The Okehu Water Scheme

Wanganui Chronicle, Volume XXXXVII, Issue 11758, 2 October 1902, Page 2

The Okehu Water Scheme

MR. MESTAYER'S REPLY TO CR

BRIDGE,

The Mayor (Mr. A. Hatjick) has kindly haaided to us the following communication from Mr. Mestayer in reply to Cr. Bridge's statfEirsuits: —

His Worship the Mayor, Wanganul. Sir, —Re the Okdhu supply. I notice from the Wamigaxui "Chronicle' that "when the report of the Water Works Committee, recommending this scheme, was laid bei'oie the GounoJl last Tuesday, Councillor Bridge criticised my figures and deductions at considerable length, pointing cut what he imagined to be discrepancies! and inconsistencies between my last report and that of March, 1901. It is a pity thati the points now raised were not submitted tc> me before the report was placed before tbo Council., .as they all admit of very simple answer:-*, Bind the assumed discrepancies merely result from, attempting to compare dissimilar Echemes, without taking into account the governing conditions of each, and without possessing the technical kclsTvledge requisite to appre■cipiiia -the effect of these conditions. In the river scheme, the supply was practically unlimited, and, after determining the maximum limit of the diaCy supply to be afforded—which was fixed at 1£ million gallons per day—the most important consideration was to render the supply availably at the least initial cost, and so as to admit of gradual extension in the future as increased population, might render desiderable. Since that report was presented the suggestion has been made to me that it was advisable to bear in view the probability of a greater Wangaami being created in the near future, and, therefore, in reporting ujon the Okehu scheme, a daily supply of' 52 million gallons was adopted as the maximum to he provided for. As thi3 -mas about the whole of the watershed capacity, my object has been t«t> enable the whole of it to bs utilised in the most economical manner. If now, my report en the. river scheme is carefully read, it will be noted that although 1£ million gallons per day was the ultimate supply. to be afiordied/it was intended to provide only ■ one pumping engine to begin with, the .engines, pumping maim, filter beds, and reservoir being added to from, time to time, as the increasing peculation rendered desirable. With, regard! to the pipe disfoharges, Cr. Bridge make a positive statement equivalent to saying that I have made a, gross bhicder in my calculations; He says: "I believe a/ 15-inch main is to be laid from Westmere to the town, but this will not carry 2^ million gallons per day, as a 16-inch pipe only carries a little more than that." There is no justification possible for euch a, statement, more especially as this point has been already very fully dealt with in my letter of the 15th instant. As a matter of fact, the 15-inch pipe could' deliver 5 million gallons per day into the present 15-inch main ■at Virginia, but witih this discharge all the surplus head would! be consumed in pipe friction; when the supjply of 2 million gallons per day is passing, there will be about 85 feet head, over the end of the present 15-inch maim.v Cr. Bridge has' fallen into the en'or of cemrardng two mains working under totally different conditions, without taking these into- account. The 16-inch; main, has a fall of 17£ feet in a mile, the 15-inoh has a fall of 109 feet in a mile, andj can consequently discharge very much more water than the larger pipe. Again, ha says: "The 10----inch main recommended more than once, in place of bringing 1£ million gallons-— the quantity said to be required—will orly bring in 1,150,000 gallons. There .seems to be some discrepancy here." The discrepancy is imaginary, and" results from a confusion of ideas, and comparing two schemes framed on different bases, without allowing for the differences. The 1£ million gallons referred to was the "ultimate" supply provided for in the river scheme; the 10-inch pipe is for "one half of the ultimate" supply provided for in the Okehu sahemc In, each case it is proposed to limit the initial cost of the works, by providing for a smaller quantity than tho maximum for a commencement, and to extend the works in Ine future as may then be found most suitable. The principal is identical, but the essential difference in the nature of the two schemes involves methods of attaining the end. I dc- not quite understand the next objection, and think there must be eoino error in the printing; the copy I have readii 'a» follows: "Mr Mestayov recommends cast iron mains for the 'Wnr.ganui scheme, and yet 'in a recent letter l:e save that at Beetaloo the joint's were cOiiStantly 'blowing out,' and he would i.ot us* 1 cast iron again under such a prat-sure." Now, if tliero is one- thing I have insisted upon more than another, it is th;e adfyisability of us;ng steel maim instead of cast iron "for this ■f.eheine. If Cr1. Bridge refers to the river •scheme, then his argument d-x-s not apply, because the maximum pressure on the 'pipes in that case did net amount to one-half of that of the 10-io.ch main; and the 'piipes would have been quite safe against blown joint?. Cr. Bridge appears to have formed a most error ec«Y!s idea of the relative pressures on the pipes in these two schemes. He goes on to my : "The Okehu is 1200 feet above Wanganui. The river reservoir would have been 400 feat above river level. Ido not know how much that would mean above Wanganui, but I should! say a considerable number of fleet more." I believe there is a flail of less than 40 feet in the river from <fhe site of the pumping station to the town, which would make ll:t reservoir 440 feet above Warga.nui, »v agalnsfc the 970 feet (not 1200) which is the reduced height of the dam- at head work*, ■w that how there can be more prß^ure on the pipes in the river scheme 1h in 111 those of the Okehu scheme passes my comprehension. The 1200 feet alluded to ik the mea.n height of the whole water.red above Wongamu, and was so give.', in my report. With regard to the (on "f pumping, it i«i said that at Nap-" it costs £750 per annum to pump 90).f''~0 gallons per day. and therefore I mast l.t greatly out in "my estimate of £3.802 per annum, to pump one million gallons per day at Aramolio. Now, a nu.v pi»faii' argument than this it is hard to (on-oivc. Tn the first place, what authority i« there for the quantity pumped at Naot;..*? 1 think this lies been very large 7 1 \-<-r estimated. I have been told that it is about 14 million gallons per week, and in tho second place no mention is made of the height to which the water is raised, a 1 though this is of equal importance with the quantity, and the consequent expense. My estimate can very easily be roughly checked by those conversant, with pumping machinery ; and I now give the full data by which it can be done. The reservoir" is 415 feet above pump?, the friction head in the pumping main; is 35 feet, making a total pro-sure of 450 feefe against pump plungers. The quantity to be pumped is one million gallons in 16 hours, or 62,500 gallons per hour. Allowing for 60 per cent, efficiency in the engnies, pumps, etc., this will require 237 horseDower. The coal consumption, allowing for waste, banking1 fires, getting up steam, etc., will not be less than 3^ lbs. per tiourse-poTver per hour, and will therefore

