Production of total dissolved gas supersaturation at hydropower facilities and its transport: A review

Total dissolved gas supersaturation (TDG) is a common issue in hydropower facilities as a result of water conveyance structures that increase the amount of air entrainment from the atmosphere and dissolved into the water. Water with TDG supersaturation can negatively impact fish, aquatic invertebrates and their habitats. This study comprehensively reviewed the physical mechanisms of TDG generation and predictive TDG generation models at various facility types. To establish TDG mitigation strategies, it is essential to develop predictive tools for TDG generation that consider both facility geometry as well as the hydrology of the downstream environment. Applications of TDG prediction at different discharge modes included plunging flows, trajectory jets, plunging jets, free-falling jets, and submerged jets were discussed. TDG transport models in downstream rivers involving mixing and dissipation were introduced, which can be integrated with TDG generation models into a platform to describe TDG distribution in river systems. Subsequently, risk ranking procedures for assessing the degree of TDG risk on fish were provided. Potential measures for mitigating TDG supersaturation were reviewed and included engineering, operational, and technical solutions. Outcomes from this review considered a diverse suite of studies on TDG issues in regulated rivers and allowed for recommendations to reduce uncertainties and improve environmental performance at facilities where TDG risks occur.