THE WATER SUPPLY OF CHRISTCHURCH.

Globe, Volume IX, Issue 1263, 5 April 1878, Page 2

THE WATER SUPPLY OF CHRISTCHURCH.

ME CLARK’S REPORT. The following report has been presented by Mr Clark To the Worshipful the Mayor and Oifcyr Council of Christchurch. April 2nd, 1878. Q-entlemen, —In answer to your ment for schemes to supply Christchurch with water, twenty-fire communications hare been received, and I am honored with your instructions to consider the same and report as to the best, together with such further suggestions for the object in view as may appear to me to be suitable to the wants and position of Christchurch. The conditions which the authors of the schemes were required to observe are as follows WATER SUPPLY. Conditions. 1. That the water shall be suitable for drinking and other domestic purposes. 2. That the force of the water shall be such that it shall throw water through the one-inch diameter nozzles used by the Fire Brigade a height of at least 100 feet in Cathedral square. 3. That the supply shall be one million gallons per [day at the least, with capabilities for an enlargement of quantity. 4. That the scheme shall be to bring water into Christchurch in such a way as to afford the, best means for distribution. 5. Each competitor must furnish reliable daSa. and a detailed estimate of the cost of the scheme. 6. That no premium will be awarded if tha scheme shall not be of sufficient merit, in the-, opinion of the judge appointed. Smallness of cost shall be considered a point of merit. 7. That thp successful scheme shall become the property of the City Council. The schemes shall be sent in to the Town Clerk, under cypher or motto, accompanied with a letter containing the competitor’s name and the motto or cypher used by him, by Tuesday, the sth day of February nest. City Council Office, Jan. 16th, 1878. v Briefly stated, these are—that one million gallons per day of water suitable for domestic purposes shall be supplied at a pressure capable of giving a jet of 100 ft. high in tho centre of the city, and accompanied by reliable data and estimate of cost. It is understood that tbs conditions limit the supply to a rate of one million gallons in twenty-four hours, or 11T4 cubic feet per minute. To meet the requirements of dom&slie service, it is usual to estimate that one-half the total daily supply may be required in four or five hours. This would necessitate that the arrangements should bo capable of delivering the water at a higher rate, or about 300 cubic feet minute. In this case, however, the supply is to be as above stated at a rate of lil'4 cubic feet. On examining the twenty-five plans, I find that—r. Ten schemes propose to draw water from the Heathcote, Seven schemes propose to draw water from the Waimakariai Five schemes propose to draw water from the Avon, aided in two cases by an artesian supply One scheme proposes to draw water from the Okuku One scheme proposes to draw water from the Hawkins One scheme proposes to draw water from. artesian supply. There arq several proposals to obtain power* fqr the water by damming tho

