A System Model for Total Dissolved Gas Risk Assessment Due to Multidam Spill Operations

Spill operations of hydropower facilities can generate supersaturated total dissolved gases (TDGs) that can negatively impact fish residing in downstream habitats by causing gas bubble disease and mortality. Assessment of such impact and management of TDG can be challenging, particularly in river systems with multiple dams, because this requires investigation of complex physical processes related to gas transfer and dissolved gas generation in spillways as well as its transport, mixing, and dissipation in the riverine environment. In this study, an integrated analytical platform was developed to model system-wide total dissolved gas levels during spill events. To test its functionality, TDG monitoring data from the lower Columbia River hydropower system was evaluated for different operational conditions. The system includes contributions from the dams on two other regulated tributaries. In addition to facility-specific estimates of TDG production, the system model provided TDG distribution in the river system, which provides rapid and accurate estimations for impact assessment or mitigation. A ranking process was developed to address cumulative TDG risk for multifacility spill operations. This involved estimation of risk scores considering severity of supersaturation level, river depth compensation, and exposure duration for a given spill event, and grouping the scores into four risk categories defined as none, low, moderate, and high. This resulted in a risk assessment framework that identifies the potential risk zones and the degree and extent of fish habitat impacted for the combined facility operations in a complex river system. This framework can be used as a tool to help inform water management decisions for regulatory compliance and environmental performance, and provide a strategic assessment of the relative need for mitigation action. (c) 2021 American Society of Civil Engineers.