IMPOUNDED WATERS

Evening Post, Volume CVIII, Issue 26, 30 July 1924, Page 7

IMPOUNDED WATERS

THE DAMS AT MANGAHAO

MAKING MOUNTAIN LAKES

(By ."The Post's" Special Reporter.)

Strictly speaking, one \vould describe the work on the Mangahao hydro-elec-tric scheme, as Caesar did his Gaul, as being divided into three parts. First, there is the dam across the Mangahao Gorge, impounding the upper waters of that important tributary of the Manawatu in the high country, before they get very far on their long journey to tile sea via the Manawatu Gorge. Secondly, there is the dam across the Arapeti basin, connected with Mangahao by a tunnel and forming therewith a sort ol Siamese twin of a mountain lake on the same level—when the waters are imprisoned—just like two reservoirs joined by an underground pipe. The arrangement by which the waters of the Orongorongo will flow via the Semple tunnel and the new pipe lino to the upper Karori reservoir is much the same. The third and last part of the Mangahao works consists of the surge chamber on the outer face of the foothills of the Tararuas, overlooking the Manawatu plain, connected itself, as the third portion of the upland lake, with Arapeti, and consequently with Mangahao, on the same level, 1250 feet above the sea. Included in the third part are the steel penstocks which convey the water steeply down a spur of the foothills to the plain and the power-house and its turbines and dynamos to which they lead.

DIVIDED INTO THREE PARTS

This division into three parts does not follow a logical principle of hydroelectric engineering; it is simply a grouping of the work and the workers for convenience on the score of proximity. The power-house is at Mangaore, three miles from Shannon, and the surge chamber is about a thousand feet above it up a spur climbed by a cable tramway alongside the big pipes. To get to the next group at Arapeti—about a couple of miles as the water will flow —one has to travel upwards of eight miles by a narrow road winding up a valley with many steep grades and hairpin bends till the ridge is crossed at a height of over 2000 feet. Obviously Arapeti is the natural home of another settlement of workers on the job. And finally the road goes on, it would seem, another four or five, miles from Arapeti to the Mangahao Gorge, though the distance by tunnel is just over a mile. At Mangahao proper, therefore, there is a third settlement. As the man in charge of work like this, as well as the body of the workers, must be on the spot, it has followed that each section of the work is under a separate engineer of the Department. Mr. G P. Anderson is in charge at Mangaore, JUr. Jj. W. Lmdup at Arapeti, and Mr. 1. A. Johnston at Mangahao, while Mr. A. K. Ulackwood is supervising the installation of the plant in the powerhouse. The whole work, including the erection of the transmission lines, really a fourth part of the scheme, is under Mr. A. Dinnie, District Electrical Engineer of the P.W.D. It is infinitely to be regretted that the brilliant electrical engineer to whom fell the duty of carry ing out the bold and comprehensive plans laid down by, Mr. Evan Parry some years ago, the late Mr. Lawrence Birks, Chief Electrical Engineer of the Department, did not live to see the complete fulfilment at Mangahao of a masterly conception in hydro-electric engineering. ' ° HEART OP THE SYSTEM The heart of the whole* system lies in the Mangahao dam. Here, at the bottom of a gorge which must be six or seven hundred feet deep, the engineers are putting a dam across the stPeam to create a reserve of water for seasons of drought, when otherwise the- powerhouse might come to a standstill. The two wings of the dam, driven deeply into the rock on each side of the gorge, including the extra wall which had to be built to cut off the old river bed, practically doubling the length of the structure, stand with vertical abutments on each side of the stream making a sort of artificial gorge within a gorge. Th c stream itself still flows untrammelled through this gateway which is soon now about to be closed up. On the further side, where the old bed lay, the Wall of the dam will be hidden by an easy-sloping revetment covered with big boulders, cemented together. This will be a double protection against the encroachments of the river. AUTOMATIC FLOODGATES The main feature of the Mangahao dam, when it is completed, will be the automatic floodgates, which will operate when the river is in flood and rising too quickly for all the water to be used for power or stored. There will be two of these gates over sixty feet wide, so designed as to come into operation when the water rises above a certain predetermined level. The high water mark in the Mangahao reservoir will be 1257 feet above sea level and about a hundred feet above the bed of the actual stream. The top of the solid dam is 1261 feet above sea .level. The principle of the automatic floodgate is that as the pressure of the Water increases on the upper portion of the big hinged gates, the gate cants over and releases enough of the pent-up waters to restore the balance, when the door automatically, by means of a concrete counterweight, swings back into its closed position. The principle of the'syphon is ingeniously applied to steady the motion.

