TAMING THE TORRENT

Evening Post, Volume CVI, Issue 105, 31 October 1923, Page 12

TAMING THE TORRENT

WILD HORSE FROM THE HILLS

BRIDLED IN ITS COURSE TO THE PLAIN

HOW MANGAHAO IS MADE TO SERVE MAN

(By M.N.)

'. By short-cirouiting a portion of the ■ IManawatu Eiver system. New Zealand • ■wilf ' add- to' its productive energies a ; tireless- and permanent worker, to re- . place in. part its -wasting and industri-j ; 3ljy r interruptible coal. '. A tributary of the Manawatu, the < iMangahaa, will in .large measure be J Hiverfed' from its natural' course on *he j eastern ■■side of the backbone range. ; Instead- of pursuing a circuitous route past of the range and round through. tho Manawatu Gorge to tho West Coast, ', the diverted- water will be taken, by • » damming and tunnelling process, • through ths-baelcboue direct from e;>st ; to West, and, thus'short-circuited, will ' isin.its,~jaEent^ri\ier'"oft the W,est: Coast t slder "3tit eniroute, when issuing from ; the tunnel on the, western side of the i range, the water will be made to fall *~"aboutr*'9o&—■feetv-"in"""on}trr-tc -generate ~t^24sßo~horse power- at-a- power house .. now being completed a few miles back '. of Shannon. ; These tremendous liberties taken with • Nature^ produce a tremendous result, . aSfTit? looKs':lUKe- a permanent:, result, ' ilrfltSSs should elect to shrug her ; shoulders, as she did in Japan the i other day. In which case "hUmaii calJ culations might possibly go astray. But ''■ xs%s.!s^-\&s..')>s\l}''i}e' can rebuild, and ; hb "generally- rebuilds Better. : "".ieeServoies-and tunnels. ; The Mangahao hydro-electric power ; scheme .will ; include two reservoirs (to >' which a third will .be added later), two .v tunnels, a pipe-line, a power house, and ! transmission ' lines: v Of ■ the ' reser- ; voirs, the-bigger (500 million gallons) will, be on the eastern river, Mangahao, and the smaller (500 million gallons) ; will be on the western stream, ■ Toko- ; ihafu. liear"its' confluence with' 'the ' Ara- ! peti Stream. 'The first is'known as the Mangahao Dam, the second as the Araj poti Dam. - The third reservoir (in --■__ri>_eyance) .. wilL. be. ..on the Mangahao •.^■J'fciY^tb.e, Mangahao Reservoir, - and its ■ -purpose will be "increased storage' (also •-trapping of silt, if *any)." . ; One of the two tunnels will connect ; the Mangahao Reservoir (on the . Mangahao) T \yith the ' Arapeti Eeser- !. voij;,(oh:-th,«i- Tokomaru).-:- The other ; tunnel will connect the Arapeti. Reser- ; -voir with the western side of the range, I . where ; the pipe-line will .take charge of ; the diverted wators, so as to give them .' their power-producing 900 feet drop. : That drop ends at. the power-house, . situated in the bed of the' Mangaore, a | ; small stream that ,has been diverted ; somewhat,- in order, to fit in with this- ; arrangement." ' ! The Mangahao to Arapeti tunnel is one mile long and seven feet in diameter, the Arapeti to pipe-line tunnel is. : a little over a mile and a'quarter km> • and eight feet in,;diamcter. Th«e are. now-four-main centres cf i constructive' effort'—the Mangahao ; Eeseryoir, the Arapeti Reservoir, the pipe-line, and the powerhouse. The '. two reservoir operations best lend ; the.njselvesto the...,ar.t of the panoramic ; photpgrapher.' a,r}<i the. large views .of them presented in this issi-e>nre worth careful "scrutiny. ' ; The photograph of ths Mangahao • Reservoir^construction i hows on the '. left-hand-side of the picture the portal of the Mangahao-to-Arapeti mile-long ; tunnel. .In.the other (righMiand) half - of the picture, at stream level, will'be "■"seen the portal'of the diversion tunnel, -—•which is eight feet in diameter and seven, chair.s long. , Now, what •is a diversion-tunnel ?. It is a tunnel driven' from ,the stream bed into the bank of the stream, at a point up-stream of the site, selected for the dam wall of the reservoir. - This tunnel emerges ■ again on the bank of the stream at a point down-stream, or below, the site of tho dam-wall. When the water is diverted through the diversion tunnel, the river bed between its two portals is thus left dry, and that means that the builders of the dam-wall can make their excavations and lay their concrete, etc unaffected by the water-flow. „ But a diversion tunnel alone will not ahert the flow. Below the upper portal of the ~aiveriio'n tunnel some sort of a dam'iaS to the constructed to turn Jlre, water put of- its ordinary bed and lrtW --the diversion tunnel. And this

