Waitaki Diverted To Aviemore Tunnel

Press, Volume CIV, Issue 30818, 2 August 1965, Page 1

Waitaki Diverted To Aviemore Tunnel

(From Our Own Reporter) OTEMATATA, August 1. The Waitaki river was blocked by a coffer dam pushed across it during the night and sent racing through the Avimore diversion tunnel.

The river, normally six times as big as the Waimakariri, was stopped and turned into the £1 million tunnel by 3.36 a.m., some four hours ahead of schedule.

To assist the damming of the river the flow from Lake Benmore was cut off, and Benmore produced no power between 10 p.m. yesterday and 9 a.m. today.

The New Zealand Electricity Department agreed to the Benmore shutdown because of the low power demand throughout th e country during that time.

The coffer dam was thrown •cross the river at Aviemore and built up to a planned height in this period. Work on the coffer dam at Aviemore went ahead at all speed during today and will proceed tonight. It was expected to reach a height at 8 a.m. tomorrow which would allow the three turbines at Benmore to be back on full load-feeding up to 240-270 megawatts into the national grid as required. The N.Z.E.D. and Ministry of Works combined to carry out the Aviemore diversion without a hitch and without power cuts. On Saturday morning, the departments obtained a special weather forecast for the whole of New Zealand. Cold weather was forecast in many places. If the forecasts had been for exceptionally severe cold, the Aviemore diversion would have been put off because Benmore power would have been needed. At Head Of Lake The Aviemore power project. to produce 120 megawatts of power in 1968 (almost enough to meet the peak demand for electricity in both Dunedin and Christchurch at present) is six miles down-

stream from Benmore, and five miles upstream from the Waitaki dam. The site is at the head of Lake Waitaki—the water backed up behind the Waitaki dam.

The Waitaki normally flows down to Aviemore at 6 m.p.h., so approximately two hours were given for the water in the riverbed below Benmore to flow past Aviemore after the Benmore had been shut down at 10 p.m. yesterday.

The head of Lake Waitaki is at the same level at Aviemore as the river coming down from Benmore. Therefore, water had to be through the Waitaki station to lower the lake level. Thus, while power was shut off at Benmore and all the water in Lake Benmore contained behind the dam, water was run through the turbines at Waitaki so they were on almost full load. The river at Aviemore dropped about five feet, and the level of Lake Waitaki was reduced by about the same figure. The river flow was reduced from 12,000 cusec to 2000. Bulldozers Begin At 11.40 p.m. yesterday a fleet of 12 bulldozers began pushing piles of clayey gravel into the river to form the upstream coffer dam at Aviemore. The piles were on the Canterbury and Otago banks of the river, and six bulldozers worked from each side. The critical period, the meeting of the moles of spoil was achieved at 3.30 a.m.—some fou. hours ahead of schedule. Once the base of the coffer dam spanned the river, bulldozers began building up. Other bulldozers worked from stockpiles 50 to 100 yards upstream to spread a blanket of silt on the gravel river bed. This will prevent seepage under the coffer dam into the Aviemore dam site proper. Below the dam site, four bulldozers worked to form a downstream coffer dam. This will prevent Lake Waitaki from backing up into the site. The downstream coffer dam was across the river at 5.5 a.m. Yesterday morning, the “dumpling” (a small mound of soil) had been removed from the outlet to the 1300 ft diversion below the site of the downstream coffer dam. The lake water backed up and flooded the floor of the diversion tunnel. Flow Through Tunnel The dumpling blocking the tunnel intake, above the upstream coffer dam, had been reduced almost to the level of the river. The dumpling was removed last night as the up-

stream coffer dam was being formed.

When the upstream coffer dam was formed, the river began to flow through the diversion tunnel. Water was flowing through the tunnel 7ft deep before the coffer dam was across the river. At 4.30 a.m. today the water was flowing through the diversion tunnel at speed and at much greater depth. .When the normal flow between Benmore and Aviemore is restored the water in the tunnel will be about 20ft deep. The tunnel, of horseshoe section, has a maximum height of 41ft and will take 30,000 cusecs of water.

During the construction of Aviemore, the level of Lake Benmore will be kept three feet below the level of the dam. This will enable any flood of more than 30,000 cusecs to be controlled at Lake Benmore. Thus, the Aviemore diversion tunnel is much smaller (and much less costly) than if it had had to be constructed to take the biggest estimated flood in the Waitaki.

The upstream coffer dam is expected to be built to a level and thickness (35ft high, base 150 ft wide, top 30ft wide) to turn 30,000 cusec of water into the diversion tunnel by August 13. Sump For Seepage The dam site proper will then be able to be excavated. The upstream and downstream coffer dams and the diversion tunnel will keep the river out of the site. Some water will seep in, but this will go into a sump below the dam foundations, and will be pumped from the sump out of the site. Already excavation work to the foot of the Otago end of the concrete dam, and for the power-house foundations at that end, has been carried out. The concrete dam will be 1100 ft long, 186 ft high, and have a content of about one million tons of cement It will be the largest concrete dam in New Zealand. From the Otago end of the concrete dam, an earth dam wing 1500 ft long, containing 1.5 million cubic yards of earth, will sweep to just below the level of the main highway (Oamaru-Kurow-Omarama). A concrete batching (mixing plant) stands high on a spur above the abutment of the concrete dam on the Canterbury side. Head towers for the flying fox cables, which will span the dam site some 200 ft above the ground, are already erected. The plant, which mixed the concrete for the Roxburgh and Benmore dams, will be ready to start pouring concrete for the Aviemore dam on Friday.