The Kaunas Reservoir was formed during construction of the Kaunas Hydroelectric Plant (Kaunas HP) on the Nemunas River in 1960. Since 1992 it has also been serving as a lower reservoir for the Kruonis Hydroelectric Pumped-Storage Plant (Kruonis HPP), which currently has four generating units with 200 MW of installed capacity each. There are plans to increase the capacity of this power plant by constructing additional four generating units. Operation of the Kruonis HPP is already changing the hydrophysical regime (water levels and flow velocities) of the Kaunas Reservoir and new units can seriously aggravate the situation by inducing bottom erosion. The objective of this study is to determine the optimal number of generating units so that the impact of the Kruonis HPP on the hydrophysical regime of the reservoir would be acceptable. The hydrodynamic model MIKE 21 was used to calculate the flow velocity and water level fluctuation in the Kaunas reservoir for different number of the Kruonis HPP generating units. The Kaunas HP was considered as operating with minimal load or under conditions equivalent to a big flood (2,000 m3/s). The modelling results indicate that the impact of four units is acceptable while simultaneous operation of the eight units at Kruonis HPP would not be desirable.

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