ROXBURGH GORGE POWER SCHEME

Press, Volume LXXXV, Issue 25859, 19 July 1949, Page 6

ROXBURGH GORGE POWER SCHEME

Cut To Divert Molyneux River MORE ELECTRICITY FROM NEW SITE OF DAM To divert the mighty Molyneux river while the massive dam and power station are being built, the diversion cut, on ■which work was begun a fortnight ago, is itself a major job. It is a key job. A rigorous programme for the completion of the cut has been set. The time for the cutting of the big race is an official secret. The target has been set. Whether the Public Works Department hits a bull’s eye remains to be seen. After years of lying in the project files and two years of investigation on the site by surveyors anti engineers, the Roxburgh Gorge hydro-electric development scheme is now under way. The scheme is one of first magnitude as an engineering job and one of first urgency for the electric power supply of the future of the South Island. Until the Roxburgh Gorge station is operating, the South Island has an uncomfortable electricity outlook, for between now and 1953 the only new stations which will feed into the South Island system are Tekapo and two stations at Waipori. The best information obtained by the State Hydro-elec-tric Department is that Roxburgh Gorge—it was originally named Coal Creek—is such a big job that it cannot be built in under six years and a half. So even with a perfect run the first 60,000 k.w. unit to be installed would not be operating until the summer of 1956. Some engineers predict that it will not be till about 1960 that the works are completed.

As it .is built and when it is completed. the Roxburgh Gorge dam will dominate the landscape. Its site is at the last bluff of the rocky and long Alexandra-Roxburgh gorge, and the dam will be in full view of travellers for four miles on the highway from Roxburgh to Coal Creek. The dam will be only a quarter of a mile, or so away from' the highway as it leaves the flat to run into the hills on the winding trip to Alexandra. 23 miles away. No other power station in New Zealand will be so accessible to a major highway. The Waitaki power station is on the Kurow-Omarama road, but it does not carry the traffic that the Otago Central highway carries.

The immensity of the future dam is difficult to visualise. Pinned into the rocky hillsides, it will have a length of nearly a quarter of a mile —from Colombo street to Madras street. At its base, deep down in the rock, the dam will be 200 feet wide —two-thirds of the distance between Worcester and Gloucester streets. From its base to the parapet the dam will be nearly as high as the Christchurch Cathedral spire. Taking the pavement in the Square as the average level of the river, the dam will tower to a height about 30 feet above the balconies of the Cathedral spire. Penstocks will be built through the great dam to the power-house, which will be placed as close as possible to the downstream base of the dam. The projected power-house has a length of 500 feet, with a width of 100 feet — more than a third bigger in area than King Edward Ban-acks. Who is to Build Dam? The dam, however, is still in the future. The problems of who is to build it, and when, have not -been solved. The Government has announced that tenders for the construction may be called overseas, but the dollar situation complexities appear to rule out any possible American contractor bringing his equipment and manpower for a job which will last several years and cost several millions in sterling, let alone dollars. Arapuni, the biggest station now operating in New Zealand, was built by an English engineering firm. 'At least one New Zealand contractor is contemplating making a tender when tenders are called. The organisation and capital needed would set a record for a New Zealand enterprise. After 26 months of survey and exploratory work, the Government decided on. the site for the Roxburgh Gorge station at the very end of the gorge and in Tamblyn’s orchard. The other site was a mile and a half upstream. in the Pleasant Valley area. The schist rock upstream was tested and the rock under the river was proved to be stable. Tunnelling downstream has not given quite as good results, but the engineers are satisfied beyond doubt of the stability of the country into which the dam will be built to hold back tremendous hydraulic pressure of a lake in the gorge running back to beyond Alexandra. Borings and tunnelling already carried out suggest that a large amount of grouting will have to be done to fill the fissures in the rock.' This engineering problem is already appreciated. More Storage and Power The main point determining the site of the dam was that, by building it at the extreme limit of the gorge, the water storage would be increased by one-sixth and the head of water five feet greater provided at the lower level would increase the maximum capacity of the station by one-fifth. Originally planned to produce 320,000 k.w., Roxburgh Gorge station will have a capacity of about 380,000 k.w., more than double the effective plant capacity now feeding into the South Island main power system. The capacity to-day is 180,300 k.w., made up of 168.560 k.w. from the hydro stations at Coleridge, Arnold. Highbank, Monowai, Waitaki, and Waipori, and 11.760 k.w. from the t fuel stations of Invercargill and Dunedin City, the Christchurch Tramway Board, the Kaiapoi Woollen Company, the Kaitangata Coal Company, and the Diesel plant at Dobson. The orchard site also offered an easier spillway construction, and a flat situation for the future 220 k.v. outdoor station, against extensive excavations in Pleasant Valley. Another important factor was that the transport of tens of thousands of tons of material for the dam and power-house is reduced by a mile and a half; a lesser factor, but still important, was that the conveyance of workers will be reduced by three miles each day. Work has begun in earnest, although not spectacularly. For the last fortnight, a fleet of eight bulldozers hauling carry-alls have been whirring round the 40 acres of terrace land which was formerly Tamblyn’s orchard, from which came some of the most noted Otago Central apricots. Most of the trees were uprooted by bulldozers and trucked away before the bulldozers hitched on the carryalls. From 3.30 a.m. until 10.30 p.m. for’ six days a week, they have been moving in a procession in circles, biting into the rich orchard loam first and then into the silt and gravel. Blasting of Rock From 10 to 40 feet of overburden have to be lifted by the bulldozers from the site of the diversion race before the solid rock is bared. In the severely frosty weather for which the district is famed, the work for the drivers of the bulldozers is unpleasant. When they have a patch of rainy weather, too. the work is also arduous. The orchard becomes a mud pool; but the work goes on in spite of the heaviest frosts and drenching rain. Altogether 250.000 cubic yards of earth and shingle will be shifted by the bulldozers before they reach the rock. As the bulldozers make their ceaseless roundabout of the orchard, the 12-yard carry-alls scoop up the material which is taken to stockpiles, for use later in building the two coffer dams.

