Eco-hydraulic modelling of the interactions between hydropeaking and river morphology

Hydropeaking related to hydropower operations produces adverse ecological effects that depend on its interaction with the channel morphology. A first quantitative attempt is proposed to investigate the eco-hydraulic response of different river morphologies to hydropeaking waves based on a two-dimensional hydraulic modelling approach. Physical habitat diversity, macroinvertebrate drift and fish stranding, all relevant for hydropeaking, are quantitatively investigated with reference to realistic hydro-morphological conditions of regulated alpine streams. Habitat diversity and fish stranding have the strongest dependence on channel morphology and show nearly opposite behaviours with increasing morphological complexity. Braided reaches are the most resilient to hydropeaking offering the highest habitat diversity and very limited base-to-peak variation of macroinvertebrate drift, while alternate bars are extremely sensitive environments to drift and offer safer regions from stranding. Transitional morphologies between single-thread and multi-thread offer the best eco-hydraulic trade-offs. The method allows quantifying to which extent same eco-hydraulic targets can be achieved by either morphological restorations or base flow increases: in transitional morphologies, identical reduction in reach-averaged stranding risk might be obtained either by halving the channel width or by a threefold base flow increase; analogously, the same improvement in macroinvertebrate-fed areas can be achieved in a channel with alternating bars either by a threefold base flow increase or by increasing 2.5 times the channel width. Such quantification of the eco-hydraulic effectiveness of complementary management strategies offers a powerful tool to support design of restoration measures in hydropeaking rivers.