WATER SUPPLY FOR THE CITY.

Globe, Volume XX, Issue 1591, 26 March 1879, Page 3

WATER SUPPLY FOR THE CITY.

A meeting of the water supply committeo of the City Council was held Tuesday morning, Messrs Cass (chairman), Ayers, Gapes, atld Wilson being present. The following reports in reference to the water supply question were read and receited i South British Ctambtfrfy Hereford street, Christchnrch, March 20th, 1879. gi r) _-vV e have the honor to acknowledge receipt of your circular letter, dated the 13th March, and to give you the information requested therein on the preS'sPre to be' obtained from an 18in pipe laid into Ohristch&rch from No. 6 well on the Waimakariri Eiver. Assuming the site of No. 6 well to be 400 ft. above the level of the railway at Cathedral Square, and the distance to be seventeen miles, the pressure upon a valve closing the main «t its lower end, as shown by a pressure gauge fixed on the pipe, would be nearly 1731 b. to the square inch. If, instead of being closed, the end of the pifa were left open, so that the water woula uncharge freely, as into a tank placed below it, the amount of water so discharged wouh 3 be at the rate of 2500 gals. per minute, and the pressure on the gauge would f .11 to zeru. This is the maximum discharge that could be obtained under the most favorable conditions, viz.. ample supply at the head, with uniform gradient and absence of sharp curves in the* pipe. Under the foregoing conditions, and with the valve closed, the 18in. main would deliver sufficient water to throw twelve jets lin, diameter, 100 ft. high, provided that the hydr-nts are attached directly to the main, and that no hose exceeds fifty yards in length. If this 18in. main is connected with the reticulation of the town, there will be a diminution ia its delivering power, due to the friction in the sub-mains and house services. In the absence of data respecting the lengths and levels of the sub-mains this cannot be calculated, but assuming it to be 10 per cent., we shall have a delivering capacity in the mains of 2250ga15. per minute.

Assuming the daily requirements of Cbristchurch and its suburbs for all purposes to be 1,000,000ga15., which is equivalent to a daily supply of 30gals. per head for a population of a little oyer 33,000 people, and assuming that of this quantity 500,000 gals. are consumed between Gam. and 11 a.m., 250 OOOgals. between 11 a.m and 6 p.m., and the remaining 250,000ga15. between 6 p.m. and 6 a.m., the greatest rate of supply—namely, from 6 a.m. to 11 a m., would be 100,000g.15. per hour, or 1666ga15. per minute, and this quantity would be delivered with a pressure in the sub-mains of about 751 b. to the square inch. This pressure is sufficient to supply a single jet 100 ft. high lin. diameter, provided the water is not throttled by the sub-mains being too small to admit of its free delivery. Considering that the height of the buildings in Christchurch is in few instances such as to render necessary the employment of a 100 ft. jet, we are of opinion that during the period of least demand, viz , from 11 a.m to 6 a.m., the proposed size of main would give ample pressure for the extinction of fires without having recourse to pumping. During the earlier part of the day, viz., from 6 am. to 11 a.m., there might probably be a deficiency of pressure for fire extinction purposes, especially if two fires occurred at the same time, unless tha ordinary supply were cut off, so as to bring full pressure on the mains. The usual system of doing this iB to shut off the mains themselves, so as to concentrate the pressure on the line of pipes leading to the site of the fire; but this method has the serious disadvantage of leaving large portions of a town unprotected, and in case of a fire breaking out in those loc ilities. immense damage may be done before a supply of water can be obtained. We consider the best plan of arranging the domestic supply is to lay the service pipes, not from the mains, but from rider pipes laid round the blocks at the edge of the footpath, each set of rider pipes b ing connected at each corner of each block, with the principal main by a 5-inch branch, with a stand pipe and tap at the street corner. In the event of a fire breaking out, the police will at once rhut off the rider pipes by turning the cocks at the street corners, whist any serious inconvenience to private consumers is obviated by the tap on the stand pipe. Under this system the whole of the available pressure is in a few minutes concentrated on the mains, which remain always charged, whilst the cost of the rider pipes is more than made up by the reduction effected in the length of the service pipes, to say nothing of the advantage of being able to lay services and cond ct repairs without either breaking up the streets or shutting off the water in the mains. It may be worth while to remark that with the existing arte ian supply, it is scarcely probable that the experimental 30gals per head will be required to be delivered by the main for many years to come. in conclusion, therefore, we beg to advise that, subject to the distance and height of No 6 well being as above assumed, an 18in main will be amply sufficient for all the present requirements of Christchurch, provided only th .t the supply at the source is equal to the estimated delivery. We have the honor to be, &c, E. Dobson and Son. Cathedral Square chambers, Christchurch, March, 1879. To the Chairman Water Committee, Christchurch City Council. Sir, —In answer to your memo, bearing date March 13th, with reference to pressure and friction to be obtained from No. 6 well through an 18-inch pipe, we have the honor to report as follows: — We find after giving the subject careful consideration that, taking the data given as correct, namely, the distance seventeen mi es in a direct line from Christchurch, the pipe 18 inches internal diameter, with a uniform inclination and free from bends, and the head 400 ft., the following results:— That the pipe would discharge 2428ga18. per minute, or 3,496,320ga15. in 24 hours, that the loss of head due to friction in the main would be 400 ft. That the hydraulic pressure with the pipe closed would bo 1731b5. to the square inch, and that the head necessary to comply with the conditions, namely, to throw a jet 100 ft. high in Cathedral square through a lin. nozzle would be 588 ft., made up in the following manner : Feet. Head to overcome friction in main ... 400 Head in hose, 200 ft. long 45 Head on jet, about 143 588 The formula used to arrive at the above results are those of the best known authoritus, and the figures have been obtained by taking the mean of several. With reference to the 143 feet for head on jet, we have used the term "about" as there are very few reliable experiments on this subject, and the laws indicated by those that have been made are extremely intricate. For this reason, it i 3 impossible to state arbitrarily what the head will be to a few feet. Although formula; have been deduced from a few careful experiments, they often give erroneous results. Again, it has been found that each particular size of jet attains its greatest height with a certain head, and that when the head is increased beyond that point, the height of the jet. instead of being increased, Is diminished. Experiments with excessive heads show an enormous loss, vide " Box on Practical Hydraulics." We have the honor to remain, &c, Thobnton ahd Bull,

