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Institut für Wasser- und Umweltsystemmodellierung - IWS



"Modelling Stranding Risk of Fish in a Norwegian Regulated River with Fluctuating Flow "

Hydropower operation may cause fluctuating flow in downstream rivers because of turbine regulation according to the electricity demand. A potential impact of such rapid flow fluctuations is stranding of fish. Juvenile salmonids show a seasonal shift in habitat use and diurnal activity triggered by water temperature and light. For this reason, seasonally adjusted operational schemes may be important for the avoidance of stranding. In the Norwegian river Surna restrictions on the rate of flow and stage decrease were introduced in the year 2006. This case study addresses three objectives: (1) Stranding risk assessment during a typical rapid flow decrease; (2) Implementation of water temperature and light into the assessment approach; (3) Evaluation if the restrictions are appropriate. Habitat-hydrodynamic modelling tools were applied in order to assess the stranding risk of juvenile Atlantic salmon (Salmo salar). The seasonal variations in habitat requirements were implemented into a fuzzy-logic based habitat model. Two scenarios for stranding risk assessment in accordance to the seasonal changes were developed (1: Temperate water and daylight; 2: Cold water and daylight). Areas with different degree of stranding risk were localised. The stranding risk was found to be highest along the shallow river bank at low temperature during daylight, while the risk was lower at temperate water and daylight. The influence of temperature and light on the stranding risk could be implemented into the model successfully. The results suggest that restrictions on higher flows may contribute to reduce the stranding risk. Rapid fluctuations should be avoided at low temperature and daylight, particularly in spring when this combination leads to the highest stranding risk.

Keywords: Stranding risk, habitat-hydrodynamic modelling, fuzzy-logic, hydropeaking, rapid flow fluctuations, regulated river.