Large Eddy Simulation of ice-covered flows in a river bend*

Ice coverage is a critical component of riverine system in cold regions. In this study, three-dimensional flow structures of a river bend are analyzed under both ice-covered and open surface conditions. Field measurements are carried out to reconstruct river bathymetry as well as two-dimensional flow structures in a river bend, which locates in a river reach in Fargo, North Dakota (United States). The Digital Elevation Model is reconstructed from the bathymetry data and the LiDAR data. An in-house Large Eddy Simulation (LES) is used to simulate the three-dimensional flow structures in the bend using the measured discharge from the on-site United States Geological Survey station. Our field measurements and LES results show two distinctively different flow modes corresponding to the open-surface and ice-covered conditions. Under open-surface condition, the secondary flow includes a well-defined circulation. Under ice-covered condition, multiple cells are observed under the ice coverage. We observe a critical role of local bank characteristics on regulating the distribution of high velocity core (HVC) by the presence of dead zones. These zones redirect HVC toward the thalweg and excite Turbulent Kinetic Energy (TKE) to form in the vicinity of the associated shear layer. The narrowing effect of dead zone induce jet-like structures to form at the bend apex and increases TKE locally throughout the water column. Our results suggest a hidden role of ice in regulating flows in meandering rivers.