MR. PEPPERCORNE ON THE AUCKLAND WATER SUPPLY.

New Zealand Herald, Volume X, Issue 2901, 16 May 1873, Page 1 (Supplement)

MR. PEPPERCORNE ON THE AUCKLAND WATER SUPPLY.

No. I. Tub filial report of the committee appointed by the City Council to obtain every information as to the best source of supply of water for the city of Auckland, having now been published, with the decision that the Nihotopu Btream be taken as the source of permanent supply, I will, with your permission, make a few remarks on the subject of gravitation works, and the supply of cities by pumping engines. And, in the first place, I would observe, that very exaggerated notions often prevail as to the cost of raising large bodies of water by pumping, while it can be shown that whoii'the cost of the power is fairly com* pared with the quantity of water raised, together with the height to which it is raised, the actual cost of pumping large bodies of water, with engines and boilers of the beat description, ami with fuel at a moderate price, is in reality very small. In fact, as Mr. Beli justly observes in his report of last year on tins subject to the City Council, the whole question at issue bet ween the two methods of gravitation works may bo divided tho annual cost, and the freedom from accident and interruption; and he further observes that 11 the freedom from accidents or interruption is a matter of judgment-, and may be differently estimated by diflerent people. It is my opinion that tho gravitation scheme from Nihotopu is the most liable to accidents from landslips, from floods, and from the bursting of pipes through the great pressure. The accidents most likeiy to happen from the pumping scheme, are the breaking down of the machinery. X do not believe that such accidents would stop work for more than a day or two, and pumping engines are known to run for 30 years without accident, and with little repairs." We have, therefore, tho deliberate opinion of a competent civil engineer in favour of pumping over the JS Tihotopu gravitation scheme, and from my own experience as to the working of pumping engine? at one of the London ater-works, X can fully endorse what. Mr. Bell says, as to their durability and freedom from accidents. I am at a loss, therefore, to understand how the committee arrived at tlie conclusion that the Iwhotopu gravitation scheme was to be preferred to the method of raising the water by pumping, at a site considerably nearer to Auckland than the Nihotopu. lhe Chairman stated that the committee had gone into the matter verv carefully, and had come to tho conclusion "that a gravitation scheme was preferable to a pumping scheme, but that they had not gone into the question of figures ; but it appears to me that the question is eminently ore of pounds, shillings, and pence, combined with an absence of any great engineering difficulties. I presume, of course, That previous to issuing their report, the gentlemen forming the committee carefully examined the country about the "Jsibotopu, particularly the line proposed by the late Mr. Weaver, and adopted, I believe, by Mr. Bell, , for the piping, as this would give them some 1 idea of the difficulties and expense to be encountered in laying the pipes ; and for the information of your readers I will give im extract from Mr. Bell's report on this point. He says, "The Nihotopu stream, just after passing the site of the proposed dam, enters a gorge, and precipitates itself down a succession of waterfalls, some of them upwards of 100 feet high. It is walled in by precipices, and the mountains ri?e from the gorge to great heights, and SO steep that many great landslips occur. The mountains are also intersected by steep ravines. Along ground more or less like this, the road for the line of pipes would liave to be made for a distance of 3i miles. It would afterwards pass along the slopes of the range for another three miles, tho ground being very steep in places, and subject to landslips. ! Thi's description will give some idea of the I great engineering difficulties that will have to I be encountered in laying a line of piping in ■ such a broken and precipitous country, and | will show that no reliable estimate can bo ! formed of the ccst of the work without an

accurate survey and plan, showing all the rises and fills along the line, lhe late Mr. Weaver, who reported on this subject in April 1565, stated that " several trial lines were run, which, from the extremely precipitous nature of the ranges, were entirely imi practicable for a pipe track, and that it was I a work of great labour and expense, owing to j the exceedingly broken nature of the ground, 1 and the dense forest with which it is covered, ! to obtain any approach to a practicable lino ] out of these gorges." He further on states that " a personal inspection of the locality has ! given me no reason to hope that, the line enn ! be materially improved by further surveys, ! but it is ulterly impossible!for any one to form tiie slightest estimate of the nature of the ground to be traversed, even for a dis- : tance of a few chains, until he commences to

cut his way through the forest. It is then I generally found that the ground which api peared to be moderately favourable is broken i up into precipitous gorges, frequently almost ! perpendicular, and varying in depth from 100 !to 150 feet." (Report of April 10th, 18G5.) ; Of course, all these facts were known to the i committee before they mudo their report, us i well as the fact that when Mr. Weaver ! gauged the Nihotopu, he estimuted the quan- | 850,00U gallons in the 24 hours, while 1 Mr. Bell, after a heavy fall of rail!, estimated ' it at 1,400,000 gallons ; but when I visited I this stream in January last, the quantity was not nearly so great as this, and I estimated it

I roughly at 750,000 gallons per diem. This j shows how -very irregular the supply is, and I how Hiush it is influenced by tlie rain-full, on | so small a gathering-area us 1200 acres ; and 1 we must bear in mind tho fact, that when we I lmve to deal with the wuter supply of a large ; city, it is the minimum quantity obtainable in dry seasons, which must equal the average duily consumption, and in the case of the !Nihot,opu 6tream, it does not do so, as it is under the slated quantity of one million of gallons in the 24 hours, although the average quantity in the 12 months may equal or even eiceetl this quantity. "Wo now eorae to the question of cost , and on thie point the chairman stated that "ho thought the entire bcheme would co ,; t from £100,000 to £115,000;" but he is not borne out in this

opinion by the report of Mr. Bell, who, in April, 1872, roughly estimated the whole cost at £131,689, exclusive of land and compensation, and when Mr. Bell was examined before the Council in March last, that gentleman Btated that ho had in 1872, estimated the cost of the iron piping at £47,896, but that at the present time (1873) itwould probably cost athird more, or asum of £63,801, making the total estimated cost amount to £147,654. This sum, it must bo understood, does not include the cost of the land required or of compensation, as Mr. Bell distinctly states in his report that this amount would not be rightly ostiinatcd. The annual charges, exclusive of interest on the outlay, he estimated at £2160, and if we add to this sum the interest on £147,654 at 5 per cent, per annum, or £7382, we hare a total annual charge of £9542. But as Mr. Bell's estimate was, as he states, compiled from that of Mr. Weaver, it can only bo considered as a very rough approximation, and I am quite convinced that, taking iuto consideration the great engineering difficulties, as well as the length of the pipe track, and the size and weight of the main pike or pipes, a sum of £200,000 will be required to carry out this

project in its entirety, including the piping for the city and suburbs of Auckland. Mr. Bell's estimate provides for a single line of pipes 12 inches in diameter, and 15a miles in length from the Nihotopu to the proposed service reservoir, in upper Symonds-streot; but I believe that a caroful of tho line, so as to avoid the low part of the Wliau Valley, would show that fully 17 miles of piping will be required. And there would be so much more safety from interruption of tho supply from the occasional bursting of a pipe, or other accident by landslip, by the use of a double line of piping, of say 10 inches in diameter, that it would be found to be expedient to provide for such a double lino. The size of the pipe here stated is larger than would be required for a Bupply of ono million gallons per diem ; but looking at tho prospective certain increase in the consumption of water, I havo preferred being on tho safe side, and every competent engineer will, I am snrc, coincide in my viows as to the necessity for a double line of pipes which, although it would no doubt, add considerably to tho cost, would secure the city from interruption to the supply.