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Transport and Transfer of Total Dissolved Gases in the Lower Columbia River Hydropower System
- Author / Creator
- Kamal, Rajib
Hydropower facilities can generate elevated or supersaturated total dissolved gases (TDGs) during spill operations that can impose environmental and ecological risk to downstream habitat, particularly to fishes causing gas bubble disease and mortality. Assessment of such impact and management of TDG can be challenging, particularly in river systems with multiple dams, as this requires investigation of complex physical processes related to gas transfer and dissolved gas generation in dam spillways as well as its transport, mixing and dissipation in riverine environment. This research aims on developing an analytical platform that can project system-wide total dissolved gas levels during spill events to evaluate the cumulative impact of multi-facility operations and assess relevant risk on fish and habitat ecosystem.
The current research has been carried out based on the comprehensive field study at the Lower Columbia River hydropower system in British Columbia, Canada. This transboundary river comprises the reach below Hugh L. Keenleyside Dam to Canada-US border and includes the confluences of Kootenay and Pend d'Oreille rivers regulated by the Brilliant Dam and Seven Mile and Waneta dams respectively. Several field work sessions were carried out in this system to measure total dissolved gas concentrations, water temperature and river hydraulics. In addition, facility-specific historical data as well as system-wide monitoring information were collected. This resulted in the collation of comprehensive system-wide field data which are extremely difficult to measure and rarely available in the literature.
TDG dissipation, a key process for transferring supersaturated dissolved gases out of the river system, was quantified directly based on field measurements during spill operations at the Hugh L. Keenleyside and Brilliant dams. To estimate the dissipation rate, an analytical approach based on modified streamtube method was utilized incorporating transverse mixing between the spill and generation flow as well as tributary inflow. This presented a methodology to model TDG distribution in dam-regulated rivers with limited measurements. The dissipation rate was quantified at two hydraulically different reaches of the Columbia River. The effect of depth-velocity ratio on direct transfer, as well as the role of bubble-mediated transfer caused by liquid phase supersaturation was discussed.
The complex gas transfer processes and corresponding generation and degassing of TDG were investigated in the ski-jump spillways of the Seven Mile Dam located on the Pend d'Oreille River. A simplified mechanistic formulation, incorporating physical processes related to air entrainment, bubble characteristics and mass transfer across free surface and bubbles, was utilized to partition gas transfer in the spillway face, free jet and plunge pool and evaluate the contribution of each regions supported by extensive field measurements. Due to gas exchange dominated by bubble-mediated transfer, substantial degassing of high TDG water was observed during spill operations with the free jet being major contributor. Gas transfer efficiency was high when pre-aeration occurred on the spillway face. The plunge pool region was found to generate additional dissolved gases as well as degas TDGs depending on the spill rate, pool geometry and bubble-penetration depth. Similar approach was adopted to predict dissolved gas levels in other facilities of the system.
The mechanistic TDG generation models of individual facilities, the generalized mixing and dissipation relationships and the streamtube-method based transport model were integrated into an analytical platform to develop a two-dimensional TDG distribution model. To test its functionality, TDG monitoring data at the Columbia River system was evaluated for different case conditions. The system model provided spatial distribution of TDG for multi-facility spill operations, which is not only physically meaningful but provides rapid and accurate estimations for impact assessment. A ranking process was developed to address cumulative TDG risk that was consistent with existing TDG management guidelines. This process involved estimation of risk scores considering severity of supersaturation level, 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 TDG risk assessment framework that anticipates potential risk in fish habitat for the combined operation of hydropower facilities in a complex river system. Results from this study can help inform water management decisions for regulatory compliance and environmental target achievement, thereby enabling sustainable hydropower generation.
- Graduation date
- Spring 2020
- Type of Item
- Doctor of Philosophy
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