Intervention strategies to mitigate hydropeaking: Two case studies from Switzerland

Water retention in reservoirs and concentrated turbine operations allow electricity to be produced on demand. Sudden opening and closing of the turbines can produce highly unsteady flow conditions in the river downstream of the tailrace channel, the so-called hydropeaking. Mitigating its adverse impacts on the aquatic ecosystems has become a mandatory requirement in today’s water policies in many countries. Existing facilities are being ‘rehabilitated’ for habitat improvement or restoration, and new facilities designed with a focus on habitat conservation. However, there are regions still suffering from a lack of experience in respect of hydropowerrelated ecological challenges and little scientific knowledge available on local ecosystems. The paper presents two selected projects of hydropeaking mitigation in Switzerland: the Mauvoisin II scheme extension according to project specific design guidelines and the Plessur hydropower scheme according to today’s mitigation policy. For both projects the context, the evaluation procedure as well as the findings are given and compared, covering a wide range of feasible solutions. Understanding the specific behaviour of local species is the starting point in order to address the unique needs for each and every project. However, the approach how to define the best performing intervention strategy can be defined and will be presented in the paper, considering the lessons learnt from hydropeaking mitigation projects.