Total Dissolved Gas
Elevated levels of total dissolved gas (TDG) in spill flows is a significant water quality concern at many dams due to the potential to induce gas bubble trauma in fish. When fish are exposed to water with elevated levels of dissolved gas, the gas pressures in their bloodstream equalize with the gasses in the water. When the fish rise in the water column, the reduction in pressure can cause bubbles to form in the fish's tissues and bloodstream; the accompanying symptoms are known as gas bubble trauma. State water quality standards typically limit TDG to 110% of saturation. However at Boundary Dam, as at many dams on waterways with sequential hydropower facilities, TDG regularly exceeds 110% when flows into the project exceed approximately 70,000 cfs.
As part of a program aiming to meet State mandated water quality objectives, Seattle City Light and a team of consulting engineers, are developing physical modifications to reduce TDG generated by spill releases. Consultants have been assisting Seattle City Light with the design, performance prediction, and evaluation of measures intended to reduce the level of TDG downstream of Boundary Dam.
Seattle City Light has been studying TDG production at Boundary Dam since 1999. From 1999 to 2007, Seattle City Light conducted a literature search, performed field studies and undertook a comparative analysis of potential TDG abatement alternatives. As a result of these activities, Seattle City Light modified its power plant operations to "last on, first off" for two turbine units that had been entraining air. This produced a significant reduction in TDG; however, the water quality standard is still exceeded when river flow exceeds about 70,000 cfs. There is considerable variability between years, but on average these flow conditions correspond to an occurrence of about 7.4 days per year.
Workshops were held in 2007 and 2008 to identify TDG abatement alternatives. Of the identified alternatives, three were selected for detailed study because they could be more quickly evaluated and implemented to reduce TDG: throttle sluice gates, roughen sluice flow and provide spillway flow splitters/aerators. These modifications are being evaluated and optimized through a combination of physical and CFD modeling and application of TDG predictive tools.
As of early 2015, modifications have been made to the Right Spillway No. 2 at Boundary Dam and work is in progress to complete similar modifications on the Left Spillway No. 1 in late 2016.
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