Continuous Long-Term Observation of Suspended Sediment Transport between Two Pumped-Storage Reservoirs

Transport of suspended sediment between the two reservoirs of a pumped-storage plant in Switzerland was monitored over an 8-month period. A turbidity probe was installed on the upstream end of the pressure shaft, in which real-time monitoring was ensured by automatic data acquisition and remote control. The suspended sediment concentration in the reservoirs varies considerably over the course of a year, with low particle loads in the winter, when the reservoirs are covered by ice and the catchment area is entirely covered by snow. In late spring, when snowmelt starts, the suspended sediment concentration increases and remains high until summer. The short-term evolution of the suspended sediment concentration was found to be correlated to the pumped-storage sequences, especially when the reservoir levels are low. Higher concentrations during pumping mode were observed for low levels in the lower reservoir, and low levels in the upper reservoir corresponded to higher sediment concentrations in turbine mode. Concentration ratios between pumping and turbine mode between 0.93 and 1.16 were observed. Periods with higher particle load were observed more frequently during pumping mode. Nevertheless, the suspended sediment balance due to pumped-storage operations is equilibrated, which indicates a high degree of correlation between the transported sediment volume and hydropower operation. Connecting the applied monitoring system in an existing plant control and management system would allow the plant operator to detect increased suspended sediment loads in the pressure shaft and to adapt the operation mode to actively manage reservoir sedimentation or to estimate and predict the long-term sediment balance.