Sediment transport below a small alpine reservoir desilted by controlled flushing: field assessment and one-dimensional numerical simulation

Abstract

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

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

“…The grain size of the flushed sediment was almost entirely below 2 mm. However, the sand content (grain size between 62.5 μm and 2 mm) varied from negligible at VR and CR, where most of the evacuated sediment was silt [28, 31], to approximately 75% at MR [34]. Additional data concerning the grain-size of the suspended sediment sampled during the CSFOs are provided in S1 Table.…”

“…In that case, the amount of the sandy deposits was 300 and 30 kg m -2 on average, at M1 and M2, respectively [32]. However, McNeil sampling in the Liro River evidenced a negligible change in the silt/clay content before and after the CSFO, related to the high Q maintained in the course of the event [34].…”

“…The sediment mass was then divided by density, estimated according to Yang [33], to quantify the volume of sediment removed during the CSFOs. At MR, the flushed sediment volume was determined by comparing before/after bathymetry [34].…”

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

“…This kind of alteration is commonly documented in mountain rivers developed for hydropower (Brignoli, Espa, & Batalla, 2017;Hauer, Holzapfel, Tonolla, Habersack, & Zolezzi, 2019;Leitner et al, 2015). In contrast, the lower reach of the Mera River (e.g., M3) was more severely aggraded, and therefore potentially affected by a heavier and more prolonged change in aquatic biota.…”

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

“…However, independent of the reservoir impacts, the transport of suspended sediments in a river environment is highly non-linear in time and space, e.g., [38] and is influenced by many factors, including the geological framework, climatic conditions, and the topography of the drainage basin [39,40]. In addition to the well-known single-cause relationships for various aspects in sediment management of reservoirs, only minor studies, e.g., [41] deal with a detailed integrative assessment of sediment management measures. An integrative view of the downstream impact on the abiotic and biotic environments (on various trophic levels) was only conducted in one comparable study, the controlled drawdown in winter of the Cancano reservoir, Italy [42].…”