Experimental and Numerical Determination of the Head Loss of a Pressure Driven Flow through an Unlined Rock-Blasted Tunnel

Jochen Aberle,Fabian Kleischmann,Ralph Eikenberg,Mari Vold,Nils Reidar Bøe Olsen,Pierre-Yves Henry,Christy Ushanth Navaratnam

doi:10.3390/w12123492

Abstract

The friction loss in a part of the rock-blasted unlined tunnel of the Litjfossen hydropower plant in Norway was determined from experimental and numerical studies. Remote sensing data from the prototype tunnel provided the input data for both the numerical model and the construction of a 1:15 scale model with an innovative milling approach. The numerical simulations were based on the solution of the Reynolds-averaged Navier–Stokes equations using the CFD program OpenFoam. Head loss measurements in the scale model were carried out by means of pressure measurements for a range of discharges and were compared against the results of the numerical model. The measured data were used to determine the Darcy–Weisbach and Manning friction factors of the investigated tunnel reach. The high-resolution remote sensing data were also used to test the applicability of existing approaches to determine the friction factor in unlined rock blasted tunnels. The results of the study show the usefulness of the chosen hybrid approach of experimental investigations and numerical simulations and that existing approaches for the determination of head losses in unlined tunnels need to be further refined.

Highlights

Hydropower is the principal source of Norway’s electricity supply with Norway being the world’s seventh-largest hydropower producer and the largest in Europe in 2019 [1]
Remote sensing data from the prototype tunnel captured by terrestrial laser scanning provided the input data for both the construction of a 1:15 scale model with an innovative milling approach and a numerical model
The high-resolution spatial data of the tunnel were used for the analysis of the performance of existing approaches to determine the friction factor in unlined tunnels

Summary

Introduction

Hydropower is the principal source of Norway’s electricity supply with Norway being the world’s seventh-largest hydropower producer and the largest in Europe in 2019 [1]. More than 50 embankment dams in Norway have closed conduit (shaft/tunnel) spillways, typically constructed as rock-blasted tunnels, which are used for flood protection. Compared to lined tunnels, unlined tunnels have a larger roughness due to irregularities in cross-sectional area and wall surface e.g., [5,6] which originates from the excavation method such as boring, drilling, and blasting. Despite the reduced conveyance capacity and larger energy losses, unlined tunnels are still considered to be economically advantageous in hard rocks because of reduced. Rock blasting represents a more rapid method for tunnel excavation than utilizing tunnel boring machines [7]. For all these reasons, unlined rock-blasted tunnels are a common feature in the Norwegian hydropower system [4]. The scope of this paper is to highlight possibilities to determine the friction losses in such conduits using state-of-the-art measurement, experimental, and numerical techniques

Methods

Results

Discussion

Conclusion