Tongariro Power Scheme Description
The Tongariro Power Scheme can be divided into four key sections the Eastern Diversion, Western Diversion, Tongariro Section and Rotoaira Section.
The Eastern Diversion extends from the Wahianoa Aqueduct on the southern flanks of Mt Ruapehu, eastward to the Moawhango Dam and north to the discharge point of the Moawhango Tunnel at the Rangipo Dam on the Tongariro River. The major structures in the Eastern Diversion and their purposes are outlined below.
Takes water from tributaries of the Whangaehu River via 22 intakes and delivers it to Lake Moawhango. The Whangaehu River itself is not intercepted as it is naturally acidic, draining from the Crater Lake on Mt Ruapehu. The aqueduct is approximately 8,400m long and is buried underground. It transports water eastward under the Whangaehu River and into the Mangaio Tunnel.
Mangaio Tunnel and Drop Structure (1.8MW)
Water from the Wahianoa aqueduct passes under the Desert Road through the Mangaio Tunnel and either into Mangaio Power Station or a drop structure to enter the Lake Moawhango via the Mangaio Stream. Construction of the Mangaio Power Station began in January 2007 and commissioning began at the end of 2008. When operating, the power station will provide additional generation to the Tongariro Power Scheme of approximately 1.8MW's of electricity.
The Moawhango Dam dams the Moawhango River and Mangaio Stream to create Lake Moawhango. Lake Moawhango has a normal operating range of approximately 15.2 metres and as such rarely spills except during the largest floods.
Water is taken from Lake Moawhango to the upper Tongariro River by a 19.2km tunnel. The tunnel flow is regulated by a discharge valve at the downstream end of the tunnel where it discharges into the Tongariro River, at Rangipo Dam.
The Western Diversion begins at the Whakapapa River where the water is routed north east via a 16.5km tunnel into Lake Te Whaiau. There are 4 smaller intakes into this tunnel which intercept water from the Tawhitikuri, Okupata, Taurewa, and Mangatepopo streams. Water is also diverted from the Whanganui River into the Te Whaiau Stream, which in turn discharges into Lake Te Whaiau. From Lake Te Whaiau water is discharged into Lake Otamangakau which is then discharged into Lake Rotoaira via the Wairehu Canal.
The Whakapapa Intake
The Whakapapa Intake comprises a concrete gravity dam set into the river bed just below the confluence of the Papamanuka Stream and the Whakapapa River. The intake has a flow capacity of 35 cubic metres per second. Any flow above this amount is passed over the spillway and down its natural course. A minimum flow of 3m3/s is maintained in the river downstream of the intake. Recreational flows are also released from the intake periodically.
Okupata, Taurewa, Mangatepopo and Tawhitikuri Intakes
Intake structures on each of these streams divert all the flow into the Whakapapa - Tawhitikuri - Whanganui Tunnel up to the following design capacities:
- Okupata 2 cubic metres per second
- Taurewa 2 cubic metres per second
- Mangatepopo 5 cubic metres per second
- Tawhitikuri 2 cubic metres per second
A minimum flow downstream of 0.5m3/s is released downstream of Mangatepopo Intake.
Whanganui Intake and Te Whaiau Culvert
The Whanganui Intake diverts water from the Whanganui River via a short tunnel into the Te Whaiau Stream, which then flows into Lake Te Whaiau. A minimum flow of 0.3 m3/s is maintained below this intake constantly.
The Te Whaiau Canal, Lake and Dam
Lake Te Whaiau Canal receives water from the Whakapapa - Tawhitikuri - Whanganui Tunnel and the Te Whaiau culvert. The dam spillway discharges into the old Te Whaiau stream bed.
The Otamangakau Canal, Lake and Dam
The Otamangakau Canal links Lake Te Whaiau to Lake Otamangakau. Lake Otamangakau provides short term storage for the western diversion flows.
The Wairehu Canal
The Wairehu Canal takes water from Lake Otamangakau to Lake Rotoaira. Here it combines with the inflows from the Eastern Diversion.
The Tongariro Section begins at the Waihohonu Intake and Tunnel, which transport water from the Waihohonu Stream to the Rangipo Dam on the Tongariro River. It then extends from the Rangipo Dam northward as far as the Poutu Intake, then North West to the Poutu Tunnel and Poutu Canal as far as the Poutu Dam. Waihohonu Intake and Tunnel takes water from the Waihohonu Stream via a 960 m long tunnel to the Rangipo head pond.
The Tongariro River creates a head pond for the Rangipo Power Station. Water is transported from here to the Rangipo Power Station via a headrace tunnel. A minimum flow of 0.6 m3/s is maintained below the dam to enhance the Blue Duck habitat. Recreational flows are also released from here periodically for sporting activities. For more information view our Rivers, Lakes and Rainfall section.
Rangipo Power Station (120MW underground)
The Rangipo Power Station is 63 m below the ground and was the second underground power station to be built in New Zealand. Rangipo was built underground to minimise the visual effect of the power station on the surrounding environment and the Tongariro River. Rangipo power station has two 60MW generators, and was commissioned in 1983.
Rangipo Tailrace Tunnel
The Rangipo Tailrace Tunnel discharges water from the Rangipo Power Station after electricity generation, through a 3 km tunnel to the Tongariro River, immediately upstream of the Poutu intake.
The Rotoaira Section is the northern-most section of the scheme. It includes Lake Rotoaira, the Tokaanu Intake and Tunnel, Tokaanu Power Station and Tokaanu Tailrace.
Lake Rotoaira lies at an altitude of approximately 564m asl between Mt Tongariro and Mt Pihanga, south west of Lake Taupo. The lake is the reservoir for the Tokaanu Power Station.
Conveys water from the Poutu Dam to Lake Rotoaira.
Tokaanu Intake and Tunnel
Tokaanu intake is at the northern end of Lake Rotoaira. Here the water enters a 6.1 km Tunnel to Tokaanu Power Station.
Tokaanu Power Station (240MW)
The Tokaanu Power Station is a 240MW capacity station situated on the base of Mount Tihia at the southern end of Lake Taupo.
Water from the power station passes into the Tokaanu Tailrace channel, a 3.8km channel that discharges into Lake Taupo at Waihi Bay, west of the Tongariro Delta.
Tokaanu Stream Diversion
The Tokaanu stream diversion was created to preserve the Tokaanu stream (an important trout spawning stream) which crossed the path of the power station tailrace. The stream still crosses the path of the tailrace, but via an aqueduct under the tailrace road bridge.