Instream flow requirements and hydropower development— A case study from the Gjengedal development in Norway

In the planning of hydropower developments the instream flow requirements are often decisive of both the project economy and environmental effects. This is especially important in many salmon rivers, where the preservation of salmon stock is of great importance. The instream flow is also of vital importance for many other users, including agriculture, water supply and recreation. In this process both economical, technical and environmental effects have to be considered and optimal balance between competing goals has to be found. The use of simulation models has proved to be helpful in this process. By combining simulations of hydropower production, river flow, physical parameters in the river and fish habitat, the effect of varying instream flow requirements can easily and accurately be investigated. A direct comparison between project economy and usable fish habitat has been possible in the case study Gjengedal. The different types of simulation models used are described, and also how these models have been used in the planning process.