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
Modeling in ice-covered rivers is limited due to added computational complexity, specifically challenges with the collection of field calibration data. Using River2D, a 2-dimensional hydrodynamic modeling software, this study simulates depth-averaged velocity and shear stress distributions under ice cover and in open-water conditions during varying flow conditions in a small, shallow riffle-pool sequence. The results demonstrated differences in velocity distribution throughout the channel and increases in discharge were found to impact the velocity magnitude under ice cover, while the spatial distribution remained consistent. A recirculating eddy found along the pool’s left bank was exacerbated under ice cover, with potential implications for silver shiner habitat suitability. Bed shear stress magnitude did not vary significantly between ice and open water, although the spatial distribution differed notably. Model validation demonstrated success in simulating water depth and velocities, and the shear stress was estimated within a reasonable margin. Using hydrodynamic models provides valuable insight into seasonal changes in velocities and shear stress when ice is present.
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