Case Study of Total Dissolved Gas Transfer and Degasification in a Prototype Ski-Jump Spillway

Gas transfer in dam spillways often leads to supersaturation of total dissolved gases (TDGs) that can cause fish mortality. Quantifying TDG associated with this process is crucial in the development of operating alternatives to minimize environmental risks to downstream aquatic habitats. In this study, the transfer of TDG in a spillway was evaluated through field observations at the Seven Mile Dam on the Pend d'Oreille River in British Columbia, Canada. The dam degasses high-TDG water due to a ski-jump design where aerated flows are generated from a flip bucket at the end of the spillway chute. A simplified mathematical formulation, incorporating physical processes related to air entrainment, bubble characteristics, and mass transfer across free surface and bubbles, was tested and verified for prototype flows to partition gas transfer in the spillway face, free jet, and plunge pool supported by extensive field measurements. Due to gas exchange dominated by bubble-mediated transfer, substantial degassing of TDG was observed during spill operations, with degassing in the free jet being considerably higher compared to the spillway face and plunge pool. The gas transfer efficiency was high when pre-aeration (air entrainment on the spillway face) occurred. Practical relationships were proposed to estimate degassing in the jet and assess overall gas transfer efficiency. Results from this study can help inform water management decisions during periods of elevated TDG, particularly in cascading hydropower systems.