Improving sustainability of sediment management in Alpine reservoirs: control of sediment flushing operations to mitigate downstream environmental impacts.

This thesis focuses on the desiltation of Alpine reservoirs in order to sustain their long-term utilization and restore the functioning of the bottom outlets, minimizing at the same time downstream environmental impacts of sediment removal operations. Different case studies of controlled sediment flushing operations (CSFs) are analysed adopting a multidisciplinary approach. In particular, sediment transport and downstream riverbed alteration, ecological impacts on benthic macroinvertebrates and fish, and performance indicators were investigated. Moreover, the experimental dataset acquired before, during, and after a sediment removal operation was used to carry out and calibrate a one-dimensional sediment transport model of the monitored event. The duration of the CSFs and the average suspended sediment concentration in the outflowing water were constrained, with the specific environmental objective of limiting the downstream fish mortality as predicted by a simple dose/response model. The biomonitoring indicated that this model allowed for a first approximation estimate of the impacts on trout. On the other hand, similar scenarios of sediment removal in terms of duration and average SSC can be identified with analogous predicted impacts on fish but non-negligible differences in flushing efficiency. Therefore, site-specific investigation of environmental impacts may be the key aspect for upgrading the trade-off between economic and environmental needs in planning further operations in the same area.