The existence of a bypass tunnel has been mentioned for the purpose of diverting the waters of the Mangahao while the foundations of the dam were being excavated. This tunnel going round the side of the dam will be a permanent feature of the .scheme. A ponderous steel gate has been frtted to close the tunnel by means of gear operated by rods from the bank down a shaft a hundred feet deep. Thia gate should soon be in working order. It will be used in the future, should it ever be desired to unwater the bed of the lake created by the dam to get rid of any accumulation of shingle or debris, or for any other reason. _ Similarly the mouth of the tunnel leading to the Arapeti reservoir can be closed by a gate so that there would be no need to drain that reservoir also and thus cause the power-house plant to shut down for lack of water.

MOST PICTURESQUE OF SCENES

Incidentally, it may be said, with due respect to the claims of tlyj other portions of the work at Arnpeti and Mangaore, that the scene at Mangahao is the most picturesque of all th o works in operation. The road coming over the ridge from Arapeti, past a. delightful bush settlement, where the married workers at Mangahao live with their families in sylvan simplicity, does not preU'nd to descend the full "depth of the gorge to river level, hut comer, to a stop many hundred feet up the side of this hill. Here there, is one. of the stecpr-u i-r.l..lrwava i o.ie could ovoi- wish to see. iliev yaU it i,he '"jig^iy" at Mauguh.au.

It is used for transporting the material trom road level to the works in the bed ot tho gorge. Once on a frosty morning the brake refused to work, and there was a nice little example of the forces ol gravity in a swift descent of the truck to the bottom. On the whole the visitor will prefer to go down to the dam by the_ zig-zag path of his own locomotion, it is a bit tiring going up again, but the workers at Mangahao are quite used to it now. The single men are spared the climb, because they mostly live in ■ the tented settlement on the other side of the gorge back of the big dam. lou can see the sources of Mangahao power in the mountain tops that wall out the gap in the upper end of the gorge, and from them comes the rain which makes work so uncertain here. The weather had as much as anything to do_ with the decision to worK the threeshift day, for more can be done in twenty-four hours continual work in fine weather than in any amount of broken time. It is a sight to see the high-candle power electric lights flash in of a winter's afternoon, for it gets dark early deep down in the canyon of the Mangahao. STRENUOUS AND EXCITING It must be a strenuous and exciting life at Mangahao. In the early stages there were many narrow escapes from sudden floods. Men Were caught in the tunnels and iiad to wade out with water up to their necks. The swirling torrent rose on one occasion fourteen feet in a few hours and swept away the1 work of weeks. Workers had to run for their lives. There is still a certain peril from rising waters, and wilT be until the cenj tral gap is filled high enough with solid concrete to create a space for the waters to fill. When the wall gets up to its topmost height the lake which it will create Will back up the waters nearly two miles up stream. The big trees that used to come down with the floods, roots and all, at full speed, carrying all before them, will float along peacefully enough then in captive waters. Much could be written about Mangahao alone. It reveals the most romantic phase of the whole construction work and constitutes a real triumph of- engineering over great odds. The resident engineer has been on the job since tho very start, living right alongside the great dam. He has seen it all and been through it all. There should be a story m it for him of the sort of pioneering Aew Zealand is doing in this second stage of her growth.