Hydropower tunnels represent an important component in Norwegian hydropower systems. They are used for both the transport of water from reservoirs to the powerhouse for energy production and to provide the controlled release of flood flows from reservoirs into downstream areas. How much water can be conveyed in a tunnel depends on its friction. Norwegian hydropower tunnels are generally unlined, i.e. the tunnel walls are left rough after excavation. The friction caused by such tunnel walls, i.e. their hydraulic resistance, is generally quantified using empirical approaches, tabulated values, or photographic methods. In recent years, the laser scanning technology has been significantly advanced and in this project we will use it to scan the topography of unlined hydropower tunnels. These data will then be used to gain detailed information on tunnel roughness, i.e. the friction associated with the tunnel walls, and hence to derive novel approaches for the determination of tunnel discharge capacity. For this purpose, we will combine physical scale model studies, computer (numerical) simulations, and analytical considerations. The laser scanning data will provide the input for physical scale model studies in which we will measure velocities and turbulence in miniature versions of the tunnels using advanced hydraulic instrumentation. From these measurements, we will determine the friction and energy losses and relate them to the structure of the tunnel roughness, which in turn will be assessed based on statistical analyses of the laser-scanning data. The laser scanning data will also be used as input for numerical models in which turbulence and friction losses will be calculated in computer simulations. The results from the physical and numerical experiments will be compared in order to validate the model-studies. The final results of the project are of high relevance for end-users as they will allow for the assessment of energy losses in unlined tunnels from laser-scanning data.

Primary objective:

• Improve the accuracy of analytical, experimental and numerical methods for the determination of energy-losses through friction in unlined hydropower tunnels.

Secondary objectives:

• Assessment of tunnel roughness using statistical analysis of tunnel topography and relating geometrical roughness characteristics to spatial scales and tunnel construction methods.

• Development of an advanced approach to link geometrical surface properties to hydraulic roughness and hence friction losses.

• Linking near-wall turbulent flow field features to tunnel roughness characteristics using innovative analytical methods.

• Assessment of the performance of numerical models for capacity calculations in tunnels.

• Improvement of physical scale modelling techniques for the simulation of unlined tunnels.