Overview
Mon. Dec. 10.
My 29th visit to Mangahao Hydro-electric Works. Up at 4.5am – very favourable morning – put through the cows +c + left after breakfast on bike at 6.17. Reached Shannon Borough bdy [body?]. at 7.15 + Powerhouse at 7.37. The roof is now being made with the Pipe Line. left bike on roadside near Powerhouse + proceeded on foot. Left road at foot of pour at 7.59 + reached the old timber tram saddle at 8.29, getting down to Arapete cookhouse at 8.42am. The quarry in end of spur below the Tokomaru dam-site the high-level “concrete platform” mounted on high-trestles is now completed with tramline running from the concrete-mixer to the hopper at the further extremity of platform + the shute leading down to the dam foundations. 1/4pl[ate]. photo (1) of concrete platform. The great wall of the dam has had many of the big block-like sections added to it since my last visit + the highest of these now rises some 15ft. or so above the top of the flume. The wooden flume is now removed and the aperture through the dam has been boxed to hour-glass shape (with the narrow neck in centre) and concreted over. The concrete
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“blocks” of the dam-wall are not only made to over-lap each other in all directions where possible but their faces are also pitted (by means of picks wen only partially set) to give a good interlocked contact with the succeeding “blocks”. On the upper surfaces of the “blocks” this effect is obtained not by pitting but by leaving boulders projecting. The coarse rubble pipes to take up any possible seepage are also being built in as decided upon. Photos (2) 1/2pl. dam-wall under construction viewed from left bank. 1/4pl. (3) face of dam with funnel-shaped aperture. 1/2pl. (4) dam wall under construction from right bank. 1/4pl. (5) back slope of dam-wall with funnel-shaped aperture. 1/4pl. (6) surface of concrete block with projecting boulders. 1/2pl. (7) dam from top of eastern end of trench (this photo a failure owing to wind vibration.) Took measurements of one of the pitch on lead slabs, two of which are set in the joints between successively-built “blocks” of concrete forming the upstream face of the dam – evidently to prevent water-seepage:- Sheet lead 6¼ft long 1½ft. wide + 1/10 inch thick with an elongated block of pitch or bitumen, 6 inches by 3 inches stuck on lengthwise. Left Tokomaru Dam-site at 9.42 + proceeded up Tokomaru River to Tunnel No1.
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After a snack, entered tunnel at 9.58 + traversed it in 18 minutes, reaching the inlet end at 10.16. The western half of the tunnel which is level + is now concrete lined contained rather less water than last time I was through, as the eastern half which has a slight slope to the Mangahao River, is now concreted along its bottom. On emerging from the tunnel it was evident that a big flood had come down recently, the river-bed being strewn with driftwood + the large quantities of sand + gravel washed down at the same time scored with ragged channels by the receding waters. Subsequently I was told that the flood commenced about 4.am on Saturday morning last (8th [inst? [instance?]]), overtopped the Diversion Dam at 6.am + rose to its maximum about 11.am – then the waters went down a little + again rose, the last being a peculiar feature common to most big floods. On reaching No1 Gorge my first impression was that the flood had done considerable damage part of the flume having gone and the boulder apron of the upstream steek=l piling coffer dam torn up + piled over the tramline + upper end of flume. It turned out, however, that very little harm had been done, the middle part of the flume having been removed prior to the flood
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to make room for construction work. Since my last visit the bed of the gorge had been completeky cleaned of detritus + any defective portions of the bed-rock also removed. A short deep trench was then cut across the bottom + into the sides of the gorge along the line of the upstream face of the dam (after the fashion of the concrete seal in the Tokomaru Dam) and a great mass of concrete up to the height of the former normal river-level has been put in to form the upstream basal part of the dam. The strength of this concrete is 6 to 1 + a little stronger on the face. This work really marks the defeat of the river + nothing in the way of floods can now do more than cause slight delays. The flood of last weekend through only a 12ft. one, rose in No1 Gorge to an unprecedented height: this was the effect of the coffer-dams + other obstructions at the dam site. Everything was completely submerged at the height of the flood + the water rose over the planking at the western end of the small suspension bridge (i.e. [missing word] ft. above former normal river level), + to within about 2ft. of the bench on which the temporary powerhouse + other buildings stand. The damage done was remarkably slight – a good
