Hydropeaking in Alpine rivers: an ecosystem services approach

Rivers provide to society many important goods and benefits. Some of these ecosystem services depend on the river flow regime, which has been deeply modified by human structures and activities. These alterations have a direct influence on biodiversity, natural habitat and on the supply of river ecosystem services. The release of water from storage hydropower plants generates rapid flow and stage fluctuations (hydropeaking) in the receiving water bodies at a variety of sub-daily time-scales. In this thesis, we describe an approach to quantify such variations, which is easy to apply, requires stream flow data at a readily available resolution, and allows for the comparison of hydropeaking flow alteration amongst several gauged stations. Hydropeaking flow alteration is quantified by adopting a rigorous statistical approach and using two indicators related to flow magnitude and rate of change. We utilised a comprehensive stream-flow dataset of 105 gauging stations from Italy, Switzerland and Norway to develop and test our method. Next, we introduce a modelling approach to evaluate the spatial and temporal variations of a discharge-related ecosystem service, the rafting. The application of hydraulic and habitat models allowed to define spatially thresholds of suitability in each river reach and the application of an hydrological model allowed to assess temporally the suitability for the rafting navigability in different discharge conditions. We applied the method to the Noce River, an Alpine River in Northern Italy affected by hydropeaking. Our analysis showed that in this river, the water releases are fundamental to maintain high flow conditions required for rafting, which can be granted only by hydropower production especially in summer months. Together with present discharge conditions, our approach allows to analyse also the effects of an additional withdrawal which locally has a negative impact on river suitability. Finally, the application of the methodology was extended to include in the analysis the fish habitat and the small hydropower production, along with the rafting. The effects of hydropeaking on these ecosystem services were assessed in space and time. Hydropeaking has a strong influence on rafting navigability and less obvious consequences on the other services. Different management scenarios of the water releases from the hydropower plants were produce, with the aim to evaluate spatially the reciprocal effects of optimizing each ecosystem services. Only the scenario of rafting optimization will significantly increase rafting navigability, while the effects of other scenarios are less evident. Moreover, two additional increasing withdrawals have been simulated to evaluate their impacts on the services. The small hydropower withdrawals will have a negative impact on rafting and fish habitat, while the preservation of requirements for rafting will greatly affect the small hydropower production. This ecosystems-services based approach can be integrated in the decision-making process to evaluate river management alternatives.