rivers Heathcote or Avon, to woiui turbines or other machinery, but in no case is there any data given whereby to estimate the power which may bo so obtained. I am however furnished with an estimate by 0. N. Bell, Esq., engineer to the District Drainage Board, by which I find that the gross power’ obtainable from the Avon by a dam of 4ft. in height is 58-horse power, while from the Heathcote, by a dam of similar height, about one-fifth of that amount, or 12horse power in the gross, giving about 40 and 8-horse power effective in each case. In the case of the Heathcote, where not less than 40-horse power will be required to lift the water to the required height, it would be manifestly undesirable to interfere with the river, and the various interests affected therein, by any attempt to obtain so small an amount of power. And in the case of tbe Avon, it is very questionable on sanitary grounds whether any further holding up of the river surface is desirable; whether, in fact, the saving of expense in working steam machinery continuously—amounting to about £I4OO per annum —would not be dearly purchased by the higher level of saturation in the subsoil which would result. The great distance, thirty miles and upwards, of the source of supply in the case of tbe Okuku and the Hawkins rivers, and the length of iron piping required, and consequent large expense, are serious disadvantages to the schemes proposing their adoption. With reference to the proposed artesian Supply, if it he true, as I am informed, that ■when the pumping machinery for the Lyttelton supply is in operation, that the water in the tube wells of the_ neighbourhood is lowered, it is very questionable whether any further extensive demands on this source, will not injuriously affect the present supply. During my inquiries, I have, in addition to the sources above enumerated, visited the springs which form the source of the Avon. These are the Avon-head springs, which Mr Bell estimates to discharge 350 cubic feet per minute. Another spring to the east of the former, which yields 200 cubic feet per minute, and the Boag Springs, yielding 700 feet per minute. These are about equidistant from Cathedral square, 4g miles, and at an elevation of 34 feet. With reference to the first condition, viz., the suitability of the water for domestic purposes, With one exception, viz., the Heathcote, no information if? given ; in this case a qualitative analysis is given, which shows the water to be of fair quality, but the source is liable to be affected by storms, and nearly all the proposals recognise that the water from ibis source should be filtered. This involves settling tanks, filters, &c., constructed on the mountain, together with a service reservoir; the cost of which, if to contain one million gallons, and filters, with spare one, if efficiently constructed with concrete or masonry lining, will amount to about £14,000, and involve an additional lift to the pump of about 25 feet. It is, therefore, manifestly desirable to avoid, if possible, the selection of a source which requires these expensive &diuncto« The Avon above Christchurch is only liable to the pollution caused by the decay of vegetable matter, which is 'abundant, but which is entirely avoided by taking the water at any of its sources. The Waimakariri, as generally considered, furnishes the water drawn from the artesian wells in Christchurch, where, after the natural filtration it undergoes, it is said to bo unexceptionable. It is a very pleasant drinking water, but I have not been able to obtain an analysis. When taken from the river it is in a somewhat milky condition, and is than said to he unwholesome, and all the schemes, with one exception, propose to settle and filter this water before sending it to Christchurch, entailing the expense of reservoirs and filters. In one scheme an ingenious proposal is made to obtain the water from cylinders sunk in an island in the river bed, which involves the carrying of a pipe crossing the channel at an elevation above flood level. An objection to this arrangement is that the filtering is limited to the sectional area of the cast iron cylinder, which is an expensive arrangement, as a great number would be repaired. Yarions expedients are proposed by sunken ffiams and otherwise in tbe other schemes to ■fores the water out of the river channel into rfche settling tanks and filters, and in one case ijby a channel cut at right angles to its course, fund which would be difficult to keep open. I am of opinion, however, that a dam or .■any other work crossing the river in the part where the water is proposed to he drawn is very undesirable. The river has there no proper banks, and the section furnished to mo ■by Mr Bell shows that the river bed is rather ‘higher than the plain on which it flows, and hence the great difficulty in keeping it Sn its channel in times of flood. With reference to the second condition, viz., the height :at which the water has to be delivered s,lt 'Cathedral square, and the arrangements necessary to give the required pressure there appears to have been a general misapprehension. ’ . , The height of the column of water winch furnish 11D4 cubic feet per minute, and a jet l(X)ft. high* is 140 ft, or 40ft. of pressure is to overcome the friction of the water in passing the water through the small nozzles of t> Fire Brigade. The water m'ust be taken at a height in the river asat 1 A in dim which shall be sufficient to force fbe water through a pipe of sufficient size to furnish 111-4 cubic feet per minute at the top <u column 80, 140 fc. Juab, at Cathedral sqm-urc, and in calculating the size of the pipe, the inclination taken into calculation must be tbe line AC anc * not AB,as appears to have been every case. A 12-incb pipe, as proposed in several of the schemes, would discharge somewhat more than the 111’4 cubic feet per minute at B if it there had an open end and mo pressure was required. By my calculation the nearest point on the river where the requisite eleva:tion can be obtained is eleven miles from •Christchurch; the elevation is there 196ffc, :.and the inclination obtained there is sufficient rto collect the water from the river and discharge it through a- pipe 15in. in diameter at ?the elevation of 140 ft. According to this calculation, there is not one of the schemes which will comply with this condition as to (quantity and pressure from the Waimakariri ass' a source of supply. At various periods of time the Waimakariri river has occupied different positions over . the entire plain, and has brought down the pebbles and sand with which its present bed is strewn, and which extend over the plain under the surface mould. The stream, as represented by what is visible, is but a small part of the entire quantity, a large portion passing through the shingle and sand, nob only in the bed of the sriver, but more or less in the plain through which it flows. If a trench be cut in the plain of sufficient (depth below tbe level of the surface of the srtream, and parallel to its course, the water will collect therein and be naturally filtered ; and if the trench so dug be continued with the fail of the plain, 20ft. to 28ft. per mile, a stream of naturally filtered water will be obtained, which can be carried by a 15in. pipe to Christchurch, and be delivered with the required pressure, if it commences eleven miles from the city. I am unable to say what would bo the length of such a trench to collect 111.4 cubic •feet per minute, but I have assumed that ihalf a mile is .amply sufficient, but, if not, it earn be extended to any required length. An implement has lately been invented — Messrs Bruce and Batho’s excavator —by which pebbles, gravel, sand* .Ac.,can bereadily and cheaply excavated under water; if this be done to a depth of 3ft. or 4ft. below the water level, and stoneware pipes with open joints he laid in the trench, covered with fine shingle, the entrance of sand will be prevented, and the water collected will be free from all ■.Vegetable and suspended impurities. The excavation of such' a trench I2ft. to Ijg fi deep, with the requisite land, I estimate ■will about £3300, or much less than the filter of ordinary construction for one million ;gallon« j/er day, with spare area; while the necessity for any dams or reservoirs or inter■ferenco with the river will be avoided. . Before actually deciding on tbe means of •obtaining the water, it; would bo desirable to •excavate a short trench, and, by pumping it mut, to ascertain Dm depth and dimension Squired, and the quantity pi wijtyr so pro-

