The effect of ice cover on velocity and shear stress in a riffle‐pool sequence

Abstract

Discharge and channel geometry control velocity and bed shear stress within a reach.Channel roughness (e.g., riffles, pools) and ice cover in winter moderates velocity and shear stress at the bed. This study evaluated velocity profiles across a channel segment with variable bed roughness (e.g., riffles, pools) to determine changes in the position of maximum velocity, maximum velocity magnitude, and resulting bed shear stress estimates when ice cover was present. Using acoustic doppler velocimeter (ADV) and acoustic doppler current profiler (ADCP) high resolution velocity profiles were collected during ice cover, open water, and open water with significant increases in vegetation cover through a riffle-pool sequence in a low-order channel in southern Ontario Canada in the first half of 2021. Key findings were that in five of the seven cross-sections, flow direction was significantly different when ice was present. Additionally, maximum velocities were closer to the bed during ice cover, a common finding in modelling and experimental work, and is confirmed in this field setting. Although maximum velocity magnitudes were not significantly different, derived bed shear stress values under ice were larger. Specifically, under ice conditions, riffle bed shear stress ranged 0-16 N/m 2 compared to 0-9 N/m 2 in ice free conditions. In the pool, bed shear stress ranged 0-6 N/m 2 under ice cover, and 0-5 N/m 2 in ice free conditions. Further, as flow levels increased through the spring