Simulation of the ice regime in a Norwegian regulated river

- A one-dimensional river Ice model was applied in a regulated, shallow and fast-flowing river.
- The simulation results of hydrodynamic, water temperature and ice conditions correspond well with observations.
- Model can simulate the general impact of hydropower on ice conditions.

River ice models are commonly used to study ice conditions in wide and low-gradient rivers in cold regions. However, examples of applications of these tools in shallow and fast-flowing rivers are rarely reported. In this study, the one-dimensional model MIKE-Ice was applied to study the ice regime of the regulated river Orkla in Norway. The river Orkla is characterized as a steep and shallow river with sequences of pools and riffles throughout its length. The model was calibrated and validated for the winters, 2010-2011 and 2011-2012 respectively. The study shows that the model is applicable in a steep river with transects of both subcritical and supercritical flow conditions, which is a different environment than most previously reported applications of river ice models. The simulated hydrodynamics and water temperature show good agreement with observations. The model was further tested on its ability to simulate the presence/absence of frazil ice and the development of an ice cover at observation points in the river using continuous monitoring with cameras. The ability to correctly simulate the ice cover extent of the entire model domain was also tested based on a number of field campaigns. The ice simulation results correspond well with observations both spatially and temporally. Further, two discharge scenarios were simulated: a) No hydropower regulation, and b) reduced hydropower operation: to assess the impact of the regulation and to explore the sensitivity of the model. The simulation results demonstrate how hydropower regulation has altered the thermal and ice regimes. The study concludes that Mike-Ice is a useful tool for predicting the river regime in the river Orkla, where dynamic ice formation is a dominant process.