Risk assessment for total dissolved gas supersaturation in the Nepali hydropower sector

Abstract The hydropower sector in Nepal is experiencing rapid growth, even though only a small part of Nepal’s economically feasible hydropower potential has been developed. The number of operating power plants is therefore expected to increase rapidly in the near future. This expansion comes along with significant engineering and environmental challenges, particularly concerning the aquatic ecology in Nepali rivers. The construction and operation of hydropower plants can disrupt natural river flow regimes, alter sediment transport, and negatively impact the aquatic biodiversity. One such threat is total dissolved gas (TDG) supersaturation, which is proven to be generated by hydropower plants all over the world, especially in connection with high run-off events. As those are common during monsoons (i.e. the wet season) in Nepal, assessing the risk of hydropower plants in Nepal to generate TDG supersaturation and thereby negatively affect the downstream waterways is crucial. TDG supersaturated water is harmful to the aquatic ecology through gas bubble disease, which is found in fish and aquatic invertebrates. A theoretical risk model developed in Norway is used to conduct the assessment. It determines the risk in dependence of power plant parameters such as turbine type, occurrence of secondary intakes, or head size. A detailed overview of operating hydropower plants and projects that obtained a construction license is assembled and subsequently used to determine the risk for each power plant. Even though the data base is not covering the salient features of all power plants, it is concluded that about half the power plants in operation and a quarter of the projects with a construction license are in the medium risk classes. Moreover, it is found that especially larger power plants are in the higher risk classes. Only one power plant to be constructed is in a high risk class. Nevertheless, it is advised to start early monitoring of the TDG saturation level at the power plants in question to avoid harming the aquatic environment.