Macro-Turbulent Flow and Its Impacts on Sediment Transport Potential of a Subarctic River during Ice-Covered and Open-Channel Conditions

Eliisa Lotsari,Elina Kasvi,Michael Dietze,Maria Kämäri,Petteri Alho

doi:10.3390/w12071874

Abstract

Macro-turbulent flows (i.e., coherent flow structures reaching through the whole water column), have not been studied widely in northern seasonally frozen rivers during both open-channel and ice-covered flow conditions. Thus, we aim: (1) to detect and compare the macro-turbulent flow, both at open-channel and ice-covered flow conditions; (2) to explore spatial variation of macro-turbulent flow characteristics within a meander bend; and (3) to detect the effects of near-bed layer velocity fluctuation on bedload transport during differing overall flow conditions. The analyses are based on 5–10 min-long acoustic Doppler current profiler (ADCP) measurements from a subarctic river. The ice-covered low flow, and open-channel higher and lower flow conditions were measured over the period of 2016 to 2020. This study found that macro-turbulent flow existed at all measurement locations under both open-channel and ice-covered flow conditions. Macro-turbulent flow was most consistent and obvious in the streamwise velocity component, and in particular at the inlet and outlet of the investigated meander bend. During all seasons, the near-bed velocities consistently exceeded the sufficient amount for sediment transport. At inlet and outlet areas, the greatest near-bed velocity fluctuation across the critical threshold for sediment transport coincided with the measurement times having frequent macro-turbulent flow.

Highlights

River ice can have a significant impact on the distribution and magnitude of flows in the river channels [1,2,3]
Further studies need to verify the impacts of these pulsating flows during both open-channel and ice-covered conditions on the sediment transport, as this study is only aware of Demers et al [9] who have studied similar conditions at a natural state sand-bed meandering river
Based on the preliminary assessment of the local flow conditions at the Pulmanki River, the flow characteristics were measured with an acoustic Doppler current profiler (ADCP)

Summary

Introduction

River ice can have a significant impact on the distribution and magnitude of flows in the river channels [1,2,3]. Under ice-covered conditions, increases are seen in the spatial variability and characteristics of depositional and erosional locations compared to the open-channel conditions [3] These observations are based on studies conducted during only one season and on measurements that are too short (less than 1 min) to capture larger pulsating structures of high and low flows. The impact of this flow pulsation on the sediment transport potential and morphology of rivers during open-channel conditions has been studied over several decades (e.g., [4]), and it has been identified that this characteristic likely plays a major role in bedload sediment transport [5,6]. Further studies need to verify the impacts of these pulsating flows during both open-channel and ice-covered conditions on the sediment transport, as this study is only aware of Demers et al [9] who have studied similar conditions at a natural state sand-bed meandering river

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