The Manapōuri Power Station

The Manapōuri Power Station is an underground hydroelectric power station located in the Fiordland National Park, which in itself is in a world heritage area (Te Wāhipounamu, South West New Zealand World Heritage Area). The man-made cavern is 200 meters below the surface of Lake Manapōuri, or more to the point 200 metres underneath a hard granite mountain.

The cavern (machine hall) is about 111 metres long, 18 metres wide, and 39 metres high. The access tunnel down to the machine hall is 2 kilometres long and 9 metres wide. There are two tailrace tunnels, which takes the water that flows out of the station into Deep Cove in Doubtful Sound, 10km away. With the exception of the second tailrace tunnel, the power station was constructed in the 1960s using ‘drill and blast’ excavation methods to carve through the hard Fiordland rock (the second tailrace tunnel was constructed between 1999-2002 using a tunnel-boring machine).

That is impressive and one of New Zealand’s greatest engineering achievements.

Why do I have this site?

Because I have a bunch of photos from my late father, Ray Williams (Dip.Surv., M.N.Z.I.S.) from when he worked on the construction of the power station. I think the photos are impressive and as I said – the Manapōuri Power Station is an impressive feat of engineering, so I thought I would share them.

Why the construction period, not the whole life of the station?

Here is the long answer… Back pressure of the water exiting the power station through the tailrace tunnels is relieved via a surge chamber which in turn vents into the access tunnel and therefore into the powerhouse itself. This means you risk flooding the powerhouse if you run the station with too much throughput. The initial tunnel had greater than anticipated friction between the water and the walls of the tunnel, which resulted in greater backpressure and therefore higher water level in the surge chamber than designed restricting the powerhouse to less than 585MW, which was well short of the designed peak capacity of 700 MW. The solution to this was to construct a second tunnel.

Work on the first tunnel began in early 1964 and breakthrough occurred four years later, in late 1968. The tunnel was built in two halves with one contractor starting from West Arm and the other contractor from Deep Cove. When the two contracts finally met in October 1968, the two tunnels were only two inches (~50mm) out of alignment. In comparison, in 1998 work began on the second tunnel and this time using a 1000 tonne Tunnel Boring Machine (TBM). The second tunnel was estimated to take only take 18 months to cut/dig, but it broke through in 2001 (the delays were caused by the poor penetration rate and utilisation of the TBM due to unexpected rock conditions).

It took 4 years to drill and blast the first tunnel, and 3 years for a modern boring machine to drill the second tunnel. Even allowing for the stricter environmental constraints and health & safety rules that existed the second time around, this is a true testament to the amazing work that took place to build the original tunnel and the actual power station.

The short answer is just that, short: I only have photos on the construction.