ARAPUNI DAM

Northern Advocate, 22 August 1929, Page 7

ARAPUNI DAM

EROSION TAKING PLACE | j TEMPORARY STOPPAGE POSSIBLE. POSITION GRAVE. (Special to “Northern Advocate.’’) WELLINGTON, This Bar. Although official statements arc cautiously worded, there is good reason to -Relieve that the position at Arapuni is so grave that the ■■Government may be coin polled to proceed almost immediately' with the second section of the scheme. That section is not scheduled to be carried into effect, for several years. It will entail a large expenditure, running up to hundreds of thousands of pounds. The gravity of the situation is such that Arapuni may have to be closed down for several months, so. that urgent work may bo undertaken to save the loss of a great portion of the existing works. At any mte the position is so serious that the mind of the Govcrmeut 1 is seriously perturbed by reason of the 1 alarmingly rapid erosion of the riverbed below the spillway, and it-may bo said that before the year is out an important .pronouncement. regarding, further extensive work at Arapuni will be made. MINISTER \S STATEMENT. Following on the statement which the Minister of‘Public Works, the Hdn; E, A. Ransom, made in the ‘House on Tuesday, on being interviewed yester-' day, ‘the Minister explained that the position —-though it might 'tie serious from ;the point of view of continuity of power supply 'from the recently opened power-house at Arapuni—.presented no danger to the lives or property of the. hihabitants of the valley -of the Waikato below the works.

“The trouble which is threatening,” said the Minister, “is not in connection with the powerhouse, nor the dam, hut in connection with what is taking' place at ,a. point on .the river-some ,distance below the spillway. To explain the position properly, it was -necessary-; to enter into a little .explanation in, order to .refresh -readers’ minds. The, generation of power at Arapuni wa-s made possible by the construction of .a. high dam, which raises the waterfrom the ordinary channel,of .the river, into the old river channel, which was abandoned. After running down the old channel for a distance of about three-quarters of a mile, a weir, known as Spillway Weir,-cheeks the water and assists in (turning dt 'into tpipedined tunnels, -which carry it through the hill -and ‘back ■•into the powerhouse, which .’Stands on ‘the banks of the rc-‘ cently diverted river. ' . The surplus water-goes over‘the spillway, continues' another quarter mile or so down -the old channel, d,hen falls over at the sitb of the old waterfall,an Waited Stream, and -.thence back in to- the -Waikato. As the -output of rthe power' station isbased approximately *on the minimum flow, it follow-s that for the greater part of the year some watyor w'ould not, bo utilised and must escape over Spill-, way Weir and dow r u the Old waterfall. Had the contractors for the powerhouse completed their work according' to programme ’between .11000 and 4000 cubic ifoet ,per second, -according to load, would have passed .through the machines, and the -surplus flow only would have ’gone down' the channel.' The , usual low underflow in the. river is approximately 0000 .ousees, sometimes dropping to 300 Q, and the mean flow round about 8000.”

“Owing to the* dam'and conversion works being completed, and the powerhouse construction dicing abandoned by the contractors, the-result-was that the' whole flow of the river has -been far over a year and a half passing down the old channel over the previously mentioned old waterfall site. At the present time the flow is well over 12,000 cubic feet per second, and the amount of water used by the one machine now in operation has very little effect in reducing this. For the first few months after the water was turned over no effect was produced on the rocks over which the water fell, the erosion being confined to a large quantity of sand lying in Waiteti Volley, which was scoured out in the course of the first three or four days after diversion. However, towards the middle of lt)28 a fall of rock took place at, the crest —nothing alarming —and from time to time since then other falls have taken place, the overage rate

of retrogression

indicating that it

would bo years before any chance of undermining the spillway need be apprehended. dm the meantime, with an increasing load on the power station, the amount of water escaping over the spillway would be progressively less and the rate of retrogression consequently slower, affording plenty of time for protective measures, which some day had to be faeed. Unfortunately, owing to the river having for many months been consistently high, the rate of erosion has accelerated, so that the question of repairs is now more urgent. On Sunday, August 18, a considerable fall from the top of the waterfall took place, making the question of repairs now much more urgent.

PliOC h’KSS OF PPOTECTIOX WOP Tv

“The Department has been makingpreparations for some time, and has works on hand to protect the concrete near the .main diversion tunnel .gates, '[’his is almost completed. It has also commenced the excavation of two of future pipe tunnels between the headrace and powerhouse. The proposal is

to use these tunnels, which will be part of ultimate development, to- divert a considerable portion of surplus water away from the channel where erosion is Taking place. This -work will, however, take about 12 months to complete. ■ln addition, preparations- are being made for concrete protection work at the actual waterfall itself to deal with the position .which- has .arisen (I), either sufficient water might be, releas-, ed'lrom the by-pass tunnel to prevent .water escaping over, the spillway, and still keep the plant operating —not only the unit which is now delivering power, but the second unit, which will be 'ready within a month, and a third one, which will be ready before the end of the year. This-proposal, even with the •assistance of the two extra tunnels, however, .would not cope with more serious floods than the present, which might do very considerable damage, and therefore cause additional expense at the site -of the works securing the waterfall. This is a risk which, perhaps, it might be justifiable toj take. There is, however, another risk connected -with this continuous operation of the by-pass tunnel under a full head. The valves controlling the tunnel are of exceptionally large size, th-isi having booh-necessary during construction of the similar size have over been operated under the head existing at Arapuui. The-gates were originally intended for closing the tunnel on completion of the idam and weir, and they carried out this work with complete -satisfaction. But it is a very different tiling to pass a continually decreasing amount of water as the head increases (which were the conditions at the end of ,1927) from operating these gates at their maximum capacity under their maximum head, or (2) as -an alternative measure it may be necessary to open the diversion tunnel gates to an extent sufficient to completely drain' the lake, which is a .contingency which wa?, considered when they were originally installed. Work on the overflow channel could then be carried out under the. best conditions without risk of possible interruptions and damage by high water periods during construction. This would, however, necessitate closing down the power plant for a period of possibly twelve months. This extreme measure must be considered, though it is -hoped that it may not be necessary. Which of these two courses will bo adopted is still subject of Consideration. Should it become necessary to divert the whole river back into its old course electrical consumers ’now supplied will be given as much notice as ‘possible, and by bringing into operation all 'the plants which were in operation prior to June, when the first unir at Arapuui commenced to deliver power, it will be possible to carry the load, but it may be 'necessary to impose restrictions against, increase of the load.