Tackling reservoir siltation by controlled sediment flushing: Impact on downstream fauna and related management issues

PLoS One. 2019 Jun 24;14(6):e0218822. doi: 10.1371/journal.pone.0218822. eCollection 2019.

Abstract

Sediment flushing can tackle reservoirs siltation and improve sediment flux through dammed rivers. However, the increase of the sediment loading below the dam can trigger a suite of undesired ecological effects in the downstream river reaches. To limit these drawbacks, sediment flushing can be controlled, by jointly regulating the sediment concentration of the evacuated water and the streamflow in the downstream channel. In this paper, we report on ten controlled sediment flushing operations (CSFOs), carried out between 2006 and 2012 in the central Italian Alps, at four hydropower reservoirs. These CSFOs displayed specific common traits: (i) Limits were set by the local environmental authorities concerning the allowable suspended sediment concentration. (ii) Reservoirs were fully drawn-down, earth-moving equipment was used to dislodge sediment, and the downstream water discharge was increased, compared to baseflow, by operating upstream intakes. (iii) Abiotic and biotic measurements in selected downstream reaches (before, during, and after the CSFOs) represented an integral part of the operations. In contrast, significant differences characterize the hydropower facilities (elevation and storage of reservoirs, in particular) as well as the basic CSFOs parameters (i.e., season, duration, mass and grain-size of the evacuated sediment, suspended sediment concentration). The macroinvertebrate assemblages resulted noticeably impacted by the CSFOs. In the short term, a significant density drop was observed, slightly influenced by the extent of the perturbation. In contrast, the latter appeared to control the assemblages contraction in terms of richness, according to the different sensitivity to sediment stress of the different taxa. The time employed to recover pre-CSFO standard ranged from few months to just under one year, and the related patterns would seem mostly correlated to the flushing season and to further site specificities. The density of trout populations was impacted as well, thus suggesting the adoption of mitigating strategies as removal by electrofishing before, and repopulation after the CSFO.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Environmental Monitoring
  • Geologic Sediments*
  • Hydrology
  • Italy
  • Power Plants
  • Rivers*
  • Trout
  • Water Movements
  • Water Resources*

Grants and funding

Most of the field sampling presented in this study was financed by the INTERREG Italy/Switzerland project ECOIDRO (Water Use and Safeguard of Environment and Biodiversity in the River Basins of Adda, Mera, Poschiavino and Inn - ID 7630754). University of Insubria financed a PhD position (MLB) and a post-doc project (SQ) concerning CSFOs. This work was also supported from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group Fluvial Dynamics Research Group - RIUS (2017 SGR 459), and within the CERCA Program. These funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors acknowledge two commercial affiliations, Graia S.r.l. and Pavia Acque S.c.a.r.l. The first one (Graia S.r.l.) was involved as sub-contractor by the University of Insubria in the INTERREG project ECOIDRO, and thus had a role in the study design, and data collection and analysis (see specific roles of GG articulated in the ‘author contributions’ section). Pavia Acque S.c.a.r.l. had no any role in all the phases of this work; in fact, it is the current affiliation of MLB, but all her work for this study (see specific roles of MLB articulated in the ‘author contributions’ section) was carried out before and during her PhD at the University of Insubria.