Methods to prevent or mitigate total dissolved gas supersaturation in the waterways downstream hydropower plants

Downstream hydropower plants, a change in water chemistry can lead to the occurrence of a widely unknown problem: total dissolved gas (TDG) supersaturation. It takes place when air is entrained in a water body and exposed to high pressures, which leads to gas dissolution in the water. Re-exposure to atmospheric pressure downstream the power plant results in TDG supersaturation. This is a potential danger for the aquatic environment living in these waters, as the increased saturation poses the risk of experiencing gas bubble disease (GBD). Studies about TDG supersaturation are found in North America (USA and Canada), China, Brazil, and Norway (minor studies include Austria, Germany, and Sweden). Yet, knowledge about the risk of the problem is not widespread, which leads to the repetition of mistakes. Moreover, shifting precipitation patterns induced by climate change are expected to lead to an increase in TDG supersaturation occurrences, as those are associated with flooding. An overview of methods to either prevent or mitigate the problem of TDG supersaturation downstream hydropower plants is presented and recent study results are disseminated. These include civil engineering, operational, and technical methods. Where hydropower plants are in a planning phase, this can contribute to preventing the occurrence of TDG supersaturation in the first place, while existing hydropower plants can implement different measures to reduce the risk of producing TDG supersaturation in the downstream waterways. This helps maintain the aquatic environment as well as local habitat for fish and invertebrates, and therefore counts towards hydropower taxonomy and increases social acceptance of hydropower.