Boundary Shear Stress in an Ice-Covered River during Breakup

River ice complicates river hydraulics and morphodynamics by adding a new boundary layer to the top of the flow. This boundary layer affects the velocity distribution throughout the depth due to increased flow resistance, and varies the local boundary shear stress on the bed (lower boundary) by adding new shear stress on the upper boundary (under surface of the ice). Variation of shear stress plays an important role in incipient motion of upper and lower boundary materials: sediment motion and transport are directly affected by local boundary shear stress, as are ice cover thickness, condition, and progression. This paper provides estimates of upper and lower boundary shear stress during stable ice cover and the important stage of ice cover breakup using available methods based on continuous field measurements of velocity profiles obtained with a bottom-mounted acoustic Doppler current profiler in the Nelson River, Canada. Boundary shear stresses varied dynamically with transformation of the ice cover, including the presence and removal of slush ice and formation of the ice jams. Estimated upper boundary shear stress varied between 0.52 and 23.87N/m2, whereas lower boundary shear stress varied between 1.51 and 2.40N/m2, with the highest values observed during breakup. These values are compared with estimates from previous laboratory experiments.