Supercooling and frazil ice development processes at Nordic rivers with different channel forms and materials

The knowledge of supercooling and related frazil ice phenomena in rivers and lakes is critical for purposes of flow control, operation of hydraulic works and estimation of the conveyance capacity of the channels, in particular, at rivers having long and severe winters. Most of the previous studies on supercooling, and related frazil ice and anchor ice development, and further on solid ice development, have been done in flumes. However, there are differences in real river and flume results of temperature effect on supercooling, such as supercooling reaching lower temperatures and establishing equilibrium faster in laboratory than in outdoor conditions. The data of supercooling is still sparse and need for observing the processes in different types of cold region rivers is crucial for also understanding their impacts on natural hazards, such as ice dams and winter flooding, and impacts on hydropower production and land-use planning (such as erosion protection). Studies including field observations would be crucial, but those have been so far rare from rivers of different channel gradient, cross-sectional form and material. Further, in addition to directly gaining more understanding of the supercooling processes, the vast field data sets could enhance the modeling of river ice development, and in particular, the calibration of the models.

 

Therefore, the aim of this study is to enhance understanding of the supercooling events and related occurrence of frazil ice based on data from Nordic rivers with different channel forms and materials. The study is based on high precision temperature sensors and close-range remote sensing, e.g. time-lapse cameras, installed in three sites: 1) Pulmankijoki River in Finland, 2) Sävarå River in Sweden, and 3) Orkla river in Norway. Also discharge, air temperature, and geomorphological data, e.g. channel form, slope and sedimentological data, are available from each site for understanding the differences in the study site characteristics.

 

The water temperature sensors and time-lapse cameras were installed to the study sites in autumn 2023, before the freezing period started. The Seabird (Swedish and Norwegian rivers) and RBR solo water temperature sensors (Finnish river) were applied. These sensors were collected from the rivers after the ice melting and flood season in spring 2024. The time-lapse cameras recorded still images and videos, and enabled connecting the frazil ice occurrences to the water temperatures and the supercooling events. These events were also analysed against the air temperature, and thus the energy balance was possible to compare between the different rivers. The preliminary results of the supercooling and frazil ice development processes are presented.