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The Cyclopedia of New Zealand [Wellington Provincial District]
Water Supply
Water Supply.
The water supplies of the City are from two opposite sources, and were, of course, secured at different dates. That commonly known as the Karori supply, taps the Kaiwarra stream at a distance of two miles south from the General Post-office, and at an elevation of 460 feet above the lowest level of the City. The reservoir covers an area of about twelve acres, and its holding capacity is 98,000,000 gallons. When this reservoir was constructed it was deemed advisable to lead the water some thirty chains to a distributing basin in Polhill Gully, by means of pipes through a tunnel. The elevation of this concrete basin is 310 feet, and its capacity is 500,000 gallons. From thence the water was led into the city by ten-inch, eight-inch, seven-inch, and five-inch mains, and distributed through the various streets by means of smaller pipes. This supply was obtained in 1877, and for a few years it was fairly efficient. As the consumption of the city increased, however, and the higher levels began to be utilised for residential purposes, the supply failed deplorably. It had cost the City £80,000, and yet the fact had to be faced that the volume of the Kaiwarra stream was quite inadequate to the requirements.
In 1881 Mr. George Fisher was first elected to the chair of the Council, and to him is largely due the credit of securing the second or Wainui-o-mata supply. This is sixteen or seventeen miles away, in a north-easterly direction. The reservoir is at an altitude of 395 feet, its area about eight acres, and its capacity 75,000,000 gallons. Only a portion of this quantity can, however, be drawn off, as the discharge is only some six feet below the level of the overflow, the remaining twenty-five feet of the depth being provided for the purposes of a settling-pond. The volume of the Wainui-o-mata river has never been known to fall below three millions of gallons per day, so it was confidently supposed that this second supply would be equal to all demands for many years; and the Karori supply was reserved for use in cases where the bursting of a pipe or other contingency necessitated the shutting off of the main supply. The water from the Wainui reservoir is led through a concrete-covered race more than a mile long into a concrete well about sixty feet deep, and thence to the city by pipes of thirty inches in diameter, reducing, after a few miles, to twenty-four inches, and then, on the City boundary, to twenty-one inches.
For a few years this supply really was sufficient; but as building on the higher levels became more general, complaints were made about the continuous absence of pressure and occasional absence of even the power to flow in these parts. The trouble did not arise because the elevation of the Wainui reservoir was insufficient, but because the capacity of the pipes connecting therewith was too small to meet the demands made upon them. Even on Sundays, when the water-engines were standing, there was little improvement. For the time being, and for some years past, the difficulty has been surmounted by cutting off the distributing basin and connecting the city mains direct with the Karori reservoir. The mains are fed principally from Wainui; but the Karori water is allowed to flow in at the opposite end of the city to make up the deficiency between the capacity of the Wainui pipes and the demands of the City for water. If the demand were to cease entirely for a few hours there would be a pressure at the low levels of 171 pounds to the square inch from the Wainui service; and if this were cut off and connection allowed only with the Karori reserve, there would be a pressure of a fraction under 200 pounds. Yet the demand is so great that the full strength of the Wainui service needs to be aided very largely by the Karori reserve to maintain a pressure of 120 pounds at the Engineer's room (upstairs) of the Corporation offices. That is the pressure aimed at, as it is sufficient to force the water to the higher levels at present utilized, namely, about 300 feet above sea level. Should higher levels be utilized, or the consumption at present levels be largely increased, the full strength of both services will be insufficient. When that time arrives, as assuredly it will, larger or additional pipes will be needed to convey an increased supply from Wainui; and to prepare for the increased consumption larger reservoirs further up the stream would be needed. When that is done it will be a convenient time for the provision of filter beds.
On the whole, the supply must be considered excellent, and it is fortunate that in case of a temporary break-down of either service, the other can be utilised to fill up the gap.