Evolution of reservoir flushing and emptying practices in a cascade of large reservoirs across several decades

Reservoir sedimentation is, at present, one of the main concerns in the operational management of dams in the Alps. For larger seasonal storage reservoirs, the main operational concern is the blockage of bottom outlets and turbine abrasion. In case of infra-seasonal storage reservoirs (daily, weekly, monthly regulation), the loss of storage becomes the main concern. Therefore, sediment monitoring should become part of the dam safety and environmental compliance routines. This paper presents a methodology to characterize flushing operations based on reservoir typology, morphology and operation. The application of this methodology to a cascade of three reservoirs in the Swiss Alps illustrates the change in practice across 70 years. The analysis focuses on sixteen flushing operations, several of them with full drawdown (emptying operations). It presents the methods used to document the operations and monitor the consequences. The results show that emptying operations have become less frequent in recent decades, save in case of dam safety concerns. This is a consequence of an increased awareness of the implications of concentrated sediment release for the downstream ecosystem.