Environmental Effects of Storage Preservation Practices: Controlled Flushing of Fine Sediment from a Small Hydropower Reservoir

Abstract

Sediment flushing may be effective in mitigating loss of reservoir storage due to siltation, but flushing must be controlled to limit the impact on the downstream environment. A reliable prediction of the environmental effects of sediment flushing is hindered by the limited scientific information currently available. Consequently, there may be some controversy as regards to management decisions, planning the work, and monitoring strategies. This paper summarizes the main results of a monitoring campaign on the stream below a small alpine hydropower reservoir subjected to annual flushing between 2006 and 2009. The removed sediment was essentially silt, and the suspended solid concentration (SSC) of the discharged water was controlled to alleviate downstream impact. Control was achieved through hydraulic regulation and mechanical digging, alternating daytime sediment evacuation, and nocturnal clear water release. The four operations lasted about two weeks each and had an average SSC of about 4 g L(-1). Maximum values of SSC were generally kept below 10 g L(-1). Downstream impact was quantified through sampling of fish fauna (brown trout) and macroinvertebrate in the final reach of the effluent stream. The benthic community was severely impaired by the flushing operations, but recovered to pre-flushing values in a few months. As expected, the impact on brown trout was heavier on juveniles. While data biasing due to fish removal and re-stocking cannot be ruled out, the fish community seems to have reached a state of equilibrium characterized by a lower density than was measured before the flushing operations.

Effects of sediment flushing from a small Alpine reservoir on downstream aquatic fauna

“…The macroinvertebrate assemblages at the BSs were congruent with those usually detected in regulated Alpine watercourses (Robinson et al, 2004;Monaghan et al, 2005;Espa et al, 2013). Most of the sampled families have adaptations enabling them to live in a disturbed environment (Matthaei et al, 1996).…”

“…Riverbed sampling and sample analyses followed the procedure described by Espa et al (2013). The deposit thickness above the armour layer was measured at two sites in the Scalcoggia Stream reach (at S1 and at a site below the drop) and eight sites (including L1) in the Liro Stream reach, approximately 0.5 km equally spaced.…”

“…Beginning in the last century, the massive exploitation of these watercourses, mainly for hydropower, has altered their hydromorphology and, as a consequence, their biology (Bundi, 2010). The density reduction of benthic macroinvertebrates and fish can be related both to the increase in streamflow and transported sediments during flushing and to the riverbed alteration due to sediment deposition (Espa et al, 2013;Espa et al, 2015). Different management strategies have been employed to reduce reservoir siltation to avoid the loss of power productivity over time.…”

Acute impacts of reservoir sediment flushing on fishes in the Yellow River

“…Case studies on impacts of reservoir sediment flushing involve water quality (Buermann et al, 1995;Garric et al, 1990;Hesse and Newcomb, 1982), benthic assemblages (Crosa et al, 2010;Gomi et al, 2010;Gray and Ward, 1982), fish mortality (Buermann et al, 1995;Espa et al, 2013;Garric et al, 1990;Kemp and Sear, 2011), and the modification of riverbed substrate (Li and Sheng, 2011;Wohl and Cenderelli, 2000).…”

Sediment Flushing of Reservoirs under Environmental Considerations

“…Case studies on negative impacts of flushing involve water quality, the increase in the suspended sediment concentration (SSC), and the change of habitats caused by scouring and sedimentation in downstream river reaches (Hesse and Newcomb 1982;Gray and Ward 1982;Garric et al 1990;Buermann et al 1995;Wohl and Cenderelli 2000;Gomi et al 2010;Kemp and Sear 2011;Li and Sheng 2011;Espa et al 2013;Baoligao et al 2016). Espa et al (2016) evaluated the efficiency of some measures such as regulating the dislodging works inside the reservoir, increasing the streamflow in the regulated tributaries, and operating an instream settling basin for controlling SSC and mitigating the downstream impact of sediment flushing.…”