Ice and thermal dynamics under hydropower operations From field evidence to model structural assessment

Hydropower (HP) operations alter the heat, momentum and mass budgets of reservoirs, affecting thermal structure, water quality and ice cover. This thesis investigates how the ice cover and thermal structure of freezing reservoirs respond to HP under different operational configurations, using two Norwegian reservoirs, Roskreppfjorden and Øyarvatn, studied within the HydroConnect project on converting conventional HP to pumped storage. First, nine winters of satellite and in-situ camera imagery show that large cracks form in early winter from bathymetric features and follow water level drops rather than temperature changes; simple mechanical models confirm that moderate drawdown can crack ice where intermediate bathymetric support is present. Second, a calibrated, ice-adapted CE-QUAL-W2 model simulated many hypothetical pumped-storage scenarios: disturbance magnitude grows with operational intensity, but persistence depends on the reservoir's seasonal state. Perturbations during freeze-up have long-lasting effects, whereas open-water ones recover quickly, which motivates recovery time as a complementary metric. Third, Delft3D experiments explain the model's failure to reproduce ice melt near the intake/outlet: laterally averaged models capture reservoir-scale dynamics but underestimate nearfield plume warming, and the hydrostatic assumption matters for vertically developing jets. Overall, the response of freezing reservoirs is governed by the interaction between operational forcing and reservoir susceptibility, and model suitability depends on the scale of the processes of interest. These findings provide a framework for assessing the persistence of pumpedstorage impacts and support the environmentally informed expansion of flexible HP in cold-region reservoirs.