Forks Stream hydro scheme

Press, 15 June 1979, Page 13

Forks Stream hydro scheme

By

OLIVER RIDDELL

As if there were not enough new hydro-elec-tricity schemes springing up in the Mackenzie country, another is proposed. The South Canterbury Electric Power Board has released a preliminary descriptive report on a development for Forks Stream at Tekapo. The initial response to this has been quite favourable. The consulting engineers, on behalf of the power board, are now seeking advice or comment before preparing their final report. The Forks Stream rises in the Grammick Range and runs in a generally southerly direction below Joseph Ridge for 38 km before joining the Tekapo River about 1 km south of Lake Tekapo. It has a total catchment of 130 square km in high tussock country, with a substantial flow except in winter. On behalf of the Ministry of Energy, the Ministry of Works and Development has been building large-scale hydro-electric stations in the Waitaki Valley since the 19502. In 1971-72, it drew up preliminary plans for the diversion of some of the waters from Forks Stream to Lake Tekapo by a 4300 m canal with discharge by penstocks into the lake. Its purpose was solely to divert water into the lake without using its peak generating potential. Since then the ministry and the South Canterbury board have discussed the possibility of generating power in a small station at Lake Tekapo. This would utilise the

available head of water and still enable the diversion of Forks Stream water into Lake Tekapo. The scheme would draw water from Forks Stream, utilising 60 metres of head available before reaching normal lake level. The installed capacity would be about 2.5-3 megawatts. The scheme is regarded as essentially “run-of-the-river” because of the variation in flow and lack of storage. In winter the entire flow would be utilised and in summer, with the increased flow, the scheme would pass a maximum of 8.5 cumecs (cubic metres of water per second).

The scheme would involve a diversion weir half a kilometre north of Tekapo military camp, a 4500-metre open canal 12 metres wide at the top, a 4 4 0-metre penstock about one metre in diameter between the end ot the canal and the powerhouse, a 440-metre overflow pipe alongside the penstock, a small powerhouse building near the ice-skat-ing pond, and two km of single-pole kV transmission line from the outdoor station to the Electricity Department switchyard at the Tekapo powerhouse.

Environmental aspects have not yet been considered in detail. But it is not expected to have much visual impact, pro-

vided care is taken in sit« ing the transmission lines. In view of the tremendous modification of the Mackenzie basin by other bigger schemes, this one should not pose major problems.

There are perhaps only two area of real concern — the' effect of the removal of Forks Stream waters on the Tekapo River spawning and wildlife areas, and the effect of this diversion on other artificial habitat proposals being considered.

Forks Stream is now the major water source for the Tekapo River after other water is used through the upper Waitaki hydro scheme. The Ministry of Agriculture and Fisheries considers this water is important to fish making for spawning water in Maryburn. Greys Stream and Irishman Creek.

This is an important area for Lake Benmore, and will become even more important if the Ahuriri River further south is harnessed for electricity. (The reverse could easily be argued for keeping the Ahuriri from development.) The waters of Forks Stream have been declared “waters of national importance” to allow State hydro development. ■Whether this local body proposal is part of that or whether it will have to apply for water rights is not being debated by the Government departments. This will be the crux of any argument for residual flow or compensating waters.