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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.

Sediment flushing is currently performed to recover the storage capacity of small-sized to medium-sized reservoirs. However, its environmental impacts are not yet adequately quantified. This work aimed to evaluate the effects of a 3-day sediment flushing from a small reservoir in the Italian Alps on downstream freshwater fauna. Biomonitoring was carried out in two streams. In the impounded stream, benthic macroinvertebrates were surveyed immediately below the flushed reservoir. In the receiving stream, where diluting flows were released to reduce the sediment concentration and deposition, fish and macroinvertebrates were monitored at two sites, one above and one below the stream junction. Above the stream junction, the only disturbance was the increase in streamflow, while the reach below the stream junction was perturbed by the increase in both streamflow and sediment load. At the site closer to the flushed reservoir, the benthic community was almost completely impaired after the operations, and its recovery was still incomplete after 1 year. In the receiving stream, the average sediment concentration of approximately 6 g l(-1) and deposition of 30 kg m(-2) were estimated at the site subjected to the sediment increase. A density reduction of 80% and a change in composition affected the macroinvertebrate assemblage, which recovered in approximately 9 months. At the same site, the fish survey showed that brown trout juveniles were affected by the sediment load from the flushing operations. Only minor effects were detected on macroinvertebrates and fish subjected only to the increase in streamflow, i.e. above the stream junction

Purpose: Sediment transport and riverbed sedimentation were investigated in an alpine stream below a small hydropower reservoir desilted by a controlled sediment flushing (CSF) operation. The term “controlled” refers to the operational tasks implemented to mitigate the downstream environmental impact of the operation. The experimental dataset acquired before, during, and after the CSF was also used to carry out and calibrate a one-dimensional sediment transport model of the monitored event. Materials and methods: The investigated reservoir is located in the central Italian Alps, and its original storage was 160,000 m3, about 30% filled by a mixture of sand and silt/clay before the CSF. Downstream sediment concentration was controlled by releasing clear water from upstream reservoirs and regulating the work of earth-moving equipment in the emptied reservoir. A 3.6-km-long reach with average slope of 0.015 was monitored: concentration and grain size of suspended sediment were measured during the CSF and the riverbed alteration was evaluated by volumetric sampling and measurements of the deposits’ thickness. Sedimentation and River Hydraulics—One Dimensional (SRH-1D) was used to simulate sediment transport during the monitored CSF. Model parameters were calibrated by comparing the computed and the observed amount of sediment deposited along the study reach. Results and discussion: Sediment flushing was carried out in October 2010 for 3 days. Ca. 16,000 m3 of sediment were evacuated, representing approximately 30% silt/clay and 70% sand. 2.4 Mm3 of clear water was released to reduce sediment concentration and increase transport capacity downstream. About 3000 m3 of sand was deposited in the study reach after the CSF, with maximum height up to 0.2 m. Although the riverbed before the CSF was simply set as mono-granular, after calibrating the parameters, good agreement was achieved between the depositional pattern computed by SRH-1D and the one observed, both in terms of deposit thickness and grain size of deposited sediment. The sensitivity analysis revealed a major role of the parameters controlling bed mixing processes in affecting the simulated deposition after the CSF. Conclusions: Sediment below 0.1 mm in diameter was not detected in river deposits after the flushing: the effects on river biota associated with substrate clogging by very fine sediment were therefore minimized. After proper calibration, 1-D sediment transport modeling can effectively support the planning of CSF operations: to minimize the downstream environmental effects, concurrently achieving acceptable flushing efficiency, the analyzed scenarios as well as the model outputs need to be carefully evaluated from a multidisciplinary perspective

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