Rock falls in selected Norwegian hydropower tunnels subjected to hydropeaking

• We have inspected ten hydropower tunnel system subjected to hydropeaking.
• The frequency and volume of rock falls were registered.
• Examined if hydropeaking can cause increase in structural instability.
• The results were compared to an earlier study.
• The costs related to head loss due to rock fall were found to have increased by 70%.

Unlined rock-blasted hydro tunnels are key components of the Norwegian hydropower system (NHS), used both as water collectors and conveyors. Long-term stability of these constructions is essential to ensure continuous and cost-effective operation. Recent trends in the energy market have led to so-called hydropeaking production patterns, which increase the frequency of unsteady flow situations in the hydro tunnels. Such production patterns can trigger structural instability in the tunnel systems in form of rock falls, which reduces the cross-sectional area and cause head loss. This work examines the quantity of rock falls in hydropower tunnels subjected to hydropeaking through a series of inspections. The results indicate that the frequency and total volume of rock falls increases if a tunnel is subjected to hydropeaking.

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