Author(s): Silvia Quadroni; Alberto Doretto; Francesca Salmaso; Livia Servanzi; Giuseppe Crosa; Paolo Espa

Linked Author(s): SILVIA QUADRONI, Francesca Salmaso, paolo espa

Keywords: Sediment flushing; Ecological impact; Alpine reservoir; Hydropower; Mesohabitat

Abstract: Sediment flushing is currently performed to counteract the loss of storage of reservoirs due to siltation. In the central Italian Alps, several controlled sediment flushing operations (CSFOs) have been carried out in the last decade. These operations are characterized by the control of the suspended sediment concentration (SSC) in the downstream watercourse, achieved through flow regulation and sediment dislodging by mechanical equipment. The control is aimed to reducing the ecological impact of the significantly increased sediment load. In this study, we assessed the effects of a CSFO from a small Alpine hydropower reservoir (storage capacity of 1.2 Mm³; top of the active pool at 1,210 m amsl) at the mesohabitat scale. Specifically, we repeatedly sampled three different mesohabitats (one pool, one riffle, and one step-pool) in the final stretch of the 7.8 km long stream subjected to the CSFO at the end of summer 2018. In each mesohabitat, we measured both the amount of deposited sediment and the related effects on the lower levels of the food web (periphyton and benthic macroinvertebrates), before and after the CSFO. The CSFO was carried out according to a consolidated protocol (i.e., two weeks of duration, and maximum permitted SSC of 4 g/L as average value over the whole operation). Evacuated sediment, predominantly silt, amounted to around 20,000 tons. As expected, after the investigated CSFO, sediment deposition was larger in the pool. Both periphyton and benthic macroinvertebrates were affected by the CSFO in all the investigated mesohabitats. The lowest contraction of benthic macroinvertebrate communities, in terms of density and richness, was observed in the riffle. Differences in the recovery trajectories between the three investigated mesohabitats were recorded up to two years after the CSFO. An improved knowledge about the ecological impact of CSFOs and the related recovery patterns is evidently required to support sediment management practices in regulated river systems, particularly in mountain catchments developed for hydropower.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022173

Year: 2022