Ice‐covered river hydraulics and bedload transport: Insights from three-dimensional modelling

In cold regions, river sediment transport during mid-winter periods is strongly influenced by the presence of ice cover. However, the effects of ice cover on bed shear stress, sediment mobility, and the longer-term evolution of fluvial geomorphology remain insufficiently understood due to the complexity of the physical mechanisms and the scarcity of relevant field measurements. To address this gap, the present study investigates mid-winter bedload transport processes in a sub-arctic river under ice-covered conditions using the Delft3D Flexible Mesh (Delft3D FM) software. The modelling approach employs a novel integration of three-dimensional (3D) hydrodynamics, sediment transport and ice to resolve interactions between hydraulics, ice-induced resistance, and sediment mobility. The Pulmankijoki River in northern Finland was selected as the study site owing to its typical sub-arctic hydrological regime and seasonal ice cover. Comprehensive field measurements, including river topography, flow discharge, water level, sediment diameters, bedload transport rate, and ice thickness, were conducted during 22–29 February 2022, 21–28 February 2023, and 17–24 February 2024.

Based on the numerical results, firstly, comparison with field measurements shows that the 3D model is more accurate than the depth-averaged (2D) approach, demonstrating its advantage in bedload transport simulations under ice-covered conditions. This highlights the importance of resolving vertical velocity gradients. Secondly, sensitivity experiments on the ice-cover roughness coefficient indicate that ice roughness has only a minor influence on bed shear stress and therefore does not significantly modify bedload transport rates. Thirdly, by mapping the ratio of local bed shear stress to critical shear stress, spatial patterns of sediment transport potential during the three mid-winter seasons were clarified, illustrating how sediment mobility persists beneath ice. This study demonstrates that Delft3D FM is an effective modelling tool for resolving sediment transport under ice-covered conditions in sub-arctic rivers. The findings contribute to an improved process-based understanding of winter river dynamics and provide insights for sediment management strategies in cold-region environments.