Overview
Nov. 8. Thurs.
My 28th visit to Mangahao hydro-electric Works. Up at 4.30, milked cows separated cream, +c. Maud gave me breakfast + I set out in gig at 6.30 for Mangahao. Reached Hydro-elec. Powerhouse at 8.10 + left gig at “1st house” in Mangatangi Gully at 8.20. Ascended spur to old timber tram saddle (8.53am) + arrived at Arapete cookhouse at 9.7. Three new shafts, 50ft or 60ft in depth, have been sunk, one each on the north, south + east sides of the cement stores respectively. The new quarry in the narrowed spur on right bank of Tokomaru below dam-site has now been extremely opened up, but so far the rock-rubble obtained is now of high quality as it contains too much soft decomposed rock + dirt. 1/2pl[ate]. photo (1) of quarry from service road at cookhouse. Electric locomotives with long trains of trucks were busily plying between the quarry + the big crushing plant but to-day no crushed rock was being hauled up the inclined tramline to the concrete-mixer at western end of dam-trench as concreting has ceased for the time being. Since my last visit a large number of the huge overlapping black-shaped sections of concrete have been put in on both sides of the flume carrying the Tokomaru River + the flume is now enclosed on either side. As the work has risen
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and the gradient of the steel concrete-delivery shute for the concrete-mixer has lessened so that the liquid concrete will not now flow rapidly enough, it has now become necessary to construct a high-level tramline on the top of high tall trestles on a level with the concrete-mixer and jutting out along the line of the dam trench; the delivery-shute will then be attached to a metal hopper set up at the end of this high-level tramline. 1/2pl. photo (3) of huge overlapping concrete blocks of lower part of dam-wall from tramline leading to the quarry. Met Mr. Lindup, resident engineer at Arapete + among other things he informed me as to the use of the three new shafts near the cement stores: these are to be used in connection with seepage experiments to be conducted in the very deep shaft (147ft) in the old buried river-channel lying between the Tokomaru dam-trench + the lower slopes of the spur of the Te Paki Ridge. The 147ft. shaft has been filled with water + the three new ones pumped dry: the first experiment was to introduce 14lbs of permanganate of potash into the 147ft. shaft + look for tinted seepage in the three new ones – his failed. Then an aniline dye of German manufacture was used but no results were obtained. Finally a substance named fluoresin [sic] is to be used – this is a
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chemical, in this instance prepared locally in the laboratories of Victoria College, Wgton, which consists of golden iridescent crystals, a minute quantity of which has the property of tinting a large bulk of water green. I was also told that the Tokomaru Dam is to have a width of 7ft. at its crest + that the line of the dam cuts throughout the upstream end of the buried ridge which divides the old and the more modern channels of the Tokomaru River. 1/2pl. (2) of dam-trench showing recent concrete work at base of dam, new high-level tramline from concrete-mixer +c. from top of east end of dam-trench. The wooden flume is now being taken out and the water of the Tokomaru River will now run through the bare concrete aperture till the dam is finished. 1/4pl. (4) of men removing flume. Left Arapete at 10.8 – entered Tunnel No1 at 10.15. The concrete lining of the western half of Tunnel No1 is now completed and while this work is being carved out in the eastern half, the water has been dammed back + is a foot deep necessitating wading. Mr. Lindup also told me that the floor of the western half of Tunnel No1 is level but that the eastern half has a slope of about 2ft. to the Mangahao River. Reached the inlet end of Tunnel
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No1 at 10.38am, doing the distance through in 23 minutes. On reaching No1 Gorge I found the space between the two coffer dams (which are each constructed of double lines of steel sheet piling with concrete between) had been pumped dry + the loose detritus on the bed of the gorge cleared out so that the gorge now has the appearance of a small rocky canyon – a most interesting sight. The water in the gorge was I believe about 12 feet deep with 6ft. of detritus down to the solid rock, The detrital material, the last of which is now being removed, was disposed of as follows: The men load the detritus which consists of huge boulders + finer material of all sizes, into the tub-shaped truck-bodies; these are raised by means of a new crane perched on the end of the downstream retaining wall above, to their chassis running on a tramline laid along the top of the flume; the trucks are then pushed up- or down-stream as required and the detritus dumped into the bed of the gorge beyond the coffer dams of steel piling. All the finer material is dumped downstream but the boulders are tipped into the dead water upstream where they act as a protecting apron to the upstream steel-piling coffer dam against floods. These coffer dams
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which are 150ft. apart have acted very effectively in keeping back the water but in order to make the space where the dam is to be perfectly water-tight a narrow concrete wall has to be built inside each of the steel-piling coffer dams. The downstream wall is now completed while the boxing for the upstm one is under construction at date. Two large 10 inch centrifugal pumps each with a capacity of 1500 gallons per minute have been installed to keep the space between the coffer dams free from water + these are located on the left bank at the downstream coffer-dam. Each of the pumps runs on a steep wooden skidway and are connected by steel ropes to winches located high up on the bank above in order that they can be raised out of reach of the floods when these come raging down. Several floods have occurred during the last week or two, overtopping the Diversion Dam + the coffer dams + re-filling the dam-site with water. October was a very wet month with 10 inches of rain + only 8 fine days whereas September had 14 fine days + only 8 inches of rain (“Weekly Press” p.38 Nov. 8 – 1923). An incidental effect of these floods is to prevent the further use of the punt with derrick + monkey which was used for driving the steel sheet
