36Hydropeaking power production has the potential to pose serious challenges towards and positions in the river bed that can be potentially relevant to ecology were investigated. 47Understanding the complexity of those processes at the fine scale from the physical point of 48 view is both important for the judgment of potential ecological impacts and for the future 49 management of such regulated systems.

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The ice conditions in a regulated river will depend on the climatic changes as well as the changes to the hydropower operation strategies in the future. The existing literature shows that very few studies have been carried out to investigate the impact of climate change on the river ice regime, which is important for operation of hydropower in cold climates. In this study, a series of modelling tools have been used to transform the climate change signal in terms of precipitation and air temperature into cross-section based river ice assessment in a basin with a complicated hydropower system. The study is based on the EURO-CORDEX climate change data extracted from a regional climate model driven by a suite of five general circulation models with three representative concentration pathways. Hydrological model simulation results show that the winter and spring flow will be increased, which will have an impact on the river ice conditions towards the middle and end of this century. Reservoir-hydropower model simulation shows that the production flows in the winter will be increased in the future. River ice model simulation shows the number of days with freezing water temperature are reduced in the future climate, and correspondingly days with frazil ice are reduced at most of the locations in the study area. The future period with ice cover will also be shortened. The paper also demonstrates a general methodology and procedure to simulate future ice conditions in a regulated river combining multiple models and data sets.

Abstract:Ice formation and related processes in rivers and lakes/reservoirs influence the operation of hydropower plants in cold regions. It is a matter of interest to the scientific community and hydropower operators alike how existing ice effects and problems will manifest themselves in a future changed climate. In this paper, we use different modeling results to investigate future freshwater ice conditions. The modeling approaches include using temperature derived winter indices, using one-dimensional (1D) hydrodynamic and ice cover model on three case study reservoirs, and using a 1D river hydrodynamic and ice cover model for a river reach. The analysis shows that changes in river and reservoir ice regimes due to climate change scenarios may have both positive and negative consequences for hydropower operation. Positive consequences emerge from reduction in ice season and reduced static ice loads. Negative consequences or challenges are attributed to unstable winters that may lead to increased frequency of freeze-thaw episodes with a shortened winter season. These aspects are discussed in more detail in the paper.

11The ice conditions in regulated rivers can be very complicated due to both intake 12 and release of water to and from hydropower plants. The optimal operational 13 strategies for the hydropower system must involve ice management in the river 14 basin and finding a balance can be a challenge for the hydropower operator. 15Issues with ice occur during both freezeup and breakup, and at certain 16 conditions both situations can occur in a basin at nearly the same time. In this 17 study, a series of modelling tools have been used to investigate the consequences 18 of a forced shutdown of a power plant in the Orkla river. The associated impacts 19 on the stability of the ice cover in a downstream bypass reach and the ice and 20 ecological conditions in the reaches upstream of the power plant have been 21 explored. The reason for this is restrictions on upstream water releases during 22 the shutdown period given in the regulation permit to prevent ice breakup in the 23 downstream bypass reach and subsequent flooding problems downstream. The 24 study demonstrates a wide application of numerical tools for environmental 25 impact assessment, providing knowledge for better decision-making and for 26 optimal operational strategies for hydropower systems during winter. 27In summary, the analysis shows that a shutdown period shorter than the travel 28 time of water from the upstream hydropower plants to the bypassed reach does 29 not guarantee a reduction of the ice problems in the bypass reach. Since the 30 intake pond is too small to store already released water from the upstream 31 power plants, spill and ice breakup will occur. It is rather found that a shutdown 32 of the upstream power plants can induce environmental problems due to the 33 rapid dewatering of the river, and an ice breakup during the restart of the 34 upstream power plants. 35 36