Ice conditions in Norwegian rivers regulated for hydropower: An assessment in the current and future climate

Abstract

This PhD is focused on the river ice conditions in regulated rivers for the current and future climate. The studies are based on a series of modelling applications, field data collections and data analysis. First a process based one dimensional river ice model Mike-Icehas been established in a shallow and fast flowing river. The analysis has been carried out in a basin with a complex hydropower system consisting of multiple reservoirs, hydropower plants and secondary intakes. The result suggests that the Mike-Ice model is a useful tool for the simulations of general hydropower impacts on river ice conditions. The study further investigates the use of the calibrated model to examine climate change impacts on river ice in detail.

Climate change impacts have been investigated on rivers and reservoirs in general at a regional scale. In the future, climate change impacts on ice have both positive and negative implication for hydropower operations. The reduction of ice loads on hydropower structures and shortened ice period in the future climate are the positive consequences. However, the unstable winters with possibility of frequent freeze-thaw periods can be problematic in future climate. The detailed simulations of climate change impacts on ice conditions show that there will probably be fewer frazil ice days, shortened ice cover periods and warmer water temperature at most of the locations in Orkla river by the middle and end of the 21th century.

The winter hydropower operational constraints given in the regulation permit in the Orkla basin have been investigated by using a series of numerical modelling tools. This is focused on the lowermost power plant intake at Svorkmo and the outlets from Grana and Brattset upstream which provides the inflow for Svorkmo. The permit states that the upstream plants should be stopped in the case of a shutdown of Svorkmo to prevent ice problems in the downstream bypass reach. The study found it very likely that the effect of this is small since water already released upstream will spill and break the ice. Further, stopping the upstream plants might introduce environmental and ice problems upstream of Svorkmo. The study concludes that the hydropower operational strategies can be improved by using the recently available modelling tools for finding a balanced operation of the hydropower system in relation to ice problems and the environment.

A field based freeze-up study has been carried out in the anchor ice dominated river Sokna. The observation shows that ice formation and a freeze-up process are very complicated and have a highly spatio-temporal variability.  Big emergent boulders and turbulent water are the locations for the formation of the dominant anchor ice dams controlling ice formation in the reach. A significant water level rise along the study reach caused by the ice formation has been observed that altered the ice free shape of the water surface. A consequence of this is a meandering flow path driven by the ice formation that moved the main flow from one bank to another and further added to the complexity of the ice surface. The finding and the data collected in the study will be useful for future development of anchor ice modelling tools, as modelling of anchor ice and anchor ice dams is still a challenge in river ice modelling.