When New Zealand prime Minister Helen Clark formally inaugurated Manapouri hydro power station’s second tailrace tunnel on 25 May 2002, the country’s largest power plant was able to operate at its rated 700MW capacity for the first time since its 1972 commissioning.
For the past 30 years Manapouri has only been able to operate to 585MW using six of its 7x100MW turbines because greater than anticipated friction inside the first tailrace tunnel had capped its flow at 460m3/sec. Manapouri has resource consents to draw 510m3/sec from the Manapouri and Te Anau lakes in Fiordland National Park on the southwestern tip of South Island in order to deliver its full capacity. But so far any flows in excess of 460m3/sec have merely backed up to flood the turbines.
For 20 years, no cost-effective solution to this situation could be found. But in 1993 state-owned project operator Meridian Energy began to explore the possibility of boring a second tailrace tunnel parallel to the first and bifurcating from it just below the tailrace manifold. This approach promised to cut Manapouri’s project-related outage to a tolerable three weeks. It is this tunnel which was inaugurated on 25 May.
The 9.8km long, 10m internal diameter tunnel 2 will increase Manapouri’s total tailrace flow capacity to 630m3/sec that would in turn enable a 760MW rated capacity. However, any drawdown of over 510m3/sec from the lakes 178m above the underground power house must obtain new resource consents.
The second tailrace tunnel project began in February 1997 for completion in August 2000 at a contracted cost of US$85M. Fletcher-Dillingham-Ilbrau joint venture (FDI) was the main contractor. It would have been commissioned in October 2000.
In the event, ground conditions – extremely hard and faulted gneisses, quartzites, gabbros, amphibolites and granites ,combined with groundwater flows of sometimes more than 1000 litres/sec at pressures of up to 550psi, caused frequent and extended downtime of the purpose-built Atlas Copco/Robbins tunnel boring machine and hence significant cost and time overruns.
Tunnelling began on 12 June 1998 and after 9.6km working upstream from the tailrace exit in Deep Cove achieved breakthrough with the 200m transition tunnel to tunnel 1 on 13 March 2001. In the meantime a watertight bulkhead had been placed in tunnel 1, with tunnel 2 also being fitted with stoplog structures at its headworks and exit so that, unlike tunnel 1, it can be closed off and pumped dry for periodic inspection.
Tunnel 2 was informally commissioned at midnight on 4 May 2002 when the bulkhead in tunnel 1 was raised to allow water to flow through the transition tunnel to tunnel 2. In initial tests prior to formal inauguration, it has given Manapouri 715MW peak capacity at 510m3/sec.
Environmental concerns have been a major issue throughout the project. Due to its unique geography and hydrology — over 6m of rainfall annually in the area provide a permanent layer of fresh water over seawater in the fiords – Fiordland National Park is also a World Heritage Site.
Consequently the tunnel boring machine which was built at Chesterfield, UK had to be examined for foreign organisms before entering the park; all water from the project was treated before being released into Deep Cove; and all spoil was piled, contoured and planted with local vegetation near the tailrace exit.
Perhaps most importantly, full use of tunnel 2 depends primarily on assurances that peak flows of 630m3/sec of diverted fresh water into Deep Cove will not disturb the present fresh/seawater balance. Studies are being conducted on the effect of 30 years of 460m3/sec flows. The impact of 510m3/sec flows will now also be measured to estimate the probable effect of 630m3/sec.
At present it seems likely that fresh consents will be granted. As New Zealand’s Ministry of Economic Development has already pointed out: ‘In the context of natural (freshwater) flood flows to Deep Cove of around 5-6000m3/sec, the (120m3) increase in discharge from tunnel 2 would be very small.’