Linking physical wall roughness of unlined tunnels to hydraulic resistance – The Tunnel Roughness project
Hydropower tunnels are a vital component in Norwegian hydropower systems, both for transporting water and controlling releases of flood flows. How much water can be conveyed depends on a number of factors such as the cross sectional area and the friction from the tunnel walls. Norwegian hydropower tunnels are generally unlined and energy losses have mostly been quantified before commissioning by using empirical approaches, tabulated values, or photographic methods. In the present paper, we review these methods, general knowledge and modeling methods of wall roughness, before introducing a recently launched research project focusing on the determination of friction factors from the statistical analysis of the surface geometry of unlined hydropower tunnels. The paper describes how innovative use of modern remote sensing techniques can support analytical considerations, experimental based research, and practical design of unlined hydropower tunnels.