Page:Harnessing the Waihopai.pdf/9

Page Seven unfortunately, died before its full completion, the work being finished by his trustees. The major portion of the headworks, comprising the dam, intake, supply tunnel, surge tank, and road diversion required round the lake, were included in one contract, and let to Mr. W. Williamson, of Christchurch, who subsequently took contracts for the concrete supports required for the pipe line, and also for the erection of the power house building.

The dam is a reinforced concrete structure of the arched type, of 90 feet radius, the top standing 120 feet above the bottom of the foundation and 107 feet above the old river bottom. The thickness at the base is 14 feet, amd this reduces till at a point 35 feet from the top it is 4 feet, this thickness continuing to the top. Naturally, the structure looked very slender whilst being erected, but it has already been severely tested by floods, and no fears need be entertained as to its stability. The dam is designed to divert surplus water over by-washes provided on side at a level of 15 feet below the top of the dam, and, owing to the nature of the country on the right bank, it has proved necessary to construct a massive concrete wall there to confine the water to an area having a goold rock foundation. The intake is a reinforced concrete chamber attached to the dam, and furnished along one side with four 6 feet x 4 feet gate openings situated some 20 below the by-wash level.

On the other side of the chamber is the entrance to the supply tunnel, which delivers the water through a spur to the surge chamber. This tunnel is 6ft. 6in. wide by 6ft. 6in. high and some 460 feet long, and is lined throughout with concrete of thickness varying from 3 inches te 12 inches.

The surge chamber links the tunnel to the pipe line and is a circular reinforced concrete structure, 45 feet high and 33 feet in diameter. It is provided with two openings, one for attaching the steel pipe now installed, and the other available for fitting a second pipe when the growth of the load makes this a necessity.

The pipe openings are cach controlled by a pair of 6ft. x 4ft. gates o ted by gear placed on the top of the tank. The pipe line which carries the water from the surge tank to the house, where the turbines are located, was supplied and erected by the Dunedin Engineering Company. It is wholly of steel, the diameter being 5ft. 3in. and the length 316 feet, the support being provided by reinforced concrete pedestals.

The power-house building is also of concrete, and is 43ft. 6in. long by 25 feet wide, the roof top being about 60 feet above river level. It has two floors, one for the turbines 8 feet above ordinary water level, and the second for the generators and

Mr. G. F. MacLEAN, A.A.I.E.E., Resident Engineer, Marlborough Power Board.

Mr. W. WILLIAMSON, of Christchurch, Contractor for the Headworks.