Modelling the downstream longitudinal effects of frequent hydropeaking on the spawning potential and stranding susceptibility of salmonids

Hydropower plant operating conditions are expected to change to be more in tandem with intermittent power production so as to meet the requirements of the Paris Agreement, which in turn may negatively impact ecological conditions downstream of the hydropower plants. The current study investigates how highly flexible hydropower operating conditions may impact several salmonid species (European grayling, Atlantic salmon and brown trout) in the River Umeälven, a major river in northern Sweden; specifically, how changes in hydropeaking frequency may affect the area of the downstream watercourse that is hydraulically suitable for spawning (potential spawning area) and how changes in spill gate closing time may affect the propensity to stranding. River hydrodynamics were modeled using the open-source solver Delft3D, with a range of hydropeaking frequencies (from 10 to 60 starts and stops per day) and a range of spill gate closing times from (1–30 min). Increasing the hydropeaking frequency caused a reduction in potential spawning area, but also a reduction in dewatering of potential spawning area at low flows. Increasing spill gate closing time caused a decrease in propensity to stranding. Effects were dependent on both species and life-stage, and declined longitudinally with distance downstream from the spillway outlet. The modelling approach used here provides an effective method for predicting likely outcomes of flexible hydropower operating conditions, taking into account fish species and life-stages present and watercourse characteristics. [Display omitted] •Hydrodynamic scenarios with very high hydropeaking frequencies were modeled.•A general method of computing dewatering time for any numerical model was provided.•Higher hydropeaking frequencies had smaller amounts of potential spawning areas.•High frequencies lowered the portion of potential spawning area at risk of dewatering.