flow patterns drastically in comparison to the free surface condition. Here we show the existence of multi-cell structures in the secondary patterns. We hypothesize that the presence of the ice cover is responsible for such an emergence of secondary flows.