Linking physical wall roughness of unlined tunnels to hydraulic resistance

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 it was used to scan the topography of unlined hydropower tunnels. These data were used to study the tunnel roughness, i.e. the friction associated with the tunnel walls, and hence to derive a novel approach for the determination of tunnel discharge capacity. This was achieved by combining physical scale model studies with computer (numerical) simulations and analytical considerations. The laser scanning data provided the input for physical scale model studies in which velocity and pressure measurements were carried out in miniature versions of the tunnels using advanced hydraulic instrumentation. Friction and energy losses were determined from these measurements and related to the structure of the tunnel roughness, which was assessed based on statistical analyses of the laser-scanning data. These data were also used as input for numerical models in which turbulence and friction losses were calculated in computer simulations. The results from the physical and numerical experiments were compared to validate the model-studies. The final results of the project are of high relevance for end-users as they show novel paths to assess of energy losses in unlined tunnels from laser-scanning data.

The project made use of technological advances in digitalization and experimental methods to advance predictive capabilities for the quantification of wall resistance in unlined hydropower tunnels and tunnel-spillways. The project results provide valuable guidance for the design of future hydraulic scale model studies using CNC-manufacturing methods. The derived formula to relate measurable topographical characteristics to the friction factor can be used in economical optimization of new tunnels as well as deciding the profitability of improving existing tunnel systems. Used on closed conduit spillway tunnels, it will be important for assessment of the safety of dam constructions, and cost analysis for necessary upgrades due to increased floods caused by climate change. The results of the numerical simulations were used to support the experimental results, i.e. the project showed the usefulness of a hybrid approach to tackle sophisticated hydraulic problems in an efficient way.

The present proposal addresses the hydraulic resistance of wall-bounded flows by focusing on the evaluation of energy losses through wall friction in unlined hydropower tunnels, an essential component in Norwegian hydropower systems used for both power production and flood control. The determination of the hydraulic capacity of unlined hydropower tunnels requires the knowledge of friction factors whose determination is mostly based on empirical approaches. Thus, despite their significance, friction factors can be considered as the weakest component in the design of tunnel waterways. This aspect is tackled in the present study through a combination of analytical considerations, physical scale model studies, and numerical simulations. Analysing high resolution digital elevation models (DEMs) of existing unlined tunnels obtained through Terrestrial Laser Scanning which were excavated by different methods, novel roughness parameters will be defined on the basis of a statistical analysis of the DEMs. Friction losses and the flow field in the tunnels will be measured in scale model studies with miniature versions of the tunnels constructed through CNC-milling. The obtained friction factors will be related to the derived roughness parameters on the basis of a theoretical framework allowing for the decomposition of the friction factor into components reflecting the physics of rough bed flows. These considerations will be supplemented through data from high resolution 3D-numerical simulations validated by the scale model data. The final results of the project are of high relevance for end-users as they will allow for the assessment of the friction factor with regard to the excavation method and improved protocols for physical scale and numerical model studies used for the design of tunnel-waterways. Moreover, the results will allow for the assessment of wall-friction in unlined tunnels from data measured in-situ during excavation in order to verify design-assumptions.