Field Investigation of Controlled Sediment Flushing at Sernio Pondage

Sernio barrage (central Italian Alps) dams the Adda river 75 km above its mouth in lake Como. The project diverts water to a run-of-the-river hydropower plant (30 MW installed capacity, 40 m3/s rated discharge). At maximum headwater, a 0.75 Mm3 pondage develops up to 1.5 km above the barrage. Controlled sediment flushing took place in 2009 and in 2010 during maximum seasonal runoff (late spring/early summer) in non-consecutive days to increase the environmental sustainability and to meet the requirements of the other river stakeholders. In 2009, about 100’000 tons of sediment (predominantly smaller than coarse sand) were flushed in 16 days. About 25’000 tons were evacuated in 6 days in 2010. During the flushing works, the typical discharge was between 70 and 100 m3/s, while minimum flow released below the barrage is of 2 m3/s. Two limits for the Suspended Solid Concentration (SSC) were fixed, i.e. 1.5 g/L (average value on the whole working day) and 3 g/L (alert threshold to adjust the ongoing activity). SSC control was successful and constraint were essentially fulfilled. About 5 km below the barrage field surveys were carried out to quantify the impact on river biota (Adda river below the barrage is characterized by suitable habitats for grayling and marble trout, both catalogued in the EU Habitats Directive and in the IUCN red list of threatened species - least concern category). The operation seems overall tolerable and the reported values could be useful to plan future sediment management in analogous contexts.