Near-bed flow directions under varying ice cover of a meander bend in 2016–2024

Abstract

Currently, in the northern hemisphere, ice cover forms annually on rivers located in high latitudes. However, in the context of future climate changes, this natural phenomenon may undergo alterations, resulting in later ice formation and earlier ice breakup compared to traditional patterns. These changes affect the flow characteristics of rivers and, consequently, impact river environments, especially sediment transport, nutrient transport, and aquatic habitats. While there has been an increase in field measurement-based studies during past decades, our understanding of the effects of river ice cover on flow dynamics within natural environments remains constrained. Specifically, the influence of ice cover on near-bed flow characteristics has been relatively underexplored due to the difficulties in obtaining accurate data in such conditions. When ice cover forms in a river, it creates an additional roughness layer compared to open-channel conditions. This roughness layer alters flow conditions by creating an asymmetrical flow structure. Under a fixed ice cover, flow velocity increases, and maximum velocities might be higher than under similar discharge conditions in an open channel. The flow gradient near the riverbed is notably different in an ice-covered channel than in an open one. In this study, we aim to 1) detect the impacts of ice cover on near-bed flow direction and 2) compare the effects of different riverbed roughness on near-bed flow conditions. The aim is to assess how ice cover and riverbed roughness alter flow characteristics and quantify the effects that might occur when river ice is not present in a similar form to before. The study site is a meander bend of the sub-arctic Pulmanki River, located in Northern Finland. The Pulmanki River undergoes annual freezing, with the ice-covered season typically extending from October through May. Field measurements have been conducted during the ice-covered low flow period spanning from 2016 to 2024 using ADCP (Acoustic Doppler Current Profiler) and ADV (Acoustic Doppler Velocimeter). Near-bed flow directions are analysed based on the comparison of different field measurements. Analysis of the effects of different riverbed forms, roughness,  and prevailing ice conditions on flow characteristics is conducted. The results of this study contribute to the understanding of river ice processes and, therefore, help improve the management of river systems under a changing climate.