| Summary: | Although run-of-river hydropower represents a key source of renewable energy, it cannot prevent stresses on river ecosystems. This is especially true in mountain regions, where the outflow of a plant is placed several kilometres downstream of the intake, inducing the depletion of river reaches of considerable length. In this thesis, multi-objective optimization is used in the design of the capacity of run-of-river plants to identify optimal trade-offs between contrasting objectives: the maximization of the profitability and the minimization of the impact induced by the plant. The latter is quantified either as the upstream/downstream changes of a set of ecologically-relevant flow metrics, or as the loss of hydrological connectivity in the impacted river reaches. Optimal plant sizes are devised for several case studies belonging to catchments in Italy and UK. Results show that the duration of economic optimal design capacity is strongly affected by the nature of the flow regime at the plant intake. In particular, the analysis emphasizes the important distinction between persistent (reduced variability) and erratic (enhanced variability) streamflow regimes. Multi-criteria optimization indicates that in persistent regimes a trade-off between profitability and hydrologic impact is achieved reducing the plant capacity below the economic optimum, whereas in erratic regimes distinct trade-offs are available depending on the relative importance of the different flow statistics. This work also confirms that water abstractions for human exploitation induce ecologically-meaningful and quantifiable impacts on the hydrologic connectivity of altered river reaches, which may limit significantly migratory movements of fish. The application of a probabilistic eco-hydrological model to reproduce the observed immigration rates of Atlantic salmon in a Scottish river shows that limitations of connectivity are more pronounced in years where exceedance probability of relatively high flow is low. The analyses conducted in this thesis ...
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