CHRISTCHURCH WATER SUPPLY.

Star (Christchurch), Issue 3360, 15 January 1879, Page 4

CHRISTCHURCH WATER SUPPLY.

Thpy j.fqUOjif/jjgg, letter, appeared in fche columns ofc oitr, contemporary this morning :— 'SO %fj&. s jß}XS<jsE'§9 TUB lyttelton timbs. Sib,— 4.f ter tad' numerous meetings of the Wat*- . Supply Committee, I observo that they Jiiave arranged for tbp oaeting of a cylinder for

the purpose of testing Mr W. White's scheme and otherwise for expending a certain portion of the ratepayers' money in making this experiment. I feel I should be wanting in my duty as a citizen, were I to remain silent upon such an important matter, although I feel rather a repugnance to offer valuable information gratis, for others to make use of. Immediately after the publication of Mr Clark's report upon the various schemes submitted, I sent to England to three or four of my friends, who are leading hydraulic engineers, with whom I was duly educated, and with whom my connection extended over a period of ten years (Messrs Bramwcll, Easton, Homersham, and Bateman, J\8.8., President Institute, C.E.s) a full description of my scheme for the supply of this city with -water, also an epitome of that of Mr Clark, more especially relating to the height and distance of source of supply above Christchurch, as recommended by him in his Waimakariri scheme. On July 24 I received their replies, quite coinciding with my views as to the extent of my scheme for the supply of such a large city as this is destined to become, and bearing me out in my calculations for the elevation of the source of supply above the city necessary to comply with the conditions issued by the Council, viz., one million gallons per day, capable of discharging a- jet 100 feet high in Cathedral Square. T My reason for making this communication f and referring to the operations of the Water Supply Committee re Mr White's scheme, is i that I condemn the Waimakariri as a source ifor a gravitation supply. My calculations \sho\v that to obtain the necessary elevation , to overcome the friction of such a large volume of water passing through the length of mains, requisite to comply with the stipulated conditions of the Council, it would be necessary to proceed too high up the river j such it distance, in fact, from Christchurch, that taking the cost of the supply main into onsideratiou, a much cheaper and equally good supply can be obtained closer to the city. Moreover, if any scheme be adopted which proposes as its source 1 the Waimakariri at tin) elevation recommended by Mr Clark's calculations, viz., 198 feet above Christchurch (which exceeds any of the schemes submitted in regard to that source according to his report), I say, without fear of contrary proof, that any scheme proposing anything approaching such an elevation as 196 feet, if adopted, will most assuredly fail to fulfil the stipulations of the Council, and end in disappointment and waste of public money. In these views I am upheld by the eminent engineers before referred to. The following extracts from my letter of August 13 last to the City Council I think will interest not a few : — " 1. That the following all-important point in preparing a water scheme must be considered. With regard to the ordinary consumption of water, it has been proved by the highest authorities that one-halt" of the daily consumption takes place during five hours ; therefore, the quantity to be provided for delivory per minute should bo bused upon this. For example, one million gallons per day is the stipulated quantity, therefore the maximum draught of water— exclusive of fire service — will bo 500.0U0 gallons in 5 hours, 100,000 gallons in 1 hour, and 1G66" — or say, in round numbers, 1700 — gallons per minute. Should a fire occur during the same period of five hours, there will be an additional draught on the pipes, according to tho number of jets brought into play ; and here it may not be out of place to say that the amount of water required lor each jet is 150 gallons per minute, and that provision should be made for at least six jets, equal to 900 gallons per minute, which would increase tho maximum draught from 1700 gallons per minute to 2600 gallons per minute.

" 2. To follow up the example I have given of delivering 2600 gallons per minute, to include six jets of 150 gallons each, to be thrown 100 ft high in case a firo should occur during the first five hours of daily consumption (and this is the point which I make no doubt has been generally overlooked), I will suppose a service reservoir is constructed on Port hills, a distance of only three miles from the city. In order to deliver the above quantity and to provide for future probable increase of daily consumption, say to 1,500,000 gallons per day, or 50 per cent increase, I should recommend an 18-inch main pipe for this distance. Now the head consumed in friction by 2600 gallons passing through three miles of 18-inch pipes is •0117233 ft per lineal yard of its length, equal to a head of 78ft, and the head consumed in friction by 150 gallons passing through 200 ft of fire brigade hoee, which is no unusual length where hydrants are judiciously and properly placed on the mains, equate 50ft. " Let x —Height of reservoir or source of supply above city. " Then x— 7Bft head due to friction in main, 50ft hoad due to friction in hose. " And five-eighths (x— 7B— so) -the height to which a jet will rise with this head. "Height of jet required 100 ft. "/. x=Height of reservoir or source of supply above city=2BBft." "To explain that portion of the above formula relating to the height of jet due to head it has been proved by those eminent engineers, " Sir J. W. Bazalgette, Engineer to the Metropolitan Board of Works, London, and Messrs Brain well, and Edward Easton, Hydraulic engineers who were appointed to make experiments for the Select Committee of the House of Commons as recently as June, 1877, upon the supply of water and water jets for the extinction of fires, that fiveeights of the available head will be the height to which the main body of water (not the more spray) will rise in a jet." Tims, from the above, it is manifest that to supply 2600 gallons per minute, and to secure a jet 100 feet high only three miles from Chriatchuvch, the elevation of the source of supply must be equal to 288 feet, and yet in tho face of these facts the Committee are about to waste money in experimenting upon a source not 200 feet above the city, and come 11 or 12 miles distant, which of 'course would require a still greater head to overcove the friction, quod est ridii-ulum ; and taking the prospective future demands of the city into consideration, which to ensure the stipulated conditions would necessitate a still greater elevation above the city, only makes the absurdity of the scheme more patent. For example, JVo. 2, if instead of one million gallons a day, it were deemed expodient to provide for half as much again in view of future needs — that is to say, 1,500,000 gallons per day— which it is not at all unlikely will bo required hero before many years have elapsed, and to give six jets of 150 gallons each, the elevation of the reservoir or heud necessary to overcome friction, if situate upon Port Hills as before, would be 432 feet. It is yerv rare indeed that an efficient gravitation supply can bo economically procured from a river runing through a plain. 1 do not hesitate for ono moment to assert that nothing short of an utter failure will be the result of any schome with a source of supply at anything approaching such an elevation as that contemplated and about to be experimented upon at that distance from the city. — I am, &c, W. F. HUBBARD, Hydraulic and Civil Engineer.