Once the rock lies bare, the first of many spectacular and dangerous works will begin. The big job will be the hewing out of the rock of the diversion

cut with a maximum depth of 100 feet, varying in width from 80 to 100 feet, and more than a quarter of a mile long. Where the river will be diverted the terrace is 60 feet above the normal level of the Clutha, and the cut there will be taken down to 20 feet below the river level. The rock will be bored in sections, the bores loaded and fired. The layers blasted out will be 15 feet deep. The carry-alls will have had their day at Roxburgh Gorge when the rock blasting begins. The rock spoil —400,000 yards of it —will be lifted out by mechanical scoops, now on their way to New Zealand. * > The rocky barrier at the inlet end of the cut will be left until the day of the diversion when engineers will blast it away with one shot. The barrier at the bottom end. a short distance below the site of the powerhouse, will also be removed quickly at the same time. The next stage will be the building of the earth coffer dams in the old bed of the river. The lower coffer dam will rise 70 feet above the bed of the nver and will have a length on the crest of 350 feet. The upper coffer dam will have a length of 600 feet and will be 95 feet high. , • When the dams are built, the pumping out of the dam site will begin. How long that will take is a question which cannot be answered. Not until the area is reasonably dry can the dam excavation work begin. 1,000,060 Yards of Concrete For the dam, the powerhouse and the spillway, for which the lower section of the diversion cut will be used, the immense quantity of 1,000,000 yards of concrete will be used. Fortunately for the Public Works Department, the aggregate is abundantly available less than a mile and a half from the dam site. From a river terrace below the the batching plant, which will take -he Dominion record for size. - . The building up of the staff is now possible. Up to the present, the accommodation available on the right bank of the river, just below TarnoIvn’s orchard has been for 140 men. The camp has been enlarged to take another 250 men and it will be in use this week. A big dining room and ablution block has been constructed. Each hut has a fireplace, which is needed in the chilly Otago Central winter. Much work remains to oe done to make the camp an all-wea-ther camp. The construction work left the roads muddy last week. Because of the long duration of the job and the all-round excellence of the climate—hot in summer and invigorating in winter for those who keep moving at work —-Roxburgh Gorge is unlikely to experience any labour shortage. So far, there have been more applicants than jobs. When the dam construction is at its peak which cannot be expected for two to three years yet—the workers will probably total 1500. . Many will be accommodated m the permanent village on the eastern bank of the river, to which access from the present camp is given by a new type of bridge, a Bailey suspension bridge. Authority for the building of the village has not been given yet Dy the Government

Transport of Materials Most materials to be used at the building of the mammoth power station will be railed to Roxburgh station; the line, which was opened in April, 1928, is about a mile to the east of the town and on the opposite bank of the river. From there it will be taken five miles by road. Major road construction to improve and widen the road from the station to the Roxburgh bridge will have to be done. The county road, merely a farmers’ access road, runs towards the gorge for three miles and a half and it is now being brought up to highway standard to take heavy traffic. The remaining section of the highway to the works will be left until the building of the permanent village is decided on. Machinery and plant too big to be taken through tunnels between Dunedin and Roxburgh will be shipped to Bluff and railed to Waipahi and then taken by lorry or transporter through to Rae s Junction and then along the main highway to Roxburgh. The steep and tortuous Devil’s Elbow on me Waipahi side of Rae’s Junction, is to be remodelled and some work will also have to be done on the tricky three miles, of hills past Rae’s Junction.

Storage Dam at Hawea The raising of the level of Lake Hawea by between 60 and 70 feet to provide storage for the June-October period at Roxburgh Gorge is a work which will be completed simultaneously with the beginning of power generation. No big engineering difficulties are expected to be met, the present intentions being to build an earth dam similar to that at Pukaki at the outlet of Hawea. No final decision has been made to dam Lake Wanaka.

Lakes Hawea, Wanaka, and Wakatipu, embracing 235 square miles, feed the Molyneux river, which has the greatest discharge of any river in New Zealand. In the winter, its normal flow is 10.000 cusecs and in the summer between 20.000 and 25.000 cusecs. In floods, the river, fast flowing in the gorge, is fearful. Last November a flood of 70.000 feet was recorded: the record was 100,000 feet in October. 1878.

Two graduates of the Canterbury College School of Engineering are in charge of the big Roxburgh Gorge scheme. The engineer in charge is Mr G. J. Halle well. who was formerly on the East Coast railway construction and the Lake Waikaremoana power schemes. Second in charge is Mr A. S. L. Fooks. whose home was at Ashburton. Mr Fooks formerly worked at Omakau and the Lindis Pass for the Public Works Department.