SUMMARY
Hydropower plants are generally considered as economical and sustainable sources of energy, yet they can create unintended disruptions to sensitive ecosystems. The objective of this study was to assess the environmental and social impact of a proposed mini-hydropower plant based on Sudu Ganga (Matale district) in Sri Lanka. Data for the environmental impact assessment were obtained via species observation and from secondary sources. Data for social impact assessment were obtained through the views of the residents and other stakeholders in the area. The regulation degree was 0.052% which indicated minor impact on the hydrological flow regime. In the project area, the study identified 28 flora species and 30 fauna species including five flora and fauna species at high risk of disappearance, and another five species at the risk of falling into a threatened category. The estimated value of the Shannon biodiversity index for flora species was 1.71 and was lower compared to other ecologically sensitive areas in surrounding districts. The environmental impact values for the physical and biological environment, and the social environment were -0.65 and 1.4, respectively, on a scale of [-5, 5]. These values indicate a slight negative impact on the physical and biological environment but a positive impact on the social environment. The proposed reforestation project and fish bypass gate were recognized as significant mitigation measures. We recommend further precautionary measures, particularly, during the construction phase to minimize disturbance to wildlife and damages to plant species, proper management of site excavation and construction waste, careful use of machinery and vehicles, environmental awareness of employees, and involvement of community and stakeholders to reduce adverse impacts.Keywords: Environmental assessment, hydropower, impact mitigation, Mahaweli project, Shannon-biodiversity index