The Hinemaiaia Scheme had its beginnings in 1939 in a quest to develop an electricity supply for Taupo. Power was eventually generated in late 1952, from the Hinemaiaia A Station (800 kW from a single synchronous generator) and fed into the national grid in 1958.
In 1982, a second machine was commissioned in the Hinemaiaia A Powerhouse, increasing output to 2.0 MW. The Hinemaiaia B (1.3 MW) and C (2.8 MW) Stations were commissioned in 1966 and 1982 respectively. Today the scheme provides an annual output of 30 GWh or close to 10% of the power requirements for Taupo.
Hinemaiaia A Lake has developed into a productive wetland with protection being offered by restricted public access. In addition, the riverbed below the Hinemaiaia B Powerhouse supports a productive trout spawning area. As a reflection of the significance of this stretch of the Hinemaiaia, TrustPower has agreed to maintain a flow of three cubic metres per second below the Hinemaiaia B Station, where inflows into the Hinemaiaia A Lake permit, to address concerns that the dams may inhibit trout migration up the Hinemaiaia River and that fluctuating levels may contribute to downstream erosion.
In partnership with the Department of Conservation, TrustPower undertakes a comprehensive trap and transfer programme releasing 200 trout and approximately 35,000 fry annually to above the Hinemaiaia B Dam. The Hinemaiaia A Lake has to be periodically dredged to maintain sufficient depth for water storage, which can generate adverse effects. To address these concerns TrustPower commissioned scientific investigations, the results of which indicated that the dredging has no major long-term effects.
New Resource Consents for the Hinemaiaia Scheme were granted during 2003 with expiry scheduled for 2036.
The 33 hectare lake of the HA Station has an average depth of only 1.2m and contains 16.6 hours of storage at mean river flow. The river carries considerable pumice debris and the original lake was fully silted in its first 25 years of existence.
Levees have been built to form future silt ponds for the time when dredging of the lake area above the weir will become necessary to maintain storage. The HA lake is a wildlife sanctuary and has become the habitat of a wide variety of wild fowl. The lake in the past has been stocked with salvelinus fontinalis brook trout by the Wildlife Conservancy.
The HA dam is comprised of an arch dam 12m deep in a narrow ignimbright gorge with an adjacent 90m long weir perched on the surface of the ignimbright rock with the intake structure situated on the left bank at the end of the weir.
A remote controlled and hydraulically operated bypass gate has been placed in the dam which is capable of passing 5 cumecs when required.
The intake chamber contains hydraulically operated bypass gates, intake screens and screen scraper mechanism. The screen scrapers cope with pumice debris and lake weed and the scraper operation is initiated by both pressure differential across the screen and time switch control.
Two 243 metre long penstocks, one 1.67m in diameter and one 1.8m in diameter, lead from the intake chamber to the power station. The power station contains two horizontal shaft machines with Francis turbines.
Situated some 3km below the HC Station, the HB Station Dam is a 12 metre high arch dam in a narrow ignimbright gorge with a 75m long spillway weir adjacent.
The 12.1 hectare lake provides independent storage from the HB station giving 7 hours storage at mean flow. Water flows from the intake structure, with its gate and intake screen, through 360-metres of concrete conduit, 2.3-metres in diameter, and a further 168-metres of steel penstock to the HB Power Station which houses a horizontal Francis turbine driving a 1350kW induction generator.
The Hinemaiaia C dam has been built 2km down river from the HA Power Station in a very narrow ignimbright gorge some 27 metres deep. The lower section of the dam extends horizontally between the gorge walls while the the upper section has been designed as a buttressed wall.
An ogee spillway has been incorporated into the dam structure to discharge flood waters and the weight counterbalanced radial gate controlling the impound waters self adjusts to spill flood waters. The 2 hectares of water confined in the lake provide negligible storage and the HC Station is operated in tandem with HA Station.