HYDRO-ELECTRIC SCHEME.

Taranaki Daily News, 15 November 1921, Page 5

HYDRO-ELECTRIC SCHEME.

INCREASING SIZE OF FOREBAY, DECISION OF THE COUNCIL. ENGINEERING ASPECTS. Another important advance in the New Plymouth hydro-electric scheme was made by the Borough Council last night, when it was decided to authorise the construction of the forebay on the larger scale. The council had previously decided to continue the tunnel at the enlarged size throughout its length, and the consulting engineers wrote asking for a decision in regard to the forebay. In the course of a lengthy letter the engineers stated: “From the report of the proceedings of your council, appearing in the Taranaki Daily News of November 8 we gather that Mr. Kassel’s report was forwarded for our consideration, because some councillors were under the impression that it contained advice re the forebay, which conflicted with advice which we had given to your council when the question cropped up at a meeting held on Thursday, August 4, at which we were present by request. To save time, therefore, we will review the advice then given. In the first place we may state that, although the one large forebay is much the better proposition, from an engineering point of view, there is no insuperable engineering difficulty in constructing one forebay now and another one later, if your council prefer this arrangement, provided certain precautions are taken to deal with any surge between the two chambers. When the station has reached the ultimate capacity for which provision has been made by the enlargement of the tunnel, however, the ultimate cost of the two forebays will be approximately £3OOO more than the cost of the one large forebay. Further, owing to the fact that provision has to be made immediately for the connections to the second forebay, and for providing against the possibility of surge, the immediate .saving over the cost of the larger size is approximately £lOOO only. That is to say, assuming the price of materials and labor remains the same, if one chamber is constructed now, and another one later on, interest on £lOOO is saved until the second forebay is constructed, but from then on, interest on £3OOO is incurred as compared with the construction of the one large forebay from the commencement. “It must be clearly understood that these figures are approximate, and are sufficiently accurate only for the purpose of the above comparison. As to which of the above is the cheaper proposition depends upon the time that elapses before the second forebay is required, the rate of interest at the time, and the price of materials and labor. Assuming that there is a market for the total output in the near future, necessitating the construction of the full forebay capacity, then of the two schemes the one large forebay would prove the cheaper. For this reason, and also because this is much the better from an engineering point of view, we advocated the larger forebay at the meeting above referred to. “We desire further to refer to the following sentence regarding No. 2 tunnel and occurring in the newspaper report before mentioned: ’A different aspect had been placed on the proposal as a result of the visit of the Government engineer. The report of this officer stated that the tunnel as at present constructed would later on mean a loss of 48 horse-power, and if this had been known there never would have been any question of the enlargement. ‘ln justice to the Council, therefore, it must be said that until Mr. Kissel’s report was received they did not know of the loss.’ “At the meeting of your council held on August 4 this question was asked, and both the borough electrical engineer and ourselves stated in reply that the increased size would mean a decrease in friction loss. We further gave, in £ SD, the average saving which' would be effected by this decrease in loss. We did not give it in horse-power, nor did we stress the point, as we are of the opinion that this is not the determining factor upon which the decision to enlarge the tunnel rests.” After the letter had been read the Mayor (Mr. F. E. Wilson) said he was in favor of the forebay built now the size it was ultimately to be—a complete job should be made of it now. They had had experience of a forebay leaking when it was enlarged, and in this case they should see that there was no chance of any leakage. Mr. Wilson declared that he was in favor of this right from the start, even at the added cost. Without further discussion it was decided to authorise the construction of the forebay on the large size, as planned by the consulting engineers.