To the Chairman of the Water Committee

Sir,--Your communication under date the f&b' March, 1879, calling for further information with a view to set at re9t the question of pressure obtainable thranpu an 1 Sin. pipe from the site shown us No. o' well in Mr White's recent experiments on the Waiinakariri, unintentionally opens up the question of hvdratdic discharges through pipes, a fertile subject for discussion and latitude of opinion. This, judging by Mr Hubbard's letter p-jblished in the "Lyttelton times" of 13th inst , was anticipated, Mr Hubbard electing to represent ono of the extremes. At the ontset I must confess my inability quite to understand the data for Mr Hubbard's calculations, yet at the same time I wish to guard against misunderstanding him, for he shows friction to be an increasing quantity under all circumstances, without any reducing causes. Consequently his theory has only to be carried to its legitimate en I to prove that if the length of pipe was sufficient, you arrive at the startling conclusion that the gravitation of water can be overcome, and that a main le iding from a never failing source having a permanent head, must entirely cease to flow Mr Clark, a recognised hydraulic engineer of European reputation, has reported to the Council that a 15iu. main at 11 miles distant from Christchnrch, and with 140 ft. head, will supply the required quantity and pressure at Cathedral square ; and t am quite at a loss to understand why that gentleman's report _ Bhould require supplementing as against the views set forth by a young and less experienced man. I have carefully calculated the question of pressure and discharge under the conditions named, and find the results to be as stated under : Discharge per minute, 2478 gals. Discharge per 24 hours, 3.568,320 gals. Pressure, 1731 b. to the square inch. Height of jet, 105 ft. I have, &c, H. B. HUDDLESTON. Christchnrch, March 19th, 1879. To the Chairman of the Water Supply Committee. Sir,—ln reply to the resolution of the water supply committee—" That the engineeis who reported on the proposed Wairoakariri water scheme of Mr White be reque-ted to set at rest the question of pressure to be obtained from the site of No. 8 well through an 18in. pipe," I beg to say that an lßin, pipe laid from No. 6 well t® Christchureh will deliver at ground level in Cathedral Square about 2500 pallons par minute, which would be equal to 3,C00.000 gallons per twenty.four hours, also that the pressure to be obtained from the 400 ft. head at disposal will be 1731bs to the square inch, which theoretically would produce a jet from a lin. nozzle of about 160 ft. in height, but practically I do riot think that a stream of water from a 1-inch nozzle would give more than 100 ft. owm'g to the resistance it meets with from the atmospbeYtf, *hich resistance causes it to burst into spray. ~ I may remark here thai too great a pressure actually reduces the height of a jet, as the great resistance it meets with from the atmosphere, owing to tre with which it travels causes the solidity of the column of water to be destroyed. I remain, &c, C. Walkden, City Surveyor, j