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

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.

Classification of Management Alternatives to Combat Reservoir Sedimentation

“…Downstream impacts to aquatic species depend on both the magnitude and duration of high sediment concentration, plus factors such as oxygen content in the released water, oxygen demand by the sediment, and the presence of contaminants. Management of drawdown rate, dilution of high concentration flow with clear water, interval between flushing events, timing to avoid spawning or other biologically sensitive periods, and post-flushing releases of clear water can all be important environmental mitigation tools [67][68][69].…”

The year after an extraordinary sedimentation event in a regulated Alpine river: The impact on benthic macroinvertebrate communities

“…Recovery times of about 2 years were also reported after small dam removal (Carlson et al, 2018;Tullos, Finn, & Walter, 2014) and after an uncontrolled sediment release (Doretto et al, 2019). Shorter recovery times of benthic communities were generally documented in regulated Alpine streams subjected to controlled sediment flushing operations, varying from a few months up to 1 year, and mostly depending to the persistence of riverbed alteration, to seasons, and to site specificities such as the provision of macroinvertebrates by undisturbed tributaries (Espa et al, 2019). aimed to protecting the safety of the system.…”

Controlled Reservoir Drawdown—Challenges for Sediment Management and Integrative Monitoring: An Austrian Case Study—Part A: Reach Scale

“…The target value for the permanent SSC immission was 1 g L −1 (as the drawdown took place in the spawning period of the brown trout), but up to 10 g L −1 was permissible in the short term, i.e., a maximum of 3 g L −1 for a period of up to 24 h, a maximum of 5 g L −1 for a period up to 6 h, and a maximum of 10 g L −1 for a period up to 2 h. Steps to reduce high SSCs were taken immediately when the guidance value was exceeded for the permanent immission. As the input into the Inn River was stopped at weekends, "recovery phases" were provided for the aquatic biota [46], which is also known from other case studies [55]. Further thresholds were set by the authorities for the dissolved oxygen concentration (permanent value above 10 mg L −1 (maximum 2 h between 10 and 6 mg L −1 ), for ammonium (permanent value below 0.3 mg L −1 , up to 0.5 mg L −1 for a maximum of 2 h) and for pH (allowed range between 6 and 9).…”

“…Within the herein presented case study, a large stretch of the Tyrolean part of the Inn River was investigated in order to assess the longitudinal effects (Figure 1). Over the last few centuries multiple hydro-morphological pressures, such as land reclamation, flood protection, establishment of infrastructure (e.g., railways and highways) as well as hydropower development have taken place along the Inn River similar to other rivers in the Alps (compare to [55,56] and 9). Within the herein presented case study, a large stretch of the Tyrolean part of the Inn River was investigated in order to assess the longitudinal effects ( Figure 1).…”