In cold and temperate climates ice and ice covers are formed in rivers annually. These affect the wintertime flow conditions in riverbeds. As the ice cover breaks in spring, the freed ice floes can form ice jams and thus cause flooding. It is predicted that due to climate warming the annual existence of stable ice covers will shorten in duration. Not all rivers in Finland will necessarily form a stable ice cover, which leads to changes in the wintertime flow conditions.

This study examines the mentioned wintertime flow conditions and the factors that affect them through the means of hydraulic modeling and time series analysis. It most notably focuses on analysing the ice cover and ice jam situations. The main object of this study is to investigate the effects these varying situations have on the flow conditions in the riverbed and the movement of the sediment. Furthermore, this study explores the ways in which the ice cover begins its movement, as well as the changes in weather from 1990 to 2020 that have affected those ways. The area of study is Uskelanjoki, to where ice control structures have been constructed in 2018 and 2019, and where a fish habitat restoration has been implemented.

This study was conducted by utilizing bathymetric modeling via an Acoustic Doppler current profiler, and digital elevation modeling via terrestrial laser scanning.The underwater geometry of the bathymetric model and the surface geometry of the digital elevation model functioned as a base for the hydraulic model, which was used to model different 1-dimensional ice cover and ice jam situations in different discharges. The results were examined for changes in water elevation, flow velocity and boundary shear stress as well as the movement of the sediment in the modeled situations. Average values were used in the time series analysis, through which ice breakup types and changes in weather conditions could be investigated.

The formation of the ice cover slowed down the flow velocity and decreased the boundary shear stress. These changes resulted in a reduced amount of moving sediment in the riverbed. On the contrary, underneath an ice jam the flow velocity fastened and the boundary shear stress increased, causing the amount of moving sediment in the riverbed to increase as well. The formation of the ice jam elevated the water level and led to flooding in the upstream. The changes in the flow conditions did not cause sediment movement in the area of the fish habitat restoration. In the investigation of the ice breakup types no specific type was found to be more common than the rest. The examination of weather conditions revealed a significant increase in the average temperatures during springtime, but no changes in precipitation were detected during the investigated time period.