This 800 MW underground hydropower station is situated the Fiordland National Park and World Heritage Area in the South Island of New Zealand (NZ). It generates electricity for about 619,000 average homes from water diverted from Lake Manapouri through vertical penstocks to seven 122 MW turbine generators located 200 m below the surface of the lake.
The original construction of the Manapouri hydropower station was a huge engineering feat. The first tailrace tunnel built was 10 kilometres long, nine metres in diameter and lined with concrete. Constructed using ‘drill and blast’ methods, it discharges water into Deep Cove in Doubtful Sound.
Shortly after the station started generating power in 1972, engineers noted the hydraulic head loss in the tailrace system was higher than predicted. This and the fact that for environmental reasons the lake was not raised to the original intended level, limited the maximum power output of the station. Feasibility studies concluded it would be viable to use a Tunnel Boring Machine (TBM) to build a second tunnel to increase the generating capacity.
Designing the Second Tunnel
WSP was engaged to create the conceptual design, detailed design and production of the contractual specifications of all ancillary works associated with the second tunnel. This included the headworks and outlet works stop logs, the dewatering system and all permanent electrical systems and modifications to Channel 1 at Deep Cove.
The headworks included the headworks stop logs, stop logs stands, maintenance access shaft, and associated crane facilities.
The dewatering system comprised a tunnel sump, connected to dewatering adit with a mass concrete plug so the pumps can be operated with wet and dry wells to use the vertical access dewatering shaft as part of the sump, thus minimising excavation costs. The six 560-kilowatt pumps can each move 800 L/s to dewater the tunnel for inspections and maintenance. An overhead crane and ventilation system was also included. The power supply was 11 kV and two 1,000 V motor control systems control the pumps with soft starters controlled by programmable logic controllers.
The outlet works included the stop logs, stop log stands and embedded framework. Lifting and placement is via a mobile crane.
During the construction phase, we reviewed the contractor's submittals, made inspections and answered contractor's information requests. The second tunnel using a 25-metre-long tunnel-boring machine. The second tunnel is 10 metres in diameter and is largely unlined.
The project was successfully carried out in a World Heritage Area, where preservation of the unique environment is of great concern. The power station was commissioned in 2002 was pronounced an outstanding success in terms of generation output and the cost-effectiveness of the additional capacity. It provided power for an additional 64,000 more homes.
In 2003, the project team won the IPENZ Electrical Systems Engineering Excellence Award for its work on the Manapouri Second Tailrace Tunnel.