Surface flow and ice rafting velocities during freezing and thawing periods of three northern rivers.

Abstract

In present climate, almost all rivers freeze in the northern high latitude regions every year. In the future, this annual freezing cycle may undergo major changes. These changes can already be observed in the current climate. The effects of climate change in cold climate areas are radical and will have a great impact on the environment: Changes in the ice formation, duration and break-up of river ice cover will affect the hydrology of the rivers. Effects will vary from flow rates to sediment transport and discharge levels. For the most part, studies of freezing and thawing have been carried out under laboratory conditions or are outnumbered by open-channel studies depicting river systems in warmer areas. More information on the processes of freezing and thawing and the effects this cycle has on river flow, and vice versa, is needed by monitoring natural sites. Consequently, this study seeks to respond to this lack research and process knowledge, and aims 1)  to examine, whether the freezing and thawing periods affect the river surface velocity and the spatial variations of speed in the partly frozen channel (and vice versa), 2) to detect the differences in the formation of ice, ice cover and the ice break-up between different type of river reaches, and 3) to examine differences in the freezing and thawing cycles and surface velocity connections in hydro-climatically different rivers. The study is conducted in three cold climate region study sites: Boreal and continental, meandering Koitajoki River (Finland), straight reach of more maritime Sävarå River, and sub-arctic Pulmanki River (straight reach of a meandering river). The data for the study has been collected between 2020-2022 using game camera images and videos with georeferenced targets. The velocity of river flow is analyzed with two different methods (STIV and PTV) from the videos. At the same time, efforts will be made to compare how well the different methods work to analyze the situation in question. Reference measurements have also been carried out in the autumn before freezing, in winter during ice cover, and in spring after the break-up of ice, and these measurements are compared with the results of the analyses. The change and duration of the ice cover is analyzed based on the image series. Regional climate data, as well as data collected from research areas (water temperature, air temperature, atmospheric pressure) is used to estimate the winter conditions. In addition for new knowledge of freezing and thawing processes within cold climate region rivers, the results from this study enable advancing hydraulic modelling approaches in ice covered season, and further specify our knowledge on freezing and thawing of river ice.