Heritage NZ

Manapouri Muscle

It was possibly the country’s greatest engineering feat, and certainly one of its most profound political milestones. HUGH DE LACY tells the story of the Manapouri power station.

There could be no denying the economic potential that such a freak of Fiordland nature offered: a great lake – in fact two of them, conveniently connected by a river – sitting 230 metres above sea-level and only 10 kilometres from the sea.

A self-regenerating volume of water that vast – 142 square kilometres in area and up to 440 metres deep –with a fall that precipitous, cried out to be used to generate hydro-electric power.

As early as 1904 there had been talk of hacking a channel between Lake Manapouri’s West Arm and the Tasman Sea at Deep Cove, and it was only the remoteness of the place and the huge scale of the undertaking in relation to New Zealand’s colonial economy that stopped it.

But the idea was never going to die, and in 1926 an outfit called the NZ Sounds Hydro-electric Concession Company acquired a right from the government to use Manapouri’s water to produce fertiliser and munitions by electrically fixing nitrogen from the atmosphere. However, the nation’s taste for war had been dampened by the catastrophe of 1914-18, and its capacity for capital investment was shortly to be crippled by the Great Depression, so the scheme got no further.

What brought a West Arm-to-Deep Cove hydro scheme back to life was the discovery in 1955 of what was then the world’s biggest bauxite deposit, near Weipa on Australia’s Cape York Peninsula in Queensland. And all that was required to economically reduce the alumina extracted from the bauxite to form the lightweight wonder metal aluminium was lots of cheap electricity.

The discovery was made by Consolidated Zinc Pty, and before the year was out it was knocking on the door of the New Zealand government asking about hydro power. It so happened that the Ministry of Works had recently reported on various applications of Manapouri’s potential. The government put one and two together, and within three years Consolidated Zinc had been granted rights to develop hydro power from Lakes Manapouri and neighbouring Te Anau to power an aluminium smelter at Tiwai Point on Bluff Harbour, 190 kilometres to the southeast.

From the start it was assumed that Lake Manapouri would have to be raised, as had occurred nearly 40 years earlier when another southern lake, Monowai, had been adapted for hydro generation. That development had left thousands of drowned trees cluttering and disfiguring Monowai’s edges, a vision that had burned itself into the public consciousness.

The Consolidated Zinc deal clearly violated the National Parks Act, and the public backlash was immediate: 25,000 people signed a petition in 1960 opposing the raising of Lake Manapouri. But the National Government of Keith Holyoake pressed ahead, retrospectively validating the agreement by a special Act of Parliament the same year.

This failed to deter the scheme’s opponents, and when Consolidated Zinc revealed plans to build dams that would raise Lake Manapouri by 30 metres and merge it with Lake Te Anau, the nationwide Save Manapouri Campaign was launched.

In 1963 Consolidated Zinc decided it couldn’t afford to build the hydro plant, so the Government agreed to build it instead, selling the electricity to it at bargain-basement rates with no provision for inflation.

Work on the project started that year under the management and using the design of international consultancy Bechtel Pacific Corporation, directed by the Ministry of Works, with the Utah Construction and Mining Company building the powerhouse and, in association with two local companies, the tailrace tunnel and the Wilmot Pass Road linking West Arm with Deep Cove.

Over the next six years public pressure to abandon the lake-raising part of the project mounted, and in 1970 a petition organised by the Save Manapouri Campaign attracted 264,907 signatures, almost a tenth of the country’s population.

In the run-up to the 1972 election, Labour Opposition leader Norman Kirk promised to freeze the lake’s level if Labour won. It did, in a landslide. Kirk kept his promise and appointed six of the protest’s leaders as guardians of Manapouri, Te Anau and Monowai to ensure it was respected. The success of the protest today is regarded as the birth of the environmental movement in New Zealand, as well as being a major contributor to such fundamental democratic reforms as the Official Information Act and proportional representation.

The political associations aside, what makes the Manapouri project so remarkable is that the power station itself was excavated out of solid granite 200 metres below the lake level. And to get down there the builders had to excavate at a one-in-10 gradient a downward-spiralling two-kilometre tunnel wide enough for two-way traffic. A 220-metre deep elevator for personnel transport was also installed, along with seven penstocks each 180 metres long.

Temporary accommodation at Deep Cove, Doubtful Sound.

More than 1.4 million tonnes of rock was excavated, and for most of the nine years of construction 1800 staff from 22 countries were employed, working eight million man-hours. Tragically, no fewer than 16 men were killed on the job which otherwise cost $135 million ($1.95 billion in 2008 dollars).

There was originally just one 10-kilometre-long tailrace taking the lake water from the seven turbines to the outlet at Deep Cove, but on completion of the project in 1972 it was discovered that a major design cock-up had been made: the friction between the water and the tailrace tunnel walls so reduced the hydrodynamic head that if power output had exceeded 585mW – against a planned 700mW – it would have flooded the powerhouse.

This is the way it stayed for 30 years, until the completion of a second tailrace bored from the Deep Cove end by a consortium comprising Dillingham Construction, Fletcher Construction and Ilbau, using a Robbins tunnelling machine. Since then the seven Francis-type vertical generating units, made by General Electric of Canada, have been refurbished at a cost of $98 million to a peak output capacity of 121.5mW each at 120rpm. Though this has delivered an installed capacity of 850mW, the resource consent limits output to 730mW. Average annual output is 4800 gigawatt hours.

Most of the power goes to feed the Tiwai point smelter’s 620mW peak demand, but there’s enough left over for Invercargill and the western half of Southland.

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