Art. VI.—On the Probability of a Water Supply being obtained for the City of Auckland from Mount Eden.
[Read before the Auckland Institute, 10th November and 8th December, 1873.]
Auckland, advantageously situated as it is on an isthmus between two fine harbours, the Manukau and the Waitemata, commanding both sides of the island, is rising fast to be a fine city, and will doubtlessly some day be one of the most magnificent in the southern hemisphere. In spite of its many advantages however, it would always remain one of secondary importance if, in its progress, it could not obtain a sufficient supply of water. Happily there is no lack of this needful commodity; perhaps, for the present, it had been better for Auckland if so many sources of supply had not been known to exist, for then the question might have been settled, and the pure element flowing through Auckland, refreshing its inhabitants, purifying its atmosphere by sweeping away all refuse into the sea, saving life and property in the extinguishing of fires, thus adding health and preserving wealth to its citizens.
Had there been only one source of supply, probably that would have been in Auckland by now, as the only delay seems to be caused by not knowing which source to choose, which, after all, is a purely financial question. The various sources are known to be good and abundant, therefore all that remains is to find out which can be most cheaply brought into this town. The Nihotupu gravitation scheme would yield more water than is required at present. The western and Onehunga springs would yield, by pumping, much more than is wanted. Lake Takapuna, North Shore, has also been talked of as a likely source, but the cost of the engineering works requisite for bringing the water over or under the Waitemata would be a sufficient hindrance for that source to be entertained at present.
All these sources, excepting the Nihotupu, arise from the volcanic formation at and adjacent to Auckland, yet this city may at any time be scourged by a pestilence or burnt to the ground for want of an available and sufficient supply of water. These varied schemes have been from time to time propounded by their supporters; it is not the intention of this paper to enter into their various merits, but to bring under notice another scheme which may prove to be as good, and which is close to Auckland, namely, from the scoria and lava beds of Mount Eden.
We all know that in the vicinity of Auckland there is a vast tract of volcanic country, consisting of extinct volcanoes, tuff cones, and lava streams, extending over twenty or thirty square miles. Almost the entire rainfall over this large tract of country is being stored by Nature in the porous lava rocks,
and being served out again through the many springs occurring on the road to the Whau (the Western Springs), at Onehunga beach, Lake Takapuna, and other places. These springs are merely the overflow of what is a natural subterranean reservoir. To prove this, one must consider how volcanic rocks occur, especially those at Mount Eden and its vicinity, and their physical construction.
Dr. Hochstetter remarks about these volcanoes that “the first outbursts, as a closer observation shows, were probably submarine; they took place at the bottom of a shallow, muddy bay, little exposed to waves and wind, and consisted of flowing mud mixed with loose masses, such as fragments of sandstone and shale, lava debris, cinders, and scoria (lapilli), which now form beds of volcanic agglomerate or tuff. The eruptions occurred, no doubt, at intervals, for in this manner alone can the fact be accounted for that the ejected material has been deposited round the point of eruption in layers one above the other, forming low hills gradually rising, and with a circular basin or dish-shaped crater in the middle; a cross section presents clearly the different layers which usually slope inwards towards the bottom of the crater, as well as outwards down the sides.” Further on, he says:—“A complete volcanic system accordingly consists of three parts: a tuff cone the base and pedestal of the whole frame, a lava cone, the chief mass of the mountain, and a scoria or cinder cone forming the top, with the crater.”
These violent outbursts and ejection of such large quantities of scoria and lava would undoubtedly produce very important effects on the surrounding country and on the rocks immediately below and through which the eruptions occurred. Before a vent could have been made through the earth's crust, it must have been somewhat upheaved and cracked to emit the molten materials below, which, as they ascended, would have enabled the crust to subside, and this it would continue to do as long as material was ejected, for the earth's crust, by its own weight and that of the piled-up scoria and lava above, would necessarily sink down and occupy, in a measure, the place of the ejected materials. Thus, below a volcanic mountain of any considerable extent, there must be a basin-like depression immediately beneath capable of holding a large quantity of water, which, combined with the water in the mountain above, retained in it as if it were a large sponge, may probably be made available for a water supply. But as the quantity in the basin and above it may be inadequate for a large supply, we must consider whether this may be helped by the adjacent waters stored up all around. An inspection of lava and scoria beds prove that they are able to hold a large amount of water from their porosity, dependent upon the amount of resistance to dam the waters back.
Before it can be decided whether a sufficient quantity of water could be
obtained from Mount Eden, it will be necessary to prove the depth of the basin or floor of the volcano, its shape, height of the rim of the basin at its lowest part, probable direction of the flow of water from it, summer level (minimum) of water, and inclination of the water from the rim of the basin to its outlet at the springs.
A part of these questions may be answered by a survey of the ground, and by obtaining levels—that is, by surface examination; and the entire question would be solved by adding to the above a series of bores. A survey of the ground would determine the probable shape of the rim of the basin, also the course of the water travelling beyond the rim. The lava streams having run in the old valleys formed by the tertiary rocks, indications may yet be obtained of the course of the bottom of that valley by the run of the lava and other indications, such as the outcropping tertiary formation; and, as the overlaying lava rocks are of a very porous nature and abounding in large cracks and cavities, any water pouring from the hills above (after rain) would undoubtedly flow through the ancient valley, but would occupy more space, have a greater inclination, and would take more time to flow than were it unimpeded.