curable. I think, however, that the provision made will be sufficient. I have ascertained that the lowest elevation which should be given to the surface reservoir for receiving the water from tbe Heathcote is 182 ft. above Cathedral square ; allowing 25ft. for pressing the water_ through the tanks and filters, that a 15in. pipe will then give the required pressure and quantity, and that 40 h.p. in duplicate should be provided. In the event of the source of the Avon being adopted, a 15in. pipe would be required, 4£ miles long, to a convenient point near the river at Christchurch, where pumping engines could be erected. These would require to be 30 h.p. in duplicate, to give the pressure of 140 ft. at Cathedral square. As a further test of the sufficiency of the various schemes, I have made estimates of the cost of supplying Christchurch from the various sources proposed, adopting the arrangements which, appear to me to bo the best, and intended to comply with the conditions in all cases. They are as follows ;

waimakariri. Land, and cost of collecting 1,000,000 gals, daily of naturally filtered water, and 11 miles 15in, pipes, including 10 per cent, for engineering and contingencies £41,626 Establishment, one man with residence at £3 per week —£156 per annum, capitalised at 6 per cent 2,600 £44,226 HEATHCOTE. Engine, pumps, settling tank, filters, clear water tank, 3 miles 15in. pipe, residence for workmen, and 10 per cent, for contingencies ... ... £42,052 Working expenses and depreciation, :£2507 per annum, capitalised at 6 per cent. 41,800 £83,852 Assuming that an artesian supply could be obtained, the cost may he, with 10 per cent, contingencies, estimated at £28,560 Working expenses as above capitalised 36,000 £61,560 ‘AVON. Supply drawn from the source of the Avon-—engine, pumps, &c., land and compensation £28,027 Annual expenses of working and depreciation, £1960 per annum at 6 per cent 32,600 £60,627 A comparison of these estimates shows that the cheapest supply will be obtained from the Waimalariri, and by far the moat expensive is the Heathcote, while the cheapest, in first cost, is from the source of the Avon, which could doubtless be still farther reduced by using the power of the river, but which I should not recommend. As a pumping scheme doubtless the sources of the Avon furnish the purest supply at the lowest expense. The height of the springs, thirty-four feet above Oathedrel square, will be quite sufficient to keep the distribution pipes through the city filled during the night, and by keeping a watchman at the engine with electrical communication from different parts of the city, it will be possible to give the full pressure in a few minutes when required for extinguishing tires. It is not actually necessary to have an ei'evated reservoir for the purpose, though apart from cost it is convenient. The engines can pump into the mains through an air vessel, and give a steady flow at the required pressure. When compared with these .results I consider the schemes which propose to draw their supplies from the Waimakariri are the most worthy of commendation, but unfortunately they all fail to comply with the conditions as to pressure and quantify by their proposed arrangements. There is not a single proposal to draw the supply direct from the source of the Avon, the nearest and best of the available sources, while the proposals to take the river water lower down in its course and to pump by power obtained from a further damming of the river is not, I think, desirable. I have therefore considerable difficulty in making an award of the premia, as there is no single scheme which I could recommend for adoption. Seeing then that the proposals are so nearly equal in merit, and being unwilling to deprive the competitors of their reward for these good intentions, I would recommend that £IOO be divided among the competitors, W. Clark. :