Case Study of Total Dissolved Gas Transfer and Degasification in a Prototype Ski-Jump Spillway
Publication: Journal of Hydraulic Engineering
Volume 146, Issue 9
Abstract
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.
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Data Availability Statement
Some or all data or models used during the study are available from the corresponding author by request.
Acknowledgments
This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada and BC Hydro. The authors would like to thank Greame Billay, Pengcheng Li, and Marco Marrello for their help in the field, and Perry Fedun for his support in the lab.
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Published In
Journal of Hydraulic Engineering
Volume 146 • Issue 9 • September 2020
Copyright
© 2020 American Society of Civil Engineers.
History
Received: Nov 15, 2019
Accepted: May 4, 2020
Published online: Jul 10, 2020
Published in print: Sep 1, 2020
Discussion open until: Dec 10, 2020
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