water, and is practically at the bottom of the reservoir. It is therefore well situated to act as a scour or as a run-off for gathering silt. But how can it be operated—opened and closed—with such a huge weight of water against it? The plan adopted at Mangahao is to select a point on the surface of the riverbank and vertically above the course of the diversion tunnel. •At this point a shaft is sunk, and a reinforced steel gafeis lot down through the shaft and set in the. course of the tunnel. It is operated from on top, so powerfully that it can be lifted to allow the scouring waters to pass, and then dropped again into position. Thus the diversion tunnel, equipped, with gate, has a permanent purpose in unwatering the reservoir, if necessary, and in scouring away the silt, if there is any. The silting of a reservoir does not affect, the water service for power purposes until the silt exceeds the level of the draw-off tunnel (in this case, the Mangahao-to-Arapeti tunnel). A scour tunnel at a lower level should be able

to remove the silt long before the drawoS tunnel is affected. It is also important to prevent the accumulation of s;lt on the up-stream side of the damwall. And that consideration brings into the discussion the wing-wall that figures so prominently in the Mangahao picture. BAD FOUNDATIONS, AND WINGi WALL. The wing-wall, it will be seen, is on the right bank (looking down-stream) of theMangahao. Unlike the dam-wall (to be), the wing-wall does not cross or dam the river, but runs practically parallel with | it. The purpose of the wing-wall is not to arrest the Mangahao, but to prevent the material on the right-hand side from falling into the main channel. If this right bank had been found.to consist of rock, there would have been no serious danger of its sliding into the river. But it was found, unexpectedly, : to -consist of a loose material, being probably part of an old bed of. the ■ liver: and that discovery has immensely increased the amount of concrete wall-work required! Some of the loose stufi has been removed, and concrete has replaced it. The bal-. ance of the loose material will, it is hopedi be consolidated with the aid of the wing-wall. It should be explainedj here that both at Arapeti (that is, in the bed of the Tokomaru) and in the bed of the Manga-' hao- there has been unexpected trouble in finding rock foundations suitable to form, the base of/ a dam-wall. It is obvious, that difficulty .hi..finding foundations increases the excavating cost, also the concrete cost. At the dam sites on-the 1 Tokbmaru and the.Mangahao, before excavation reveal-

dam is shown clearly in the picture as the "temporary diversion dam," lying, immediately down-stream of. the diversion, tunnel's..portal. ',"Jhe diversion.tunnel 1 can by-pass 1200----cubic feet of water per second, but. cannot nearly take a Mangahao flood. Hence flood interruptions of the work. "~^n"#£l-&ltj:.f'6r Louring • ... . REgERVOIft. . • ' '/he purpose of the diversion tunnel £?i<i. temporal}' diversion dam, in so far as'triey allow the building of the permanent dam-wall-to proceed^ is plain enough. But has this tunnel a permanent purpose.? It has. And that purpose may-as. Weii be-described at once; though it concerns the completed, not the constructing, undertaking. Imagine- the. Mangahao -reservoir complete.' The^ater has risen high enough to flow through the Mangahao-to-Arapeti tunnel.:- That •rrieans'that the diversion tunnel, driven at the old stream-bed *evel, is deep below ths surjacs of th*

