A modeling approach for the quantification of fish stranding risk: the case of Lundesokna River (Norway)

River reaches downstream hydropower plants regularly experience severe discharge reductions due to production shutdowns. Such rapid dewatering processes result in harmful, even lethal conditions for some biological species. Focusing on fish community, several eco-hydraulic features, e.g. wetted area variation, dewatering rate, grain size heterogeneity, habitat availability, are recognized to determine the stranding risk. The goal of the present work is to explore a model approach based on a two-dimensional unsteady numerical tool for the quantification of two crucial hydro-morphological controls on fish stranding risk, namely the dewatering rate and the wetted area variation. The approach is tested on the case-study of Lundesokna River (Norway), which is regularly subjected to rapid hydropower plant shutdowns, resulting in severe problems of stranding for the altlantic salmon community. Results show that the river morphology has a relevant role in finding effective dewatering scenarios that offer the best ecological-economical tradeoffs. Moreover the outcomes underline the need of fully unsteady modeling when aiming at accurately estimate hydraulic features under hydropeaking conditions.