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In hydro comparisons, Mokihinui will be to the West Coast what Wairau is to Marlborough. HUGH DE LACY looks at the challenges Meridian faces with its hydro project in a very under-powered region of the country.
Hence the importance to the region of Meridian Energy’s proposed 65-85MW hydro project on the Mokihinui River, 40 kilometres north of Westport. At present the West Coast is supplied from the Waitaki hydro system, with power being sent from the Benmore dam through Christchurch to Kikiwa and Inangahua, with resultant transmission losses of up to 50 percent at peak times. Before the Wairau project cleared the resource consent hurdles, Marlborough faced transmission losses nearly as great, with 90 percent of its power coming through the national grid either from Islington in Christchurch or from the 32MW Cobb plant near Nelson. It was something of a public relations triumph for Trustpower to get Wairau approved, given the objections from environmentalists and trout fishers, and no less a challenge faces Meridian with the Mokihinui scheme. The advantage of Mokihinui to the West Coast is even greater than that of Wairau to Marlborough. Mokihinui could meet all the Coast’s current and immediate future electricity needs, and provide a security of supply that is otherwise under constant threat from the elongated transmission line.
But Meridian, still smarting from the Environment Court’s rejection in early November of its giant Project Hayes wind farm project on Otago’s Lammermoor Range, might find itself in an even bigger tussle with objectors to Mokihinui than Trustpower had with Wairau. The consent hearings for Mokihinui ended seven months ago, and Meridian is still awaiting a decision – and the signs are not good. For example, the West Coast Conservation Board in November rejected an offer from Meridian of an exchange of 794 hectares of land near the river for the 225 hectares that would be flooded by the dam. A final decision on the land-swap offer will be made by the Department of Conservation, but if the locals are chary of something despite its demonstrable economic benefits, the bureaucrats in Wellington are likely to take heed. Not that that would necessarily doom the project, but it wouldn’t help. After publicly notifying the scheme in March of last year, Meridian has faced bills of $886,000 and $184,000 respectively from the West Coast Regional Council and the Buller District Council, so the rejection of another major scheme would be a further blow to the pocket of the state-owned generator, if not its policy of backing only renewable energy projects.
The consenting challenge with Mokihinui is that it’s a pristine wild river popular with white-water rafters and kayakers, and it’s also a well-regarded whitebait and trout fishery. Mountain-bikers, hunters and trampers also frequent the area though the main access track is in such a poor state that boaters and fishers alike usually have to be flown in by helicopter. The river has a catchment area of 68,000 hectares in the Glasgow, Matiri, Lyell, Allen and Radiant Ranges, and it has an average flow of 90 cubic metres a second (cumecs). It’s a relatively short river, flowing just 27 kilometres from the Mokihinui Forks where the catchments converge, through the naturally formed Lake Perrine and the precipitous Mokihinui Gorge to the sea. Except where it emerges onto the narrow coastal plain east of Seddonville, it’s typically gorge-bound, and some of the heaviest rainfalls in the country can send its flow rocketing to 1840 cumecs every couple of years, though most of the time it stays below 105 cumecs. The volatility of the Mokihinui’s flow is such that the system of managing it has yet to be decided. The Mokihinui was the site of two goldrushes, the first around the mouth of the river in 1866 and the second about 30 kilometres upstream in 1873. The second, focussing on the high-quality quartz in the Maori Creek and Jones Creek areas, saw mining continue well into the 1920s.
The main impact of dam construction, apart from the flooding of the gorge, would be in ground clearance for the site works, including buildings and workshops, aggregate crushing and concrete batching plants. As well as the dam, powerhouse, switchyard and the 29 kilometre 110kV transmission line connecting to the Inangahua-Waimangaroa line near Cedar Creek, construction would include a diversion channel. Meridian is adamant that, despite the height of the dam and the length of the lake, the scheme would have relatively little visual impact from the main vantage points. Indeed the company argues that restoration and enhancement works it would carry out when construction is complete would make the area more attractive and accessible to visitors than it is at the moment. Of the nine mostly mining-related archaeological sites identified in the area, four will be drowned by the lake, but features like the remains of the old iron bridge in the Mokihinui Gorge would be relocated, and the old packhorse track upgraded to a walking track.
Mokihinui is a much-needed facility in a remote corner of the country few people had heard of until Meridian suggested building a hydro station there. Whether need, remoteness and economic viability win out over environmental considerations is yet to be seen.
Energy NZ No.11 Summer 2009 |
With Marlborough soon to be lit up by the 70.5MW Wairau River hydro scheme, the West Coast of the South Island stands out as the last significant region in the country lacking its own electricity generation capacity.
The Coast’s electricity demand presently peaks at 65MW, and is expected to increase to 80-88MW by 2012. Mokihinui would produce 310-360 gigawatt hours of power a year for a region growing rapidly on the back of its dairying, mining and tourism industries.
The Mokihinui scheme involves the construction of an 85-metre-high dam on the river 11 kilometres from the coast and three kilometres upstream from the village of Seddonville. The resultant dam would be 300 metres wide and extend 14 kilometres inland, with an operating range of three metres. The dam would be built by the roller-compacted concrete method which is similar to the layered method of road-building, only using concrete as well as aggregates.
The area’s ecology is fairly typical of the West Coast, and there are none of the endangered bird species nesting on river flats that so concerned environmentalists with the Wairau scheme, but the loss of the white water stretch through the Mokihinui Gorge appears to be the main objection to the scheme. Whitebait and trout spawning grounds can be accessed past the proposed dam by a catch-and-carry scheme, Meridian says.
It’s ironic that, despite its famously high rainfall, the West Coast boasts not a single hydro power station, aside from a few mini-generators built to service individual mining operations. Nor do the economics of hydro power generation on the Mokihinui run into the cost questions Meridian encountered with Project Hayes in Otago, where the unit price of windpower was cited by objectors as being double the going rate.