Ed hidden things,' it looked as if each stream had an ordinary -Y-shaped bed. No one would havebelieved that each of them had an old bed parallel with the present- bed. Yet'such seems to have been the case. Flanking the present Vbed is an old- V-bed which had bitten deep into the rock. The'old bed had -been, filled ,up by. water action or landslip. ,o'r",'other..cause, 1 butf*tardy excavation .revealed it, {and showed, on the flunk.of either stream, a. deep gravel trench where hard rock should have beon. Anyone'with an X-ray eye would have seen that the V-shaped" bed was really in the nature of ■■» W, or that the V, if not exactly a "\V, had, at any rate, a deep side-pocket. No one expects an engineer to have aji X-ray eye, but the opinion is held that if there had been sufficient prospectirig, moie accurate" reservoir estimates could have been made. As the: Minister of Public Works liaß admitted a failure to do sufficient pros-pect-ing of dam sties at the ou.tset, tlig

point need not be laboured. It is sufficient to say that both at Mangahao and' at Arapeti the dam-walls have prored bigger undertakings than was expected, . requiring . more . conorete and more time. It is not certain whether Arapeti dam-wall will be completed next June. It is certain that.Mangahao will not be. And it. is certain that the ! estimate of cost all round will be exceeded, perhaps doubled. : DAM-WALL MEASUREMENTS. i The wing-wair,in'the Mangahao pic- I ture has an added .interest for the read- ' er, as it • marks the -height that the ' dam-wall now being erected will at-' I tain." This dam-wall -will be 100 feet 1 high, will be 70'feet thick at the bot- < tom j; ,and_ will be 118 feet long at its s widest point. It may be mentioned here, I for comparison purposes, that the Ara- i peti dam-wall will be of the same height, i will be 66 feet thick at the bottom and i will be 430 feet long at its widest point. Mangahao dam-wall' should take about i 22,000 yards of concrete; Arapeti about i 23,000. . • i The impounded waters at Mangahao 1 will form a lake 1$ miles in length. Any 1 silting would occur at the top end first. ( This lake will be 12C0 feet aboye sea 1 level. ,< Another feature of the Mangahao pic- I ture is the jig tramway, 500 feet high, < down which all material'is lowered from 1 the road-head. I The other of the two bigger pictures i shows the_ Arapcti reservoir work. High . I in the middle stand the stone-crushers, \ ) which prepare material for concrete; The l site of the dam-Wall is on the left of the i picture. A little above it is the tram- ] way connecting with the' Arapeti poi- < tal of the Mangahao-to-Arapeti tunnel. I On the right hand side is the tramway ' leading to the Arapeti portal of the Ara- l peti-to-pipe-line tunnel. The cottages of i the Arapeti • workmen's settlement, the i site of which will be submerged by the full reservoir, are a feature of the view, i For obvious reasons, the Arapeti dam 1

will not be nearly so subject to silting as the dam on the much more powerful Mangahao River. The Tokomaru andArapeti streams.are'not main sources of power, though they are useful. Mangahao, operating through reservoirs and tunnela, is the driving force. The pipe-line giving the 900 feet drop consists of four large diameter pipes. The power house will contain five units giving a total of 24,000 horse power— three units of 6COO h.p. each, and two units of 3000 h.p. each.

which has an average efficiency of 82 per cent. Thus to get 1. horse-power on the belt more than .746 kilowatt has j to. be used. The amount of electric ! energy actually is about .91 kilowatt. | ■• POWER FACTOR, | Another electrical term commonly seen in print is "Power Factor." This is associated with the use of "alternating current," which term itself requires explanation. In the case of continuous or direct current the direction of the current remains uniform. In the case of alternating current the current starts-in one wire—say a mile long— and goes through a lamp and back through the return wire to the source of supply. Then it dies to zero and starts again in t'ie opposite direction in | tho other wire ind repeats the opera- j tion. This is called a "cycle"—two alternations—and the process is repeated 50 times per second. In alternating current the volts and amperes do not always work in exact synchronism, but in general the current measured in amperes lags behind the pressure expressed in volts. ' Where the current is used in a lamp, radiator,, iron, toaster, etc., which consist of heating resistances,, the current is in step or keeps pace with the volts, hence the power factor is called unity and volts multiplied by amperes equal watts. But in all. motors the current lags behind the volts. The average motor at full load has a power factor of .85, so that if the volts and amperes are multiplied together and divided by 1000, the result is K.V.A. But the true power in kilowatts is 85 per cent, of th*s K-V.A. result. For purposes of rough estimating, therefore, in practice it is usually assumed that a kilowatt will develop a horse-power at the belt of the motor after allowing for losses in the motor and transmission line. Current is supplied to the consumer at 400 and' 230 volts. A motor needs three wires run to it to operate ■ on the system of three-phase .supply, and all motors—