REPORT ON THE WATER SUPPLY OF DUNEDIN,

By J. T. Thomson, Civil Engineer.

BEFORE entering on the subject of works for Dunedin, a few preliminary remarks seem called for. The sources from whence water supplies are obtained for towns are, rivers, springs, artesian wells, and surface collections. The works necessary for conveying the supplies are modified by the nature of the topography of the district, and superficial contour of the town and suburbs. Rivers generally afford the most ample supplies; springs and artesian wells seldom afford more than limited quantities, and the power of steam in this case is for the most part held in requisition to raise the water to the proper level; surface collections, on the other hand, while affording extensive supplies, are frequently available by the force of gravity unaided by steam power, and, if the area of collection be within a reasonable distance of the town, are to be preferred, as the expense of steam power, with wear and tear of machinery, together with the cost of attendance, is avoided.

In determining plans for a town water supply, purity is the most important consideration. Generally speaking, the comparative purity of water may be said to stand in the following order:
1st, rain water; 2nd, water from mountain brooks; 3rd, soil drainage and water from rivers, and lastly, water from deeply-seated springs or subterranean sources. The impurities of water are four kinds: mechanical, animal, vegetable, and mineral or saline. The quality of water is of great importance for economical purposes, not to mention the health of man, especially in manufacturing towns; the action of hard water having a perceptible influence in the production of steam and effects on boilers. In breweries, bleaching-grounds, washeries, &c., much depends on the quality of water supplied.

The several purposes for which water is required in a town are—1st, ordinary domestic uses, including drinking, washing of persons, clothes, utensils, houses, yards, watering gardens, &c.; second, manufactures; third, supply of public buildings, baths, wash-houses, &c.; fourth, extinction of fires; fifth, cleansing and watering of streets and thoroughfares; sixth, supplying fountains, public gardens, and pleasure grounds.

In projecting water-works the number of the population is the datum from whence the calculations are made as to the quantity required. The quantity varies in different towns from 30 down to 10 gallons per head per diem—towns possessing trades and manufactures having greater requirements than others. The quantities supplied by the four leading companies of London are as follows:

East London . . . . 100 gals. per house per diem
New River . . . . . 114 " " "
West Middlesex . . 150 " " "
Chelsea . . . . . . 154 " " "

This will average 20 to 30 gallons for each individual.

The cost of water supply in London, amounts to £3 per individual supplied, and in the provincial towns of Great Britain the cost varies from the above down to fifteen shillings, according as the difficulties attending the supply are greater or smaller.

With the above preliminary remarks, I will now proceed to the subject in hand. The site of Dunedin fronting the harbour, as laid out for occupation, extends two and a half miles in length, and five furlongs in breadth, and its surface rises to an altitude of about 350 feet above the sea level. Immediately behind the town a ridge runs, whose greatest elevation—Bellevue Hill—is 700 feet, and whose lowest depressions are 404 and 378 feet, the former at Balmaceuen, and the latter at Lookout Point. Rising beyond this ridge, and closing in the town site in a westerly and northerly direction, are a range of hills of the following elevations:

Kaikorai . . . . . . . . . . 1096 feet
Waikari . . . . . . . . . . 2096 "
Cargill . . . . . . . . . . 2255 "
Signal . . . . . . . . . . 1278 "

Out of the ridge immediately behind the town several springs issue, and their waters flow through the town. The three principal of these have been gauged, and their capacity will be noted in Tables A and B hereto appended. From the higher range of hills three streams issue, namely, the North-east Valley stream, the Water of Leith or

*The expense of machinery or capital invested in the arrangements for supplying the metropolis with water—exclusive of the communication-pipes to the houses, the tenants’ water-butts, tanks, &c.—amounts to about £3,810,343, or about £3 per individual supplied.—Dempsey on Water-works.