Headloss Measurements and Particle Image Velocimetry in a 3D-milled Scaled Model of a Rock-blasted Hydropower Tunnel

Tunnels are a common component in hydraulic systems and are used to convey water for energy production, flood control and water supply. In places where the rock masses are stable enough, tunnels are often rock-blasted and left unlined to reduce the construction costs. However, the rough nature of unlined rock blasted tunnels increases energy losses and hence hydraulic resistance, which goes hand in hand with the reduction of the conveance capacity of such conduits. This aspect is hence of particular importance for hydropower production and the design of tunnel spillways.

The headlosses in unlined tunnels depend on a number of factors such as variations in the tunnel cros-sectional-area, tunnel shape, and the irregular roughness patterns of the tunnel walls. Until today, different approaches have been used to estimate head-losses in unlined tunnels, ranging from the selection of an arbitratry Manning coefficient to the characterisation of the variability of the longitudinal profiles and cross-sectional areas (so-called IBA method) or the fitting of an equivalent sinusoidal roughness based on the spectral analysis of longitudinal tunnel profiles (so-called Pegram and Penningon’s method). Since such approaches are based on simplifications of both the stochastic properties and the 3-dimensionality of the rock-roughness, they remain associated with large uncertainties. Moreover, until today there exists no deterministic relation between the geometric roughness of rock blasted surfaces and their hydraulic resistance.

It is against this background that the research project TunnelRoughness at the Norwegian University of Science and Technology has been established to develop new analytical, experimental and numerical methods to enhance the understanding of the interplay between the near-wall turbulent flow field and the tunnel roughness characteristics. In this paper, results from headlosses and Particle Image Velocimetry (PIV) measurements are presented, which have been conducted in a pressurised scaled model of a real-world rock-blasted hydropower tunnel, reproduced accurately at the 1:15 scale with a 3D-milling technology. The tunnel model was connected to a closed re-circulating loop and equiped with pressure transducers for the determination of headlosses in the pressurized tunnel and transparent windows for the implementation of PIV at two sections. Results of the headloss measurements are presented for a range of discharges together with preliminary analyses of velocity and turbulence characteristics at the two PIV sections. The results are discussed with regard to roughness characteristics of the tunnels which have been derived by statistical methods.