Experiences of controlled sediment flushing from four alpine reservoirs

Ten controlled sediment flushing operations in four Alpine reservoirs were monitored between and 2012. Reservoirs were completely draw-down and mechanical equipment was employed to dislodge bottom sediments during daytime. Flushing durations ranged from 3 to 53 days. Different seasons were scheduled to perform the operations mainly due to the size of outlet facilities and the needs of other river stakeholders. Mitigation measures, such as suspended sediment concentration thresholds and dilution of sluiced water, were employed to limit downstream environmental impacts. Despite technical difficulties and some critical phases, the thresholds were generally respected. Sediments deposited along the study reaches varied in terms of grain size and mass per unit area. When flushed material was primarily silt, deposition rates of few kg/m² were measured in the riverbed wetted by minimum flow. Deposits up to 300–450 kg/m² were detected when an important fraction of the flushed sediments was sand.