10. Wanganui-Manawatu Region
10.1 Catchment #5: Whanganui
This catchment has:
- diversion of headwaters to Tongariro Power Scheme / Waikato catchment,
- existing hydro-electric power station (Piriaka),
- no controlled lake storage.
Table 16: Hydropower Opportunities in the Whanganui Catchment with High to Medium Confidence of Proceeding in the Next 20 Years.| Name | MW | Average Energy Production (GWh p.a.) | Public Reference |
| Mangawhero to Wanganui Diversion | 60 | 260 | MOC 1990 |
| Waimiha Stream | 6 | 27 | T&T 1985 |
| Waimarino Stream | 6 | 24 | T&T 1985 |
| Waikoura/ Mangakahu Diversion | 8 | 35 | MWD 1986 |
| Tokiahuru | 30 | 130 | MOC 1990 |
10.1.1 Mangawhero to Wanganui Diversion
The Mangawhero River, a tributary of the Whangaehu, also has its headwaters on Mt Ruapehu, but does not receive any of the acidic water from the Crater Lake.
A possibility of a hydroelectric scheme exists downstream utilising the height difference between the Mangawhero and the neighbouring Whanganui River. A dam on the Mangawhero in the vicinity of Orore, and tunnel to a powerhouse on the Whanganui River would develop a head of some 300m and with an installed capacity of 60MW, would produce approximately 260GWh p.a. (50% load factor).
10.1.2 Waimiha Stream (A/6/6)
The Waimiha Stream falls some 130m as it crosses the Ohura fault north-east of Waimiha settlement, and this gives potential for a 15 m dam with a canal and pipeline configuration supplying a power station on Waimiha Stream. A mean flow of 3.6m³/s gives an installed capacity of 6MW, which would produce approximately 27GWh p.a. (50% plant factor).
10.1.3 Waimarino Stream (A/9/4)
This scheme utilises the steep section of the Waimarino Stream between the Erua Swamp and the Makatote River confluence. This fall would be enhanced by a 25m high earthfill dam which would flood the swamp creating a reservoir providing significant storage. A 2-3 km lined canal would feed into a power station located on the right bank of the Makatote River just upstream of the confluence. A mean flow of 4.2m³/s with an installed capacity of 6MW, would give approximately 24GWh p.a. (50% plant factor).
10.1.4 Waikoura / Mangakahu Diversion
The Waikoura scheme is based on a dam in the high level Waikoura Valley feeding a power station in the Mangakahu Valley with a head of about 350m. It appears possible to divert a string of catchments into the Waikoura, and the initial assessment rated the scheme at 66MW. Several reassessments have drastically reduced this output to 8MW and the scheme need no longer be considered in the "large" class.
The dam in the Waikoura Valley would probably be earth of about 150,000m3, and about 20m high with a spillway chute on one side.
Catchment area of 15.2km² with a mean annual flow of 0.3m³/s and 316m nett head would, with an installed capacity of 8MW give approximately 35GWh p.a. (plant factor 50%).
10.1.5 Tokiahuru
The Whangaehu River has its headwaters on Mt Ruapehu and flows to the Tasman Sea south of Wanganui. The waters received from Crater Lake periodically give the river a relatively high acidity, conditions not favourable to fish and wildlife. The acidity would also require the use of special materials in generation machinery and structures. Some water from freshwater tributaries e.g. Wahianoa River, are diverted to the Taupo/Waikato catchment for use in the Tongariro Power Scheme / Waikato River power stations.
There are possible hydro-electric schemes which have been identified on the Whangaehu River. In the upper catchment there is an opportunity to develop a scheme which diverts the Tokiahuru Stream tributary via a 2km long canal and then on to a 10MW power station on the Whangaehu River via a 2km canal/pipeline.
Just downstream of the powerhouse for the above scheme is a possible scheme which bypasses a U-shaped section of the Whangaehu River with a 2.5km long tunnel to a 20MW power station.
When combined, this 30MW scheme would deliver approximately 130GWh p.a. (50% load factor).
10.2 Catchment #15: Rangitikei
This catchment has:
- diversion of headwaters (Moawhango) to Tongariro Power Scheme / Waikato catchment,
- no existing hydro-electric power stations,
- no controlled lake storage (Lake Moawhango excepted).
Table 17: Hydropower Opportunities in the Rangitikei Catchment with High to Medium Confidence of Proceeding in the Next 20 Years.| Name | MW | Average Energy Production (GWh p.a.) | Public Reference |
| Hautapu | 9 | 41 | T&T 1985 |
10.2.1 Hautapu (C/1/1)
The Hautapu River flows past Taihape to join the Rangitikei River near Utiku. It offers worthwhile hydro potential downstream of the Turangarere where the river falls 240m in 22km.
Studies have identified some 9MW potential (approximately 41GWh p.a. with 50% plant factor) which could be developed by a diversion type scheme with a combination of canal, tunnel, pipeline and a power station near Ngawaka Road deviation.
10.3 Catchment #7: Manawatu
This catchment has:
- existing hydro-electric power station (Mangahao),
- controlled lake storage (Mangahao Reservoirs).
Table 18: Hydropower Opportunities in the Manawatu Catchment with High to Medium Confidence of Proceeding in the Next 20 Years.| Name | MW | Average Energy Production (GWh p.a.) | Public Reference |
| Pohangina | 10 | 41 | T&T 1985 |
Note: A 3MW power station at Mangahao No. 1 dam (previously included in assessments of potential hydro) is presently being constructed / commissioned (January 2004).
10.3.1 Pohangina (D/3/1A)
As the Pohangina River flows out from the Ruahine Ranges it falls steeply from above elevation 1000m to elevation 300m and this fall affords the potential for two schemes.
One scheme involves an intake structure with a 4km concrete pipeline and a 750m penstock to the powerhouse. An installed capacity of 10MW would generate approximately 40GWh p.a. (50% load factor).
10.4 Other Opportunities in this Region
10.4.1 Prohibited
#5 Manganui a te Ao / Mangaturuturu - 8MW (National Water Conservation (Manganuioteao River) Order 1989)
10.4.2 Undefined
#5 Whanganui catchment
Whakapapanui I Makahikatoa - 5MW
Waimarino - 6MW
#44 Whangaehu catchment
Makotuku - 10MW (Diversion weirs on Makotuku and Mangawhero, just south of Raetihi)
Tokiahuru / Mangaturuturu - 10MW
#231 Otaki - 10MW
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