TRICKING NATURE

Evening Post, Volume CIII, Issue 53, 4 March 1922, Page 9

TRICKING NATURE

A DIVERTED RIVER

MANGAHAO LED FROM BAST TO WEST AND, INCIDENTALLY, TO A 819 DROP.

(Contributed.)

Only a few years ago there was no hydro-electrical construction service in New Zealand. To-day it is a service which, though still an affiliation of the Public Works Department, threatens to oui-grow the parent. A service, moreover, that is leaving on New Zealand's bevelopraent" an impress second to none. SPYING 03T THE WATERS. Once in the back-blocks I met two business-like looking men who were taking an extraordinary interest in lakes and rivers, a-ud who travelled from point to point, over impossible roads, in anirresistible motor-car, which Used to go backwards uphill when the angle of the benzine tank would not let it go forwards. They were young men in a hurry, though the hurry was not on the surface. At any rate, they had time to mend my push-bicycle. Acquaintance ripened, and it turned out that they were making a rapid inventory of the water-giants. They were, in fact, expert judges of strong waters, though their only brand (so far as I know) was H2O. I didn't test them on anything else.

Armed with an aneroid barometer and other things, they took out levels and distances, examined possible reservoir sites, and took observation of rapidity of. flow and volume, which sometimes necessitated swimming exercise in snaggjr rivers. Essential data were entered in a field book, to be worked out and appreciated with the aid of the midnight oil. Some power propositions were easily turned down; others were treated with respect and held for deliberation. These pioneers were not spying out the land. They were spying out the water, and what it might do for the land. They belonged to the intelligence staff of the army of civil progress. < In just such manner, I imagine, some piorieeil of the hydro-electrical service— a scientific John the Baptist .in the wilderness—visited that part of the Tararua backbone range where springs the, Mangahao tributary of the Manawatu. Probably he had noted beforehand the peculiar geographical and geological feat performed by the Manawatu, which drains a huge area of land east of the backbone. range, and instead of carrying;, this eastern water to the less battlemented East Coast, makes a direct assault upon the Tamparts and citadel of the backbon.B range and carries the flood —through the Manawatu. Gorge—to an outflow on the West Coast. Drainage of the eastern country not to its own coast, but to the other seaboard, through the only real gap in a mountain range that runs from the East Cape almost to Cook Strait, is a fact of great, geological and economic _ importance. Economic, because- it provides about- the only avenue for the road and railway that are, and for the east and west power lines that are to be.

CONDUCT OF THE MANAWATU.

: As to the geological aspect, Dr. Marshall, Professor of Geology and Minerology, Otago University, has pointed out in "Geology of New Zealand" that though, as a rule, "the direction of the flow of streams is the result of the direction of the steepest initial slope in the country over which they flow," there are numerous instances'in which "a river seems to flow in opposition to the general slope of the cotratry, or, rather, instances in which rivers leave level country or mature valleys and enter tortuous gorges through mountain ranges. The Manawatu river leaves the flat country at Woodvilleto enter the deep gorge between the Tararua and Ruahine mountains." Dr. Marshall cites similar conduct of the Waikato and Mataura rivers, and adds :—

"In each of these cases the gorge portion of the coarse must lie regarded as a new valley that has been cut by the activity of some stream after the main valleys had been formed. The river flowing through the new valley had a shorter course to the sea than the older rivers. This shorter course, to the sea gave it greater velocity and hence a greater'- erosive power. It was therefore able to deepen its yalleya and extend its head-waters backwards, thus beheading the valleys of the main streams. A tributary of the Nokomai took the waters of the Mataura. A tributary of the Pohangina committed piracy on the waters of the Manawatu, -which must have flowed out to the east coast previously, but 1 in this case elevation of the Puketo.' Hills, to the east possibly slackened the flow of the Manawatu in its upper- reaches." ANOTHER PIEACY.

Let -us |jo back to our pioneer of hydro-electricity. As "the Pohangina committed piracy on the waters of the Manawatu," he decided to commit piracy on the waters of the Mangahao tributary of the Manawatu. He decided the dam at Mangahao, and divert the water by tunnel through -the backbone xange to the western side, thus bringing the upper Mangahao water from east to west far more directly and rapidly than the same transference is'accomplished by Nature. The water of the upper Mangahao will now take a short' cut to the lower Manawatu instead of circumscribing a lone course round ']by Woodville. To take this liberty .with. Nature was not, however, the pioneer's, purpose. His purpose in diverting the water by tunnel was to bring it to a place (inland of Shannon) where it can fall 900 feet and give him at least 24,000 horse power. For this 24,000 horse the Mangahao is being short-cir-cuited.

