Second Manapouri Tailrace Tunnel
We are generating energy
The completion of the Manapouri Power Station in 1972 was hailed as one of the greatest engineering feats this country had ever seen. 1800 men had worked for eight years to drill and blast the 10 kilometre tailrace tunnel through the hard granite of the Fiordland mountains from the power station at Lake Manapouri to Doubtful Sound.
But shortly after commissioning, it became clear that the power station was only operating at 85% capacity – friction between the water and the walls of the tailrace tunnel causing a loss of hydraulic head and with it, productivity.
To allow the water to flow through the power station more quickly and increase hydraulic head, Meridian engaged the Fletcher Dillingham Ilbau consortium, to construct a second tail race tunnel.
Though a modern Tunnel Boring Machine replaced the drill and blast methodology of the 1960’s, constructing the second tunnel was no less challenging. Fiordland National Park is a UN World Heritage Site, and treading lightly in the pristine natural habitat was essential.
Fiordland National Park is a UN World Heritage Site, and treading lightly in the pristine natural habitat was essential.
A bespoke TBM, specifically designed for the hard rock environment, was used to construct the 10km long, 10 metre wide (about the width of a two lane road) tunnel. Muck from the tunnelling operation was placed on top of spoil from the original tunnel excavation, and the area extensively planted to return it to its natural state.
The new tunnel created enough energy to supply another 64,000 homes – or a town the size of Rotorua - without using an additional drop of water.