amount to about 2166 tons /per"annum. (The quality cf-ccal is assumed as equal 'to Westpcrt steam coal; .if > an" inferior qualify is used the quantity will be greater.) The cost of coal delivered at the pumping station will be 265.-p,er ton, and fchias the annual cok<t of coal alone will be £2,816, leaving ~dß6 p.?r annum to cover the cost of wages* for two shifts of engin-. eers and stokers, oil, waste, repairs, stores, lighting1, et?. (In my estimate I had only allowed for 210 horse-power, but these en--1 gines could be easily worked up to the higher figure.) Cr. Bridge is in error in assuming that I have allowed for two reservciis at Aramoho. 'Ine £4000 he refers to covers the cost of the land required for the reservoir itse'if, but the cost of .tine', rerervou* itself is given separately. With regard to the size of the reservoir, a gi-eat deal is m»c!ie of the assumed discrepancy between that included in the Aramoho scheme and tlie one proposed!, last year for theNriver scheme. Here, again, a careful consideration.' of the two reports would have cleared up the difficulty. I say in my report i(pa-ge 6 of the printed copy): "To institute a fair co-miparison between the two schemes —the Okehu and that-at Aramoho—it is necessary to put them, co far as can possibly be done, on an equality as to quality, quantity, and pressure. Therefore, since five million gallons storage had been provided for in the estimate of the Okehu scheme, I provided for the same quantity in the Aramoho scheme. Had I been comparing the latter with the river scheme, I should have aclofpted the same principle, and allowed for a reservoir similar to that in the latter. I would here call attention to the fact of the one and a half! million gallon' reservoir for the liver scheme, being only the first instalment of a total storage of three 'niiilion gailons. The different conditions of the Okehu and river schemes fully .-justified, in my opinion, the increase of storage to five millions in the case of the former. I do not consider that the natural conditions oif the two svtes would render the river site so much more costly than that at Aramoho. I have seen them both and have no alteration to make in my estimate on this score. Cr. Bridge also calls attention to the fact of the pumping main at Ararncho being £900 mjore than that at the river site; but he omits to inquire what isl the length of each. There is a great difference, and it is tills wiMch accounts for the price. With regard* to capitalising- tlie cost of pumping, and not having done so in the case of the river scheme last year, I contend that the course I adopted was correct. - TiDi comparing two pumping schemes, where the quantity pumped!, and the height to "which it waei delivered would be the same, it was unnecessary to capitalise the cost of pumping, as it would have been the same for bath. But when comparing a pumping scheme with one of gravitation, a fair comparison could not be made withiout capitalising the cost of the pumping. Cr. Bridge states that the flow of the four-inch well has been measured ard found tot be 640 gallons a minute. How was it measured? I have measured flowing water too often not to be aware of the difficuties in obtaining accurate results ; and I am certain that at the time I visited the well the discharge was never up to 500 gallons a, minute. There is only one way by which this point can be^ placed beyond dispute, and that was clearly pointed out in my report. With regard to the spiral riyetted pipes, I do not agree with Cr. Bridge's anticipations as to the difncuilties in laying them. There are more ways of dealing with this than by allowing any "give' in the joints, which would infallibly mwlt in the blowing of the joints when! the full pressure cam© upon them. This isl one of the practical details whic hmust be left tct practical men to defflgn, and each' case will have to be dealt with on its own merits. As pointed out in last Thursday's "Chronicle," I have not debited the Okehu scheme with the annual interest on the ciaipital cost of the work. Tlie reason is that I regarded this asi com*mgi within the province cif the Finance Committee-, rather t'hari within mine, my duty being to place the 'particulars of the two- schemes fully before .them. For the flame reason, I did n"»t debit the Aramoho scheme, with the annual interest on its capital cost either; but no attention was called to this omission. The only other point toi which I need refer is the difference in the cost of the two pumping stations. Here again a close attention to the two reports would explain the reason. The o^pital cost or the one at Aramoho is considerably larger than that for the river scheme, because the daily supply is larger, and steam instead of oil is used aa fuel. In, view of the greater cost of coal and labour up the liver, I thought that the saving in first cost by using oil would justify the increased-running cost. At Ararmoho, on the other hand/, I considered that as the coal cculd be readily delivered at the works it would be more economical in the lomg run to save running cost at the expense of an ircrease in the capital expenditure. Again, it will be noticed that 1 proposed to commence the river scheme with only one engine; but the quantity of water pumped would then be much* less than that delivered by the engines at Aramolio, with which the ccniparison lias been made. After carefully considering all the criticisms advanced, and re-checking all my calculations, I do not see reason to make any alteration';, except that I should have painted out that when; the Okehu scheme is completed it will probably be necessary for the Council to pro/'de an additional man to loolc after the head works and the main to Westmere. I have the honour to be, Sir, Your obedient servant, B. L. MESTAYER.