The Mangahao dam, the excavation of which, has now been begun, is the lower of two. A higher one, to give increased storage, will follow. From the loivor dam a tunnel {partly driven) penetrates the backbone to the Tokomaru Valley, where another dam (making three in all) is now being started. This dam will receive ■ a certain amount of fresh power from the Tokomaru stream, but it is in the main a storage dam fo» Mangahao, in the sense that the great bulk of the power comes from the Mangahao and only a small pi'oportion from Tokomaru. Another tunnel through a parallel ridge will bring the water to the surge chamber, from which it will make its 900 feet fall per pipe line to the power house, the concrete foundations of which have just been started. THE WORK SECTIONISED. The scheme has been described from the main water-supply outwards to the power-house, but the visitor Bees it in reverse order. Ho sees, as ho crosses the plain from Shannon towards th£ mountains, tha outlet point of ' the partly-driven tunnel that will connect the Tokomaru (Arapete) dam with the surge chamber, and he sees the line that the pipes will follow dov/n the hill. (The lift machinery to take up the hill material for the tunnel outlet and surge chamber, and to take up the hill material for the pipe-line foundations, is installed and working. The excavation for the surge chamber is well under

•way.) He also sees the- gullies (along tho line of the tunnel) at which adits will be driven so that tho tunnel can be worked on from extra faces. The engineer directing this section of. the work is Mr. G. P. Anderson.

The visitor passes the power-hoUse site as he motors up the steep mountain road.. Then he crosses the divide into tho Tokomaru Valloy, and soon arrives at Arapetc, tho largest village of workers, situated between the two tunnels and on the site of tho Tokomaru dam. Arapete is building for its own abolition, as the vising waters will occupy its site. From Arapete the' two tunnels are being worked on (except at stop-work times) in opposite directions, 'and in tho spoil they yield. is a big proportion of blue stone suitable for con- I crete. A huge heap of this stone has been saved, for the foundations and walls of the clam, which will presently be in hand. Crushing plant is well forward; so is. the excavation at the dam site. The. engineer in charge of this section of the work is Mr. W.R. Miller; MANGAHAO: ITS VERTICAL APPROACH. *

Another motor ride over the main ridge, and then from the road-end one looks down, from a great height, to the deep-set bed of the Mangahao itself, now a comparatively small stream of bluish water, but a giant in potentiality. Down the steep face underfoot has been built an almost perpendicular tramway of the "jig" order. For steepness it is in a class by itself, along with Mr. Norman Campbell's timber tramway at Reikorangi, and a new tram on tho Upper Akatarawa lately constructed by Messrs. W. T. Strand, G. Fothergill, and W. T. Somerville, as part of a new sawmilling venture. . Mr. P. A. Johnston (who is engineer in direct charge of this Mangahao end of the work, and in general charge of the whole during the absence of the District Engineer, Mr. A. Dinnie) states that, the "jig" has worked well, and has not been unduly costly in ropes or in any other item of working expense. The principle of a "jig" is that gravitation supplies the lighting power, and the only labour expense seems to be a. brakesman, who controls a drum-like brake in order to prevent the down-load having all its own way. The brake and the brakesman must be effective, because a seven-ton boiler came down this almost perpendicular tramway, and is on duty now at the Mangahao dam site. ■

A zig-zag, footpath connects the end of the road with the riverside below. The great trench in which the dam wall is to be founded has been opened out, and a by-pass tunnel is being driven. The outline of this grand barrage of the primary power is now plainly visible, and in anticipation one sees a.great lake, backing .up the narrow valley '(almost a ravine) to a length of ; a mile and ahalf, and rising to the level of the new bridge that is now under construction (the old one being below the line of submergence). The harnessing of Mangahao is proceeding on the scale of grandeur that the wonderful operation involves. The sight is one to gaze on and admire. HONOUR TO THE SERVICE. Hats off to the men who pioneer and plan these works, and hats off to the men who come after and construct them. A constructing engineer is much more than a dealer in dry science. He must also cdpe with the incalculable , human factor. To be a manager of men at any 'tim© is not easy. '-In these-.times it is worse. The battle at Mangahao between man and nature, and sometimes between man and man, is ho email battle. But it is going to be won. Let this article-conclude with the very pertinent statement of the secretary of the Central Progress' League .that, out of 13,068 feet of tunnelling Ufhe donej 4562 feet is finished, and ' there is no reason to doubt the Department's ability to have the power ready by the estimated time (December, 1923)-^-provided-: of course, that labour does not leave the State employer in the lurch. If workers are available, eight tunnel forces will be working instead of, as hitherto, six. Financial and industrial troubles riotwithstanding, the hydro-electrical service bids fair to attain its goal—a- power supply created with no lost time and at no excessive capital 1 cost, and therefore capable of rendering New Zealand an. economic service unweighted with that bugbear of hydro-electrical engineers—a top-heavy capitalisation. The best wishes of the New Zealazid public should go forth to the men on whom 'devolves this heavy responsibility.