OHAKURI DIVERSION DATE APPROACHES

Press, Volume XCVII, Issue 28638, 15 July 1958, Page 11

OHAKURI DIVERSION DATE APPROACHES

Preparations for diversion of the Waikato river at the Ohakuri hydro-electric project are now well in hand and should be completed within the next few weeks, said the Minister of Works and Minister of State Electricity (Mr Watt). The doncretelined diversion tunnel is 1300 feet long and 25 feet in diameter. The contractors have finished the major part of their work, and are now grouting inside the tun-nel-forcing a thin strong mix of cement and water under pressure into all cavities, joints and cracks behind the tunnel lining. The diversion tunnel sluice gate, which will be hydraulically operated through a 120-feet deep shaft in the rock, has been manufactured at Port Chalmers during the last year and is now being assembled in the gate shaft. A hydraulic operating cylinder 35 feet long by 21 inches internal diameter has also been made at Port Chalmers and is now on the site ready to be installed. It is expected that by the time this control equipment has been installed, painted and tested, the contractors will have completed grouting and concrete clean-up within the tunnel, and will have brought excavation to a point where only a minimum thickness of rock (a “dumpling”) remains at each portal to keep out the river. Diversion procedure will be first to fill the tunnel with water to river level and to close the control gate so that when the “dumplings” are removed there will be no flow of water to carry debris into the tunnel The upstream “dumpling” will be blown up first and the remaining spoil removed to provide the river with an unobstructed channel into the tunnel: then the downstream “dumpling” will be similarly dealt with. Difficult Phase With both “dumplings” removed, the gate will be raised and part of the river will then flow through the tunnel, the remainder continuing to flow through the existing channel. Then comes the most difficult phase of the operation—construction of an upstream coffer dam to divert the whole of the river through the tunnel. Large quantities of rock material conveniently stockpiled alongside the work will be bulldozed into the river, progressively forcing more and more water to flow into the tunnel until eventually only a narrow gap with a fast-flowing current remains. Final closure will be effected by using rock gabions, many of which have already been made by wiring rock into “baskets” some 10 feet long and three feet square. 'Die whole closure operation, though requiring many days’ preparation, will be completed witran 24 hours. The Ministry of Works will arrange matters so that the last critical period coincides with a time of low generation requirements. This will be done so that the State Electricity Department can co-operate by providing a brief period of low flow in the river. Placing a downstream coffer dam, to prevent backflow of the river into its old bed, will be a relatively simple operation, carried out in still water. Finally, the pond left between the two coffer dams will be pumped out

> and excavation of the exposed river bed will begin in preparation for placing the earth dam. Gravel up to 20 feet deep will have to be removed, to an average J width of 80 feet over a length of 500 feet, so that the watertight earth dam core will have an impervious rock foundation. Spectacular Sight Ohakuri will be essentially an earth dam, with concrete intake and spillway structures. This spillway structure is designed to safely handle the maximum flood expected in the river. Flow will be controlled by two gates and will then pass into a chute 35 feet wide and 20 feet deep, discharging back into the river well downstream. Studies of scale models in the Ministry of Works hydraulics laboratory in Wellington show that the spillway in operation will be a spectacular sight. ' This station, the sixth in the Waikato chain, will have an installed capacity of 84,000 kilowatts, with provision for a fourth unit which could bring peak capacity to 112,000 kw. It will be the next hydro-electric station to be completed after Atiamuri (84,000 kw and will be followed by Waipapa (50,000 kw Ohakuri and Waipapa are both programmed to produce first electricity in 1961.