Having completed the survey, it will then be advisable to put down a series of bores between the site of the reservoir at Gilfillan's corner and Mount Eden. These bores, put down between the limit of the scoria and half way to the centre of the hill, would probably be sufficient to give the depth and shape of the basin. This will be important to ascertain accurately, as on that side the necessary shaft would be sunk, and a correct knowledge alone could be a guide as to the depth of shaft and distance of a drive to tap the water. Then, at a point to the westward of the hill facing the lava flow, another series of bores should be started, beginning at about half-way down the hill, over the rim of the basin, and thence along the probable course of the subterranean stream to its outlet at the springs. These bores would determine the depth of the basin, the permanent level of water, and the inclination of the water towards the springs. Having all these data, it can now be calculated, without much trouble, what available water there is; and by the height of the rim of the basin may be determined how much extra supply can be obtained from the outward flowing stream; for the lower the rim is, the better will it be, as then, by pumping at Mount Eden, any insufficiency of water in the basin would be supplied by infiltration. Having arrived at this stage, it can be positively ascertained if there be an adequate supply or not; and, if there be, a shaft will have to be sunk, and pumping machinery erected for supplying the reservoir.
There are two available sites for a shaft, one being at the proposed reservoir. If this be fixed upon, the shaft should be sunk to the level of the bottom of the basin under Mount Eden, unless that exceeds high-water mark,
in which case the shaft need not be deeper; from the bottom of the shaft a drive should be put in towards Mount Eden, till the lava or scoria beds are reached, when an abundant supply of water would be sure to be met with. This, flowing through the drive to the bottom of the shaft, would be pumped up into the reservoir. By this method no piping would be required for transmission of water to the reservoir, beyond that necessary for the pumping mains.
The other site, and perhaps the better one, would be by the side of the proposed railway to Riverhead, at the bottom of the valley between the proposed site of the reservoir and Mount Eden. The advantages here would be, that the railway would be available for carriage of machinery and other material necessary for the erection of works, and coals would be cheaply conveyed for boiler purposes. The depth of this shaft would be less, and the drive towards the basin would be materially shortened, which latter advantages alone would compensate for the pipes which will be necessary to convey the water to the reservoir. The shaft, in this case, would probably penetrate a layer or two of lava: it will be advisable to avoid it as much as possible, to save cost, and sink on the verge of the lava, and thus get all the shafting and driving through the soft tertiary rocks.
That water may be obtained from Mount Eden is already proved, independently of theory, by the success of Mr Seccombe's well, which supplies his brewery on the Kyber Pass Road. This well is only a moderate depth down.
There is yet another point to be touched upon, and that is, the volcanic cracks in the earth's crust, which must necessarily exist with a series of volcanoes such as occur here; for it is probable that, after the first outburst, other volcanoes started along the cracks, and the number kept increasing till the number of vents created were sufficient for the emission of the pent up gases and molten lava. Perhaps it is owing to the large number of volcanic centres that have existed near Auckland, that they have been so short-lived, and that none of them are now active—many have been the fires, but they have burnt themselves out the sooner. That the cracks existing between these craters are capable of acting as water channels is proved by the existence of Lake Takapuna (an old crater), North Shore; for how otherwise can this lake be supplied with water than from its connection with other volcanic centres? To test this, a drive should be started from the pumping shaft at right angles to a line between two craters; this would be sure to cut the connecting channel between them, and drain them of their waters and others connected with them, and who knows but we may yet bring Lake Takapuna waters into Auckland by this means.
The advantage of using these volcanic waters (if we may term them thus),
provided always that experiments have satisfactorily proved them to be available; the cheapness with which they can be utilised, being so close to the proposed reservoir, and the waters being so pure, there will be no necessity for settling-tanks or filters.
Having shown that Mount Eden contains a large quantity of water, and sketched a scheme whereby the same might be proved and rendered available for supplying Auckland, I will now make a few remarks as to its sufficiency and probable cost.
Supposing that the basin under Mount Eden would draw its supply of water from an area of about five square miles, and accepting twenty—four inches of rainfall as available, it will give the large yield of four million five hundred thousand gallons per day; or, reducing the yield to one—fourth of that quantity to allow for any over-estimate of the area of the gathering ground—it being impossible at present, without boring and other investigations, to determine the exact area of supply—there would still be left over one million gallons per day, which would be more than sufficient to supply thirty-three thousand inhabitants with thirty gallons daily per head. These results depend entirely upon the depth and circumference of the basin, which, when ascertained, will give reliabledata.
It seems natural that by pumping at the centre of such a supply, before it had time to distribute itself, the full amount of rainfall percolating through the gathering area may be raised, and a larger quantity could be obtained than from a similar area at the Western Springs, or from those at Onehunga, where only comparatively small quantities flowing in particular directions can be used, the natural outlets being numerous.
The cost of such a scheme would be less than one from Onehunga or from the Western Springs, as not only would a great saving be effected in transit pipes, but also in cost of pumping, as the water might be obtained at a considerably higher level than at either of the above-mentioned places. By examining the level of the outflow of the water at the Western Springs and the water standing in a well sunk by Mr.Edgecombe—the distance between these two places being about a quarter of a mile—it will be found that the latter level is twenty feet above the former, which would give a rise to the centre of Mount Eden of about one hundred feet. This evidence is further corroborated by the large quantity of water obtained at a high level in the well of the Northern Brewery, on the Kyber Pass Road.
The height to which the water would have to be lifted would be under two hundred feet, to a reservoir at Gilfillan's corner, which point is nearly three hundred feet above the sea level.
The cost of plant capable of raising a million gallons daily will be as follows:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Engines, boilers, engine-house, workmen's cottages||£10,000|
|Mains (including laying), 70 tons at £15 10s.||1,085|
|Shaft, 100 feet||500|
|Air cocks, check valves, etc.||250|
|Contingencies, 10 per cent.||1,583|
|The yearly cost will be:|
|Wages, coals, oil, etc.||£1,860|
|Interest, at 8 per cent.||1,393|
|Cost per million gallons||£8 18s. 3d.|