Problem for Engineers

Sun (Auckland), Volume III, Issue 748, 22 August 1929, Page 6

Problem for Engineers

DEALING WITH EROSION (The SUN'S Parliamentary Reporter) PARLIAMENT BLDGS., Today. TWO alternatives face the Public Works Department in the course of action to be adopted in combating the erosion that is taking place belotv the spillway weir at Arapuni. According to a statement made by the Minister of Public Works, the lion. E. A. Ransom, either the reservoir could be emptied through the diversion tunnel, thus cutting out the power supply until the remedial work is completed, or an effort could be made to continue feeding the power station from the reservoir by taking the surplus water from the reservoir through the diversion tunnel under pressure. This operation, it is said, will be attended with some risk, particularly in the event of the river flooding.

A further explanation of the posi-. tion at Arapuni was given by the Minister in a statement to the House | of Representatives yesterday afternoon. “With further reference to the statement which I gave the House yesterday concerning the position at Arapuni, the erosion which has been taking place on the new course of the Waikato River below the spillway which diverts the required water into the penstocks of the power station has been proceeding at an increasing rate lately,” said the Minister. “For the first year or so the erosion was proceeding at such a rate as indicated that many years would be available during which any of the remedial measures required could be taken, and as the load on the powerhouse increased the amount of water discharged over the spillway and therefore in a position to affect the erosion, would be eontiuually decreasing. FLOW MUST BE REDUCED “It might be well to explain here that had the contractors, Armstrong, Whitworth and Company, completed their contract, the full flow of the river would never have passed over the spillway. At times of low river practically none would have passed over, and under normal conditions somewhere about half, this amount decreasing gradually as the powerhouse was extended until, when the scheme was fully developed, there would be the whole low-water flow passing from the turbines with nothing going over the spillway or the fall, which is the scene of the erosion. “As you are aware, the contractors completed the dam but not the power-house, and as a result for 18 months the whole flow of the river was passing down the old channel and over the waterfall leading into the Waitete stream. “At the present time the river Is in flood, well over 12,000 cubic feet a second passing. In the reduction of this the one machine which has been operating since June has little effect. “Some considerable time ago the engineer-in-chief, Mr. F. W. Furkert, reported the matter, and recommended that remedial measures be taken without delay. The department communicated with London and obtained prices for the special plant which would be necessary for carrying out the work. To reduce the amount of water flowing over the crest arrangements were made to drive two more of the penstock tunnels which would be utilised in further extensions and these have made a fair amount of progress. Also, in view of the possibility of having to open the by-pass tunnel gates, arrangements were made to protect with steel gates the portion of the concrete on which the experience gained through the filling of the dam indicated that the maximum erosion was to be expected. This steel projection is now nearing completion. TWO ALTERNATIVES “It has been the intention to carry out the remedial measures at the waterfall without the necessity of

actually laying the river bed dry continuously. The very extensive fall which took place last week, culminating in 200 ft of retrogression of the waterfall on Sunday, has made it evident that the water should not be allowed to fall until the place has been secured. If the water is carried off this will enable the work to be more easily carried out, will make it less extensive, and also less expensive, but laying the river dry involves two alternatives: (1) The more simple is to open the diversion tunnel gates and empty the reservoir, thereafter allowing the river to flow through the tunnel without pressure, as it did during the time the contractors were building the dam. This involves a less amount of risk, bat on the other hand it prevents the generation of any power at the station until the works at the fall are completed and the dam filled and the water diverted as it is at present. “(2) It is possible to so regulate the flow through, the by-pass tunnel as to allow enough water to enter the head race to operate the present unit, the one which will be ready within a month, and the third which will be ready before the end of the year, and to allow the balance of the river flow (within limits) to pass through the by-pass tunnel. I say ‘within limits' because the capacity is limited, and if the river rose much above the point at which it stands at present it would not be possible to prevent the excess passing over the spillway and down the fall. “In the event of a very large flood, the result might be very destructive to the remedial measures in progress and also to the crest of the fall itself. “Commercial considerations suggest endeavouring to carry on the powerstation with the attendant risk I have indicated, and engineering caution dictates emptying the reservoir and carrying out the work as necessary under the best conditions.” POWER BOARD’S CONCERN VISIT TO ARAPUNI SUGGESTED The problem of Arapuni, and its effect upon the Auckland supply, will be discussed by the Auckland Power Board at its meeting on Monday. A suggestion will be put forward by the chairman, Mr. W. J. Holdsworth, that members visit Arapuni and discuss the question on the spot. To a Sun representative this morniqg Mr. Holdsworth said the board’s principal concern was to see that Auckland was safeguarded In regard to power supply. He said the board had gone to considerable expense at its end, and any delay would be serious. He thought members of the board would obtain a better knowledge of the situation if they examined the position for themselves.