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Gas bubble disease (GBD) is a condition that adversely affects aquatic animals exposed to supersaturated water. Fish mortality attributed to GBD has been observed in the tailrace of the Colider Dam on the Teles Pires River in Brazil during spillway releases. Gas supersaturation downstream of dams typically is caused by the dissolution of air bubbles entrained at depth in the stilling basin. The most common strategy to reduce total dissolved gas (TDG) concentration, and the risk of GBD, is to install spillway flow deflectors that redirect plunging flows horizontally and thus minimize the bubble transport to a depth at which dissolution is enhanced. This paper presents a numerical study, based on the open-source code OpenFOAM, assessing the design of spillway deflectors at Colider Dam and determining their effectiveness at reducing TDG. A two-fluid flow model was implemented in a free-surface solver to predict spillway jet regimes and TDG distribution for 900 m of the Colider tailrace. Residence time and TDG exposure in the tailrace were estimated using a particle tracking technique. A rigid-lid model was used to predict TDG distribution in the Teles Pires River 10 km downstream of the dam. Reasonable agreement was found between numerical model results and velocity data measured in a 1:120 reduced-scale hydraulic model. Predicted spillway jet regimes were consistent with the deflector performance curve obtained in a 1:60 reduced-scale sectional model. After calibration and validation against TDG field data, numerical simulations were performed to better understand the effect of the deflector curvature radius, length, and elevation on TDG production. Fish mortality or injury due to GBD was not observed in Colider during spill events after deflectors recommended in this study were installed in 2021. Field measurements indicate that TDG reduction by the deflectors is consistent with values predicted during the design of the deflectors. Fish mortality with gas bubble disease symptoms due to elevated total dissolved gas concentrations have been observed downstream of Colider Dam since the first spillway operation in 2018. The most common strategy to reduce TDG production downstream of hydropower dams is to install flow deflectors on the spillway face. This paper presents a numerical study to assess the performance of flow deflectors for Colider Dam. A computational fluid dynamic (CFD) model, comprising the air and water phases, and bubbles in the water phase, was developed to predict spillway jet regimes and TDG concentration. The model included the main features of the dam and a 10-km reach of the Teles Pires River. After calibration and validation, 28 simulations were performed to evaluate and improve the deflector design. A final deflector design was recommended based on TDG production and TDG exposure time. No fish mortality has been observed since the deflector construction was completed in 2021, even under operational conditions fatal to fish in the past. [ABSTRACT FROM AUTHOR]

Recent disasters involving upstream raised tailings dams in Brazil have caused regulatory agencies to prohibit the construction of new upstream dams and demand the decommissioning of existing ones. As such, new alternatives are being sought to safely dispose of iron ore exploitation byproducts. The dry stacking of filtered tailings is an option to diminish the risks associated with tailings storage facilities and cope with new legislation. Accordingly, this paper studies new solutions for enhancing the performance of tailings for dry stacking purposes. The behavior of reinforced and nonreinforced iron ore tailings-alkaline-activated cement (AAC) blends was studied through unconfined compressive strength tests and conventional triaxial testing. The AAC is produced from the residues of iron ore exploitation (waste and tailings). The addition of fibers has enhanced the performance of cemented iron ore tailings with increased strength regardless of the fiber content and ductility. Also, reinforcement effectiveness values were greater than those reported for other cement agents, due to the lower stiffness of the cemented matrix from AAC compared with others. A comparative analysis between using only AAC and ACC plus 0.5% of fibers within the η/Biv framework was proposed. The dosage curves for the same qu value yielded an AAC=20.80% for AAC only and AAC=3.80% when using 0.5% of fibers. Therefore, using fibers can collaborate with the feasibility of using a new waste-based AAC to construct dry stacking facilities. [ABSTRACT FROM AUTHOR]

Severe droughts, water-use conflicts, and minimum outflow requirements have caused infeasibilities in modeling large-scale complex hydrothermal systems. This paper presents and discusses a new formulation of objective function and constraints to better represent multiple water uses and deal with infeasibilities in the HIDROTERM model, a nonlinear programming (NLP) model developed to optimize the operation of the Brazilian interconnected hydrothermal system. The system is one of the largest in the world. In the last 10 years, approximately 72% of electricity consumed in Brazil has been supplied by hydropower plants, but with increasing shares from thermal, wind, and, more recently, solar power generation. Because of hydrological changes in recent years and the additional operational constraints imposed by environmental agencies, the use of models for decision making, such as HIDROTERM, frequently encounters the problem of infeasibility. To avoid this problem, a new objective function is proposed in HIDROTERM that incorporates economic penalties when the preset requirements are not met. Based on economic considerations, some hard constraints are removed from the constraint set and added to the objective function as penalty terms. These constraints are tightly linked with multiple water uses, including environmental protection, flood control, consumptive uses, and navigation purposes. The proposed change in the objective function and constraints avoids the problem of infeasibilities and makes it possible to apply the model to extreme conditions. Additionally, this formulation helps identify and minimize the risk, duration, and intensity of unavoidable non-compliance occurrences related to multiple water uses. The presented new version of the HIDROTERM model can be used to study and support decision making in the management and operation of large-scale hydrothermal systems. These systems can be formed by a set of hydraulically connected individual hydropower plants and reservoirs, located within a set of electrically connected subsystems. Each subsystem contains demands and other power plants, including thermal, wind, nuclear, and solar plants, and exchanges. The model has been developed with continued improvements since 2008 and applied to large systems such as the Brazilian Interconnected Power System (BIPS) with over 150 reservoirs. A graphical user interface is provided. The objective function is to minimize the total costs of additional thermal dispatch and exchanges, as well as avoid or minimize deficits in energy supply in subsystems, minimum releases, consumptive uses, and minimum storages in reservoirs. The decision variables are the power and nonpower releases for each hydropower plant and the thermal dispatches and exchanges for each subsystem. Some typical applications include (1) monthly planning operation of the system, considering a planning horizon from 3 to 5 years; (2) diagnostic studies about reliability and resilience of existing systems, processing the same system configuration to a set of different hydrological or energy demand scenarios; (3) impact analysis of changes in operational rules and constraints related to multiple uses of water; and (4) planning and evaluation of expansion alternatives. The model can be applied to a complete hydrothermal system or subsets of it, including subsystems, basins, cascades, or a single hydropower plant. [ABSTRACT FROM AUTHOR]