MANGAHAO DAM CLOSED.

Shannon News, 2 September 1924, Page 3

MANGAHAO DAM CLOSED.

THE FINAL SCENES. BY-PASS GATE TESTED. {By our Special Reporter). • The finale for which those who have been engaged in the work at the Mungah’uo undertaking have been striving for the past 4$ years is rapidly nearing accomplishment. Yesterday at 1.15 p.m. the by-pass tunnel at the

Mangahao dam—which during tiie ' past several months has taken the water of the Mangahao stream to allow of the dam in the old stream bed being constructed—was closed and the first test made of the working of Che by-pass gate under pressure of the banking-up of the waters behind the artificial block of the partly completed dam. r The level of the centre portion of the dam, which? had been left open in order to allow of flood waters escaping, has now been .raised to a height of some 70ft above the level of the old stream bed and is growing at the rate of 7ft every three working days. The top of the dam is still 30ft above this level.

At Arapeti, the great dam, is practically completed, and the surge chamber at the outlet of the No. 2 tunnel, on the Shannon slope of the range, is ready. Below in the powerhouse some small details .required adjustment, but with this exception ail was in readiness for the trial which was to prove if those who had laboured for the past 4£ years had budded well pr ill. At Mangahao at, noon when our “News” representative reached the camp, there -was an atmosphere of mild excitement. The work of filling in. the “paddock,” or area of concrete under construction at the (lam still proceeded apace. From Arapeti and from Mangahao, engineers were in telephonic communication with the “Top Camp,” at Mangahao, watching the progress of events. Very quietly everything was made ready, though still beneath it there \va s that feeling of slight tenseness, surety warranted by file momentous occasion. Should anything go. wrong—for despite the greatest care, some unexpected fault might appear—someone would be held responsible, and eagerly as the test was awaited the. result was still fraught Avith anxiety. At 1.15 a dozen men gathered at file top of the by-pass control shaft where through a 100 ft shaft the gate Avhich was to close the by-pass was suspended. Ultimately tq he controlled by electricity, for this occasion it was to be lowered by hand, a windlass operating the screw attachment toy which the gale is raised or lowered. A Half inch tliread oil file central upright rod attested that the lowering-of the seven-foot door would be more than a matter of moments, and slowly ft curled downwards under the sturdy sAving of those who worked the windlass.

Suddenly those \vliq watched the entrance of the by-pass tunnel 50yds above,, saw that file steady flow of the river, through file by-pass had been arrested. Nothing much at first but us, the door closed it swirled highei' in tiie entrance, ’covering the beams one after another, and then pouring in a sip ay stream o.ver the “crito-dam,” composed of logs and boulders, which had diverted its waters into the bypass. Doav.ll beloAV the AVater avus still Avelling out of the loAver end of the by-pass anil .a quick shift avos made to see. the river toelow. go dry,- Ten minutes passed and still . the AVater run. Was file gate going to fail under the test?' Then- came tiie oomic relief. A log was found to toe jumped. beneath the by-pass gate and preventing its being lowered to its full extent. Some manipulation removed the obstruction and then file gate was loavered.

This time there was no mistake. .The river flowing at the rate of *IOO cubic feet to the second flooded into the by-pass tunnel, blocked there, and swung outwards, flooding back over the gravel in a miniature, wave. .Swirling round from there as it found its level' it swept over the crib-dam, a loot* deep ,and banked up against the darn proper in. a steadily rising flood, which every second crept appreciably further up the solid concrete wall, arid further back tire river-bed, to swell info a, miniature lake. In half an hour the water was 15 leet deep- against the dam and stretching several hundred feet 'Up-istr-eam, was beginning to flow into the No. 1 tunnel, the gate .of which, however, prevented its further progress towards Arapeti and its ultimate objective, the power-house. The first object of the test had been achieved. The by-pass gate bad worked. Now tire problem was to find if, under the pressure of the banked-up waters, they .would rise—as later it might be necessary to raise them in flood-time with the pressure of the accumulated w.ater behind them.

At four o’clock the emptying oi the water began. The gates rose perfectly and with an escape vent of twelve times the capacity of the water in the river-, in a very short time the water was flowing once again through the by-pass, witih only the water still lying between the crib-flam and the loot of the wall to show that the first test had been achieved;

At 6 o’clock the water was again turned on and everything working perfectly, the dam was flooded to a height, sufficient to .send the water through No. 1 tunnel to the Arapeti Dam. Here since Friday last the waters of the Tokomaru stream bad been slowly backing up, until at 4 p.m. they were first beginning to find their way into No. 2 tunnel.

To the Arapeti dam only it was intended that they should be admitted that night, the "gates into No. 2 tunnel being dosed against their further progress. 'This morning it was proposed that these gates be raised and 'the water be given access through No. % tunnel into the “surge chamber.” From 9 a.m. until noon the pipes leadin e- to the power house would be slowlv filled from the surge chamberslowly and carefully to prevent alr-

pockets forming in the pipes, these being the source oi innumerable vexations and delays to the engineers if not watched .carefully. By early 'afternoon it was expected that the first water will be admitted to the turbines, and the first power—although generated only for test-purposes—will be derived from the Mangahao hydro-elec-tric power scheme. The proposal so far is that the Mangahao dam shall be flooded only sufficiently to allow of the water being lorced- through into Arapeti and so into the pipes for the purpose of generating enough power to run one turbine, this power tq> be used for testing and drying out the remaining machines. Until tiie Mangahao dam. is completed it would not he desirable to raise the water beyond a level where there would stifi be room between its level and the top of the dam to accommodate an ordinary flood—with such relief as could he given by raising the by-.pass gate. Until the dam is completed therefore and the spill-way gates erected, only sufficient water will be sent through to run one turbine. This, however, will be ample to supply the requirements of the Horowhenua Power Board’s district, and within the .space of the next few weeks it i§ confidently expected that power and light for this area will be available.