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deal of mud and shingle was swept into the bed of the gorge not yet concreted, and one of the centrifugal pumps was wrenched sideways and the skidway smashed. To-day a gang were engaged in the work of removing the detritus which was being lifted out of the river-bottom by means of the crane + dumped downstream below the lower coffer-dam + a few other men were repairing the pump skidway. 1/2pl. photo (8) of the network of minor faults in the rock-wall of the left bank of the gorge. Mr. Johnston, the engineer, put in an appearance + gave me some points re[garding] depths of old and present channel of the Mangahao: General depth of bedrock in No1 Gorge (below former normal river-level) 18½ft. but there were two deeper places, evidently of the nature of pot-holes and filled with large boulders + some finer detritus, with a depth of 22½ft. The width of No1 Gorge at face of [crossed out word] dam is 36ft. In order to give me full details re depth, +c, of the old channel of the Mangahao, Mr. Johnston took me up to his office + introduced me to the draughtsman, a young man named Hudson, who showed me a large scale section of the old + new channels with the rock-rib between + according to this the bottom of the old channel was found to be 79ft. below the
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highest part of the high-level terrace (excavation depth for core-wall 5ft. more) + 1½ft. shallower than the average rock-bottom in No1 Gorge. This was a great surprise to me as the indications at Mangahao + in the Ohau other river-valleys of the Tararuas are in favour of a greater depth for the Pleistocene channels than the present channel or river-bed. The fact is, however, that (at the Mangahao dam-site for example) the old channel has a flatter bottom than I supposed + the steep depth below the present river-bed, suddenly flatten out to form a rather flat bottom for the old channel. Mr. Hudson also showed me the “progress” plans, sections, +c, of the other parts of the hydro-electric works + by request gave me the distances along the various gradients of the Pipe Line bed – these number 13 between the tunnel at the Sruge Chamber + the H.E. Powerhouse. One plan showed the finished appearance of the earth ridge which is to cap the concrete core-wall in the ligh-level terrace at No1 Gorge – the ridge is to be faced with rubble on the upstream side + grassed on the other slope. Instructed by Mr. Johnston, Mr. Hudson took me to dinner at the
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new staff cookhouse + here I met Bill Bird who showed me a big lot of his own photos of the works. Most of these were intensely interesting, perhaps most of all being a series of views of the flood of Saturday last which were marvellous. Returning to No1 Gorge I took: 1/4pl. photo (9) of No1 Gorge from suspension bridge showing coffer-dams +c + debris left by flood. 1/2pl. (10) of bed of Gorge from foot of downstm coffer-dam showing men removing debris deposited by flood + with the huge mass of concrete forming the frontal base of the great dam in background. 1/2pl. (11) of the same from top of same coffer dam. 1/2pl. (12) of right wall of gorge with wing walls + crane lifting debris out of gorge. 1/4pl. (13) looking into gorge showing portion not yet concreted from small zig-zag track on left bank. 1/2pl. (14) looking into gorge showing stage of work at date from hauler at top of inclined tramline on left bank. 1.4pl. (15) of Mangahao river-bed above gorge showing driftwood + channels left by recent flood. Mr. Johnston then took me up the river to where a new quarry (1/4pl. panoramic photo (16)) is being opened up on the sharp spur-end between the Mangahao River + Blackwood Creek + their junction. This has
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involved the destruction of a good deal of scenic beauty but the view up the Mangahao from this spot is still very fine indeed. The loose rubble and dirt on the quarry-face overlying the good clean rock is to be sluiced off + the pipe-line (4”) for this purpose has already been laid. Just below Blackwater junction a fine new supply of sand has been deposited as the result of last Saturdays flood + a number of men were busily engaged in heaping this up + loading into trucks which are hauled up a short length of inclined tramline, by a small hauler engine, to the main tramline running down left bank, over pier bridge + then down right bank to the bins near the concrete-mixers. 1/4pl. photo (17) of men loading trucks with the sand. 1/2pl. (18) Comprehensive view of Mangahao R[iver]. showing present state of work +c. from hillside above pier suspension bridge. Again visited No1 Gorge + took 1/2pl. (19) of one of the centrifugal pumps. Left No1 Gorge at 2.37 + returned to Arapete via Tunnel No1. Here I boarded a motor-lorry + at 3.12 went over the hill via the road to save the climb but had a painful experience, the driver being desperately reckless + fairly crashed down the torturous road
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missing sudden death by inches + fairly shook his five passengers to pieces. Left Powerhouse at 4.7 + cycled home, arriving at 5.27pm.