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piling. Apparently by the interaction of the flood waters + the powerful discharge of water from the outlet end of the By-pass Tunnel, a bar of gravel has formed across the lower end of No1 Gorge, shoaling the punt so effectively that it will not be able to be moved again. Since my last visit the section of No1 Gorge between the coffer dams has been pumped dry + the detritus removed as described above. The rocky bed + sides of the gorge are thus exposed + opened to detailed inspection, disclosing many interesting features: The most striking of these is the narrowness of the bottom of the gorge which has an acute V-shaped form at a depth of 18ft. below the former normal river-level. Another feature probably indicating extreme youth; the rock-surface itself is remarkably smooth + water-worn but as a whole presents a concourse of rounded bosses + hollows each some two or three yards across. On the right bank side some 5ft or 6ft below the old normal river level there is a sort of irregular ledge while the left bank has a steeper + more uniform slope. The smooth irregularity of the
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gorge bottom is probably evidence of the youthful stage reached by the gorge in its development: except during the most powerful floods the gorge bottom was partly covered with + protected by detritus which would tend to preserve the youthful “gnarled” contours, the actual shaping of which was probably executed only by the most powerful floods. Another feature of the rocks exposed ( which consist principally of a fine grained rather slatey greywacke, through red rock, +c, also occurs) is the presence of a large number of minor faults, bounding triangular (or rather [pricnalic?]) masses of rock and indicating the tremendous crumpling (crushes) + shearing which the strata sustained during the period of ancient folding (post-Hokonui deformation). Under erosion the fault planes have been etched into grooves, some of no small depth, and containing tightly wedged cobbles + pebbles. 1/2pl. photo (5) of gorge bed +c from top of upstream steel piling coffer dam looking downstream. 1/2pl. (6) gorge looking upstm from bed of gorge at foot of downstream coffer dam. 1/4pl. (7) similar view to last. 1/4pl. (8) two men filling truck with detritus ready to be hoisted out with crane. On this occasion, Mr. Johnston, the resident engineer, was inspecting
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the work in the gorge-bed, and was accompanied by Mr. A. Dinnie, chief engineer of the Mangahao Scheme, + by Mr. F. J. M. Kissel, electrical engineer, Mr. Johnston introduced me to these gentlemen, + we got into conversation re[garding] the faults, rock-formation, + other features of the gorge bottom. 1/4pl. photo (9) of the three engineers. The dam in No1 Gorge is to have a width at its base of 90ft., + will present a vertical face upstream and a very steep batter except at its foot which will be less steep, downstream. The slope of the downstream face has been outlined on the rocky left wall of the gorge by a line of white paint. The base of the dam will be dovetailed into the bed of the gorge by means of a few transverse trenches + on this occasion one of the men was engaged in the excavation of one of these, using a pneumatic hand drilling-machine to make the holes for blowing out the trench with explosives – 1/4.pl. photo (10) of this operation. 1/4pl. (11) of the centrifugal pumps. [Here Adkin indicates below * section to be inserted at this point and read as such] The camp on the high level terrace has been augmented by a large number of newly erected tents + the number of men on the job has evidently increased. A staff cookhouse for engineers, overseers +c, has also been erected.
*[Down in the bed of No1 Gorge I also
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took 1/2pl. photo (12) of gorge looking downstm from foot of upstm. coffer dam showing a truck load of detritus being lifted out by the crane – Mr. Dinnie + Mr. Johnston in foreground.]
The concrete seal-wall or core-wall, as it is perhaps more correctly termed, which has been built up in the deep trench in the high-level terrace across the old channel of the Mangahao has now been carried up practically to the terrace level. I also met Mr. Cooper and Mr. Tennant, the latter being the chief mechanic who sees that all the machinery on this part of the works is kept in running order. [Here Adkin indicates below ** section to be inserted at this point and read as such] At the suggestion of Mr. Johnston, Mr. Dinnie invited me to accompany him in his car, along with Mr. Kissel, back to Mangaore.
**[1/2pl. photo (13) general view of No1 Gorge from left bank showing stage of work therein at date.]
We left No1 Gorge at 1.30pm + on the way up zig-zag track the subject of the upper dam + reservoir on the Mangahao River cropped up. I pointed out that the same valley-structure as at No1 Gorge undoubtedly occurs there also, the only different being that the high-level terrace is on the other side of the present channel of the river. Mr. Kissel, however, expressed the opinion that owing to the larger size of the
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high-level terrace at the upper site (No3 Gorge), a concrete core-wall would be necessary. In enquired if they had thought of a site for the upper dam at the head of the transverse gorges, where, I stated, it was possible that there the double structure of the valley does not exist as I hold the theory that even in the Early Pleistocene, the river in this transverse part of its course occupied a rock gorge + not a more maturely-developed + comparatively widely-opened valley as then existed along the longitudinal parts of its course. Mr. Dinnie objected to a site above No3 Gorge on the grounds that the capacity of the storage reservoir would be decreased, as the valley was narrowed above the wide part which lies immediately above the No3 site. He listened with much attention, however, when I said that a reservoir of huge size would be impounded in a very much wider part of the Mangahao Valley immediately above a dam situated at the head of the transverse gorges, and he agreed with Mr. Kissel that a man ought to be sent up to inspect the locality I referred to. On arrival at Arapete we alighted + the engineers in company with Mr. Lindup inspected the concrete work in
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the Tokomaru Dam. It seemed to be that the quality of the latest “blocks” of concrete added to the dam-structure are causing the engineers some concern, the cause of the trouble apparently being the inferior and dirty nature of the rubble from the new quarry down the river. The new concrete has not set properly + was able to be picked away although it had had time to set fairly hard if the quality had been up to standard. Throughout the works the shortage of good material for concrete making has been one of the greatest difficulties encountered. We continued over the hill + I alighted at “1st house” and set out for home in gig at 2.50pm – passed powerhouse at 3.pm + arrived at Woodside at 4.45pm.