12
The minimum available supply of the Tytam gathering-ground will therefore be
Millione.
70
45.5 X 15.84 ×
100
40
504-49 millions.
5.9 X 15'84
100
37-20
JJ
Total
541-69 millions.
Or, about one million five hundred thousand gallons a day.
MEANS OF AUGMENTING SUPPLY.
47. Without claiming great accuracy for these calculations, I think it is clear that the supply which the Tytam gathering-ground can afford, may, in some seasons, be very materially reduced, and that it will be desirable, at no distant date, to take steps to obtain a supply from some additional gathering- ground.
48. To some extent this has been already done, by intercepting streams flowing down the hill sides, and by connecting the Pokfulum aqueduct with the Tytam filter-beds. The Pokfulum reservoir is much smaller in proportion to its gathering-ground than the Tytam. Consequently a relatively small rainfall causes it to overflow; and the full supply which the gathering-ground affords could not be utilised if the consumption were limited to the amount that can be filtered and distributed from the Pokfulum works. Now, however, the excess can be carried to the Tytam beds and filtered there, taking the place of water that would otherwise have to be drawn from the Tytam reservoir, and therefore allowing more to be stored than would otherwise be the case.
49. Numerous mountain streams, which give an abundance of water during wet weather, have been partially intercepted and turned into the conduits. The original idea which led to this step being taken, was to enable filtration to be carried on without interruption. It was found that after heavy rain the waters of both reservoirs continued to be so turbid for a long time, as to be unfilterable. The streams, however, became bright within a few hours after the rain ceased. The interception of the streams has not only made filtration possible, but has also made a most important addition to the water supply; and by diminishing the draught on the Tytam reservoir, has increased the amount available for storage. The Director of Public Works informs me that the amount obtainable from both these sources may be materially increased. In 1891, 213 millions of gallons were collected in this manner along the Tytam aqueduct, and 13 millions along the Pokfulum aqueduct in 1892, 228 millions and 13 millions respectively, or rather more than two-thirds of the whole contents of the Tytam reservoir.
:
50. The limit of the additional quantity that can thus be collected, is the full day's consumption during the five wet months. The water intercepted from streams by the conduits will not How back to the reservoirs. So the maximum that can be obtained would be the day's consumption, the whole of
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13
the water being derived in this manner, none being drawn from the reservoir during the whole five months of the rainy season; 450 million gallons, or about 250 millions more than is now taken from the streams, might be in stock in Tytam, if there were room to receive it. There is therefore no doubt that a considerably larger reservoir could be filled.
51. The Tytam dam, as originally designed, was to have been ten feet higher than it is at present. Provision was made, however, for raising it to the full height if experience shewed it to be desirable. Not having the drawings of this reservoir, I cannot say how much additional storage would be obtained. If we take the capacity to vary as the cube of the depth an extra ten feet would give about 100 millions of gallons, which would be a valuable addition to the stock at the commencement of the dry season, and which in 1893 would have averted the necessity for giving an intermittent supply.
52. For the same reason, I cannot calculate the height to which it will be safe to raise the Tytam dam. The Director of Public Works tells me that he has investigated this question, and, to the best of his recollection, he found that the dam may be raised ten feet. I therefore recommend that immediate steps be taken to raise the Tytam dam to the greatest height which calculation shews to be prudent.
53. In a private letter Mr. Danby suggests that an additional supply could be obtained from a new reservoir, which is under construction for Messrs. Butterfield and Swire. The top-water of this reservoir is practically at the same level as that of the Tytam reservoir. Its waters could therefore be carried by a pipe into the Tytam conduit. Its capacity is stated to be 150 millions of gallons, and the gathering-ground 300 acres; roughly one-half of that of the existing Tytam reservoir, in each instance. The daily supply which it will afford may therefore be taken at about one-half of that of Tytam, say, one million gallons. The question therefore arises--How much water can the owners of this dam supply, and what price will they demand for it? I do not, however, consider it desirable that any part of the public water supply should be in private hands. I have, moreover, no information as to the condition of the gathering-ground of this reservoir. On the whole, I am of opinion that the Government would do well to develop to the utmost the supply derivable from the present gathering-ground.
54. In the same letter the construction of two new reservoirs is suggested in the drainage area of the existing Tytam dam above the level thereof. Now the construction of these reservoirs would not increase the total available supply, the gathering-ground remaining the same. They would, however, by increasing the storage, make a greater proportion of the supply available for It is open to doubt whether the total storage which would be afforded by these two reservoirs and by the existing reservoir, increased by raising the water-level, would not be in excess of the requirements of the case. If, how- ever, it appears to be unsafe to raise the existing dam, sufficiently to increase the capacity of the reservoir to the desired extent, then this project merits
use.
attention.
[159430]
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