Frequency regulation and its effects on fish stranding : A hydrological modelling example of a Swedish river

Sammanfattning: This is a master thesis aiming to answer how frequency regulation at a specific hydropower plant affect the risk of stranding for two common Swedish fish species: Trout and Grayling (Salmo trutta and Thymallus thymallus). The study comprises a literature review and a hydrological modelling in primarily Hec-Ras. Frequency regulation is necessary in order to keep the electrical power system in balance. Frequency regulation in Sweden is handled to 99 % by hydropower. The energy company Fortum has several hydropower plants, distributed all over Sweden. In collaboration with Fortum a hydrological model was created for one of its plants and the downstream river reach. Reference scenarios for both summer and winter were then simulated and the result showed that the fish most likely were at risk of becoming stranded at certain times. The risk of stranding was more linked to the normal regulations taking place du to for example increased consumption ofelectricity in the river rather than isolated frequency control. An extreme scenario was also investigated and the water level change rate in that scenario was beyond the limits identified as what fish may withstand. When comparing the outcome of the simulations to the preferences of fish it becomes clear that mainly spawning and juvenile fish are at risk of becoming stranded.