Fig 1.
Colored circles indicate the four flushed reservoirs. Monitoring reaches (colored lines) and hydropower facilities (dashed lines indicate the main water-ducts, white circles indicate hydropower plants—HPPs) are reported in the Upper Adda River basin (a) and in the Liro River basin (b). Here and in the next figures, we adopted different colors to differentiate reservoirs and related measurements: red (CR), green (VR), yellow (SR), and purple (MR).
Table 1.
Main characteristics of the four flushed reservoirs and of the supplied hydropower plants (HPPs).
Table 2.
CSFO parameters for the four flushed reservoirs at planning stage.
Table 3.
Main characteristics of the monitoring reaches.
Table 4.
Main results of the physical monitoring of the CSFOs.
Fig 2.
Examples of SSC time-series during the CSFOs.
a) SSC detected at V1 during the first week of the 13-days CSFO at VR in 2009. The bottom outlet of the dam was opened on the first day of the CSFO (24-Aug), and SSC peaked around 50 g L-1; in the following days, the system was under control, and regular SSC pulses were gauged downstream. b) SSC detected at C1 (dark red) and C2 (light red) during the seventh week of the 53-days CSFO at CR in 2011. The bottom outlet of the dam was opened on 6-Apr, after 47 days of work, and SSC peaked around 70 g L-1; in the previous days, sediment was by-passed through the dam and the system was under control, with downstream gauged SSC signals characterized by regular pulses.
Fig 3.
Stream reaches photographed few days before and after CSFOs.
Up: Adda River few hundred meters downstream from C1 looking upstream, a) before, and b) after the 2011 CSFO at CR. Middle: pool of the Roasco Stream located some hundred meters upstream from V1, c) before, and d) after the 2008 CSFO at VR (flow is from left to right). Down: Liro Stream at M2 looking upstream, e) before, and f) after the 2010 CSFO at MR.
Fig 4.
Flushing efficiency (FE) for the reported CSFOs compared to further operations reported in the literature.
Sources: Cohen and Briod [48] for thirteen events at Verbois Reservoir, from 1945 to 1981. Morris and Fan [15] for ten events at Sefid-Rud Reservoir, from 1980 to 1990, and for twelve events at Gebidem Reservoir (a), from 1982 to 1993. Sumi and Kanazawa [44] for nine events at Dashidaira Reservoir, from 1995 to 2004. Meile et al. [49] for ten events at Gebidem Reservoir (b), from 2004 to 2013. Peteuil et al. [40] and Guertault et al. [39] for two events at Génissiat Reservoir, in 2003 and 2012.
Fig 5.
Impact of the CSFOs on trout, and comparison between measured apparent mortality and computed fish mortality.
Computation was carried out by data reported in Table 4, according to the model by Newcombe and Jensen [27], and adopting the coefficients reported in S1 File for salmonids: all ages (a), adults only (b), juveniles only (c). The SEVs predicted by the model ranged between 9 and 11, implying 0, 0–20, 20–40% fish mortality, respectively. SEV = 9 was obtained only in case (b), for 2010 and 2011 CSFOs at CR. Trout monitoring was carried out by electrofishing at V1 (green), C2 (light red), and M2 (light purple). Different CSFOs years are reported on the pictures.
Fig 6.
Benthos contraction in terms of density (up) and richness (down) short after the CSFOs as a function of sediment dose. Percentage reduction was computed by comparing the first post-CSFO sample to the corresponding pre-CSFO one. In accordance to Newcombe and Jensen [27], the sediment dose was computed as the natural logarithm of the product between SSCAVE (mg L-1) and ED (hours).
Fig 7.
Temporal evolution of density (left) and richness (right) of the benthic communities after the CSFOs. Both density and richness are expressed as the ratio of the current value to the corresponding value detected in the pre-CSFO sampling.