THE SIDE-TRACKED WAIKATO

Dominion, Volume 19, Issue 168, 13 April 1926, Page 10

THE SIDE-TRACKED WAIKATO

MAN’S CHALLENGE TO NATURE FOREST GHOSTS AT ARAPUNI NEW ARMSTRONG WEAPON: THE WATER-GUN On Saturday, Mr. E. J- Darby, a pioneer settler in the vicinity of the Arapuni hydro-electric station, now m course of construction by the Arm-strong-Whitworth firm under contract, made to a Dominion reporter some interesting statements, from which are summarised the following points:— The main principle of the Arapuni hydro-electric power scheme is to dam tlie Waikato River to a height that will compel it to run into its own old bed, at an elevation of about 150 feet above its present bed. The old bed is being improved by about 75 chains of channelling, making* a channel about 60 feet wide and 12 to 15 feet deep. When the old bed, thus improved, receives the waters of the dammed Waikato, it will be called the "head race.” The river will be wholly diverted into this old bed; but at a point in the old bed a "penstock weir” is being built, for the purpose of again side-tracking the river (or as much of its waters as the engineers covet) into the penstock tunnels.

Artificial Torrents Produce Power. The water that goes oyer the weir continues its course till it rejoins its old bed. But the water that enters the penstock tunnels goes back to the old bed by a much shorter and steeper route- The penstock tunnels drop 175 feet in 400 feet, and it is thus that the required power is developed. Three penstock tunnels are included in the present scheme, and the water in each will operate an electric set equal to 20,000 horse-power. Approximately, three-eighths of the river will be taken into the. penstock tunnels, generating 60,000 horse-power. lhe ultimate development would comprise five more of these tunnels and five more sets, or 160,000 horse-power all fold. Meanwhile, the bulk of the Waikato will cohtinue ter pass over the weir, and will gravitate back to the old bed by a course much less precipitous than 175 feet in 400 feet. Nevertheless, its course will be steep enough to require special provision against erosion—such provision as concreting of the floor and sides of the channel in parts, and constructing the spillway in such a manner that the water, finally falls into, a basin speciallv designed to stand ine shock. The 'basin will provide a "water cushion,” which seems to be the best sort of surface on which to receive every second a few thousand tons of falling liquid. The penstock tunnels, which take the power torrents, are circular tubes built of concrete with a steel lining. The diameter of each tube is 12 feet Standing Dead Forest and a SuperSluice.

The main dam that is to lift the Waikato back into its old bed will be a wonderful piece of engineering skill, but after all a dam .is just a dam. Even more wonderful in a way was the discovery of a dead but till standing forest covered by silt in the old bed Apparently the once live forest was killed.and partly covered by volcanic action, for a great abundance of charcoal was found in places, and the trunks, -whether' still standing or prostrate, were partly charred. Then waterborne (not necessarily river-borne) silt accumulated, and covered oyer the slain forest without upsetting its perpendicularity. There it stood for gtoodness knows how many years until Armstrong Whitworth’s navvies,. intent on channelling the old bed to fit it for further service, embedded their picks in the ancient trees. The discovery was of great geological interest, but the navvies, being neither geologists nor archaeologists, were not pleased. Nor was the ArmstrongWhitworth management pleased when it found that excavating a buried forest was likely to be much more costly than removing' mere earth Somebody with a brain-wave came along and suggested sluicing away both silt and trees by means of a super-powered hydraulic outfit. Under big pressure a 9-incb pipe combined with a 2-inch nozzle gave a jet of water that tore down everything in its course. It is estimated that the pressure was 1301 b. to the square meh In the portion of the channel obstructed by the dead forest this water-gun cleared a course at a cost of about one-twelfth of that which would have been entailed by ordinary manual methods of excavation. Biggest Diameter Tunnel South# of Line. One important work lias not been mentioned. It is the diversion tunnel. The purpose of the diversion tunnel, is to take the waters of the Waikato while the main dam is being built. The diversion tunnel will .be 720 ft. long, but its other dimensions are much more impressive than its length. . It is 28ft. bv 28ft. unlined, and 24ft. by 24ft lined. Tn this respect it is the biggest tunnel in the Southern Hemisphere. It is wide enough to take horizontally an awrage cottage, and is high enough to take vertically a two-storeyed house. It is arch-shaped.

Mr. Darby claims that, except the Niagara and the Zambesi,’no river in the world can develop, in a distance of 70 miles, as much hydro-electric power as can the Waikato between Taupo and Cambridge. He pays a tribute to the high engineering skill, labour-saving devices, and general horse-sense of the Armstrong- Whitworth staff. The honour of designing the whole undertaking belongs to " the Public Works Department and its Hydro-Elec-tric Branch. Throughout the',work the Dominion’s interests are being carefully watched by the departmental officers.